Relationship Between Volume Fraction of Strain-Induced Martensite Under Tension and Electrical Resistivity in a Ti-Ni SMA Wire

Mechanical Engineering, Yamaguchi University, Ube city, Yamaguchi-pref., Japan

Abstract

Relationships among plastic strain, volume fraction of SIM, stress-strain curve, electrical resistivity of a Ti• 50.5 at.% Ni wire are investigated under tension at temperature from 30 °C to -20 °C and also under repeated tensile cycles up to N=200 at 20 °C. The relative electrical resistivity Δρ/ρ₀ decreases as the volume fraction of SIM increases to nearly 90%, resulting in the more decrement in Δρ/ρ₀ the lower the temperature and after that Δρ/ρ₀ increases with a steep gradient. The decrement stage is supposedly due to the formation and deformation of SIM by twinning and the succeeding increment stage is due to the deformation by dislocation. The decrementai change is nicely fitted by an equation of a logarithmic mixture-rule (LMR) for electrical resistivity. The relation between Δρ/ρ₀ and plastic strain at various temperatures also shows the decremental curve within the thermo-elastic strain range, after that it increases as the plastic strain increases. Effects of repeated cycles to those relations give the similar experimental results as the above where the major test variable is repeated cycles instead of test temperature in this case.