The formation of plate martensite in a Fe-High Ni alloy: Crystallography and Variant Selection

Mainly two different morphologies of martensite can be obtained in steels depending on the amount of alloying elements. The first morphology, referred to as lath martensite, forms in low alloy, low carbon steels. It is, by far, the most extensively studied form of martensite due to its industrial applications. The second morphology of martensite, referred to as plate martensite, forms in highly alloyed and in high carbon steels and in particular in Fe-High Ni alloys. In this case, the transformation product is disc shaped and internally twinned. This morphology is the only form of martensite that has the potential to exhibit shape memory properties. It is therefore of great interest to understand the mechanisms of its formation. This is investigated in the present dissertation through the study of the martensitic transformation occurring in a Fe-30.5%Ni-0.155%C alloy. More precisely, the influence of stress and grain size on the crystallography of plate martensite is discussed in the general framework of the phenomenological theory of martensite crystallography. This theory allows associating a unique shape deformation to each orientational variant. In this way, the experimental observations carried out at different length scales by means of optical microscopy, EBSD and TEM can be used to infer the transformation path followed under different conditions. Firstly, the burst configurations of variants observed in coarse-grained austenite under stress free conditions are rationalized by considering the mechanical couplings between the variants. It is shown that self-accommodating and autocatalytic couplings are responsible for the formation of hierarchical configurations of variants. More precisely, the transformation is shown to occur through the alternate formation of perpendicular plate groups of variants. Self-accommodation is the dominant coupling between variants of the same plate group while autocatalytic couplings are responsible for the transfer of the transformation from one generation to the next. It is suggested that the plastic accommodation of the shape deformation plays a dominant role in propagating the transformation to a lower length scales. Secondly, the influence of a uniaxial stress state on the transformation is studied. It is seen experimentally that only the most favoured variants are systematically formed in coarse Cube grains while coarse non-Cube grains generally transform into plate groups of variants that are only moderately favoured by the stress. These observations are well explained by considering the interaction energy between the applied stress and the shape deformation associated with the transformation. Thirdly, the influence of the austenitic grain size on the transformation is also studied. A decrease in grain size is seen to decrease the martensite start temperature. For a grain size below about 10µm, the thermal transformation in liquid nitrogen is indeed suppressed in the present alloy. This observation is related to the increasing yield strength of austenite as the grain size is reduced. A noticeable change in the morphology of martensite also accompanies the decrease in grain size. Indeed, martensite forming in coarse-grained austenite is mostly lens shaped and partially twinned while it appears plate shaped and fully twinned in smaller grains. Furthermore, martensite forming in fine-grained austenite develops self-accommodating configurations suggesting that most of the transformation deformations are elastically accommodated in this case. This is believed to be related to the observance of a shape memory effect in the present alloy in its fine-grained condition.Doctorat en Sciences de l'ingénieur et technologie