Depletion Induced Grain Growth in Alloy 751 After Long Term Aging and Its Effect on Fatigue

Expected service lifetimes for high temperature components can be many thousands of hours, and during this time microstructural degradation including surface oxidation are major concerns for the designer. Nickel based alloy 751 in two different grain sizes was subjected to exposure at 875 ℃ for up to 4,000 h in air. Fatigue testing was performed at 840 ℃ after exposure and the microstructures and fracture surfaces were characterized. Both grain sizes had similar amounts of oxide and alloy depletion, but it was found that the fine-grained material experienced significant depletion induced grain growth in the near-surface region. While 875 ℃ is significantly below the temperatures where grain growth is active for the bulk alloy composition, the depletion of Al, Ti, and Cr in the near surface region decreased the γ′ solvus and increased grain boundary mobility. Property testing was performed on the as-oxidized surfaces as well as post-machined surfaces to study the effect of this microstructural change. The experimental results show that the coarse-grained material had longer fatigue life after 4000 h, and that surface degradation had a more significant effect on the life of the fine-grained material.