Effect of high-temperature long-term aging on microstructure and properties of HT700R alloy
The study was conducted by employing scanning electron microscopy(SEM)and transmission electron microscopy(TEM),in conjunction with a universal tensile testing machine,to investigate the microstructure evolution and mechanical properties of HT700R superalloy were investigated during both short-term and long-term aging at elevated temperatures.The results show that,after short-time aging,fine γ′ phases are uniformly dispersed within the grains of HT700R alloy,while granular σ phases and rod-like γ′ phases are observed scattered along grain boundaries,and its ultimate tensile strength,yield strength,and elongation are 638 MPa,1042 MPa,and 24.5%,respectively,and the fracture type is a combination of toughness and quasi-cleavage fracture.After long-term aging,the intragranular γ′ phase is gradually coarsened and the volume percentage increases,Cr-rich phases transforms into M23C6 phases,TiC phase is firstly coarsened and then partly degraded to form M23C6 phase;the discontinuous γ′ phase,M23C6 phase,and TiC phase at the grain boundaries are gradually coarsened to be connected,resulting in the formation of localized precipitate-free zones(PFZs).The mechanical properties of the HT700R alloy after long-term service show an increase in tensile and yield strengths of 7.2%and 16.6%,respectively,but a sharp decrease in plasticity and impact absorbing energy of 24.1%and 48.4%,respectively.The larger-sized chain-like precipitates(M23C6,TiC,etc.)at grain boundaries are connected and broadened,leading to the occurrence of brittle fracture.
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