熱機處理對富鎳鈦鎳形狀記憶合金多階相變態行為之研究

Abstract

本研究對Ti49.2Ni50.8及Ti49.4Ni50.6SMAs冷加工35%後時效處理之，以得不同之晶粒大小。時效後兩合金都產生Ti3Ni4析出物，但其分布會因晶粒大小之不同及時效條件之不同而影響到合金之多階相變態行為。Ti49.2Ni50.8合金在500℃及550℃時效者因為時效過程中合金的再結晶與晶粒成長到約50µm，使得晶粒中心的Ni原子來不及擴散到晶界，故於晶粒內產生了析出物大小不同的三個區域：區域Ⅰ(晶粒中心)、區域Ⅱ(ⅠⅢ中間)及區域Ⅲ(晶界處)，因此形成B2ⅠⅡⅢ→RⅠⅡⅢ,RⅠⅢ→B19’ ⅠⅢ及RⅡ→B19’ Ⅱ三階相變態。Ti49.4Ni50.6合金在500℃時效者晶粒大小約為40μm，因為Ti3Ni4的均勻析出，且析出物與析出物之間距大於1µm，故可依成分或應力場之分布而分成兩個區域：Ti3Ni4析出物附近(區域Ⅰ)及析出物與析出物中間的區域(區域Ⅱ)，使其變態順序為B2→R(區域Ⅰ)，B2→R(區域Ⅱ)，R→B19’(區域Ⅰ)，R→B19’(區域Ⅱ)的四階相變態。Ti49.4Ni50.6合金在600℃時效者(晶粒大小約為60µm)出現三階相變態，此因600℃的時效溫度很高，時效初期Ti3Ni4析出物成核的位置較少，所以析出物和析出物間的距離大，使得析出物和基地間的整合應力場無法相互重疊，如此造成三階相變態，為B2→R→B19’(Ti3Ni4析出物與基材之界面附近區域)及B2→B19’(析出物與析出物間尚未變態之基材部分)。

Transformation behavior of Ti49.2Ni50.8 and Ti49.4Ni50.6 shape memory alloys(SMAs) after 35% cold rolling and subsequent aging are investigated. The distribution of Ti3Ni4 precipitates directly affect the transformation behavior of these SMAs. After 500℃×12~24hr and 550℃×40min~1hr aging for Ti49.2Ni50.8 SMA, the supersaturated Ni atoms in the grain center can not diffuse to grain boundaries due to the grain size of Ti49.2Ni50.8 is large enough(about 50µm) and thus, there are three distanct regions resulted in the grain, i.e., region Ⅰ(grain interior), region Ⅲ(grain boundary) and region Ⅱ(region between Ⅰ and Ⅲ). Three-step transformation of B2ⅠⅡⅢ→RⅠⅡⅢ, RⅠⅢ→B19’ⅠⅢ, and RⅡ→B19’Ⅱ is induced from these regions. After 500℃×10~14hr aging, the Ti3Ni4 precipitates in Ti49.4Ni50.6 SMA distribute homogeneously in the grain but their mean interparticle spacing is in excess of 1μm. In this case, there are two regions resulted in the grain, i.e., region Ⅰ(near the precipitate/matrix interface which is affected by the coherent stress field) and Ⅱ(the other region in the matrix). Four-step transformation of B2Ⅰ→RⅠ, B2Ⅱ→RⅡ, RⅠ→B19’Ⅰ and RⅡ→B19’Ⅱ is induced from these two regions. After 600℃×7~10min aging in Ti49.4Ni50.6 SMA, the nucleation sites of Ti3Ni4 is much less so the coherent stress field could not overlap each other. Three-step transformation of B2ⅠⅡ→RⅠⅡ, RⅠ→B19’Ⅰ and RⅡ→B19’Ⅱ is induced from these regions.