Annika Borgenstam

DICTRA, a tool for simulation of diffusional transformations in alloys

In the present paper, a general survey of the diffusion-controlled transformations (DICTRA) software is given. DICTRA is an engineering tool for diffusion simulations in multicomponent alloys. The simulations are based on multicomponent diffusion and thermodynamic data, both obtained by analyzing and assessing experimental information. This allows for many different cases to be studied as soon as the underlying data are available. DICTRA is not a complete simulation tool because only geometries that can be transformed into one space variable can be treated, but many well posed problems of practical interest may be solved. The program contains several different models, which are discussed in the present paper. Each model has its own applications and several examples from recent simulations are given in order to demonstrate the usage of the particular models.

Activation energy for isothermal martensite in ferrous alloys

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. ...

Driving force for f.c.c. → b.c.c. martensites in FeX alloys

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.

Direct Observation that Bainite can Grow Below M-S

In situ simultaneous synchrotron X-ray diffraction and laser scanning confocal microscopy have confirmed that bainite in steels can grow below the martensite start temperature. This observation suggests that the formation curves for bainite in time-temperature-transformation diagrams should be extended below the martensite start temperature. Furthermore, the implication of this observation on the growth mechanism of bainitic ferrite is discussed.

Deformation Microstructure and Deformation-Induced Martensite in Austenitic Fe-Cr-Ni Alloys Depending on Stacking Fault Energy

The deformation microstructure of austenitic Fe-18Cr-(10-12)Ni (wt pct) alloys with low stacking fault energies, estimated by first-principles calculations, was investigated after cold rolling. The ɛ-martensite was found to play a key role in the nucleation of α′-martensite, and at low SFE, ɛ formation is frequent and facilitates nucleation of α′ at individual shear bands, whereas shear band intersections become the dominant nucleation sites for α′ when SFE increases and mechanical twinning becomes frequent.

Thermodynamically Based Prediction of the Martensite Start Temperature for Commercial Steels

A thermodynamic method for predicting the martensite start temperature of commercial steels is developed. It is based mainly on information on Ms from binary Fe-X systems obtained from experiments with very rapid cooling, and Ms values for lath and plate martensite are treated separately. Comparison with the experimental Ms of several sets of commercial steels indicates that the predictive ability is comparable to models based on experimental information of Ms from commercial steels.

Nucleation of isothermal martensite

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.

Critical temperature for growth of martensite

Ms may be defined as the temperature below which the formation of martensite starts upon cooling. It may also be useful to define Mg, the temperature below which martensite can grow if it is alr ...

Phase-field modelling of martensitic transformation : the effects of grain and twin boundaries

In this work, we present the non-linear elasto-plastic phase-field model and simulation of the martensitic transformation in a polycrystalline material including the effects of grain and twin bound ...

Some remarks on the nucleation and growth of martensite

Abstract It is generally agreed that the difficult step in the formation of isothermal as well as athermal martensite is the nucleation since the growth occurs at a high speed. Previously the difference between M s and M g , the temperature below which martensite can grow if it is already nucleated, was determined for an Fe-1.62%C alloy. The difference was surprisingly small indicating that the difference in driving force for nucleation and growth is small. This experimental result is here compared with results on both athermal and isothermal martensite. A discussion about the critical step for the formation of lath and plate martensite formed by rapid continuous cooling is also given.