Microstructure and mechanical properties in Co-Ni-Ga-Al shape memory alloys with two-phase structure

The fee second phase (gamma) is commonly encountered in some new magnetic shape memory alloys, such as Ni-Mn-Fe-Ga, Co-Ni-Ga, Co-Ni-Al and Ni-Fe-Ga systems. The introduction of the gamma phase is greatly favored to the enhancement of ductility and its volume fraction is generally determined by the content of Ga or Al. In this study, we attempt to change the amount of the gamma phase by partially substituting Al for Ga in Co-Ni-Ga alloys. The results show that not only the amount but also the phase structure and the solidified morphology of the second phase are strongly affected by the addition of Al. The non-facet gamma phase is transitioned to the sharp-edge gamma' ordered-structure phase when 2 at.% Ga is replaced by Al. Additionally, the martensitic transformation and compression stress-strain behavior in two-phase structure Co-Ni-Ga-Al alloys are investigated. The martensitic transformation temperatures increase with increasing Al content and decreasing the second phase. However the ultimate strength and ductility decrease by the decrease of the second phase. (C) 2007 Elsevier B.V All rights reserved.