Characterization of intergranular phases in precipitation hardening Ni alloy UNS N07725

Abstract Elucidating the hydrogen embrittlement (HE) mechanisms of precipitation-hardened (pH) Ni-based alloys, such us Alloy 725 (UNS N07725), is of growing interest due to severe failures experienced in oil and gas service. Recent findings have linked the HE susceptibility of Alloy 725 to the coverage of the grain boundaries (GBs) by a nanoscale and brittle precipitate. However, none of the phases expected in Alloy 725 could explain the extent and the deleterious effect of the observed intergranular precipitates. This investigation successfully identified the predominant intergranular phases as M23C6 carbide and F phase—a σ-related topologically close-packed (TCP) phase not previously reported in Alloy 725—in two independent commercial Alloy 725 batches. F phase has a hexagonal P6/mmm structure with a = 12.5 A and c = 4.6 A and shared an orientation relationship with the γ-matrix described as 01 1 ¯ γ / / 2 1 ¯ 1 ¯ 0 F and (111)γ ∕  ∕ (0001)F. Due to the similarities in structure and composition, F phase could be erroneously identified as either σ phase or M23C6. F phase precipitated as approximately 20 nm thick sheets; hence, it could entirely cover the favorably oriented GBs, even at low volume fractions. Both Alloy 725 batches presented a dominant phase at GBs. In this regard, F phase was predominant in the commercial batch that was considered prone to HE in oil and gas service, while M23C6 was the main precipitate in the second independent batch. Our results suggested that the thermomechanical processing of Alloy 725 affects the extent of intergranular precipitation as well as the F phase/M23C6 ratio.