Microstructural stability and strengthening mechanism of a Ni3Al-Mo based single-crystal superalloy containing Re element during long-time thermal aging

AbstractThe stability of microstructure and mechanical property in Ni3Al-Mo based single-crystal superalloy aged at 1100°C for long time was investigated. The grown-dynamic law of γ strengthening phase and strengthening mechanism of the alloy were discussed deeply. The observation of microstructure and analysis of composition were conducted by using Scan Electron Microscope (SEM) with Electron Probe (EPMA/EDS) and the size of the phase was measured by quantitative metallographic technology. The experimental results showed that there were some short rod-like Y-(Ni,Re)Mo particles precipitated in the alloy aged at 1100°C for 100h and some needle-like Y-(Ni,Re)Mo particles fromed in the alloy aged at 1100°C for more than 250h. The amount and size of Y-(Ni,Re)Mo particles increased obviously with the increment of aging time, but there was no any TCP phase formed duing the aging treatment. The γ phase in the alloy was strengthened due to the Mo and Re elements solid-solution. Since the volume fraction of γ phase in the alloy is relatively low, the superior high-temperature strengthening effect in the alloy was achieved by adding a small amount of Mo and Re strengthening elements in the alloy. When the amount of Mo and Re elements in the alloy was increased, the Y-(Ni,Re)Mo phase will precipitate from the alloy, which decrease the high temperature properties of the alloy. The hardness of the alloy aged at 1100°C gradually reduced about 7% with the increase of aging time which might be due to the precipitating of Y-(Ni,Re)Mo particles from the alloy and coarsening of γ phase in the alloy. Through designing the alloy composition and optimizing heat treatment process, a superior strengthening effect can be obtained by the best amount of strengthening element addition and proper heat treatment process, which guaranteed the stability of microstructure, mechanical property and the low cost of the alloy