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 Ni2+xMn1-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 Ni2.26Mn0.74Ga, 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.

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