EPJ Special Topics

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

Microstructure changes in two phase ${\beta + \gamma}$ Co-Ni-Al ferromagnetic shape memory alloys in relation to Al/Co ratio

W. Maziarz¹, J. Dutkiewicz¹, R. Santamarta² and E. Cesari²

¹ Institute of Metallurgy and Materials Science of the Polish Academy of Sciences, 25 Reymonta Str., 30-059 Krakow, Poland
² Departament de Física, Universitat de les Illes Balears, Cra. de Valldemossa, km 7.5, 07122 Palma de Mallorca (Illes Balears), Spain

(Published online: 1 May 2008)

Abstract
The microstructure of ${\rm Co}_{35 + {\rm x}}-{\rm Ni}_{40 - {\rm x}}-{\rm Al}_$ (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 $\gamma$ phase. Annealing twins were found frequently in the $\gamma$ phase. The volume fraction of $\gamma$ phase has been determined to be about 40%. TEM observations show twinned L1₀ martensite with -type twinning plane and tetragonality c/a = 0.85 in all investigated alloys. Determination of the composition of $\beta$ 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 $\beta$ phase causes a decrease of the e/a ratio, which promotes a decrease of martensitic transformation temperatures.

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

Microstructure changes in two phase ${\beta + \gamma}$ Co-Ni-Al ferromagnetic shape memory alloys in relation to Al/Co ratio