Martensitic transformation under multiaxial deformation

Martensitic transformation upon deformation results in unique effects on the mechanical properties of metals, including transformation-induced plasticity and superelasticity. These properties have drawn significant technological interest and therefore phase transforming materials are nowadays used in various applications. During service such materials are often subjected to multiaxial loading conditions and strain path changes. It is however unclear how the martensitic transformation is affected under such complex loading conditions, preventing further optimization of such materials.

Within the framework of an advanced ERC grant MULTIAX we have investigated the link between martensitic transformation, microstructural evolution and the mechanical behavior under multiaxial deformation for a commercial NiTi alloy by a unique combination in situ Digital Image Correlation (DIC), in situ X-ray diffraction and electron microscopy characterization. Transformation is observed to occur within nanoscaled subgrains in a collective way. Furthermore a reverse transformation from the martensite phase to the austenite phase is observed when changing the strain path from a uniaxial stress state to an equibiaxial stress state. Finally, it was shown that different martensite variants appear depending on the loading direction.