TiAl预合金粉末热等静压致密化机理及热处理对微观组织的影响

DENSIFICATION MECHANISM OF TiAl PRE-ALLOY POWDERS CONSOLIDATED BY HOT ISOSTATIC PRESSING AND EFFECTS OF HEAT TREATMENT ON THE MICROSTRUCTURE OF Ti Al POWDER COMPACTS

采用感应熔炼气体雾化法（EIGA）制备了Ti-47Al-2Cr-2Nb-0.2W-0.15B（原子分数,%,下同）和Ti-45Al-8Nb-0.2Si-0.3B 2种Ti Al预合金粉末,应用SEM,OM和DSC对预合金粉末进行表征.对Ti Al预合金粉末进行热等静压致密化处理,随后对致密化所得Ti Al合金进行热处理,研究了不同时效温度和冷却速率对Ti Al合金微观组织的影响.结果表明,预合金粉末的冷却速率在10^5~10^6K/s之间,随着冷却速率的增加,预合金粉末雾化过程中出现β→α＇的马氏体转变.DSC曲线表明,升温过程中在700~800℃之间发生亚稳α2相→γ相的转变.在热等静压过程中,预合金粉末初始阶段随机堆积,通过粉末颗粒流动、转动和重排实现致密度的提高.随着温度升高α2相转变为γ相;温度进一步升高,粉末颗粒发生显著塑性变形,颗粒间形成烧结颈.随着保温时间的延长,粉末间孔隙主要通过表面扩散、体积扩散和扩散蠕变连接方式完成闭合.Ti-47Al-2Cr-2Nb-0.2W-0.15B预合金粉末热等静压致密化后,其微观组织主要为细小等轴的γ相组织,以及少量的α2相和β相.Ti-45Al-8Nb-0.2Si-0.3B预合金粉末热等静压致密化后,其微观组织主要为细小等轴的γ相组织,以及少量的α2相和弥散分布的硅化物ζ-Nb5Si3.时效温度不同,等轴γ相、等轴α2相和α2/γ片层之间面积分数发生变化,其变化规律主要取决于各相的Gibbs自由能变化.冷却速率对Ti-47Al-2Cr-2Nb-0.2W-0.15B和Ti-45Al-8Nb-0.2Si-0.3B合金连续冷却相变有较大的影响.对于Ti-47Al-2Cr-2Nb-0.2W-0.15B合金,水冷主要形成等轴α2相,油冷、空冷和炉冷都形成全片层组织.对于Ti-45Al-8Nb-0.2Si-0.3B合金,水冷形成α2相和γm相,油冷和空冷形成羽毛状、Widmanst？atten片层和α2/γ片层混合组织,炉冷形成全片层组织.对比2种Ti Al合金连续冷却曲线可知,Nb元素的增加使得连续冷却曲线向无扩散型转变方向发展.

Owing to the low density, high strength, good creep properties at elevated temperatures, TiAl alloy is considered for high temperature applications in aerospace industries. However, a major issue to the industrial ap- plications is the alloy＇s intrinsic brittleness at room temperature. Therefore, extensive efforts have been made to overcome this defect by near net shape fabrication techniques. An alternative for fabricating TiAl alloy is the pow- der metallurgy processing of pre-alloyed powders, and by this technique TiAl alloy with fine and homogenous mi- crostructure can be obtained. In this work, TiAl pre-alloyed powders with nominal composition Ti-47Al-2Cr-2Nb- 0.2W-0.15B （atomic fraction, %） and Ti-45Al-8Nb-0.2Si-0.3B are produced by electrode induction melting gas at- omization （EIGA）. The pre- alloyed powders are consolidated by hot isostatic pressing （HIP）. The effects of heat treatment on the microstructure of TiAl compacts and the influence of cooling rate on the solid-state transforma- tions which occurs during continuous cooling of the TiAl compacts have been studied. It is found that the cooling rate of the pre-alloyed powders is between 10^5-10^6 K/s. As the cooling rate increases, the martensitic transforma- tion, i.e.,β→α＇ occurs in some fine pre-alloyed powders. The heating DSC curves indicate that the transformation from α2 phase to yphase takes place between 700-800 ℃. During the HIP processing, the pre-alloyed powders par- ticles randomly accumulate, and the relative density of HIP compact is increased by the particles moving, rotating and rearranging at the initial stage. As the temperature increases, α2 phase transforms into ）,phase. With further tem- perature increasing, significant plastic deformation and the following formation of sintering necks occur in the pow- der particles. With the annealing time increasing, the pores between the particles are closed by means of surface dif- fusion, volume diffusion and diffusion creep. The microstructure of Ti-47Al-2Cr-2Nb-0.2W-0.15B powder com- pacts consists of fine y and a small number of α2 and ,6; and the microstructure of Ti-45Al-8Nb-0.2Si-0.3B powder compacts consists of fine γphase, a small number of α2 phase and dispersed ζ-Nb5Si3 phase. The area fractions of y phase, α2 phase and α2/γ lamellar structures vary with the annealing temperatures, depending on the Gibbs free en- ergies of the phases. The cooling rate has a significant effect on the continuous cooling transformation of both TiAl powder compacts. For Ti- 47Al- 2Cr- 2Nb- 0.2W- 0.15B alloy, the microstructure is composed of predominant equiaxed α2 phase after water cooling, but of lamellar structures after air, oil or furnace cooling. For Ti-45Al-8Nb- 0.2Si-0.3B alloy, the microstructure is composed of γmphase and large α2 phase after water cooling; after oil and air cooling the alloy consists of a mix of feathery like structures, Widmanstatten laths and lamellar structures; while furnace cooling leads to fully lamellar structures. Comparing the continuous cooling transformation curves, the in- crease of Nb can effectively extend the continuous cooling transformation to the diffusionless area.