Effect of Solution Temperature on the Microstructure Evolution and Tensile Properties of GH4175 Superalloy

In the present study, GH4175 superalloy was treated in the solution temperature range of 1100 ℃–1160 ℃ for 2 h, followed by unified aging treatment at 760 ℃ for 20 h. The microstructure of one step (only solution) and two-step (solution and age) treatment was compared to analyze the change of microstructure characteristics at different solution temperatures. The specimens after two-step treatment were subjected to tensile tests at 750 ℃, and corresponding fracture morphologies were observed to reveal the fracture behavior of GH4175 superalloy. The results show that the grain size of γ matrix increases from 4.6 μm to 6.8 μm as solution temperature increases from 1100 ℃ to 1140 ℃, but the volume fraction of primary γ´ particles decreases from 18.7% to 7.2%. The grain size of γ matrix significantly coarsens to 78.8 μm after solution treatment at 1160 ℃ because the dissolution of primary γ´ particles weakens the pinning effect. The secondary γ´ particles are refined after two-step treatment. An optimal yield strength of 1166 MPa is obtained by solution treatment at 1140 ℃ for 2 h followed by aging at 760 ℃ for 20 h. The re-precipitated secondary γ´ particles of GH4175 provide the major strengthening effect. However, as the solution temperature increases to 1160 ℃, the yield strength and elongation decrease to 1049 MPa and 2.88% respectively, which is attributed to the combined brittle intergranular fracture effect of coarsen γ grains and the distribution of undissolved carbides.