Effect of randomness on ferroelastic transitions: Disorder-induced hysteresis loop rounding in Ti-Nb-O martensitic alloy

We have studied the effect of disorder on the martensitic transformation (MT), especially on the hysteretic behavior of Ti-Nb-O alloy. This system allows arbitrary tuning of the amount of disorder by the introduction of interstitial oxygen atoms into elastically soft host lattice [$\text{Ti-}26\text{Nb}\text{ }(\text{at}\text{.}\text{ }\mathrm{%})$ alloy], which results in the formation of randomly oriented nanometer-sized martensite as confirmed by dark-field transmission electron microscopy images. In the clean system without interstitial oxygen doping, we observe a sharp hysteresis loop with plateau region in both a stress-strain curve and a strain-temperature curve, whereas a slim hysteresis loop without plateau appears by introducing disorder. The rounding of hysteretic behavior can be understood in random-field Ising model regime, providing the possibility of a new physical picture for MT in a dirty system, where the small size of martensite homogeneously nucleates and percolates the whole system.