Dependence of Transformation Temperatures of Shape Memory Alloys on the Number and Concentration of Valence Electrons, pp. 339-360
Authors: Mehrdad Zarinejad and Yong Liu
Abstract: Dependence of transformation temperatures of shape memory alloys on the number (ev/a) and concentration (cv) of valence electrons is investigated. The alloys are categorized into low (ev/a<5), medium (5≤ev/a≤7.50), and high (ev/a>7.50) valence electron groups. For majority of alloys in medium and high valence electron group, clear correlations between transformation temperatures and cv are found. Ms and As both decrease from 1100 and 1150˚C to as low as -206˚C and -153˚C respectively, with increasing cv from 0.10 to around 0.30. When ev/a>7.50, the transformation temperatures show much less dependence on cv and can be lower than the medium group temperatures even at medium or low cv values. High electron NiMnX (X=Ga, In, Al) Heusler alloys containing Mn (>30 at%) show high transformation temperatures at high cv values due, most likely, to antiferromagnetism of excess Mn which reduces the elastic moduli. The low valence electron group alloys show significantly lower transformation temperatures for their cv values compared to the main group. The influence of cv is discussed on the basis of the variations of elastic constants due to composition change.