CRITICAL EXPONENTS OF THE TRANSPORT PROPERTIES AT THE B2⇔IC⇔C(R) TRANSITIONS IN Ti-Ni-Me SHAPE MEMORY ALLOYS

Abstract

Critical behaviour of the physical properties at the B2⇔IC ⇔C(R) phase transitions in TiNi-based shape memory alloys has been analyzed in the frame of the charge density wave (CDW) model. Variation of total resistance at the Peierls-type B2⇔IC ⇔C(R) transition in TiNiMe (Me=Cr, Fe, Al, Ge) alloys has been found to be a sum of the β-phase normal contribution, fluctuating CDW resistance ρ f (T) in the incommensurate state and resistance change due to the energy gap formation ρc(T) in the commensurate state. The fitting parameters such as the energy gap at saturation Δ(0) and the number of electrons involved in the process of the CDW's formation ψ(0) have been determined as a function of the alloy chemical composition and thermal treatment at moderate temperature. The critical resistive fluctuations in the incommensurate phase follow a power law dρ f /dt*~t*m with critical exponent m=-1. In the frame of the CDW model this means that the process of electron scattering from periodic distortion is strongly limited to a definite plane of the crystal and system is two-dimensional. The change of ρ c with temperature is controlled by the activation energy law corresponding to electron single excitations through the gap Δ(T), with a varying ψ(T) effective number of the electrons involved in the process. The total enthalpy measured during cooling is compared with the heat calculated for the energy gap opening at the Fermi level during the IC⇒C(R) transition in the frame of the Shottky anomaly approximation. Both values are of the same order. When hydrostatic pressure is applied to the material, a small drop in the conductivity is observed around P~2 GPa and interpreted as CDW pinning by commensurability locking at a temperature higher than the transition temperature at normal pressure.