Martensite Nucleation Research Articles

On the nucleation of martensite

On the nucleation of martensite

The effect of high pressure on the martensitic reaction in iron-carbon alloys

The effect of a hydrostatic pressure of 42 kb on the kinetics of the athermal transformation of austenite to martensite have been investigated for high-purity iron-carbon alloys containing 0.3–1.23 wt.% carbon. At this pressure, depending on the composition, the martensite start temperature ( M s ) is depressed 150–200°C below that at atmospheric pressure. The measured depression of M s is in agreement with that calculated from a consideration of the effect of pressure on the thermodynamics of the martensitic transformation. The implications of these observations to the theory of martensitic nucleation is discussed. At high pressure, in contrast with the behavior at atmospheric pressure, the extent of the transformation as a function of the temperature of cooling below M s is dependent on composition. In addition, there is some evidence that high pressure reduces the amount of transformation attainable in the alloys of higher carbon content.

The mode and morphology of massive transformations in CuGa, CuZn, CuZnGa and CuGaGe alloys

This paper reports on a study of the mode and morphology of massive transformations in the β phases of CuGa, CuZnGa, CuGaGe and CuZn alloys. Metallographic and X-ray work show that in extreme cases massive phases form by a very rapid growth of incoherent grain boundaries, and possess no crystallographic relationship with respect to the parent phase. In less extreme cases partially coherent nucleation, resembling martensitic nucleation, is probably taking place. The relationship between massive-type transformations and nucleation-and-growth, and martensitic transformations is discussed.

The formation of stacking faults in austenite and its relation to martensite

The occurrence of stacking faults (SFs) in FCC austenite has been investigated by X-ray and metallographie techniques. Alloys in the austenitic field of the systems FeNi, FeNiCr, FeNiC and FeMnC were examined for SFs after quenching and after deformation at various temperatures. An assessment of the susceptibility to faulting of the various austenites in these systems as well as in the systems FeMn and FeCrC leads to the conclusion that Cr and Mn lower the SF energy of austenite appreciably and Ni less so; C was difficult to evaluate. Brief considerations on the nucleation of (BCC) martensite suggest that SFs are not likely to be a source of nuclei.

Martensite Nucleation in Substitutional Iron Alloys

Nucleation theory is applied to martensite nucleation in substitutional iron alloys. Composition fluctuarions are neglected, and a steady rate of nucleation is predicted for any composition and temperature. The maximum rate of nucleation (as a function of temperature) is shown to be measurable only for an extremefy narrow composition range, being too great or too small outside this range. A number of experimental observations, including completely isothermal transformation in some alloys and composition-dependence of M 8 temperature in others, are compared with the calculated behavior.

Influence of stress on martensite nucleation

The stresses produced by plastic deformation and by prior transformation are known to stimulate the transformation of austenite to martensite. Recent experiments have also revealed the quantitative effect of homogeneous tensile and compressive stresses. In this report the previously developed theory of martensite nucleation is extended to include the influence of external stresses. It is found that there is an effect both due to the volume change in the transformation and due to the shear associated therewith. Both effects combine additively to alter the change in volume free energy associated with the transformation. Quantitative predictions of the influence of simple tensile and compressive stresses upon the temperature at which the martensite transformation begins in carbon steels are in, reasonable agreement with the experiments of Kulin, Cohen, and Averbach. The importance of stresses produced by plastic deformation, by prior transformation and by the presence of a few dislocations in annealed austenite is discussed qualitatively.

Isothermal Mode of the Martensitic Transformation

The isothermal formation of martensite in a 71 pet Fe# 29 pet Ni alloy is found to take place mainly by the nucleation of new plates rather than by the growth of existing ones# and is dependent on the temperature and time of the isothermal hold, the amount of athermal martensite present, the state of internal strain, and the application of tensile stress. The conclusion is reached that the isothermal nucleation is activated by thermal fluctuations superimposed on localized regions of very high strain. The reaction-path concept, involving martensite nucleation at strain embryos by athermal activation on cooling, or by thermal fluctuations on holding, re-conciles the athermal and isothermal modes of transformation, and offers a unified picture of the kinetics of martensite formation.

Habit Phenomenon in the Martensitic Transformation

The martensite reaction in single crystals and polycrystals of 70 pct Fe-30 pct Ni alloys, is shown to be autocatalytic in nature, producing bursts of transformation during cooling. The temperature of the first burst of transformation, called M b , occurs below M s in these alloys. Experiments were devised to test the athermal embryo and strain embryo theories of martensite nucleation. The results indicate that internal strains, either within the virgin austenite or around existing martensitic plates, control the nucleation process in these alloys. Furthermore, the growth of martensitic plates is not limited by the attainment of an elastic balance with the austenitic matrix, but by the occurrence of plastic deformation at the martensite boundaries which interferes with the propagation mechanism.