Mechanisms of Microstructure Control Using Heat Treatments of TiAl Alloy Castings
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 of γ grains precipitate inside the lamellar colony grains and break the original coarse lamellar colony grains.The nucleation of γ nuclei in the lamellar colony grains comes from Al element segregation and continuous coarsening of γ lamellae.The grain refinement mechanism of NL microstructure heat treatment is that the γ→α transformation takes place in the α single phase region for a short time which destroys the original coarse lamellar colony grains,and the equiaxed γ grains can pin the growth of α grains,so that the lamellar colony grains formed after cooling are smaller.
TiAl alloy castingsHeat treatmentDuplex microstructureNearly lamellar microstructureGrain refinement
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