TiAl合金铸件的热处理组织调控机制

Mechanisms of Microstructure Control Using Heat Treatments of TiAl Alloy Castings

TiAl合金铸态组织粗大,致使其铸件强度低、塑性差,必须通过热处理细化铸态组织。利用OM和SEM研究了Ti-47Al-2Cr-2Nb合金铸件不同热处理组织演变规律,优化制定了双态和近片层组织热处理工艺,实现了晶粒细化,揭示了细化机理。结果表明,1185℃/6 h/炉冷双态组织热处理和1185℃/6 h/炉冷+1330℃/0.25 h/炉冷近片层组织热处理可分别将铸态组织晶粒尺寸细化75.51%和40.21%。双态组织热处理晶粒细化机制是在片层团晶粒内部析出大量γ晶粒打断原始粗大的片层团晶粒,γ晶核在片层团晶粒内形核来源于Al元素偏析和γ片层连续粗化。近片层组织热处理晶粒细化机制是在α单相区短时保温时发生了γ→α转变,破坏了原始粗大的片层团晶粒。等轴γ晶粒对α晶粒长大有钉扎作用,使得冷却后形成的片层团晶粒尺寸较小。

The as-cast microstructures of TiAl alloys have low strength and poor plasticity due to their coarse grains,and they must be refined by heat treatment.The microstructure evolutions of Ti-47Al-2Cr-2Nb alloy castings after different heat treatments were studied using OM and SEM.The heat treatments of duplex(DP)and nearly lamellar(NL)microstructures were optimized and established,which realized the grain refinement.The mechanisms of grain refinement were revealed.The results show that the DP microstructure heat treatment of 1185℃/6 h/furnace cooling and the NL microstructure heat treatment of 1185℃/6 h/furnace cooling+1330℃/0.25 h/furnace cooling can refine the as-cast grain size by 75.51% and 40.21%,respectively.The grain refinement mechanism of DP microstructure heat treatment is that a large number ofγgrains precipitate inside the lamellar colony grains and break the original coarse lamellar colony grains.The nucleation ofγnuclei in the lamellar colony grains comes from Al element segregation and continuous coarsening ofγlamellae.The grain refinement mechanism of NL microstructure heat treatment is that the γ→αtransformation takes place in theαsingle phase region for a short time which destroys the original coarse lamellar colony grains,and the equiaxedγgrains can pin the growth ofαgrains,so that the lamellar colony grains formed after cooling are smaller.