The role of nano-sized intergranular phases on nickel alloy 725 brittle failure

Abstract

Although precipitation-hardened nickel alloys, such as Alloy 725, are extensively used in the most aggressive oil and gas production conditions given their a priori superior environmentally assisted cracking resistance, recent failures associated with hydrogen embrittlement (HE) cast doubts about their performance in H-bearing environments. These high-profile incidents have led to extensive research to elucidate the failure mechanism and develop methods to reduce risks. However, the microstructural features that made only specific heats of Alloy 725 susceptible to HE have yet to be agreed upon by the scientific community. Herein, our results showed that the abundant precipitation of F phase, i.e., a recently discovered nano-sized and embrittling phase, at grain boundaries increased cracking susceptibility. A continuous F phase network offered a low-energy path for crack opening by matrix-precipitate interfacial decohesion. Additionally, we correlated Alloy 725 microstructure with HE resistance, which is essential for thermomechanical processing optimization and the design of alternative qualification tests to obtain embrittlement-resistant nickel alloys.