A study of Thermo-mechanical treatment of TiNiCr Ternary Shape Memory Alloys

• Main Authors: chen chan homn, 陳建宏
• Other Authors: Hsieh shin feng
• Format: Others
• Language: zh-TW
• Published: 2008
• Online Access: http://ndltd.ncl.edu.tw/handle/37189386918400706126

碩士 === 國立高雄應用科技大學 === 模具工程系碩士班 === 96 === This study investigates the effects of the thermo-mechanical treatment on Ti50Ni49.5Cr0.5 and Ti50Ni49Cr1 ternary shape memory alloys. Ti50Ni49.5Cr0.5 alloy’s plates were cold-rolled at room temperature to15, 25 and 35% reduction in thickness, and then annealed at various temperature, 350C ~650C, for different time intervals (10 s ~ 24 hours). Effects of grain size on transformation behavior of this alloy after cold rolling and subsequent annealing are studied. The transformation behaviors and shape memory characteristics of Ti50Ni49Cr1 alloy will also be studied under various aging and thermal-cycling treatment processes. Ti50Ni49.5Cr0.5 ternary shape memory alloy cold-rolled and annealed at specific conditions can exhibit multiple-stage martensitic transformation. This feature may arise from the grain size distribution along the specimen’s thickness direction. Optical microscopic results reveal the size of grains after various annealing conditions. From these results, the activation energy of recrystallization is calculated as 71 KJ/mole, which is larger than that of Ti50Ni50 alloy, 51 KJ/mole, due to the solid solution of Cr atoms. The experimental grain-growth exponents at 500C and 650C are 0.12 and 0.15 for cold-rolled specimens, respectively. Shape recovery of cold-rolled alloy increases with increasing the amounts of thickness reduction. The solution-treated Ti50Ni49Cr1 alloy undergoes two-stage B2RB19’ martensitic transformation. M*, R* and A* decrease with increasing aging time at 400C because the Cr atoms diffuse slightly from the Ti2Ni to the B2 matrix. At the same time, M* and A* of this alloy can be more depressed by thermal cycling than those of the Ti50Ni50 alloy. This feature results from the fact that this alloy has a higher inherent hardness due to the solid solution of the Cr atoms. The strengthening effect of thermal-cycling on the M* ( Ms) temperature of this alloy follows the expression Ms=T0  Ky, in which K values are affected by different strengthening process. It is found that the higher the inherent hardness of the Ti51Ni49 and Ti50Ni49Cr1alloys, the higher the K values they have.