Quantitative description of the role of Cr on the ordering characteristics of a single phase Ni-16 wt.% Mo- 7wt.% Cr alloy

Highlights: • Effect of Cr addition on the order-disorder phase transformation behaviour of Ni-Mo alloys is investigated to assess the suitability of Hastelloy N as a structural material for molten-salt breeder reactor. • EXAFS study of the SRO state of as-quenched Ni-29.5Mo alloy shows that no Mo-Mo bonds exist in the first coordination shell. • Thermal ageing leads to formation of D1_a type coherent LRO precipitates in Ni-29.5Mo, but not in Ni-16Mo-7Cr alloy. • EXAFS analysis establish that addition of Cr in Ni-Mo alloys destabilizes not only the D1_a LRO phase but also the SRO state by preferentially bonding with Ni in the first coordination shell in the fcc lattice replacing Mo. • First-principles calculated nearest neighbour interatomic interaction energies for Ni-Mo and Ni-Cr systems, which govern the ordering tendency, clearly indicate that addition of Cr suppresses the formation of D1_a phase. Binary and ternary Ni-Mo based alloys under the service conditions of high temperature show precipitation and chemical ordering which influence their microstructure, mechanical and corrosion properties. In the present work, the phase transformation behaviour of Ni-Mo and Ni-Mo-Cr alloys is studied to identify the role of Cr in altering the stability of brittle ordered intermetallic phases. TEM characterization demonstrates that the addition of Cr destabilizes not only the long-range ordered D1_a phase but also the short-range order represented by ordering wave vectors. EXAFS studies on Ni-Mo and Ni-Mo-Cr alloys establish preferential bonding of Cr with Ni as the first nearest neighbour in the fcc lattice replacing Mo. The cluster-expansion based calculations on Ni-Mo, Ni-Cr and Cr-Mo systems show that the nearest neighbour pair and multisite interaction energies are changed in magnitude as well as in sign for Ni-Mo and Ni-Cr systems signifying that the D1_a type ordering tendency is reduced with Cr addition and the disordered fcc phase is stabilised. A combination of experimental and theoretical studies unequivocally establishes the stability of the disordered fcc structure, which is crucial for the long-term use of the selected Ni-Mo-Cr alloy at elevated temperatures.