Textural ordering in NiTi, Ni-Fe-Ti, and Ni-Mn-Ga shape memory alloys - kinetics of intra- and inter-domain processes

Abstract

A detailed kinetics study of the first-order structural transition in virgin NiTi, Ni₄₇Fe₃Ti₅0, and Ni_2+xMn_1-xGa (x= 0 and 0.26) manifests the varying role of renucleation-driven austenitic growth with the doping-induced disorder and the magnetization state. The austenite transitions were investigated using differential scanning calorimeter (DSC) at heating rates spanning over a decade. They revealed the existence of two Arrhenius processes, with their relative presence, nucleationbarrier energies, and validity-timescales suggesting that both intra- and inter-domain texturalorderings undergo de-structuring. In the stoichiometric Ni₂MnGa, a single low-energy barrier ( ) fast kinetics observed may be attributed wholly to the short-distance textural order-disorder (a near absence of bigger, inter-domain interactions). On the other hand, two distinct Arrhenicities are found in equal strength in NITINOL (NiTi) and Ni₄₇Fe₃Ti₅0, and in unequal proportion in Ni_2.26Mn_0.74Ga, over the full range of temperature scanning rates covered (q= 2.5 to 50°C/min). The relatively fast nucleation-driven growth dominates higher T-scanning rates, with lower barrier activation (qhi) (albeit > , due to a change in the twins’ character). Another kinetics with higher barrier energy (qlo) manifests at slow heatings. The crossover in Ni47Fe3Ti50 is interpreted as increase in the (disorder-induced) A-domain-size dispersion, which also causes a broadening of the transition. Parameters characterizing the kinetics of various specimens are examined; comparisons of the relative energy/time scales of inter- and intra-domain processes made, and their transition/crossover temperature discussed.