Eur. Phys. J. Special Topics 158, 137-142 (2008)
https://doi.org/10.1140/epjst/e2008-00666-2

Microstructure changes in two phase β+ γ Co-Ni-Al ferromagnetic shape memory alloys in relation to Al/Co ratio

¹ Institute of Metallurgy and Materials Science of the Polish Academy of Sciences, 25 Reymonta Str., 30059 Krakow, Poland
² Departament de Física, Universitat de les Illes Balears, Cra. de Valldemossa, km 7.5, (Illes Balears), 07122 Palma de Mallorca, Spain

Abstract

The microstructure of Co_{35 + x}-Ni_{40 - x}-Al₂₅ (x = 0, 2.5, 5.0) ferromagnetic shape memory alloys annealed at 1200 °C and water quenched has been investigated by optical (OM) and analytical transmission electron microscopy (TEM). The microstructure consists of elongated grains of martensite and γ phase. Annealing twins were found frequently in the γ phase. The volume fraction of γ phase has been determined to be about 40%. TEM observations show twinned L1₀ martensite with {111}-type twinning plane and tetragonality c/a = 0.85 in all investigated alloys. Determination of the composition of β phase, allows to calculate Al/Co and e/a ratio in the range 7.68–8.40 in correlation with its Ms temperatures between 66 and 167 °C, respectively. An increase in the Al/Co ratio together with a decrease of Ni content in β phase causes a decrease of the e/a ratio, which promotes a decrease of martensitic transformation temperatures.