Nickel-rhenium compound sheds light on the potency of rhenium as a strengthener in high-temperature nickel alloys

Abstract

For many decades, it has been known that rhenium imparts a tremendous resistance to creep to the nickel-based high-temperature alloys colloquially known as superalloys. This effect is so pronounced that is has been dubbed "the rhenium effect." Its origins are ill-understood, even though it is so critical to the performance of these high-temperature alloys. In this paper we show that the currently known phase diagram is inaccurate, and neglects a stoichiometric compound at 20 at.% Re (Ni 4 Re). The presence of this precipitate at low temperatures and the short-range ordering of Re in fcc-Ni observed at higher temperatures have important ramifications for the Ni-based superalloys. The Ni 4 Re compound is shown to be stable by quantum mechanical high-throughput calculations at 0 K. Monte Carlo simulations show that it is thermally persistent up to ≈930 K when considering configurational entropy. The existence of this compound is investigated using extended x-ray absorption fine spectroscopy on a Ni 96.62 Re 3.38 alloy.

What is the role of rhenium in single crystal superalloys?

“…Considering the activation energy of Re diffusion in Ni, it is reasonable to expect this phase not to form except after very long heat treatments. Above the solvus temperature of the Ni 4 Re (D1 a ) intermetallic, the Monte Carlo simulations predict that relatively low concentrations of Re (0-7 at.%) can form a solid solution with Ni [33]. Simulated x-ray diffuse scattering data reveals remnant short-range ordering of Re in Ni, which gradually reduces as temperature is increased due to configurational entropy [33].…”

“…Simulated x-ray diffuse scattering data reveals remnant short-range ordering of Re in Ni, which gradually reduces as temperature is increased due to configurational entropy [33]. Clustering of Re atoms in the Ni lattice is predicted to be energetically unfavourable at all temperatures [33].…”

“…Results indicate that the Ni 4 Re (D1 a ) intermetallic would not be stable above 600-700 • C due to the effects of configurational entropy. Including vibrational entropy would result in a further reduction of the Ni 4 Re (D1 a ) solvus temperature [33]. Considering the activation energy of Re diffusion in Ni, it is reasonable to expect this phase not to form except after very long heat treatments.…”

“…Above the solvus temperature of the Ni 4 Re (D1 a ) intermetallic, the Monte Carlo simulations predict that relatively low concentrations of Re (0-7 at.%) can form a solid solution with Ni [33]. Simulated x-ray diffuse scattering data reveals remnant short-range ordering of Re in Ni, which gradually reduces as temperature is increased due to configurational entropy [33]. Clustering of Re atoms in the Ni lattice is predicted to be energetically unfavourable at all temperatures [33].…”

“…Investigation of the Ni-Re 0 K formation energy hull indicates that Ni 4 Re (D1 a ) and NiRe 3 (D0 19 ) should be stable at absolute zero [31,32]. High-throughput DFT calculations were also used to parameterise a cluster expansion to determine the effect of configurational entropy on the free energy of the relevant phases through a Monte Carlo approach [31,33]. Results indicate that the Ni 4 Re (D1 a ) intermetallic would not be stable above 600-700 • C due to the effects of configurational entropy.…”