The effect of electronic and magnetic valences on the martensitic transformation of CoNiGa shape memory alloys
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Martensite start temperatures (M s) and magnetic properties of several Co-Ni-Ga shape memory alloys (SMAs) with B2 austenite structure were investigated as a function of composition. It was found that, in addition to the well-known effect of the valence electron concentration (e/a ratio) in increasing M s, the magnetic character of the alloy also plays a decisive role in determining M s. These results were further corroborated through electronic structure calculations based on density functional theory. Experiments and calculations suggest that, for a given composition of Ga, the higher e/a ratios result in the higher M s. Moreover, at a constant e/a ratio, the higher the magnetic valence number (Z m) of the alloy is, the lower the M s becomes. It was concluded that Z m can be used as an indicator for the compositional trends in M s in the present ferromagnetic SMAs in addition to the e/a ratio. Statistical analysis of the experimental results suggests that an increase in e/a of 0.1 yields an increase in M s of about 190 K, while a change in Z m of +0.1 results in a decrease in M s of about 160 K. The calculations also confirmed that ferromagnetism may ultimately result in entropic and/or energetic stabilization of the austenite (cubic) phase, yielding lower M s. These findings seem to be valid for other ferromagnetic SMA systems, where different M s temperatures were reported for a given e/a ratio. 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
