H. Sehitoglu, R. F. Hamilton, C. Efstathiou, University of Illinois, Urbana, IL; Y. I. Chumlyakov, Siberian Physical Technical Institute, Tomsk, Russia; H. J. Maier, University of Paderborn, Paderborn, Germany
The mechanical hysteresis for single crystals of conventional NiTi shape memory alloys and the promising magnetic shape memory alloys, CoNiAl and FeNiGa is studied through rigorous experimentation. We demonstrate the variation in the stress hysteresis with increasing isothermal test temperature for martensitic transformations. The hysteresis was measured in isothermal stress-strain experiments. To further our investigation, we present the strain dependence of the hysteresis for the CoNiAl and NiFeGa alloys under pseudoelastic conditions. We interpret the role of composition for aged NiTi alloys under tensile loads and focus primarily on tension-compression asymmetry for an aged CoNiAl alloy. For the NiFeGa alloys, we compare the aged and unaged conditions as well as tension-compression asymmetry. The evolution of the stress hysteresis for NiTi alloys exhibits distinct composition dependence. The results show that the stress hysteresis expands with increasing test temperature for lower Ni alloys while it contracts for higher Ni alloys. For the CoNiAl alloys, the stress hysteresis grows under tensile and compressive loads; however, the trends for hysteresis behavior display stark tension-compression asymmetry. At low temperatures, pseudoleasticity is absent in tension while it is prevalent in compression. Stress state and heat treatment significantly influence the hysteresis behavior for the NiFeGa alloys. The hysteresis behavior is rationalized considering dissipation of elastic strain energy due to relaxation of coherency strains at martensite-austenite interfaces. The results provide insight into the influence of frictional work. A micro-mechanical model based on reversible thermodynamics was modified to account for plastic relaxation of otherwise coherent transforming interface. Based on the model, we qualitatively interpret energetic contributions that cause the mechanical hysteresis.
Summary: The mechanical hysteresis for single crystals of conventional NiTi shape memory alloys and the promising magnetic shape memory alloys, CoNiAl and FeNiGa is studied through rigorous experimentation. We demonstrate the variation in the stress hysteresis with increasing isothermal test temperature for martensitic transformations. The hysteresis was measured in isothermal stress-strain experiments. To further our investigation, we present the strain dependence of the hysteresis for the CoNiAl and NiFeGa alloys under pseudoelastic conditions. We interpret the role of composition for aged NiTi alloys under tensile loads and focus primarily on tension-compression asymmetry for an aged CoNiAl alloy. For the NiFeGa alloys, we compare the aged and unaged conditions as well as tension-compression asymmetry. The evolution of the stress hysteresis for NiTi alloys exhibits distinct composition dependence. The results show that the stress hysteresis expands with increasing test temperature for lower Ni alloys while it contracts for higher Ni alloys. For the CoNiAl alloys, the stress hysteresis grows under tensile and compressive loads; however, the trends for hysteresis behavior display stark tension-compression asymmetry. At low temperatures, pseudoleasticity is absent in tension while it is prevalent in compression. Stress state and heat treatment significantly influence the hysteresis behavior for the NiFeGa alloys. The hysteresis behavior is rationalized considering dissipation of elastic strain energy due to relaxation of coherency strains at martensite-austenite interfaces. The results provide insight into the influence of frictional work. A micro-mechanical model based on reversible thermodynamics was modified to account for plastic relaxation of otherwise coherent transforming interface. Based on the model, we qualitatively interpret energetic contributions that cause the mechanical hysteresis.
