Effect of γ′ Size on the Stress‐Rupture Behaviour of Inconel 740H Superalloy

Stress‐rupture behaviour of the polycrystalline Ni‐based superalloy, Inconel 740H, at temperatures of 750°C and 800°C, in its peak‐aged and two long‐term aged conditions (800°C for 3200 and 10 250 h) is studied. Long‐term ageing leads to the growth of γ′ precipitates as well as grain boundary carbides, which form a continuous network along grain boundaries. The rupture time, t r , at 750°C, is highest for the peak‐aged sample but lower for the long‐term aged samples for higher rupture stresses, σ r , whereas a reversed trend in variations of t r of the three samples is observed at lower σ r . At 800°C, only the decrease in t r of the long‐term aged samples at higher σ r is observed. Larson Miller parameter (LMP) vs. σ r plots confirm the insensitivity of t r to the ageing duration for σ r < 290 MPa and a decrease in t r with increasing ageing duration above this σ r . Postfacto microstructural imaging of ruptured samples reveals the presence of stacking faults, besides dislocations, in the matrix and within the significantly coarsened γ′ precipitates. The relative insensitivity of t r in the three samples at lower σ r is attributed to the significant contribution to creep resistance via dislocation trapping by γ′ precipitates, whereas the sensitivity of t r to the aged state, at higher σ r , is attributed to variations in the precipitate size‐dependent hindrance to dislocation glide. Implications of these results in the context of stress‐rupture behaviour and creep resistance of 740H‐based components in service are discussed.