熱處理對Ti48.5Ni49.5Fe2形狀記憶合金變態行為與HfNbTiZr高熵合金機械性質影響之研究

Abstract

本研究探討富鎳Ti48.5Ni49.5Fe2形狀記憶合金(SMA)在固溶處理後及時效於300ºC~600ºC之相變態行為。固溶處理後之Ti48.5Ni49.5Fe2具有應變玻璃轉變特性，而時效後之Ti48.5Ni49.5Fe2則隨不同的時效溫度及時間呈現不同的相變態行為。當Ti48.5Ni49.5Fe2時效於300ºC及350 ºC時，呈現B2↔R的一階相變態，而此變態主要與Ti3Ni4之析出有關。當Ti48.5Ni49.5Fe2時效於400ºC, 450 ºC及500 ºC時，則呈現三階相變態，變態溫度由高至低依序為：B2↔R1、B2↔R2以及R↔B19’。其中R1相變態一樣是由Ti3Ni4之析出而誘發；而R2相變態主要與遠離Ti3Ni4之基地相關。當時效溫度提升至550 ºC及600 ºC時，短時間時效下並不會有任何相變態產生，但經長時間時效後則有一階之B2↔R相變態。此變態之產生主要是因550 ºC及600 ºC長時間時效下會析出富鎳但含鐵量少之Ti3Ni4以及Ti2Ni3，因此使基地之鎳含量降低但鐵含量提升。本研究同時探討熱處理對等原子比HfNbTiZr高熵合金(HEA)機械性質之影響。HfNbTiZr在室溫下進行80%冷滾軋以及滾軋後於900ºC以上溫度做再結晶熱處理，發現皆呈現單一BCC相之固溶體，且其表面微硬度隨再結晶溫度的升高而提高。滾軋後之HfNbTiZr若經350ºC及450ºC之低溫時效處理，其硬度提升量相較於時效前可達~48%；然時效於550ºC時，由於會有回復現象使時效硬化的效果不明顯。雖然HfNbTiZr冷軋後經低溫時效，會產生B2, BCC及HCP等不同相之析出物，但產生時效析出強化效果者主要為HCP相之析出物。

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.