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Transformation and twinning processes in CoNiAl alloy single crystals

Silvia Ignacova ,  Vaclav Novak ,  Petr Sittner 

Institute of Physics ASCR, v.v.i., Na Slovance 2, Prague CZ-18221, Czech Republic


Ferromagnetic shape memory alloy CoNiAl exhibiting paramagnetic/ferromagnetic transition besides a thermoelastic martensitic transformation is a promising material for high temperature and magnetic shape memory applications. Due to the two-phase structure (β+γ) of the alloy, the Curie temperature and the martensitic transformation temperatures can be adjusted in the temperature range -150°C to 150°C by varying chemical composition of the β-phase or through annealing.

In this work, Co38Ni33Al29 shape memory alloy single crystals grown by Bridgman method were heat treated to obtain desired functional behavior. Stress induced transformation and twinning processes in this alloy were investigated on prism shaped samples deformed in compression in wide temperature range. Pseudoelastic stress-strain response at room temperature was characterized by a curious softening curve with narrow hysteresis (around 50 MPa) and 4% transformation strain for [001] load axis orientation. Twinning processes in L10 tetragonal martensite (test temperatures –5°C to –50°C ) take place at very low stress ~1MPa, in contrast to available literature reports. Experimental results are analyzed and discussed in respect to twinning processes in studied alloy.

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Presentation: Poster at E-MRS Fall Meeting 2007, Symposium E, by Silvia Ignacova
See On-line Journal of E-MRS Fall Meeting 2007

Submitted: 2007-05-21 22:09
Revised:   2009-06-07 00:44