以粉末冶金製備富鈦TiNi形狀記憶合金及其變態性質之研究

Abstract

以粉末冶金(PM)製備富鈦TiNi形狀記憶合金，於900℃以下之溫度燒結時，因Kirkendall Effect於Ni粒附近留有≦5μm之孔洞，當在900℃持溫1小時後，因共晶反應使原本的Ti粒處形成大約20~50μm之孔洞，但Ti55.08at%-Ni者因有多量的Ti2Ni於高溫時成為液相，可將孔洞補平。PM Ti50at%-Ni於900℃以下燒結時，其變態溫度較同成份之VAR製備者高，且與PM Ti52at%-Ni和PM Ti55.08at%-Ni者相近，此應與其內均是富鈦TiNi B2相有關；當燒結溫度提升至1000℃以上時，PM Ti50at%-Ni的變態溫度則下降至比VAR者低，此應與TiNi B2相已由偏富Ti變為偏富Ni有關，且此時會固溶較多之C、O原子。在不同燒結流程的研究中，使用兩階段燒結法之Ti50at%-Ni的試片，相較於一階段燒結法，其密度可提升約10%，主因於900℃持溫2小時時，Ti和Ni間會先形成Ti2Ni、TiNi和TiNi3等中間相，因此再升溫至942℃時，其共晶反應不如一階段燒結者激烈。二階段燒結之Ti50at%-Ni其形狀回復率只有76%，較同成份VAR之90%小，應為PM試片中有許多孔洞之故。使用兩階段燒結法燒結之Ti52at%-Ni試片，相較於一階段燒結者，其密度可從76%提升至97%，其形狀回復率因有較多Ti2Ni相而為70%，但與同成份VAR者相近，但經5次之形狀回復測試後其回復率可達97%。PM Ti52at%-Ni的tanδ之峰值較同成分VAR者略大，顯示PM試片中的孔洞有助於吸收震動之能量。本文同時由Ti-Ni二元平衡圖，對富鈦TiNi形狀記憶合金之變態溫度常在出現M*≒60℃，A*≒100℃之現象提出解釋。

Martensitic transformation behaviors of Ti-rich TiNi shape memory alloys (SMAs) fabricated by powder metallurgy (PM) are investigated. When sintering at ＜900℃, PM SMAs have small pores(≦5μm) formed near Ni particles due to Kirkendall Effect, but sintering at 900℃ for 1 h, they have big pores(20~50μm) formed at the prior site of Ti particles due to eurtectic reaction. Ti55.08at%-Ni contains lots of Ti2Ni which becomes liquid phase at 984℃ and fills up the pore. As sintering at ＜900℃, the forward and reverse transformation peak temperatures (M* and A*) of PM Ti50at%-Ni are higher than those of VAR one, but similar to those of Ti52at%-Ni and Ti55.08at% -Ni due to the formation of Ti-rich TiNi B2 phase. As sintering at ＞1000℃, M* and A* of PM Ti50at%-Ni are lower than those of VAR one due to the formation of Ni-rich TiNi B2 phase in which lots of carbon and oxygen atoms are dissolved simultaneously. Different sintering processes are also examined in this study. The density of Ti50at%-Ni sintered by 2-step process is 10% higher than that sintered by 1-step process. Because Ti2Ni, TiNi and TiNi3 are formed in Ti-rich PM SMAs during the holding time at 900℃, the eutectic reaction in 2-step process is not so violent as that in 1-step process. The shape recovery of PM Ti50at%-Ni is 76% which is lower than that of VAR one, 90%, due to the pores existed in PM alloy. Ti52at%-Ni alloy’s density sintered by 2-step process is 20% higher than that sintered by 1-step process, but its shape recovery (70%) is near to that of VAR one (72%) due to both have lots of Ti2Ni and the PM alloy has high density (92%). The damping capacity (tanδ) of PM Ti52at%-Ni is higher than that of VAR Ti52at%-Ni due to the pores in PM alloy can absorb the damping energy. In this study, PM Ti-rich TiNi SMAs exhibit M*≒60℃ and A*≒100℃ and this characteristic is explained from the viewpoint of Ti-Ni phase diagram.