Dependence of shape memory properties on changes in crystallography in Ni(Mn,Ga) alloys

Based on previous research at Michigan Technological University, a family of Ni(Mn,Ga) alloys with gallium ranging from 0-25 atomic percent have been studied as examples of variable transformation temperature shape memory alloys. The focus of this work has been to link changes in the crystallography of the martensite to the shape memory behavior of these alloys. Two primary shape memory properties have been examined: transformation behavior using differential scanning caloriometry and mechanical behavior in compression.; It has been found that increases in gallium decrease the transformation temperature and improve shape recovery. The decrease in temperature is linked to the increased stability of the austenite phase, while the improved shape recovery can be explained by an increased twin boundary mobility and the suppression of dislocation slip caused by the onset of Heusler ordering. The results show clearly that the transformation temperature and shape recovery behavior can be manipulated by selective alloying to alter the crystallography. These observations are important to the quest for higher transition temperature shape memory alloys.