Abstract: Evolution of microstructure during heat treatment and the formation mechanism of incipient melting microstructure of DD6 single crystal superalloy were studied. The effects of different solution and aging treatments on morphology, size distribution and volume fraction of γ′ phase were studied. The microhardness and tensile properties of the alloy after complete heat treatment were analyzed to determine the optimum heat treatment process. The results show that the incipient melting temperature of the alloy determined by differential scanning calorimetry(DSC) and the metallographic observation method is 1300-1310 ℃. The sizes of dendritic core and inter dendritic of γ′ phase are equivalent after solution treatment at 1315 ℃ for 4 h, and γ′ phase is uniformly arranged in cubic form. The melting γ/γ′ eutectic produces irregular incipient melting microstructure during the solution treatment. Under different primary aging processes, γ′ phase has better cubic degree and more uniform size distribution after aging at 1120 ℃ for 4 h and air cooling (AC). The optimum heat treatment of the alloy is 1290 ℃×1 h+1300 ℃×2 h+1315 ℃×4 h, AC+1120 ℃×4 h, AC+870 ℃×32 h, AC. After complete heat treatment, the strength of the alloy reaches the peak as the tensile temperature increases from the room temperature to 850 ℃. As the tensile temperature further increases, the strength decreases. The plasticity is the worst at 760 ℃, and as the tensile temperature raises from 760 ℃ to 950 ℃, the plasticity improves.

Key words: DD6 single crystal superalloy, heat treatment, incipient melting microstructure, evolution of γ′ phase, mechanical properties