Effect of long-term thermal exposure on microstructure evolution and high temperature tensile properties of Mar-M247 alloy
Microstructure evolution and 800℃ high temperature tensile strength and plasticity of equiaxed Mar-M247 alloy during thermal exposure at 800℃ for up to 24000 h were investigated.The results show that before 5000 h of thermal exposure,the size of the dendrite secondary and tertiary γ'phases in the alloy increases significantly.However,after 5000-24000 h of thermal exposure,there is no significant change in the size of the dendrite secondary γ'phase,while the size of the dendrite tertiary γ'phase slightly increases.With the increase of thermal exposure time,the volume fraction of the dendrite secondary y'phase increases,while the volume fraction of the dendrite tertiary γ'phase decreases.According to the precipitate size distribution(PSD)of the two γ'phases,the coarsening of both y'phases conforms to the the trans-interface diffusion-controlled(TIDC)coarsening theory.The MC type carbides at the grain boundaries gradually decompose through the MC+γ→M23C6+γ'reaction,forming γ'phase film at the grain boundaries,enveloping the MC type carbides and M23C6 type carbides.The results of high-temperature tensile tests show that with the increase of thermal exposure time,the yield strength and tensile strength of the alloy first decrease and then remain basically unchanged,while the elongation and reduction of area first increase and then decrease and then increase again.After 5000 h of thermal exposure,the yield strength and tensile strength decrease by 164 and 76 MPa,respectively.However,after 5000-24000 h of thermal exposure,the yield strength and tensile strength remain stable at 750 and 950 MPa,respectively.
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