Microstructural study of high-temperature Cr–Ni–Al–Ti alloys supported by first-principles calculations

Abstract Refractory metals present a potential for development of future high-temperature structural materials due to their high melting temperature and good high-temperature strength. However, their poor high-temperature oxidation resistance, creep resistance, and low fracture toughness at low temperatures have to be addressed. The microstructure of two Cr–Ni–Ti–Al alloys is characterized using XRD, LOM, SEM, and TEM. The microstructure of both alloys is comprised of very dilute Cr solid solution grains, intergranular L21 Ni2AlTi phase, and fine NiTi-rich B2 precipitates dispersed in the Cr-rich grains. The small spherical precipitates have a well-defined orientation relationship with the Cr-matrix, and the lattice mismatch between the precipitates and the Cr-alloy matrix is accommodated by the interface dislocations. In order to explain measured composition of L21 and B2, the site preference in these phases is further studied using the first-principles density functional theory (DFT) method at the low temperature limit. The theoretical calculations predict that Al prefers substituting for the Ti site in B2 NiTi. Structure analysis reveals that Al substituting for Ti results in the formation of strong Al–Ni bonds that have much shorter bond lengths than the Al substitution for the Ni site. The predicted Al site or Cr site preference is consistent with established Al–Ni–Ti ternary experiments. As for the L21-Ni2AlTi phase, DFT calculations predict that Cr prefers substitution with the Ni site over Al or Ti sites. This is related to the smallest relaxation effect and formation of stronger Al–Cr and Ti–Cr bonds that have much shorter nearest neighbor bond length compared to other substitution scenarios.