Valence Electron Ratio for Design of Shape Memory Alloys with Desired Phase Transformation Temperatures

Dependence of phase transformation temperatures of TiPd- and TiPt-based shape memory alloys on valence electron ratio (VER), number (ev/a), and average atomic number of the alloys (Z) are investigated. The alloys have medium numbers of valence electrons (6.6 ≤ ev/a ≤ 7.3) that are near 7 and a wide range of the average atomic number (Z = 25–54). The forward and reverse phase transformation temperatures, Ms and (As), increase with average atomic number of the alloys, respectively. Clear correlations between transformation temperatures and VER are found. Ms and (As) both decrease from around 1184 °C (1175 °C) to as low as 20 °C (32 °C), respectively, with increasing VER from 0.134 to 0.270. Temperature hysteresis of the thermoelastic transformation in these alloys tends to decrease with increasing VER. Dependence of transformation temperatures and temperature hysteresis of these shape memory alloys on VER is discussed based on the variations of elastic moduli and atomic bonding due to composition change.