Quaternary alloying of copper with Ti50Ni25Pd25 high temperature shape memory alloys

Abstract

High temperature shape memory alloys with four different compositions i.e. Ti50Ni25-xPd25Cux (x = 0, 5, 10, 15 at.%) were developed, characterized and tested for mechanical and shape memory properties. For simplicity, the alloys were named as 0Cu, 5Cu, 10Cu and 15Cu alloys depending upon the concentration of Cu. Addition of Cu in place of Ni by 5%, 10% and 15% resulted in the improvement of transformation temperatures and thermal hysteresis. Martensite start temperatures increased by 9%, 19% and 42% whereas thermal hysteres decreased by 6%, 8% and 35% respectively. The mechanical properties of 5Cu, 10Cu and 15Cu alloys also improved as compared to 0Cu alloy. The yield and fracture stresses increased by 11%, 19%, 26% and 6%, 10%, 12% respectively in martensite phase. Similarly in austenite phase the same stresses increased by14%, 23%, 28% and 3%, 7%, 8% respectively. At the same time, the hardness of quaternary alloys also improved by 3%, 4% and 7%. The shape memory properties enhanced up to addition of 10% Cu and then slightly decreased at 15%. The strain recovery of 5Cu, 10Cu and 15Cu increased by 2%, 8% and 6%, whereas the irrecoverable strain decreased by 23%, 27% and 23%, respectively. Similarly the work output of quaternary alloys improved by 2%, 8% and 6% respectively as compared to 0Cu alloy. The overall results suggested that by quaternary alloying of Cu in place of Ni enhanced the phase transformation temperatures, improved the mechanical properties in terms of yield stress and fracture stress. Moreover, the shape memory properties like recovered strain, irrecoverable strain and work output improved by addition of Cu up to 10% and then slightly decreased by alloying of Cu at 15%.