Martensite stabilization in shape memory alloys – Experimental evidence for short-range ordering

• The principle of symmetry-conforming short-range order found experimental verification.
• Neutron scattering experimentally revealed differences in the degree of order in Co-Ni-Ga.
• Austenitic phase shows partial B2 ordering being transferred to martensite upon quenching.
• Co–Ni–Ga following SIM-aging shows an almost random occupation probability of atoms.

Thermal stabilization of martensite in shape memory alloys is known to strongly affect functional properties due to changes in transformation temperatures. As martensite stabilization in many alloys proceeds in an uncontrollable fashion, it has been treated as a detrimental mechanism in the past. In a recent study it was found that martensite stabilization can be controlled by aging of stress-induced martensite, allowing development of a new class of high-temperature shape memory alloys. Symmetry-conforming adaptation of short-range order during thermal treatment has been stated to be the mechanism responsible for this phenomenon. However, direct experimental evidence for changes in short-range ordering has not been presented. The current study has been conducted in order to fill this gap. A Co–Ni–Ga shape memory alloy has been studied by neutron diffraction in different conditions, i.e. as-grown austenite, quenched martensite, heat-treated austenite and stabilized stress-induced martensite. The results obtained unequivocally reveal that martensite stabilization is triggered by a chemical disordering mechanism. Thus, the concept of symmetry-conforming short-range order proposed 1997 by Ren and Otsuka has finally found experimental verification for a Co–Ni–Ga alloy.