The effect of electronic and magnetic valences on the martensitic transformation of CoNiGa shape memory alloys

Martensite start temperatures (Ms) 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 Ms, the magnetic character of the alloy also plays a decisive role in determining Ms. 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 Ms. Moreover, at a constant e/a ratio, the higher the magnetic valence number (Zm) of the alloy is, the lower the Ms becomes. It was concluded that Zm can be used as an indicator for the compositional trends in Ms 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 Ms of about 190 K, while a change in Zm of +0.1 results in a decrease in Ms 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 Ms. These findings seem to be valid for other ferromagnetic SMA systems, where different Ms temperatures were reported for a given e/a ratio.