| 1. |
- Andersson, Magnus, et al.
(författare)
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Unified thermodynamic analysis of the stress-assisted γ→ε martensitic transformation in Fe–Mn–Si alloys
- 1998
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 46:11, s. 3883-3891
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Tidskriftsartikel (refereegranskat)abstract
- A unified thermodynamic analysis has been developed which allows the combined complex effects of applied stresses and alloy composition on the martensite start temperature, Ms(σ), to be explained and predicted. In this unified analysis, a thermodynamic analysis of the effect of applied stress on the relative stability of the high and low temperature phase is combined with existing thermodynamic descriptions of the alloy system. The calculated results are compared with experimental results for the γ→ martensitic transformation in Fe–Mn–Si shape memory alloys. For single crystals, the comparison shows that the influence of applied stresses can be directly predicted with a reasonable accuracy for a wide composition range. By integrating a polycrystallinity parameter in the analysis it is shown that the calculations are also valid for polycrystalline alloys. Application of this analysis to experimental values of Ms(σ) can also generate useful data for future optimizations of the thermodynamic description of several alloy systems.
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| 2. |
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| 3. |
- Borgenstam, Annika, et al.
(författare)
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Activation energy for isothermal martensite in ferrous alloys
- 1997
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 45:2, s. 651-662
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Tidskriftsartikel (refereegranskat)abstract
- The experimental information on isothermal α martensite in ferrous alloys is reviewed. From the kinetics one can clearly distinguish between three groups of alloys yielding isothermal martensite. The first group contains high alloy steels with a low Ms temperature. They form isothermal martensite with a temperature dependence corresponding to a very low activation energy, possibly 7 kJ/mol. The second group contains steels with an appreciable amount of carbon. Its rate of formation of isothermal martensite can be explained by assuming that it is triggered by submicroscopic plates of bainite formed with a rate controlled by carbon diffusion. The third group contains Fe---Ni alloys with up to about 25% Ni. There the temperature dependence corresponds to an activation energy of about 80 kJ/mol. It is proposed that their transformation is related to the transformation causing plateau II in experiments with very rapid cooling, a transformation which has previously been proposed to be related to the formation of bainite.
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| 4. |
- Borgenstam, Annika, et al.
(författare)
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Driving force for f.c.c.→b.c.c. martensites in Fe-X alloys
- 1997
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 45:5, s. 2079-2091
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Tidskriftsartikel (refereegranskat)abstract
- Information on Ms, the starting temperature for formation of martensite, is reviewed and one Ms line each for lath and plate martensite are drawn in a number of Fe-X phase diagrams. A reasonable interpretation of the data indicates the possibility that the distance between the two lines may vary linearly with temperature and be independent of the choice of alloying element. Using thermodynamic descriptions of the binary systems, the driving force for the start of the formation of the two kinds of martensite is calculated from the same interpretation of data. When plotted against temperature the results indicate that the driving force for martensite may not be much affected by solution hardening but may mainly be a function of temperature. For plate martensite it may have a fairly constant value of about 2100 J/mol. For lath martensite it may vary linearly, possibly from 500 J/mol at 800°C to 2100 J/mol at 250°C.
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| 5. |
- Borgenstam, Annika, et al.
(författare)
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Massive transformation in the Fe-Ni system
- 2000
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 48:11, s. 2765-2775
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Tidskriftsartikel (refereegranskat)abstract
- The critical limit for the massive gamma-->alpha transformation in the Fe-Ni system has been measured by isothermal heat treatment of diffusion couples. The position of the alpha/gamma interface at the end of the treatment could be identified but some growth occurred during the quench. Growth is probably hindered at the beginning of the quench by a redistribution of Fe and Ni at the interface during the isothermal treatment. During the quench it often happens that the alpha/gamma interfaces develop jagged shapes and even very fine plates although there should usually be no orientation relationship. The critical limit agrees fairly well with classical results for the formation of equiaxed ferrite from continuous cooling of homogeneous specimens. At 1023 K it coincides with the alpha/alpha + gamma phase boundary. At lower temperatures it moves inside the alpha + gamma two-phase field. The driving force for the massive transformation increases at decreasing temperature and the limiting composition never approaches the T-0 line.
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| 6. |
- Borgenstam, Annika, et al.
(författare)
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Nucleation of isothermal martensite
- 2000
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 48:11, s. 2777-2785
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Tidskriftsartikel (refereegranskat)abstract
- Two methods of evaluating an activation energy for the nucleation of martensite from kinetic information on isothermal martensite are discussed. In both methods the nucleation is described as thermally activated growth of an embryo up to a size where rapid growth can get started. The first method is based on the absolute Value of the rate of reaction at each separate temperature and depends on the neglect of the reverse reaction. The second method is based on the temperature dependence of the rate at low temperatures and includes the reverse reaction and it typically yields a lower activation energy. It can be used to define the region of low net driving force where the reverse reaction, which is neglected in the first method, is important and that method should not be used. A unified description of the nucleation of athermal and isothermal martensite is also presented.
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| 7. |
- Carlsson, S., et al.
(författare)
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On the determination of residual stress and strain fields by sharp indentation testing. Part I : Theoretical and numerical analysis
- 2001
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 49:12, s. 2179-2191
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Tidskriftsartikel (refereegranskat)abstract
- Sharp indentation tests, presently represented by cone and Vickers indenters, are analysed theoretically and numerically in order to explore how equi-biaxial residual stress and strain fields can be determined from the global properties, i.e. the size of the contact area between indenter and material and the hardness. given by such tests. It is shown that the residual strain fields can be accurately correlated with the hardness value while residual stresses are related to the size of the contact area. The latter feature is explained by the Fact that the size of the: contact area is sensitive to elastic effects. The results are summarized in simple closed form relations. well-suited to be used in an experimental situation, and the range of validity for the resulting formulae is discussed. The predictions are compared with corresponding results taken from the literature and good agreement is found. An experimental scheme for determination of residual fields by indentation is also suggested.
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| 8. |
- Carlsson, S., et al.
(författare)
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On the determination of residual stress and strain fields by sharp indentation testing. Part II : Experimental investigation
- 2001
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 49:12, s. 2193-2203
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Tidskriftsartikel (refereegranskat)abstract
- An experimental investigation has been carried out in order to study how residual stress and strain fields can be determined by sharp indentation testing. Aiming at a thorough understanding of the influence from general residual surface stress: and strain fields on global indentation parameters, i.e. hardness and contact area, Vickers indentation tests have been performed on specimens first exposed to standard four-point bend (4PB) or single edge notch bend (SEN(B)) loading. The experimental results have been evaluated based on the findings in a parallel theoretical/numerical investigation and are compared with finite element simulations as well as with corresponding results taken from the literature. Good agreement between experiments and numerical results has been found, particularly in a situation with no or little plastic deformation due to preloading.
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| 9. |
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| 10. |
- Chen, Q., et al.
(författare)
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Calculation of Debye temperature for crystalline structures - A case study on Ti, Zr, and Hf
- 2001
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 49:6, s. 947-961
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Tidskriftsartikel (refereegranskat)abstract
- The methods to calculate the Debye temperature from elastic moduli have been reviewed. The approximation approach due to Moruzzi e al was critically examined by considering experimental elastic constant data for all the cubic elements. It was found that many cubic elements are exceptions with regard to the assumed constant scaling factor for the expression of the average sound velocity in terms of the bulk modulus, and consequently the Debye temperature of a cubic element must be calculated from the knowledge of all the elastic constants of the system. On the other hand, a fairly constant scaling factor has been found to exist fur the hexagonal elements. Through the study of experimental data, some empirical relationships have been observed between the high temperature entropy-Debye temperature theta (D)(0) and the low temperature limit of the Debye temperature theta (D)(-3). For those structures that are dynamically unstable at low temperatures, we proposed a way to obtain their theta (D)(0) from the calculated isotropic bulk moduli. The methods have been applied to calculate the Debye temperatures of hcp, bcc, and fee Ti, Zr, and Hf from their elastic moduli derived from ah initio calculations. The calculated results agree very well with the experimental data.
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| 11. |
- Ekroth, M., et al.
(författare)
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Gradient zones in WC-Ti(C,N)-Co-based cemented carbides : Experimental study and computer simulations
- 2000
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 48:9, s. 2177-2185
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Tidskriftsartikel (refereegranskat)abstract
- The formation of surface zones with a composition gradient during sintering of WC-Ti(C,N)-Co cemented carbides has been studied experimentally and by computer simulations. The microstructure has been investigated with SEM and EPMA. The simulations are based on a solution of the multicomponent diffusion equations, coupled with thermodynamic calculations using thermodynamic descriptions of the individual phases. The results from the simulations are in good agreement with the experimental results, indicating that diffusion and the thermodynamic properties are the two major factors that control the gradient structure formation.
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| 12. |
- Frykholm, R., et al.
(författare)
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A new labyrinth factor for modelling the effect of binder volume fraction on gradient sintering of cemented carbides
- 2003
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 51:4, s. 1115-1121
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Tidskriftsartikel (refereegranskat)abstract
- To increase the cutting performance of WC-MC-Co cemented carbide tools, it is common to use a high temperature CVD process to coat them with thin wear resistant layers. During the process cracks are unavoidably introduced in the coating. To prevent crack propagation it is of interest to create a tough surface zone in the substrate, enriched in WC and binder phase. A way to create such a zone is to sinter a nitrogen-containing cemented carbide in a nitrogen free atmosphere. This formation of gradient structures has been extensively studied using microscopy and simulations, and it has been shown that the process is driven by diffusion in the binder phase. However, the diffusion paths are partly blocked by the dispersed particles. This effect can be formally handled by considering effective diffusivities by introducing a so-called labyrinth factor, lambda. In prior work it has been assumed that lambda =f(2) where f is the volume fraction of the binder. The validity of this assumption has been studied by simulations and experimental analysis of gradient sintered WC-Ti(C,N)-Co cemented carbides containing 5.0, 6.7, 10.0 and 20.0 vol% binder phase. It was found that by using the labyrinth factor lambda = f(2) instead of a better correspondence between experiments and simulations can be achieved.
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| 13. |
- Göthelid, Mats, et al.
(författare)
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Influence of O and Co on the early stages of sintering of Wc-Co : A surface study by AES and STM
- 2000
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 48:17, s. 4357-4362
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Tidskriftsartikel (refereegranskat)abstract
- The influence of oxygen on the sintering behavior of WC-Co has been investigated by Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). Deposition of Co on the WC(0001) surface and subsequent annealing at 650 degreesC results in a 2x2 reconstructed pre-cursor layer on top of which Co grows in weakly bound islands which can be moved on the surface by the STM rip. Annealing at 850 degreesC removes excess Co and leaves only the 2x2 surface. Oxygen exposure of the 2x2 surface results in a clustered cobalt oxide overlayer which on annealing at 750 degreesC breaks up and restores the 2x2 structure as the metallic Co wets the surface.
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| 14. |
- Hillert, Mats
(författare)
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A model-based continuum treatment of ordering and spinodal decomposition
- 2001
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 49:13, s. 2491-2497
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Tidskriftsartikel (refereegranskat)abstract
- Using the absolute reaction rate theory, a continuum treatment of ordering and spinodal decomposition is derived from the original planar discrete lattice model which was based on a Bragg-Williams type of approach. For small amplitudes the result agrees with Cahn's linearized equation for spinodal decomposition. The kinetic equation for ordering has a factor not included in the Allen-Cahn equation which depends on the local degree of order and may have important consequences.
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| 15. |
- Hillert, Mats, et al.
(författare)
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Diffusion-controlled lengthening of Widmanstatten plates
- 2003
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 51:7, s. 2089-2095
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Tidskriftsartikel (refereegranskat)abstract
- Various treatments of the diffusion-controlled lengthening of plates during precipitation from a supersaturated solid solution are compared numerically. Serious doubts are raised about the validity of Trivedi's treatment which has been generally accepted. A recent treatment based directly on Ivantsov's solution gives results in between those of Trivedi's equation and Zener's equation, as modified by Hillert.
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| 16. |
- Hillert, M., et al.
(författare)
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Effect of surface free energy and surface stress on phase equilibria
- 2002
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 50:9, s. 2429-2441
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Tidskriftsartikel (refereegranskat)abstract
- Due to a call for modification of the graphical method of describing phase equilibria for systems with a second-phase inclusion using a common tangent, as previously presented, the thermodynamic basis for such diagrams has now been examined in detail but only for isotropic surface proper-ties. For an incompressible second phase with composition independent surface properties there seems to be no need of modification. For compressible liquids and solids under constant pressure the common tangent construction can still be used but the evaluation of how much the Gibbs energy curve for the second phase should be lifted is more complicated. For spherical inclusions of compressible solids there will be an additional factor containing the compressibility. The equilibrium condition is modified for all cases if the surface properties change with composition and that will cause severe complications for the common tangent construction. There may be a particular composition dependence of the specific surface free energy for an interstitial phase but it can be treated as any other phase. Although the partial volumes appear in the equilibrium conditions for each component, they play only a minor role for the two-phase equilibrium in binary systems. An exception is the equilibrium between a particle and its own vapor.
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| 17. |
- Hillert, Mats, et al.
(författare)
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Modeling of solute drag in the massive phase transformation
- 2000
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 48:2, s. 461-468
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Tidskriftsartikel (refereegranskat)abstract
- The role of solute drag in the massive phase transformation is evaluated through the dissipation of Gibbs energy by diffusion. As an introduction, the solute drag in grain boundary migration is first examined using the wedge-shaped energy function considered by Cahn and by Lucke and Stuwe. The effect of a diffusivity varying from a low value in the bulk to a high value in the center of the boundary is examined. It decreases the solute drag drastically. For the massive phase transformation it is demonstrated how the solute drag increases by the tendency for segregation and by a high diffusivity in the interface but it decreases if the diffusivity is lower than in the parent phase. The most important contribution to solute drag comes from the spike of solute atoms in the parent phase being pushed forward by the advancing interface. The spike is thus an obstacle for growth that must be broken through in order for diffusion-controlled growth to turn partitionless. The possibility of dynamic nucleation is also discussed.
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| 18. |
- Höglund, Lars, et al.
(författare)
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Analysis of the Kirkendall effect, marker migration and pore formation
- 2001
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 49:8, s. 1311-1317
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Tidskriftsartikel (refereegranskat)abstract
- Some aspects of Kirkendall and lattice plane migration are studied by means of numerical simulations. The simulations are performed using individual mobilities assessed in this work and thermodynamic data available in literature. The simulations show that the position and stability of the Kirkendall plane as well as a number of other interesting features can be predicted using numerical simulation software in combination with assessed thermodynamic and kinetic data.
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| 19. |
- Jayaram, Vikram, et al.
(författare)
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Dislocation pile-up model for the yield stress of a composite
- 1999
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 47:5, s. 1635-1643
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Tidskriftsartikel (refereegranskat)abstract
- A numerical model to simulate yielding in a composite is developed for the transmission of slip across a dissimilar interface through the formation of co-planar dislocation arrays in both phases. A pile-up of dislocations in the soft phase is assumed to nucleate dislocations in the hard phase in which movement is dictated by lattice friction stress. The polycrystalline composite yield stress is calculated by determining the equilibrium positions of the dislocation arrays as a function of the length scales, elastic constants and Burgers vectors in the two phases, with particular reference to melt oxidized Al-Al2O3, in which homophase boundaries are absent, and to the commercially important system Co-WC. The hardness values predicted from this model are in good agreement with experimentally measured values in the above systems. The implications of these results for the design of hard composite microstructures are elucidated.
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| 20. |
- Johansson (Moverare), Johan, 1973-, et al.
(författare)
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Evolution of the residual stress state in a duplex stainless steel during loading
- 1999
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 47:9, s. 2669-2684
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Tidskriftsartikel (refereegranskat)abstract
- The evolution of micro- and macrostresses in a duplex stainless steel during loading has been investigated in situ by X-ray diffraction. A 1.5 mm cold-rolled sheet of alloy SAF 2304 solution treated at 1050°C was studied. Owing to differences in the coefficient of thermal expansion between the two phases, compressive residual microstresses were found in the ferritic phase and balancing tensile microstresses in the austenitic phase. The initial microstresses were almost two times higher in the transverse direction compared to the rolling direction. During loading the microstresses increase in the macroscopic elastic regime but start to decrease slightly with increasing load in the macroscopic plastic regime. For instance, the microstresses along the rolling direction in the austenite increase from 60 MPa, at zero applied load, to 110 MPa, at an applied load of 530 MPa. At the applied load of 620 MPa a decrease of the microstress to 90 MPa was observed. During unloading from the plastic regime the microstresses increase by approximately 35 MPa in the direction of applied load but remain constant in the other directions. The initial stress state influences the stress evolution and even after 2.5% plastic strain the main contribution to the microstresses originates from the initial thermal stresses. Finite element simulations show stress variations within one phase and a strong influence of both the elastic and plastic anisotropy of the individual phases on the simulated stress state.
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| 21. |
- Karlsson, Lennart, et al.
(författare)
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The influence of thermal annealing on residual stresses and mechanical properties of arc-evaporated TiCxN1−x (x=0, 0,15 and 0,45) thin films
- 2002
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 50:20, s. 5103-5114
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Tidskriftsartikel (refereegranskat)abstract
- We report the stress relaxation behavior of arc-evaporated TiCxN1−x thin films during isothermal annealing between 350 and 900°C. Films with x=0, 0,15 and 0,45 each having an initial compressive intrinsic stress σint = -5.4 GPa were deposited by varying the substrate bias Vs and the gas composition. Annealing above the deposition temperature leads to a steep decrease in the magnitude of σint to a saturation stress value, which is a function of the annealing temperature. The corresponding apparent activation energies for stress relaxation are Ea=2.4, 2.9, and 3.1 eV, for x=0, 0,15 and 0,45 respectively. TiC0.45N0.55 films with a lower initial stress σint = -3 GPa obtained using a high substrate bias, show a higher activation energy Ea=4.2 eV.In all the films, stress relaxation is accompanied by a decrease in defect density indicated by the decreased width of X-ray diffraction peaks and decreased strain contrast in transmission electron micrographs. Correlation of these results with film hardness and microstructure measurements indicates that the stress relaxation is a result of point-defect annihilation taking place both during short-lived metal-ion surface collision cascades during deposition, and during post-deposition annealing by thermally activated processes. The difference in Ea for the films of the same composition deposited at different Vs suggests the existence of different types of point-defect configurations and recombination mechanisms.
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| 22. |
- Loginova, I., et al.
(författare)
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Phase-field simulations of non-isothermal binary alloy solidification
- 2001
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 49:4, s. 573-581
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Tidskriftsartikel (refereegranskat)abstract
- A phase-field method for two-dimensional simulations of binary alloy solidification is studied. Phase-field equations that involve both temperature and solute redistribution are formulated. The equations are solved using the finite element method with triangular elements on unstructured meshes, which are adapted to the solution. Dendritic growth into a supersaturated melt is simulated for two temperature regimes: (a) the temperature is prescribed on the boundary of the computational domain; and (b) the heat is extracted through the domain boundary at a constant rate. In the former regime the solute redistribution is compared with the one given by an isothermal model. In the latter case the influence of the size of the computational domain and of the heat extraction rate on dendritic structure is investigated. It is shown that at high cooling rates the supersaturation is replaced by thermal undercooling as the driving force for growth.
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| 23. |
- Loginova, I., et al.
(författare)
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The phase-field approach and solute drag modeling of the transition to massive gamma ->alpha transformation in binary Fe-C alloys
- 2003
-
Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 51:5, s. 1327-1339
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Tidskriftsartikel (refereegranskat)abstract
- The transition between diffusion controlled and massive transformation gamma --> alpha in Fe-C alloys is investigated by means of phase-field simulations and thermodynamic functions assessed by the Calphad technique as well as diffusional mobilities available in the literature. A gradual variation in properties over an incoherent interface, having a thickness around 1 nm, is assumed. The phase-field simulations are compared with a newly developed technique to model solute drag during phase transformations. Both approaches show qualitatively the same behavior and predict a transition to a massive transformation at a critical temperature below the T-0 line and close to the alpha/alpha + gamma phase boundary. It is concluded that the quantitative difference between the two predictions stems from different assumptions on how the properties vary across the phase interface yielding a lower dissipation of Gibbs energy by diffusion in the phase-field simulations. The need for more detailed information about the actual variation in interfacial properties is emphasized.
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| 24. |
- Odqvist, J., et al.
(författare)
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A general method for calculating deviation from local equilibrium at phase interfaces
- 2003
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 51:4, s. 1035-1043
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Tidskriftsartikel (refereegranskat)abstract
- A general method to calculate the deviation from local equilibrium at phase interfaces in multicomponent systems is suggested. The deviation is caused by solute drag and finite interfacial mobility. In the limit of low transformation rates the new method degenerates to the well-known local equilibrium condition. The phase interface is divided into three zones, each with a finite thickness. In each zone a variation in thermodynamic properties and diffusional mobilities is assumed.
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| 25. |
- Ohnuma, I., et al.
(författare)
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Phase equilibria in the Fe-Co binary system
- 2002
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 50:2, s. 379-393
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Tidskriftsartikel (refereegranskat)abstract
- alpha (A2)/gamma (A1) phase equilibria of the Fe-Co system between 400 and 800degreesC were determined by means of lattice parameter measurement using thin film specimens. Bulk specimens were also analyzed to compare the extent of attainment to the equilibrium. The thin film technique was found to be greatly advantageous for obtaining the phase equilibria at lower temperatures where solid-state reactions are too slow to reach the equilibrium state in the conventional methods using bulk specimens. It was confirmed that the alpha + gamma two-phase region extends below the temperature at which the alpha (A2)/alpha' (B2) transus meets the alpha/alpha + gamma boundary. Thermodynamic analysis was also conducted by taking the magnetic and chemical ordering contributions of the B2 structure into account, findings of which confirmed the extension of the alpha + gamma two-phase region below the alpha/alpha' ordering temperature.
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| 26. |
- Schwind, M., et al.
(författare)
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A random walk approach to Ostwald ripening
- 2001
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 49:18, s. 3821-3828
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Tidskriftsartikel (refereegranskat)abstract
- We investigate a numerical model of Ostwald ripening in which matter is discretised. The model is based on transport by random walk and certain rules at the phase interfaces which mimic the continuous solution of the diffusion and moving boundary problem. In the limit of low volume fractions the results from our simulations agree quantitatively with the results predicted by the Lifshitz-Slyosov-Wagner theory. In contrast to the LSW model, the volume fraction of particles enters our model as part of the diffusion problem and it may therefore be confidently used to investigate what happens when the volume fraction of particles increases.
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| 27. |
- Schwind, M., et al.
(författare)
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sigma-phase precipitation in stabilized austenitic stainless steels
- 2000
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 48:10, s. 2473-2481
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Tidskriftsartikel (refereegranskat)abstract
- Experimental observations of sigma-phase precipitation in two stabilized austenitic stainless steers, AISI 321 and AISI 347, aged up to 80,000 h at temperatures between 500 and 800 degrees C, are compared with the results obtained from a simple model of the process. The model is based on the assumption that diffusion of alloying elements from inside austenite grains to sigma-phase particles at grain boundaries limits the rate of formation, and this is supported by the presence of zones depleted in chromium at grain boundaries revealed by TEM/EDS. The full multicomponent thermodynamic behaviour of the system is taken into account using the DICTRA software and the influence of grain shape is discussed. The predictions of the model are in fair agreement with experimentally estimated Volume fractions of sigma-phase.
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| 28. |
- Wang, Y D, et al.
(författare)
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Grain-orientation-dependent residual stress and the effect of annealing in cold-rolled stainless steel
- 2002
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 50:7, s. 1717-1734
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Tidskriftsartikel (refereegranskat)abstract
- Cold rolling leads to a residual stress that is dependent not only on the specimen directions but also on the orientation of the grain. Neutron diffraction was used to investigate residual stresses and the effect of annealing in cold-rolled stainless steel, a two-phase material consisting of 62 vol% austenite and the rest deformation-induced martensite. The specimens were prepared by cold rolling of AISI 301 stainless steel with 48% reduction. The grain-orientation-dependent residual stress, or inter-granular stress, was determined by constructing the stress orientation distribution function, a recently developed concept, from the residual strains measured along various crystallographic directions. For the cold-rolled sample, a strong grain orientation anisotropy was observed for residual stresses in both phases. Detailed analysis of the experimental stress and texture data indicates that the observed orientation anisotropy was caused by the selective phase transformation that occurred during cold rolling. Annealing at 500░C leads to recovery, which significantly reduces the orientation anisotropy of the residual stress. The experimental data show that the recovery dynamics in the austenite and martensite phases are quite different. It appears that the overall recovery behavior in this two-phase material is driven by the martensite phase. ⌐ 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.
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| 29. |
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| 30. |
- Wikström, Adam, et al.
(författare)
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Stresses in passivated lines from curvature measurements
- 2000
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 48:10, s. 2429-2434
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Tidskriftsartikel (refereegranskat)abstract
- An exact expression is presented which enables the determination of volume-averaged thermoelastic stresses in passivated lines based on information from curvature measurements. This method, which is Valid for lines of arbitrary in-plane shape, requires knowledge of the material properties of the line and passivation. The sensitivity of the line stress estimates to uncertainties in curvature data for a typical line/passivation geometry is investigated in detail. It is concluded that the present approach is well suited for Al or Cu lines embedded in SiO2 passivation. In the particular case of coinciding shear moduli or Poisson ratios for the line and passivation, it is shown that the method breaks down and only certain linear combinations of the line stress components may be determined.
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| 31. |
- Abdoshahi, Neda, et al.
(författare)
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On the energetics of the cubic-to-hexagonal transformations in TiAl plus Mo alloys
- 2022
-
Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 240
-
Tidskriftsartikel (refereegranskat)abstract
- Diffusionless transformations allow access to metastable phases and enrich the materials design portfolio. They are well suited for atomistic modeling; nonetheless, they are challenging when involving disordered systems or alloys with complex compositions. This work presents a comprehensive study of transforma-tion energetics between bcc and hcp ordered and disordered phases in the TiAl+Mo model alloy system. By employing two complementary techniques I. VASP-SQS, and II. EMTO-CPA, we can show that chemical disorder flattens the energy landscape but may introduce a small barrier. Unlike that, the energetics of ordered phases are barrier-less and hence would suggest a spontaneous transformation. Finally, we show that Mo stabilizes the bcc phases, leading to a barrier-less transformation hcp -+ bcc for both ordered and disordered states when Mo content exceeds ti 12 at.%.
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| 32. |
- Abdoshahi, Neda, et al.
(författare)
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Structural stability and mechanical properties of TiAl plus Mo alloys : A comprehensive ab initio study
- 2021
-
Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 221, s. 117427-
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Tidskriftsartikel (refereegranskat)abstract
- Titanium aluminides are technologically important intermetallic alloys with also many curious properties interesting from a basic research point of view. When alloyed with Mo, several (meta)stable phases have been reported; their properties are, however, largely unknown due to the alloy processing (e.g. fast cooling) and/or non-existence as a single-phase material. Here we employ first principles calculations to study compositional trends in structural and mechanical properties. We could show that Mo increases the density of all studied phases, leads to their chemical destabilization with the exception of the ordered bcc /3o phase, increases their ductility, and enhances the elastic anisotropy. Discrepancies between two employed ab initio methods (special quasi-random structures vs. coherent potential approximation) in the case of the /3o and B19 phases are rationalized with significant local distortions which may eventually facilitate a spontaneous phase transformation. Predictions of ordering temperatures solely based on the configurational entropy do not yield values in the experimentally expected ranges.
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| 33. |
- Abu-Odeh, A., et al.
(författare)
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Efficient exploration of the High Entropy Alloy composition-phase space
- 2018
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Ingår i: Acta Materialia. - : Acta Materialia Inc. - 1359-6454 .- 1873-2453. ; 152, s. 41-57
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Tidskriftsartikel (refereegranskat)abstract
- High Entropy Alloys (HEAs), Multi-principal Component Alloys (MCA), or Compositionally Complex Alloys (CCAs) are alloys that contain multiple principal alloying elements. While many HEAs have been shown to have unique properties, their discovery has been largely done through costly and time-consuming trial-and-error approaches, with only an infinitesimally small fraction of the entire possible composition space having been explored. In this work, the exploration of the HEA composition space is framed as a Continuous Constraint Satisfaction Problem (CCSP) and solved using a novel Constraint Satisfaction Algorithm (CSA) for the rapid and robust exploration of alloy thermodynamic spaces. The algorithm is used to discover regions in the HEA Composition-Temperature space that satisfy desired phase constitution requirements. The algorithm is demonstrated against a new (TCHEA1) CALPHAD HEA thermodynamic database. The database is first validated by comparing phase stability predictions against experiments and then the CSA is deployed and tested against design tasks consisting of identifying not only single phase solid solution regions in ternary, quaternary and quinary composition spaces but also the identification of regions that are likely to yield precipitation-strengthened HEAs.
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| 34. |
- Aghda, Soheil Karimi, et al.
(författare)
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Ion kinetic energy- and ion flux-dependent mechanical properties and thermal stability of (Ti,Al)N thin films
- 2023
-
Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 250
-
Tidskriftsartikel (refereegranskat)abstract
- Ion-irradiation-induced changes in structure, elastic properties, and thermal stability of metastable c-(Ti,Al)N thin films synthesized by high-power pulsed magnetron sputtering (HPPMS) and cathodic arc deposition (CAD) are systematically investigated by experiments and density functional theory (DFT) simulations. While films deposited by HPPMS show a random orientation at ion kinetic energies (Ek)>105 eV, an evolution towards (111) orientation is observed in CAD films for Ek>144 eV. The measured ion energy flux at the growing film surface is 3.3 times larger for CAD compared to HPPMS. Hence, it is inferred that formation of the strong (111) texture in CAD films is caused by the ion flux-and ion energy-induced strain energy minimization in defective c-(Ti,Al)N. The ion energy-dependent elastic modulus can be rationalized by considering the ion energy-and orientation -dependent formation of point defects from DFT predictions: The balancing effects of bombardment-induced Frenkel defects formation and the concurrent evolution of compressive intrinsic stress result in the apparent independence of the elastic modulus from Ek for HPPMS films without preferential orientation. However, an ion energy-dependent elastic modulus reduction of similar to 18% for the CAD films can be understood by considering the 34% higher Frenkel pair concentration formed at Ek=182 eV upon irradiation of the experimentally observed (111)-oriented (Ti,Al)N in comparison to the (200)-configuration at similar Ek. Moreover, the effect of Frenkel pair concentration on the thermal stability of metastable c-(Ti,Al)N is investigated by differential scanning calorimetry: Ion-irradiation-induced increase in Frenkel pairs concentration retards the wurtzite formation temperature by up to 206 degrees C.
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| 35. |
- Aghda, Soheil Karimi, et al.
(författare)
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Valence electron concentration- and N vacancy-induced elasticity in cubic early transition metal nitrides
- 2023
-
Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 255
-
Tidskriftsartikel (refereegranskat)abstract
- Motivated by frequently reported deviations from stoichiometry in cubic transition metal nitride (TMNx) thin films, the effect of N-vacancy concentration on the elastic properties of cubic TiNx, ZrNx, VNx, NbNx, and MoNx (0.72 & LE; x & LE; 1.00) is systematically studied by density functional theory (DFT) calculations. The predictions are validated experimentally for VNx (0.77 & LE; x & LE; 0.97). The DFT results indicate that the elastic behavior of the TMNx depends on both the N-vacancy concentration and the valence electron concentration (VEC) of the transition metal: While TiNx and ZrNx exhibit vacancy-induced reductions in elastic modulus, VNx and NbNx show an increase. These trends can be rationalized by considering vacancy-induced changes in elastic anisotropy and bonding. While introduction of N-vacancies in TiNx results in a significant reduction of elastic modulus along all directions and a lower average bond strength of Ti-N, the vacancy-induced reduction in [001] direction of VNx is overcompensated by the higher stiffness along [011] and [111] directions, resulting in a higher average bond strength of V-N. To validate the predicted vacancy-induced changes in elasticity experimentally, close-to-singlecrystal VNx (0.77 & LE; x & LE; 0.97) are grown on MgO(001) substrates. As the N-content is reduced, the relaxed lattice parameter a0, as probed by X-ray diffraction, decreases from 4.128 & ANGS; to 4.096 & ANGS;. This reduction in lattice parameter is accompanied by an anomalous 11% increase in elastic modulus, as determined by nanoindentation. As the experimental data agree with the predictions, the elasticity enhancement in VNx upon N-vacancy formation can be understood based on the concomitant changes in elastic anisotropy and bonding.
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| 36. |
- Akhtar, Farid, et al.
(författare)
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On the processing, microstructure, mechanical and wear properties of cermet/stainless steel layer composites
- 2007
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 55:4, s. 1467-1477
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Tidskriftsartikel (refereegranskat)abstract
- This study deals with layer composites of carbide reinforcements and stainless steel prepared successfully by powder technology. The layer material consisted of two layers. The top layer consisted of reinforcements (TiC and NbC) and 465 stainless steel as the binder material for the carbides. The bottom layer was entirely of binder material (465 stainless steel). The microstructure of the composite was characterized by scanning electron microscopy. The microstructural study revealed that the top layer (TiC–NbC/465 stainless steel) showed the typical core–rim microstructure of conventional steel bonded cermets and the bottom layer showed the structure of sintered steel. An intermediate layer was found with a gradient microstructure, having a higher carbide content towards the cermet layer and lower carbide content towards the stainless steel layer. The bending strength of the layered material measured in the direction perpendicular to the layer alignment was remarkably high. The variation of strength as a function of the thickness of the bottom layer revealed that the character of the material changed from the cermet, to a layer composite and then towards metallic materials. The wear resistance of the top layer was studied against high speed steel. The wear mechanisms were discussed by means of microscopical observations on the worn surfaces. The wear was severe at higher wear loads and lower TiC content. Microploughing of the stainless steel matrix was found to be the dominant wear mechanism. Heavy microploughing and rapid removal of material from the wear surface was observed at high wear load. The fracture morphologies of the top, bottom and intermediate layers are reported
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| 37. |
- Al-Mukadam, R., et al.
(författare)
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Viscosity of metallic glass-forming liquids based on Zr by fast-scanning calorimetry
- 2021
-
Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 221
-
Tidskriftsartikel (refereegranskat)abstract
- Fast-scanning calorimetry was applied to retrieve the viscosity of supercooled liquids of the Zr-based bulk metallic glasses (BMGs) Vitreloy 105 and AMZ4 for temperatures from standard glass transition down to similar to 0.78T(g)/T. Characteristic temperatures of the glass transition were translated into viscosity values by means of composition-independent shift factors based on the equivalency between structural relaxation and viscous flow. The extended MYEGA model with a fragile term dominant at high-temperatures and a strong term dominant at low-temperatures describes the entire viscous range. The analysis revealed that Vitreloy 105 and AMZ4 are strong liquids for log(10) eta >= 4.9-5.5. In turn, the fragile-to-strong crossover is centred on 0.69T(g)/T for Vitreloy 105 and on 0.66T(g)/T for AMZ4. The extent of the fragile-to-strong transition was found to be larger for Vitreloy 105 than for AMZ4, while their values agreed well with the inverse relation between transition factor and kinetic fragility of the strong regime established for BMG-forming liquids. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 38. |
- Alzweighi, Mossab, et al.
(författare)
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The influence of structural variations on the constitutive response and strain variations in thin fibrous materials
- 2021
-
Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 203
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Tidskriftsartikel (refereegranskat)abstract
- The stochastic variations in the structural properties of thin fiber networks govern to a great extent their mechanical performance. To assess the influence of local structural variability on the local strain and mechanical response of the network, we propose a multiscale approach combining modeling, numerical simulation and experimental measurements. Based on micro-mechanical fiber network simulations, a continuum model describing the response at the mesoscale level is first developed. Experimentally measured spatial fields of thickness, density, fiber orientation and anisotropy are thereafter used as input to a macroscale finite-element model. The latter is used to simulate the impact of spatial variability of each of the studied structural properties. In addition, this work brings novelty by including the influence of the drying condition during the production process on the fiber properties. The proposed approach is experimentally validated by comparison to measured strain fields and uniaxial responses. The results suggest that the spatial variability in density presents the highest impact on the local strain field followed by thickness and fiber orientation. Meanwhile, for the mechanical response, the fiber orientation angle with respect to the drying restraints is the key influencer and its contribution to the anisotropy of the mechanical properties is greater than the contribution of the fiber anisotropy developed during the fiber sheet-making.
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| 39. |
- Ananthanarayanan, Durga, et al.
(författare)
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Phase-field study of IMC growth in Sn-Cu/Cu solder joints including elastoplastic effects
- 2020
-
Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 188, s. 241-258
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Tidskriftsartikel (refereegranskat)abstract
- In this article, we aim to study the problem of the growth of intermetallic phases in solder joints undergoing mechanical deformation, using a phase-field model for multi-phase systems that can treat diffusion, elastic and plastic deformation. A suitable model is formulated and applied to Sn-Cu/Cu lead-free solder joints. The growth of the intermetallic layers during solid-state annealing is simulated for different strain states. We assess the values of stiffness tensors available in literature and perform ab initio calculations to support the selection of reasonable values from literature. We also perform a parametric study with different eigenstrain values and applied strains. We find that there is a significant effect of the considered eigenstrains and applied strains on the growth kinetics of the system and parabolic growth kinetics is followed in cases where the intermetallic layers grow. We thereby establish the importance of strain in the growth of intermetallic layers and the need for more targeted experiments on the role of strain in the reliability of the solder joint.
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| 40. |
- Andersson, Linnéa, et al.
(författare)
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Permeability, pore connectivity and critical pore throat control of expandable polymeric sphere templated macroporous alumina
- 2011
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 59:3, s. 1239-1248
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Tidskriftsartikel (refereegranskat)abstract
- We have regulated the permeability in macroporous alumina materials by manipulating the connectivity of the pore phase and the sizes of the smallest constrictions between connected pores. Templating with particle-coated expandable polymeric spheres (EPS) significantly increased the fraction of isolated pore clusters, and reduced both the sizes and the number of connections with neighboring pores, as determined by three-dimensional evaluation with X-ray micro-computed tomography. The stable particle coating, applied onto the EPS surfaces using polyelectrolyte multilayers, reduced the volume expansion and the coalescence of the EPS at elevated temperatures, which reduced the simulated permeability by as much as two orders of magnitude compared to templating with uncoated EPS in materials of similar porosities. We show that the Katz-Thompson model accurately predicts the permeability for the macroporous alumina materials with porosities of 46-76%. This suggests that the permeability to fluid flow in these materials is governed by the smallest constrictions between connected pores: the critical pore throat diameter.
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| 41. |
- Asp Grönhagen, Klara, et al.
(författare)
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Phase-field simulation of sintering and related phenomena : A vacancy diffusion approach
- 2006
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 54, s. 1241-1248
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Tidskriftsartikel (refereegranskat)abstract
- A phase-field model of sintering and related phenomena in a two-phase system and in a multi-phase system is presented. We consider diffusion of vacancies as the atomic mechanism for redistribution of material and we will use the familiar model of thermal vacancies in a crystal as our energy formulation. The solid material will thus be characterized by a low vacancy content and the surroundings by a very high vacancy content and a very low content of atoms. The surface of the solid body will be characterized by a continuous variation in vacancy content. The temporal development of particles during solid state sintering with effects such as wetting is shown in various simulations.
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| 42. |
- Bakhit, Babak, et al.
(författare)
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Improving the high-temperature oxidation resistance of TiB2 thin films by alloying with Al
- 2020
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Ingår i: Acta Materialia. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454 .- 1873-2453. ; 196, s. 677-689
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Tidskriftsartikel (refereegranskat)abstract
- Refractory transition-metal diborides (TMB2) are candidates for extreme environments due to melting points above 3000 degrees C, excellent hardness, good chemical stability, and thermal and electrical conductivity. However, they typically suffer from rapid high-temperature oxidation. Here, we study the effect of Al addition on the oxidation properties of sputter-deposited TiB2-rich Ti1-xAlxBy thin films and demonstrate that alloying the films with Al significantly increases the oxidation resistance with a slight decrease in hardness. TiB2.4 layers are deposited by dc magnetron sputtering (DCMS) from a TiB2 target, while Ti1-xAlxBy alloy films are grown by hybrid high-power impulse and dc magnetron co-sputtering (Al-HiPIMS/TiB2-DCMS). All as-deposited films exhibit columnar structure. The column boundaries of TiB2.4 are B-rich, while Ti0.68Al0.32B1.35 alloys have Ti-rich columns surrounded by a Ti(1-x)Al(x)By tissue phase which is predominantly Al rich. Air-annealing TiB2.4 at temperatures above 500 degrees C leads to the formation of oxide scales that do not contain B and mostly consist of a rutile-TiO2 (s) phase. The resulting oxidation products are highly porous due to the evaporation of B2O3 (g) phase as well as the coarsening of TiO2 crystallites. This poor oxidation resistance is significantly improved by alloying with Al. While air-annealing at 800 degrees C for 0.5 h results in the formation of an similar to 1900-nm oxide scale on TiB2.4, the thickness of the scale formed on the Ti0.68Al0.32B1.35 alloys is similar to 470 nm. The enhanced oxidation resistance is attributed to the formation of a dense, protective Al-containing oxide scale that considerably decreases the oxygen diffusion rate by suppressing the oxide-crystallites coarsening. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 43. |
- Balakrishnan, A., et al.
(författare)
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Effect of particle size in aggregated and agglomerated ceramic powders
- 2010
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Ingår i: Acta Materialia. - 1359-6454 .- 1873-2453. ; 58:3, s. 802-812
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Tidskriftsartikel (refereegranskat)abstract
- This work describes the compaction of agglomerated and aggregated ceramic powders with special emphasis on the role of primary particle size. Discrete element simulations are used to model weakly bonded agglomerates as well as strongly bonded aggregates. Crushing tests are carried out to obtain the characteristic strength of single agglomerate and aggregate. Microstructure evolution and stress-strain curves indicate that aggregates undergo a brittle to plastic-like transition as particle size decreases below 50 nm. It is shown that agglomerates made of nanoparticles exhibit much greater strength than those made of micron-sized particles, with an approximately inverse linear relationship with primary particle size. Simulation of the uniaxial compaction of a representative volume element of powder demonstrates that adhesive effects are responsible for the difficulty to compact nanopowders and for the heterogeneity of microstructure prior to sintering. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 44. |
- Bate, Pete, et al.
(författare)
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Application of laser-ultrasonics to texture measurements in metal processing
- 2017
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 123, s. 329-336
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Tidskriftsartikel (refereegranskat)abstract
- The paper describes in principle how information about textures can be obtained through the application of laser-ultrasonics (LUS) which can be carried out at elevated temperatures, for example in connection with hot rolling. The benefits from getting a measure of texture in this way are explained together with the proposed methodology which is based on the elastic anisotropic properties of the textured material. Measurements are made using only a single laser pulse and in real-time. Two approaches are presented to modelling the propagation of elastic waves, ray tracing and finite difference modelling. These give consistent results but the latter provides a more complete prediction of the ultrasonic spectrum that can be quantitatively related to measured signals through a cross-correlation procedure. Some experimental results are presented for room temperature measurements on a sheet of interstitial-free steel. Agreement between experimental data and modelling results is good and allows estimation of the 4th order coefficients of the orientation distribution function.
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| 45. |
- Bjurenstedt, Anton, et al.
(författare)
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In-situ study of morphology and growth of primary α-Al(FeMnCr)Si intermetallics in an Al-Si alloy
- 2017
-
Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 130, s. 1-9
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Tidskriftsartikel (refereegranskat)abstract
- Morphology and growth of primary α-Al(FeMnCr)Si intermetallics have been studied in-situ during solidification of a commercial secondary aluminum alloy employing X-radiographic imaging combined with deep-etching. The α-Al(FeMnCr)Si intermetallics were found to nucleate primarily on surface oxides, and the continued growth yielded both rhombic dodecahedrons and elongated rod-like morphologies. Both morphologies were observed as hopper and massive types, where the hopper intermetallics had the higher growth rates. The growth rate, which determines the type, appears to be linked to nucleation frequency; higher nucleation frequency promoted massive types and lower nucleation frequency promoted hopper intermetallics.
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| 46. |
- Boesenberg, Ulrike, et al.
(författare)
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Hydrogen sorption properties of MgH2-LiBH4 composites
- 2007
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Ingår i: Acta Materialia. - : Elsevier BV. - 1873-2453 .- 1359-6454. ; 55:11, s. 3951-3958
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Tidskriftsartikel (refereegranskat)abstract
- A detailed analysis of the reaction mechanism of the reactive hydride composite (RHC) MgH2 + 2LiBH(4) <-> MgB2 + 2LiH + 4H(2) was performed using high-pressure differential scanning calorimetry (HP-DSC) measurements and in situ synchrotron powder X-ray diffraction (XRD) measurements along with kinetic investigations using a Sievert-type apparatus. For the desorption the following two-step reaction has been observed: MgH2 + 2LiBH(4) <-> Mg + 2LiBH(4) + H-2 <-> MgB2 + 2LiH + 4H(2). However, this reaction is kinetically restricted and proceeds only at elevated temperatures. In contrast to the desorption reaction, LiBH4 and MgH2 are found to form simultaneously under fairly moderate conditions of 50 bar hydrogen pressure in the temperature range of 250-300 degrees C. As found in pure light metal hydrides, significant improvement of sorption kinetics is possible if suitable additives are used. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 47. |
- Bonvalet, Manon, et al.
(författare)
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Intragranular nucleation of tetrahedral precipitates and discontinuous precipitation in Cu-5wt%Ag
- 2019
-
Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 164, s. 454-463
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Tidskriftsartikel (refereegranskat)abstract
- Both continuous and discontinuous precipitation is known to occur in CuAg alloys. The precipitation of Ag-rich phase has been experimentally investigated by atom probe tomography and transmission electron microscopy after ageing treatment of Cu-5%wtAg at 440 °C during 30’. Both continuously and discontinuously formed precipitates have been observed. The precipitates located inside the grains exhibit two different faceted shapes: tetrahedral and platelet-shaped precipitates. Dislocations accommodating the high misfit at the interface between the two phases have also been evidenced. Based on these experimental observations, we examine the thermodynamic effect of these dislocations on the nucleation barrier and show that the peculiar shapes are due to the interfacial anisotropy. The appropriate number of misfit dislocations relaxes the elastic stress and lead to energetically favorable precipitates. However, due to the large misfit between the parent and precipitate phases, discontinuous precipitation that is often reported for CuAg alloys can be a lower energetic path to transform the supersaturated solid solution. We suggest that the presence of vacancy clusters may assist intragranular nucleation and decrease the continuous nucleation barrier. We eventually propose qualitative thermodynamic and kinetic justifications accounting for the relative importance of homogeneous and discontinuous precipitation modes as a function of temperature.
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| 48. |
- Bonvalet-Rolland, Manon, et al.
(författare)
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Kinetic theory of nucleation in multicomponent systems : An application of the thermodynamic extremum principle
- 2019
-
Ingår i: Acta Materialia. - : Acta Materialia Inc. - 1359-6454 .- 1873-2453. ; 171, s. 1-7
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Tidskriftsartikel (refereegranskat)abstract
- Nucleation kinetics in a multicomponent supersaturated solid solution is examined. Attachment rate of atoms to a nucleus of a size close to the critical one is determined combining a thermodynamic extremum principle and the Fokker-Planck equation. Two limiting cases are examined; when bulk diffusion controls the nucleation kinetics and when the process is limited by the interfacial mobility. The mixed regime is also treated. Moreover, the growth law in multicomponent alloys is derived in the general case, when both mechanisms are considered. Additionally, the attachment rate is derived, in the classical framework, from a new macroscopic growth equations and the fundamental role of the interfacial mobility is examined. These new general expressions, for the attachment rates and the growth laws, determined either applying the thermodynamic extremum principle or derived from the classical formalism are found to be consistent.
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| 49. |
- Borgenstam, Annika, et al.
(författare)
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Eutectoids with cementite as the major constituent in Fe-C-M alloys
- 2016
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 103, s. 280-289
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Tidskriftsartikel (refereegranskat)abstract
- The addition of a third element to hypereutectoid Fe-C alloys may cause inclusions of a minor constituent in the precipitate of cementite from austenite even if the initial austenite is supersaturated only with cementite. Cementite will often become the major constituent of this kind of microstructure. For Fe-C-Cu alloys this has been explained as the result of precipitation from supersaturated cementite. An alternative mechanism could be that the mixture of cementite and a minor constituent forms by simultaneous and cooperative growth of the two phases, i.e., by a reaction that may be regarded as eutectoid. This mechanism has already been applied to explain the occurrence of eutectoid colonies with cementite as the major constituent and a minor constituent for which there was no supersaturation initially. This phenomenon has been observed in hypereutectoid ternary Fe-C alloys with Al, Mn or Si. The necessary requirements on the ordinary isothermal phase diagram are now examined with a graphical method based on the slopes of tie-lines. It predicts the phenomenon in all cases where it has been observed, including Fe-C-Cu and not in the Fe-C-Ni and Fe-C-Cr systems where it has not been observed. The requirements become more evident when the calculated phase equilibria are plotted as an isothermal phase diagram with the alloy content as a function of the carbon activity instead of carbon content. Finally, a comparison is made with bainite in Fe-C alloys where ferrite is the major and cementite the minor constituent. The same two explanations have been proposed for that case.
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| 50. |
- Borgenstam, Annika, et al.
(författare)
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Metallographic evidence of carbon diffusion in the growth of bainite
- 2009
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Ingår i: Acta Materialia. - : Elsevier BV. - 1359-6454 .- 1873-2453. ; 57:11, s. 3242-3252
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Tidskriftsartikel (refereegranskat)abstract
- There are two paradigms regarding the formation of bainite. One is based on the first stage being rapid, diffusionless growth or acicular ferrite and the subsequent formation of carbide occurring by precipitation from the supersaturated ferrite. All assumption that the first stage occurs as a series of subsequent rapid steps resulting in sub-units plays an important role as an explanation of the not so rapid growth observed macroscopically. The other paradigm is based on the first stage being the formation of acicular ferrite under carbon diffusion and on the subsequent growth of carbide and ferrite side by side. Metallographic observations are presented that support the second paradigm. It is difficult to see how they can be accounted for by the first paradigm, in particular the observation of the shapes of sub-units.
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