自由落體條件下Ti-6Al-4V合金微液滴的快速凝固研究

自由落體條件下Ti-6Al-4V合金微液滴的快速凝固研究Rapid solidification of Ti-6Al-4V alloy micro-droplets under free fall condition

在自由落體條件下實(shí)現(xiàn)了Ti-6Al-4V合金微液滴的深過冷與快速凝固，研究了合金的相組成、凝固組織和顯微硬度。計(jì)算出落管中不同直徑微液滴的過冷度和冷卻速率，發(fā)現(xiàn)了Ti-6Al-4V合金凝固組織隨過冷度及冷卻速率的變化規(guī)律。結(jié)果表明，深過冷與大冷速的耦合作用使凝固組織不斷細(xì)化且形貌發(fā)生演變：層片α+β→枝晶α→網(wǎng)籃狀α′+β→針狀α′→針狀α′+不規(guī)則β。當(dāng)液滴直徑小于400 μm時(shí)，位于原始等軸β晶的晶界及晶內(nèi)的針狀馬氏體α′轉(zhuǎn)化為大量連續(xù)分布且形狀不規(guī)則的次生β相，發(fā)生α′→β固態(tài)轉(zhuǎn)變。不同直徑范圍內(nèi)的Ti-6Al-4V合金凝固組織的顯微硬度與組織形貌相關(guān)，“層片組織”、“ 針狀α′組織”和“針狀α′+不規(guī)則β組織”的顯微硬度隨液滴直徑的減小而增大，“網(wǎng)籃組織”的顯微硬度隨液滴直徑的減小而減小。其中，枝晶組織具有較大的顯微硬度，其值可達(dá)到785 kg/mm2，是母合金硬度的2.6倍。

Especially in the past decades, Ti-6Al-4V alloy has received much attention, not only due to its high melting temperature, good corrosion resistance, low density and high hardness, but also because of the diverse and complicated microstructures formed under different condition. This makes Ti-6Al-4V a potential candidate in both aerospace industries and fundamental science. It is well known that the solidified microstructures of alloy have a great influence on their mechanical properties. Generally, heat treatment is required after the solidification of Ti-6Al-4V alloy so as to improve its mechanical property. However, it is noted that most research on the solidification of Ti-6Al-4V alloy was carried out under equilibrium condition. With respect to Ti-6Al-4V alloy solidified under substantial undercooling conditions, few studies could be found. Thus, it is interesting to study two points: (i) the feature of the microstructure of Ti-6Al-4V alloy solidified under highly undercooled conditions and large cooling rate. (ii) the influence of undercooling and cooling rate on the mechanical property of Ti-6Al-4V alloy. To address these two problems, Ti-6Al-4V alloy was rapidly solidified in a drop tube. The main results are summarized as follows. The microstructure of the Ti-6Al-4V alloy solidified under free fall condition display “l(fā)amellar α+β → α dendrites → basket-weave α′+β → needle-like α′→ needle-like α′+ anomalous β” transformation with decreasing the droplets diameter. And the needle-like α′ phase in the original boundaries of equiaxed β grains is transformed into a continuous distribution and anomalous structure of β phase when the droplet size is less than about 400μm. The microhardness of this alloy ranges from 506 to 785 kg/mm2 when the droplet diameter decreases from 1420 μm to 88 μm, which is much larger than that of the master alloy. For “l(fā)amellar structure of α+β phases”, “needle-like α′ phase” and “needle-like α′ phase+ anomalous β phase”, the microhardness increases with the decrease of droplet diameter. But for “basket-weave” microstructure, the microhardness diminishes with the decrease of droplet diameter.

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