Deformation-induced martensitic transformation under multiaxial loading

The deformation-induced martensitic transformation observed in specific materials can enhance their mechanical properties or result in functional properties. For example, the transformation induced plasticity (TRIP) effect results in a good combination of ductility and strength in some stainless steels, and the superelasticity in NiTi alloys enables the materials to experience large deformation yet recover to the original shape. These unique properties have attracted significant technological interests for various structural and functional applications. Since the multiaxial deformation is expected during manufacturing and service of these components, it is crucial to understand the transformation behavior under complex loading conditions in addition to uniaxial loading. This study aims to establish the link between microstructural evolutions and the martensitic transformation induced by multiaxial deformation, by combining cruciform multiaxial mechanical tests with in situ X-ray and neutron diffraction, high-resolution digital image correlation (HRDIC) and electron microscopy characterizations. Several alloys exhibiting martensitic transformations are studied: a nanostructured superelastic NiTi alloy, a coarse-grained NiTi, and metastable austenitic stainless steels 201 and 304.

In the nanostructured superelastic NiTi, the distinctive transformation characteristics of nanoscaled subgrains under different loading directions are revealed, showing the path dependency of the martensitic variant selection. Moreover, it is found that multiaxial mechanical cycling leads to faster degradation in the superelasticity than the uniaxial cycling. The mechanisms for the material degradation, including the dislocation accumulation and the retained martensite, are discussed in terms of the crystallographic orientation and the load path.

In the coarse-grained NiTi, HRDIC captures the activation of high-Schmid-factor martensite variants and the contribution of each variant to strain accommodation under uniaxial loading. In addition to the Schmid law, it is observed that shear transmission across grain boundaries influence the martensite variant selection. A post-mortem transmission electron microscopy (TEM) investigation reveals deformation twinning occurring within the martensite as an additional strain accommodation mechanism.

In austenitic stainless steel 201, uniaxial loading is found to facilitate the martensitic transformation comparing to equibiaxial loading. However, 304 stainless steel exhibits the opposite trend: equibiaxial loading yields more martensite than uniaxial loading. The discrepancy is rationalized by the distinct deformation mechanisms depending on the evolving deformation textures under different load paths as well as the stacking fault energies of the materials. Furthermore, the HRDIC study on 304 steel shows the dependency of the slip activity on the loading direction at the grain level, supporting the nucleation mechanism of martensite proposed in literature.