A Fundamental Study Of Elemental Sublimation During Solution Heat Treatment Of Ni-Base Alloys

Ni-base superalloys are currently the most important material for high-temperature applications used in gas turbines. High amount of refractory elements used in the latest generations superalloys requires heat treatment for dissolution of inter-dendritic phases. However, occurrence of surface defects during heat treatment significantly reduced high-temperature performance of superalloys. To understand the mechanism of defect formation, a fundamental study on discontinuous precipitation and elemental sublimation is carried out. Discontinuous precipitation in single crystal superalloys during solution heat treatment has been examined. It is found that discontinuous precipitation is induced by local chemistry change near the casting surface and is dominated by the loss of Ni through sublimation. A phenomenological model is proposed to illustrate the phase evolution near the casting surface during solution heat treatment of single crystal alloys. Model Ni-Cr-Al model alloys were designed and made for measuring partial vapour pressure and thermodynamic activities of major elements Ni, Al and Cr in Ni-base alloys. Partial vapour pressures of Ni, Cr and Al in the γ phase were measured over the temperature range of 1473-1650K using Knudsen Effusion Mass Spectrometry. The partial vapour pressure of Al is about two orders of magnitude lower than that of Ni, and five times lower than that of Cr, suggesting that the sublimation of Al is almost negligible compared with those of Ni and Cr at solutioning temperatures. The kinetics of Cr sublimation was measured in long-term isothermal experiments at 1573K and it is found that Cr partial vapour pressure reduces significantly during the first 20 hours. Equations for calculating partial vapour pressure and thermodynamic activity at high-temperature range have been obtained. This fundamental study provides an enhanced understanding on microstructure instability of Ni-base alloys during heat treatment, and is useful for optimisation of heat treatment and alloy design of single crystal alloys.