Effect of Heat Treatment on Transformation Behaviors of Ti48.5Ni49.5Fe2 Shape Memory Alloy and Mechanical Properties of HfNbTiZr High Entropy Alloy

• Main Authors: Chun-Hsiang Tu, 凃竣翔
• Other Authors: Shyi-Kaan Wu
• Format: Others
• Language: en_US
• Published: 2019
• Online Access: http://ndltd.ncl.edu.tw/handle/3a9y7m

碩士 === 國立臺灣大學 === 材料科學與工程學研究所 === 107 === In this study, the effect of heat treatment on transformation behaviors of Ni-rich Ti48.5Ni49.5Fe2 shape memory alloy (SMA) and on mechanical properties of equiatomic HfNbTiZr high entropy alloy (HEA) are systematically studied. Ti48.5Ni49.5Fe2 SMA exhibits strain glass transition in as solution-treated state, and a B2↔R transformation induced by Ti3Ni4 ppts when the specimens aged at 300 ºC/ 350 ºC. When the specimens aged at 400 ºC/500 ºC, a two-stage R-phase transformation, which was followed by a R↔B19’ transformation occurs, in which the R-phase transformation occurs at higher temperature is also induced by Ti3Ni4 ppts whereas that at lower temperature is related to the matrix away from ppts. If the ageing temperature is raised to 550 ºC/ 600 ºC, a B2↔R transformation occurs for long ageing time due to the lower Ni and higher Fe contents in the matrix. As for HfNbTiZr HEA, a single BCC-phase solid solution is identified in as cold-rolled state and in recrystallized state at above 900 ºC, and its hardness increases as the recrystallization temperature increases When 80% cold-rolled HfNbTiZr HEA aged at 350 ºC/450 ºC, the hardness increment of ~48% can be attained, while the hardening effect on 550 ºC aged specimens is not so pronounced due to the recovery effect. Though multiple phases are observed after aged at 350 ºC~550 ºC, the HEA’s age hardening is mainly contributed from HCP ppts.