Characterization of Stresses and Strains Involved in the Martensitic Phase Transformations

Abstract

Transformation-induced plasticity (TRIP) steels are an example of steels used in the automotive industry, where strain-induced martensitic transformations, associated plasticity and work hardening, enhance both the strength and ductility of the material. This has enhanced both the passenger safety (improved crash-performance) and fuel efficiency as less material is consumed (lighter structure). To gain insights into these strain-induced transformations, it is crucial to understand the impact the applied stress/strain on the martensitic transformation and how the resulting strain fields affect the further deformation and transformations. This PhD dissertation reports a series of experimental measurements on how the applied deformation affects strain-induced transformations, using three techniques, namely: electron backscatter diffraction (EBSD), high-resolution digital image correlation (HRDIC) and in situ neutron diffraction. It is shown here that applied stress favours the formation of strain-induced martensite in certain orientations of austenite. Crystallographic information gathered by EBSD and HRDIC indicates that the formation of martensite is governed by prior slip in the parent austenite. HRDIC measurements showed that strain heterogeneity is found not only between different grains within the microstructure, but even within individual austenite grains, suggesting that input parameters of macro stress strain properties are inadequate for variant selection models. EBSD, HRDIC and neutron diffraction measurements at ambient temperature confirm that the transformation is preceded by plastic deformation of the austenite crystal lattice and subsequent formation of nucleation sites. Here, it was shown that the intensity of those diffraction peaks from austenite grain families most affected by plastic deformation, decreased most due to martensitic transformation. Whereas, at the lower temperature deformation regimes, slip is suppressed, this is not the case. This dissertation illustrates how the above-mentioned techniques may be used to probe material phenomenon at various length scales, stress states and temperature regimes of interest.