Anelasticity of B19′ martensitic phase in NiTi and NiTiCu alloys

Anelastic properties of the B19′ martensitic phase have been studied in two polycrystalline alloys: NiTi with nearly equiatomic composition and NiTiCu (about 49 at.% Ti, 46 at.% Ni, and 5 at.% Cu). Internal friction, dynamic shear modulus and torsional strain at zero applied stress were measured in a forced torsion pendulum for strain amplitudes of 2 × 10−5 to 2 × 10−4, temperatures of 6–300 K, temperature change rates of 1–5 K/min and frequencies of 0.001–10 Hz. Several peculiarities have been found during thermal cycling: dependence of the internal friction on the temperature change rate and frequency of oscillation, which disappears with isothermal exposure, temperature hysteresis of the internal friction and dynamic shear modulus, and low-temperature anomalies in the temperature dependence of the dynamic shear modulus. The observed peculiarities result from reversible microplastic straining of the martensitic phase under the action of thermal stresses arising from anisotropy of thermal expansion of the B19′ phase. A relaxation peak in internal friction has been found in both alloys at temperatures of about 200 K. The temperature and the shape of the peak are influenced by the thermal stresses. The activation enthalpy of the relaxation has been evaluated both in isothermal and in nonisothermal conditions.