Effect of stabilizing treatment on microstructure and stress rupture properties of phosphorus microalloyed Inconel 706 alloy

The microstructure and stress rupture properties of Inconel 706 alloy microalloyed with phosphorus are examined under stabilizing and unstabilizing heat treatment conditions. It was found that applying the stabilizing treatment resulted in a 98% increment in the stress rupture life and a 215% increment in the elongation tested at 650 °C/690 MPa for the alloy compared to that under the unstabilizing heat treatment condition. The stabilizing treatment led to the precipitation of rod-shaped and needle-shaped η phases at the grain boundaries. Morphologies of γ′–γ″ co-precipitates in the grain interior were noncompact form and compact form for the alloy under unstabilizing and stabilizing heat treatment conditions, respectively. Based on the microstructure characterizations, the improvement of stress rupture properties by the stabilizing treatment was attributed to the precipitation of η phases at the grain boundaries, which can hinder cracks initiation and propagation and relieve the stress concentration.