Influence of alloy chemistry on the transformation temperatures and local atomic structure of shape memory alloys : with a focus on NiTi-based intermetallics

Abstract

VI Abstract The dependence of transformation temperatures of shape memory alloys on the number and concentration of valence electrons are studied. This dependence is further analyzed for NiTi-based alloys followed by an investigation of the effect of precipitation on the Ms temperature, and the influence of composition change on the local atomic structure and transformation temperature of NiTiHf alloys. The bond evolution and change of transformation temperature during the local chemistry change of a Ni25Ti50Cu25 melt-spun ribbon during crystallization is also studied. The shape memory alloys are categorized into low (ev/a 7.50) valence electron groups. For majority of alloys in medium and high valence electron group, clear correlations between transformation temperatures and their valence electron concentration (cv) are found. The 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 the number of valence electrons per atom is high (ev/a>7.50), the transformation temperatures show much less dependence on cv. High electron NiMnX (X=Ga, In, Al) Heusler alloys containing Mn (>30 at%) show high transformation temperatures at high cv values most likely due to antiferromagnetism of excess Mn which reduces the elastic moduli. The low valence electron group shows significantly lower transformation temperatures for their cv values compared to the main group. The influence of valence electron concentration is discussed on the basis of the variations of elastic moduli of the alloys as a result of composition change. For NiTi-based alloys, two distinct trends of transformation temperatures with respect to the number of valence electrons per atom are found depending on whether ev/a = 7 or ev/a ≠ 7. Clear correlations between transformation temperatures and cv exist. Ms and As decrease consistently with increasing cv. The possible influence of alloying elements atomic size on transformation hysteresis is also introduced. Furthermore, for these alloys, the changes of martensite start temperature (Ms) as a result of second phase precipitation are studied. The alteration of Ms as a result of precipitation is mainly due to the change of electron concentration of the matrix because of its composition change. ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library