ABSTRACT
Shape memory alloys occupy a niche position in the domain of advanced materials
that have great relevance to medical, engineering, commercial as well as
miscellaneous applications. The transformation temperatures of the alloys are
immensely important since they decide the specific applications of the alloys in a
field. These temperatures are influenced by a host of factors, including composition
and thermo-mechanical history of the alloy. In fact, due to continued usage also the
transformation temperatures are shifted due to what is known as functional fatigue,
which is caused by the build-up of defects within the alloys during use. The horizon of
usage of shape memory alloys is ever widening. In the present study, therefore, the
effects of aging on the transformation temperatures of NiTi alloys are explored. The
martensitic transformation temperatures of the alloys were measured by differential
scanning calorimetry, while their microstructures were investigated by optical
microscopy. The results show that the aging temperatures as well as the holding
times influence the microstructure, which directly affects the transformation
temperatures. Aging at high temperature induces precipitation, leading to an increase
in the Ti content of the matrix. This in turn caused an increase in the transformation
temperatures. On the other hand, aging at low temperature caused stress relief
leading to a shift in the transformation temperatures.