Comparative study of physical properties of Ti50−xNi48+xFe2 (x=0.0−2.0) shape memory alloys after solution treating and aging at 450∘C

We report a comprehensive study of the physical properties of two series of ${\mathrm{Ti}}_{50\ensuremath{-}x}{\mathrm{Ni}}_{48+x}{\mathrm{Fe}}_{2}$ $(x=0.0\ensuremath{-}2.0)$ alloys synthesized after solution treated and aged at $450{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ using temperature $(T)$-dependent electrical resistivity $(\ensuremath{\rho})$, Seebeck coefficient $(S)$, specific heat $({C}_{\mathrm{p}})$, and thermal conductivity $(\ensuremath{\kappa})$ measurements. All measured physical properties indicated that point defects with Ni addition altered martensitic transformation (MT) characteristics from two-stage $B2\ensuremath{\leftrightarrow}R\ensuremath{\leftrightarrow}B{19}^{\ensuremath{'}}$ to strain-glass (STG) type in solution-treated alloys, whereas precipitation of ${\mathrm{Ti}}_{3}{\mathrm{Ni}}_{4}$ with excess Ni in aged alloys exhibited another $R$-phase transition. These alloys exhibit metallic behavior in $\ensuremath{\rho}(T)$ and $S(T)$; however, their electrical properties vary significantly with increasing $x$, due to the increase in point defects and precipitates. Despite this, solution-treated alloys with $x\ensuremath{\ge}1.0$ exhibit a negative temperature coefficient of resistivity below the STG phase due to their continuous formation and growth of $R$ domains. The scattering of charge carriers for both series of alloys in the $B2$ phase appears to be composition independent, although the electronic band structure near Fermi levels varies considerably at each MT. An analysis of $\ensuremath{\kappa}(T)$ data reveals that charge carriers contribute more to the MT features than phonons. As Ni content increases, electron-phonon coupling weakens, and phonon modes become harder in these alloys. Furthermore, the entropy change associated with the two intermediate $R$-phase MTs induced by point defects and ${\mathrm{Ti}}_{3}{\mathrm{Ni}}_{4}$ precipitates is estimated using ${C}_{\mathrm{p}}(T)$, which decreases with lowering transition temperatures and can be explained by theoretical ferroelastic models.