Abstract
It is shown that the mechanical properties of single-crystal superalloys can be optimized through appropriate solidification conditions and heat treatments. A comparison of the creep behaviour of low- and high-gradient processed single crystals of CMSX-2 alloy shows that minimizing porosity improves the creep strength at intermediate temperatures. The optimum creep resistance is achieved by aligned γ′-precipitates, about 0·5 μm in size. It is shown that in high-cycle fatigue the solidification conditions have a dramatic effect on the fatigue strength. The improvement in fatigue behaviour is attributed to the extremely low density and small size of pores achieved by using a very high-gradient process. The strain–controlled low–cycle fatigue properties of CMSX-2 at intermediate temperatures are also greatly improved by using the high-gradient process. Elimination of porosity in ‘industrial’ single crystals by hot isostatic pressing restores their creep and especially their fatigue performance to the level of the high-gradient solidified material.
MST/224
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