| 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. |
- 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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| 6. |
- 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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| 8. |
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| 9. |
- Li, XZ, et al.
(författare)
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HREM study and structure modeling of the η′ phase, the hardening precipitates in commercial Al-Zn-Mg alloys
- 1999
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Ingår i: Acta Materialia. - 1359-6454. ; 47:9, s. 2651-2659
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Tidskriftsartikel (refereegranskat)abstract
- The structure of the η′ phase, one of the most important age-hardening precipitates in commercial Al-Zn-Mg alloys, has been studied at the atomic level by means of high-resolution electron microscopy (HREM). A structural model of the η′ phase has been constructed on the basis of the structural characteristics in the observed images and the structure of the η-MgZn2 phase. Image simulation of this model shows a good agreement between calculated and experimental images. Comparison of this model with the early existing model on the basis of the X-ray diffraction is also given.
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| 10. |
- Korzhavyi, PA, et al.
(författare)
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Theoretical investigation of sulfur solubility in pure copper and dilute copper-based alloys
- 1999
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Ingår i: ACTA MATERIALIA. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454. ; 47:5, s. 1417-1424
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- Dilute Cu-based alloys containing S, P, and Ag impurities and also vacancies are studied theoretically on the basis of total energy calculations. This is done within the supercell approach by using the locally self-consistent Green's function (LSGF) metho
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