Abstract
The phase-transformation behavior of a TiNi shape-memory alloy (SMA) under constraint of a constant strain is experimentally investigated by means of mechanical testing and differential scanning calorimetry (DSC) measurements. It is indicated that the reverse-transformation-temperature span under constraint is much larger than that of the unconstrained state. After an incomplete constrained transformation cycle, a two-stage recovery-stress in the constrained state as well as a two-stage recovery-strain in the unconstrained state emerges upon subsequent heating. This is rationalized on the basis of a mechanism which takes into account the influence of stress on the formation of the austenite and the plastic deformation of martensite and austenite during constrained heating. Most importantly, the constrained reverse and forward transformations corresponding to the redeformation of the oriented martensite and the formation of the stress-induced martensite and thermal martensite, respectively, lead to the subsequent two-stage recovery-strain and recovery-stress characteristics. Both the prestrain level and the constrained heating temperature play important roles in the phase transformations and thermomechanical characteristics of the TiNi SMA.
Published Version
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