RELATIONSHIP BETWEEN VOLUME FRACTION OF STRAIN-INDUCED MARTENSITE UNDER TENSION AND ELECTRICAL-RESISTIVITY IN A TI-NI SMA WIRE

Relationships among plastic strain, volume fraction of SIM, stress-str ain curve, electrical resistivity of a Ti . 50.5 at.% Ni mire are inve stigated under tension at temperature from 30 degrees C to -20 degrees C and also under repeated tensile cycles up to N=200 at 20 degrees C. The relative electrical resistivity Delta rho/rho(0) decreases as the volume fraction of SIM increases to nearly 90%, resulting in the more decrement in Delta rho/rho(0) the lower the temperature and after tha t Delta rho/rho(0) increases with a steep gradient. The decrement stag e is supposedly due to the formation and deformation of SIM by twinnin g and the succeeding increment stage is due to the deformation by disl ocation. The decremental change is nicely fitted by an equation of a l ogarithmic mixture-rule (LMR) for electrical resistivity [1]. The rela tion between Delta rho/rho(0) and plastic strain at various temperatur es also shows the decremental curve within the thermo-elastic strain r ange, after that it increases as the plastic strain increases. Effects of repeated cycles to those relations give the similar experimental r esults as the above where the major test variable is repeated cycles i nstead of test temperature in this case.