Advanced Engineering Materials: 高Nb-TiAl合金中亚稳α2相在800°C下的演变行为

Rapid cooling from high temperature is a common way in heat treatment and hot processing, which could introduce excess metastable α2 phase into the microstructure. In this paper, the evolution of metastable α2 phase in a water-quenched Ti-45Al-8.5Nb-(W, B, Y) alloy from different single phase regions were experimentally studied. After water-quenching from β single-phase region, Widmanstätten α2, as well as residual βo, γ grains formed from β phase and thin γ laths in α2 phase were observed. However, the water-quenched microstructure from α single-phase region consists of equiaxed α2 grains, residual βo and massive γ phase. When annealing the quenched samples at 800 °C, similar microstructural evolutions were found in both samples quenched from different temperatures: (1) α2 phase transformed into α2/γ nano-lamellar structure immediately. (2) Recrystallization and discontinuous coarsening took place at the colony boundaries. (3) The nanometer-scale γ/γT lamellae transformed from α2 grains are also unstable, which would merge into thicker γ lamellae and the interfaces of coarsened γ lamellae became curve and blurry.

高Nb-TiAl合金作为轻质高温结构材料,在航空发动机上极具应用潜力。通过快速凝固和固态相变能有效细化合金组织、消除偏析、改善室温塑性,从而受到广泛关注。高Nb-TiAl合金快速凝固后,组织中会残留大量亚稳相,特别是低温下的过饱和α2相。在700-900 °C时(TiAl合金的期望服役温度),α2相会发生分解,从而影响材料的组织高温稳定性。

西北工业大学材料学院胡锐教授课题组关注了如何利用快速凝固和固态相变对高Nb-TiAl合金的组织进行调控、优化,同时,通过在不同单相区淬火,对高Nb-TiAl合金非平衡组织和典型的亚稳相(α2相)在中温区的演变进行了系统研究。

本篇报道主要介绍了Ti-45Al-8.5Nb-(W, B, Y)合金从单相区(β和α)淬火获得的亚稳态α2相在800 °C下的演变行为。当合金从β单相区淬火后,组织主要由魏氏体α2相、仅数层原子厚度的γ相板条、α2晶界处的等轴γ晶粒和残余βo相组成;从α单相区淬火后,组织主要由粗大的等轴α2相、块状转变的γ相和少量残余βo相组成。淬火样品在800 °C时效时:一方面,无论是魏氏体α2相还是等轴α2相,都会迅速转变成为层片间距为纳米级的α2/γ片层团;另一方面,由于较高的界面能,这些α2/γ片层团在热力学上也是不稳定的,随着时效时间延长而不断粗化。同时,在能量状态较高的片层团边界处,还会发生不连续粗化和再结晶产生的等轴γ晶粒。此外,组织中的残余βo相在800 °C分解成为ωo和γ晶粒。相关论文以题为“Evolution of Metastable α2 Phase in a Quenched high Nb-containing TiAl Alloy at 800 °C”发表在Advanced Engineering Materials (DOI: 10.1002/adem.201901539)上。(国家自然科学基金51401168和51971176资助。)