Characteristics of martensitic and strain-glass transitions of the Fe-substituted TiNi shape memory alloys probed by transport and thermal measurements

Abstract

The electrical resistivity, Seebeck coefficient, thermal conductivity, and specific heat of Ti₅₀Ni_50-x Fe _x (x = 2.0-10.0 at.%) shape memory alloys (SMAs) were measured to investigate the influence of point defects (Fe) on the martensitic transformation characteristics. Our results show that the Ti₅₀Ni₄₈Fe₂ and Ti₅₀Ni₄₇Fe₃ SMAs have a two-step martensitic transformation (B2 → R and R → B19'), while the Ti₅₀Ni₄₆Fe₄, Ti₅₀Ni_44.5Fe_5.5, and Ti₅₀Ni₄₄Fe₆ SMAs display a one-step martensitic transition (B2 → R). However, the compounds Ti₅₀Ni₄₂Fe₈ and Ti₅₀Ni₄₀Fe₁₀ show strain glass features (frozen strain-ordered state). Importantly, the induced point defects significantly alter the martensitic transformation characteristics, namely transition temperature and width of thermal hysteresis during the transition. This can be explained by the stabilization of austenite B2 phase upon Fe substitution, which ultimately leads to the decrease in enthalpy that associated to the martensitic transition. To determine the boundary composition that separates the R-phase and strain glass systems in this series of SMAs, a Ni-rich specimen Ti₄₉Ni₄₅Fe₆ was fabricated. Remarkably, a slight change in Ti/Ni ratio converts Ti₄₉Ni₄₅Fe₆ SMA into a strain glass system. Overall, the evolution of phase transformation in the Fe-substituted TiNi SMAs is presumably caused by the changes in local lattice structure via the induced local strain fields by Fe point defects.