Shape memory alloys are intermetallic materials with a unique ability to revert to a predefined physical shape by virtue of diffusionless transformations. Recent interest by aerospace and
automotive industries to exploit the functionalities of these materials in future energy efficient designs has renewed scientific research in this field. However, the current understanding of
transformation hysteresis is inhibited by experimental difficulties associated with viewing the
transformations and therefore most of our knowledge is confined to symptomatic bulk properties
such as those accessible from calorimetry and dilatometry.
In the current study, in situ synchrotron X-ray diffraction (SXRD) was used to accurately
document the adaptability of unit cells of participating phases during transformation in a series
of high temperature shape memory alloys (HTSMAs). Selected alloys based on NiTi, ZrCu
and NiMnGa systems were prepared in vacuum arc melter, homogenized and rolled to grain
size adequate for SXRD experiments. The resulting diffraction patterns were Rietveld refined and the evolution of unit cell parameters of participating phases were recorded as a function of
temperature.
It was observed that the lattices of participating phases undergo a significant overall dilation
during transformation. The lower symmetry martensite unit cell was observed to undergo unprecedented
anisotropic strains, reaching as high as 1.2% in certain alloys. The high temperature
higher symmetry austenite was observed to complement the changes in martensite lattice during
heating and vice versa was observed during cooling. These changes were mostly observed in final stages of transformation. Surprisingly, a negative coefficient of thermal expansion was observed
in the b lattice parameter of the monoclinic martensite in NiTi based alloys. The implications
of such strains on the current phenomenological martensitic transformation models that takes
into consideration lattice parameters away from the transformation regime for prediction of
orientation between participating phases has been discussed
