Effect of ternary alloying elements additions on the structural and mechanical properties of B2 NiAl-X intermetallics

Abstract

NiAl intermetallics have been widely studied due to their interesting properties, such as low density, high melting point, thermal conductivity and most importantly, very good oxidation resistance for structural applications at high temperatures [1]. The B2-NiAl system is a thermodynamically stable, ordered intermetallic that accepts ternary elements additions (e.g. Co, Cr, Mo, Pt, among others) in a wide range of compositions. Since B2-NiAl is a brittle material at low temperature ternary alloying elements additions have been intensively studied. For instance, ductility of B2-NiAl is improved by small additions of Fe, whose strengthening mechanism has been related to preferential deformation along specific slip planes directions [2]. However, a general approach about the effect of alloying elements on the structural stability and mechanical properties (e.g., phase transformations and elastic constants) of B2-NiAl during oxidation remains to be understood. In this work, a set of B2 NiAl-X (X=0, 3, 5, 7, 10, 15 and 34 at% Cr) samples with compositions were processed by high-energy ball milling from Al, Ni and Cr precursor powders. Bulk coupons were obtained by hot pressing (HP) and characterized by X-ray diffraction, scanning electron microscopy and nanoindentation. The superlattice (100), (111), (210) and fundamental (110), (200) and (211) peaks of the binary and ternary B2 phases were observed. The hardness and reduced elastic modulus were determined by nanoindentation. This work was extended on the basis of a theoretical approach to study the stability and elastic-plastic behavior of the B2-phase as a function of Co, Cr, Fe, Mo, and Pt additions in the range of their solubility limits. The theoretical calculations were performed by means of electronic structure first principle calculations of NiAl-X (X=Cr, Fe, Pt, Co, Mo) using the Spin Polarized Relativistic Koringa-Kohn Rostoker (SPRKKR) code. The lattice parameter was obtained by evaluating the calculated total energy of the crystal. The main elastic constants for cubic systems, as well as the bulk modulus were estimated by applying volume-conserving orthorhombic (C11, C12) and monoclinic (C44) deformations of the B2 lattice and fitting the total energy to the Bich-Murnaghan equation of state. The calculations show good agreement between the experimentally determined ordering and elastic-plastic behavior of the B2 phase. [1] Noebe, R.D., Bowman, R.R., and Nathal, M.V., International Materials Reviews, 1993, vol. 38, no 4, p. 193-232. [2] Darolia, R., Larman, D., and Field, R.D., Scripta Metall Mater, 1992, vol. 26, no. 7, p. 1007-1012