| Inconel 718 alloy is a precipitation strengthened nickel-based superalloy,which is operated successfully in aerospace industry due to its excellent mechanical properties at the temperature of-253～650 0C.The γ" phase is the major strengthening phase and the metastableγ" phase will transform into an orthogonal thermodynamic equilibrium δ(Ni3Nb)phase when the alloys are long term service at the temperature above 650 ℃,which will cause the strength and performance rapidly decrease and affect the stability of the microstructures.The components made by Inconel 718 alloy are able to withstand the sudden change of working temperature.Oxidation of the alloy surface at high temperature can reduce the mechanical properties and change the surface condition,which will reduce the safety and reliability of Inconel 718 alloy.Therefore,the effect of different heat treatments on the microstructure and mechanical properties of Inconel 718 alloy were studied,which will provide data reference and science support for the safe use of important components of Inconel 718 alloy.According to the TTT curve,designed four heat treatments of Inconel 718 alloy within the temperature of 720～1040 0C:①720℃×8 h+620 ℃x×8(DA);②960 ℃×1 h+720 ℃×8 h+620 ℃×8(ST);③960℃×2 h+720℃×8 h+620 ℃×8(HST);④1040 ℃×1 h+720 ℃×8 h+620℃×8(GST).Then the direct aging Inconel 718 alloys were aging at 680 ℃ for 20～100 h,and the thermal shock test were conducted between 960 ℃ and room temperature for 40 times on ST,HST,DA alloy.The OM,SEM,EDS and XRD were used to analyze the change regular of microstructure,composition and phase.In addition,the effect of different heat treatments on mechanical properties of Inconel 718 alloy was investigated by using hardness tester and tensile testing machine.Based on the above research,the following conclusions were drawn:(1)The influence of four heat treatments on micro structure and mechanical properties of Inconel 718 alloy was studied.The experimental results shown that δ phase distribute along the boundary can prevent the grain growth,which caused the grain size remains unchang after solide solution under 960 ℃.When the solid solution temperature rised to 1040 ℃ for 1h,the grain coarsening occurd due to the dissolution of 8 phase into the matrix,and the mechanical properties were significantly reduced.The alloy obtained the maximum strength and hardness after direct aging(DA)treatment.After solid solution at 960 ℃ for 2 h(HST),the alloy obtained a favorable match between the grain strength and intragranular strength,which improved the plasticity obviously.Fractural analysis by SEM demonstrated that the fracture of alloys after four heat treatments have mainly characteristics of dimples.(2)After aging at 680 ℃ for 20～100 h,the microstructures of Inconel 718 alloy were all isoaxial crystal,and the grain sizes were basically unchanged.The γ" phase was gradually changing to δ phase,and the rate was slow.The amount of δ phase increased continuously.The morphology of the 8 phase on the grain boundary changed from granular and rod-like to parallel needle-like,and the intra-grained δ phase appeared needle-like.During the process of aging at 680 ℃ for 20～100 h,the strength of the alloy decreased and elongation increased.After aging at 680 ℃ for 100 h,the microstructure and properties were also stable.The tensile fracture modes of all alloys were ductile fractures and its fracture mechanism were all microporous aggregate fracture.The δ phase and carbides is the core of micropore during the formation of the fracture.(3)After thermal shock test between 960 ℃ and room temperature for 40 times,black gray oxide films were formed on the surface of Inconel 718 alloys(ST,HST,DA),which composed of Cr2O3,Al0.5CNi3Ti0.5 and Cr1.3Fe0.7O3.The average thickness of oxide films increased with the thermal shock cycle.The thermal shock behavior changed the distribution and quantities of δ phases,which have significantly reduced the strength and increased the elongation of Inconel 718 alloy.The above analysis shows that the thermal shock test is detrimental to the mechanical properties of Inconel 718 alloy and is not conducive to long-term use of the alloy in an environment with a temperature gradient. |
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