| 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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