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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105~106K/s之间,随着冷却速率的增加,预合金粉末雾化过程中出现b→a’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a2相→g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a2相转变为g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g相组织,以及少量的a2相和b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g相组织,以及少量的a2相和弥散分布的硅化物x-Nb5Si3.时效温度不同,等轴g相、等轴a2相和a2/g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a2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a2相和gm相,油冷和空冷形成羽毛状、Widmanst?tten片层和a2/g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.
Ti-47Al-2Cr-2Nb-0.2W-0.15B (atomic fraction, the same below) and Ti-Al-8Nb-0.2Si-0.3B two kinds of TiAl were prepared by induction melting gas atomization method (EIGA) Pre-alloyed powders were characterized by SEM, OM and DSC. The hot-isostatic pressing of TiAl pre-alloyed powders was carried out, and then the densified TiAl alloy was heat-treated. The effects of different aging temperature and cooling The results show that the cooling rate of the prealloyed powders is in the range of 105 ~ 106K / s. With the increase of cooling rate, the martensite of b → a ’appears during the atomization of the prealloyed powders The DSC curves show that the metastable a2 phase → g phase transition occurs between 700 and 800 ℃ during the heating process.In the process of hot isostatic pressing, the initial phase of the prealloyed powder accumulates randomly, and through the flow of powder particles, And rearrangement to achieve the increase of density.When the temperature rises, the phase a2 changes to the phase g, and the temperature increases further, the plastic deformation of the powder particles occurs, and the sintering neck forms between the particles.With the holding time prolonging, Closed by surface diffusion, volume diffusion and diffusion creep connection .Ti-47A The microstructure of l-2Cr-2Nb-0.2W-0.15B pre-alloyed powders was mainly composed of fine isometric g-phase microstructure and a few a2 and b phases after hot isostatic pressing 0.2Si-0.3B pre-alloyed powders after hot isostatic pressing densification, the microstructure mainly for the small equiaxed g-phase microstructure, and a small amount of a2 phase and dispersed distribution of silicide x-Nb5Si3 aging temperature is different from the equiaxed g-phase, the fraction of the area between the equiaxed a2 phase and the a2 / g lamella changed, and its variation mainly depends on the Gibbs free energy of each phase. The cooling rate of Ti-47Al-2Cr-2Nb-0.2W-0.15B And Ti-45Al-8Nb-0.2Si-0.3B alloy has a significant effect on the continuous cooling transformation.For the Ti-47Al-2Cr-2Nb-0.2W-0.15B alloy, the main cooling water forms the equiaxed a2 phase, For the Ti-45Al-8Nb-0.2Si-0.3B alloy, a2 phase and gm phase were formed by water cooling, oil-cooled and air-cooled to form feathering, Widmanstatten sheet and a2 / g The results show that the addition of Nb increases the continuous cooling curve to non-diffusive transitions compared with the continuous cooling curves of two TiAl alloys.