Research On Microstructure And Properties Of FGH97 Powder High Temperature Allo

FGH97 alloy is a nickel-based superalloy which was developed in China.And its chemical composition is similar with EP741 NP alloy,made by Russian.FGH97 alloy is one of the leading candidate materials for manufacturing aero-engine turbine discs and other hot components due to its superior low-cycle fatigue performance and creep strength.The excellent performance mainly depends on the quantity,morphology and particle size distribution of γ′ strengthen phase.And it is worth to investigate the dissolution and precipitation behavior of γ′ phase during solution heat treatment due to its theoretical significance and engineering value.As one of the three main defects of powder met allurgy(P/M)superalloys,thermal induced porosity(TIP)is the main factor that deteriorate the performance of P/M superalloys and lead to the deformation during heat treatment.Hence,the dissolution and precipitation behavior of γ′ phase,evolution of microstructures,microhardness and TIP growth regulation of FGH97 alloy during solution heat treatment were studied in this paper,which can be used to provide theoretical basis and experimental reference for the selection of solution heat treatment process of FGH97 alloy.Dissolution behavior of γ′ phase of hot isostatic pressed FGH97 nickelbased P/M superalloy was systemically studied at solution temperatures ranging from 1150 to 1200 °C and dwell time ranging from 60 to 480 s by thermophysical simulation.The results showed that the dissolution behavior of γ′ phase was obviously affected by solution temperatures and dwell time.When the temperature was low,γ′ phase was not completely dissolved in matrix.And the dissolution rate of secondary γ′ phase was higher than primary γ′ phase.When the temperature was close to or higher than the temperatures of γ′ solves,γ′ phase was completely dissolved in the matrix.The area fraction of γ′ phase decreased with the increase of dwell time and then gradually approached to a thermodynamic equilibrium.The dissolution kinetics model of γ′ phase and the grain growth model of austenite grain of FGH97 alloy were established by JMAK model and Beck model.According to the experimental data,it was found that dissolution rate decreased rapidly at the beginning stage with the increase of dwell time,then approached to 0.Meanwhile,the dissolution active energy increased with the increase of dwell time.Thermophysical simulation was used to investigate the precipitation behavior of γ′ phase of FGH97 alloy during continuous cooling at the cooling rates ranging from 0.1 to 100 °C/s.And the effect of cooling rates on the microstructure and microhardness were discussed.It was found that nucleation density,morphology,and particle size distribution of secondaryγ′ phase were sensitive to the cooling rates.As the cooling rate decreased,secondary γ′ phase morphologies evolved from sphere or cuboids,concave cuboids and octets which affected by elastic strain energy and interfa cial energy.Also,the nucleation density of secondary γ′ phase increased with the increase of cooling rates.However,the size of secondary γ′ phase decreased with the increase of cooling rate.The particle size distribution of secondary γ′ phase presented in a bimodal distribution when the cooling rate is lower than 1 °C/s.With the increase of cooling rates,the size of secondary γ′ phase exhibited a unimodal distribution.And,it was found that both secondary γ′phase and tertiary γ′ phase exited in matrix under the change of matrix supersaturation and element diffusion two joint action at a lower cooling rate.Also,primary γ′ phase gradually coarsened and its leaded to the formation of serrated grain boundary.Based on the statistical results of microstructures,the linear model between cooling rates and the average size of secondary γ′ phase was established in logarithmic coordinate system.In the end,effects of solution heat treatment parameters on the growth of the TIP of FGH97 nickel-based P/M superalloy were investigated.Holding temperatures were ranging from 1150 to 1200 ° C,and holding time was ranging from 0.5 to 8 h.The results showed that the holding time,temperatures,and the initial volume of porosity were the primary factors affecting TIP growth behavior,and the volume fraction of TIPs increased by increasing the temperatures or extending the holding time.Compared with the experimental data,the porosity growth models implying that the proposed TIP growth models have greater accuracy and can be used to illustrate TIP behavior in solution heat treatment were constructed based on the porosity statistics at different solution heat treatment processes combined with a nonlinear fitting method.