Effect of γ′ precipitation during hot isostatic pressing on the mechanical property of a nickel-based superalloy

The effect of the precipitation of γ′ phase during hot isostatic pressing (HIPing) on the mechanical property of a nickel-based superalloy, GTD-111, was evaluated by conducting tensile and creep-rupture tests at 871 °C. In the 4-h two-step HIP process, the coupons were isostatically compressed (at 120 MPa) and heated to 1230 °C, well above the dissolution temperature of γ′ precipitates into the γ matrix, for the first 2 h, and cooled down to a temperature to induce the precipitation of γ′ phase and held for the last 2 h at 120 MPa or at ambient pressure. The precipitates were controlled in size by varying the temperature for the last half of the process. According to the result of the tensile test, the mechanical properties of the alloy were varied upon the microstructural evolution, and improved more than 40%, compared to those of the untreated ones. The precipitation of γ′ phase under high pressure further improved in the properties, suggesting that the precipitation of γ′ phase at high pressure provides an advantage for the rigidity of the structure. Based on these findings, a 6-h three-step HIP process was tried, and proved to be an effective substitute for the normal heat treatment, especially in terms of creep properties. This feature was mostly attributed to the homogenized microstructure of HIPed ones, as evidenced by the X-ray diffraction patterns.