Thermomechanical characterization of NiTiCu and NiTi SMA actuators: influence of plastic strains

Abstract

The focus of this study is the thermomechanical characterization and comparison between two different shape memory alloys (SMAs) quantifying the effect of plastic strain on the transformation characteristics of SMA actuators. In this study, the thermomechanical response and transformation characteristics of a NiTiCu and a NiTi SMA are studied as a function of the induced plastic strain for four different loading paths: 1) an elastic-plastic loading of the austenitic phase, 2) a stress-induced martensitic phase transformation, 3) an elastic-detwinningplastic loading of the martensitic phase and 4) thermally-induced phase transformation under a constant applied stress. Each loading path is repeated multiple times, with an incremental change of the total applied strain, to determine the effect of accumulated plastic strain on the phase transformation characteristics of the two SMA material systems. The effects of plastic strain are quantified by measurements of recoverable strain during a thermal cycle under zero applied stress, measurements of heat of transformation from a differential scanning calorimeter and microstructural evaluations.

Aerospace applications of shape memory alloys

“…2 and also shown experimentally in Fig. 4 [13]. Such a path is similar to the one previously described, though in this particular case a constant stress is maintained throughout the thermal cycle.…”

Influence of Dynamic Compression on Phase Transformation of Martensitic NiTi Shape Memory Alloys

“…[12][13][14][15][16][17][18] The characteristic transformation temperatures for the austenite-to-martensite phase transformation as well as the mechanical response can be modified to meet application requirements through (i) thermo-mechanical processing, [19][20][21] (ii) slight variation from the equi-atomic NiTi chemical composition, [22,23] or (iii) addition of alloying elements. [24][25][26][27][28] This phase transformation generally involves an austenitic cubic B2 structure transforming to and from either a martensitic monoclinic B19 or orthorhombic B19¢ structure. Depending on the thermo-mechanical processing, intermediate phase transformations may occur as well; one of the particular notes is the commonly observed R-phase transformation, which involves a slight distortion of the austenitic cubic B2 structure.…”

Transformation-induced plasticity in high-temperature shape memory alloys: a one-dimensional continuum model

“…The coupling of plasticity and transformation in SMAs produces irrecoverable strain and reduces the work output under cyclic loading. The poor stability of SMAs under thermal cyclic loading is experimentally illustrated in [1,2], and the instability during superelasticity is reported in [3,4]. Furthermore, the creep mechanism is usually activated in HTSMAs because the martensitic transformation temperatures lie between 0.3 and 0.5 of HTSMA melting temperatures where visco-plastic behavior in metallic alloys occurs.…”