过冷Ti-46Al-7Nb亚包晶合金的相选择

Phase selection for undercooled Ti-46Al-7Nb hypo-peritectic alloy

采用电磁悬浮方法,通过原位观察再辉曲线进行过冷Ti-46Al-7Nb亚包晶合金的快速凝固研究,获得的最大过冷度为240 K。在一定过冷度下对悬浮的熔体进行铜基底悬淬,进而对凝固合金的微观组织进行分析。超过一定的临界过冷度(ΔT*=205 K),凝固模式将从具有包晶转变特征向包晶转变被抑制转化。当熔体初始过冷度ΔT≤ΔT*时,遵循包晶合金的典型凝固规律,β相作为初生相析出,在随后的冷却过程中包晶相α以包晶反应、包晶转变的方式析出。当ΔT>ΔT*时,β相直接凝固,包晶相α的析出被抑制。包晶反应能否发生取决于包晶相α的孕育时间τP与再辉后熔体完全β相凝固所需的时间tβ的相对大小。当过冷度相差不大时,通过改变凝固过程的冷速,组织中获得β相向α"相的马氏体转变。

The rapid solidification of undercooled Ti-46Al-7Nb alloy was investigated by in situ observation of recalescence events during electromagnetic levitation. A maximum melt undercooling up to 240 K has been achieved. Levitated drops of controlled undercooling were quenched onto chill copper substrate and subjected to phase and microstructure analysis. Beyond a critical undercooling of 205 K, solidification mode changes from peritectic transformation into suppression of peritectic transformation. When the intial undercooling （ΔT） is lower than a critical value （ΔT＊）, the solidification follows a typical manner of peritectic alloys, i.e.βphase solidifies as primary phase, after which, peritectic α phase forms through peritectic growth which is controlled by peritectic reaction, peritectic transformation. IfΔT〉ΔT＊, the formation of peritecticαphase will be suppressed, and the solidification of singleβphase will prevail.This phenomenon can be explained by comparing the relationship between incubation time for peritectic reaction （τP） and post-recalescence time corresponding to pure β phase solidification （tβ）.When the undercooling is no different,microstructure can obtain theβ/α＂ martensitic transformation by changing the cooling rate.