On the effects on concentration gradients on martensitic transformations in NiTi shape memory alloys

The phase transformation behavior of NiTi shape memory alloys (SMAs) strongly depends on microstructures. It has been documented on various occasions in the literature that point defects, dislocations, internal interfaces, and precipitates affect elementary processes of the martensitic transformation and thus functional properties of the material. In the present work, binary NiTi diffusion couples with different concentration profiles/gradients were subjected to in-situ cooling in a scanning electron microscope (SEM). The experiments allowed to directly monitor the propagation of martensite transformation fronts into microstructural regions with higher Ni-concentrations / lower transformation temperatures. As a striking result, we could observe that a higher degree of undercooling is required to transform the graded material as one would expect for chemically homogenous material states with identical (local) composition. Our data allow to derive that a direct correlation exists between the steepness of the concentration profile and the thermal driving force which is needed to push the transformation front forward, towards a region where the low temperature phase is thermodynamically unstable. These findings can be interpreted such that chemical gradients impose “chemical constraints” which limit the available space for twinning accommodation and thus result in higher barriers for the transformation. The observed effect is relevant for various cases where chemical compositions locally change in SMAs, e.g. for precipitates which are surrounded by matrix regions with chemical gradients.