**1. Introduction**

High-temperature structural materials have attracted considerable interest in the world of materials research for many years. There is a huge demand for materials that can resist extreme mechanical, thermal and chemical environments. Ni-based super-alloys (NBSA) are currently used for high-temperature application due to their phenomenal properties such as high creep strength, good ductility at elevated and room temperature environments, low density and high melting points. Despite the accomplishment of NBSA, 90% of Ni's melting point have already been exploited [1]. Many metal alloys are currently being studied [2] as potential alternatives to NBSAs.

Currently, ruthenium (Ru) based alloys have been under intense study [3, 4] due to their attractive combination of physical and mechanical properties, including high melting point and good oxidation and corrosion resistance. Furthermore, Ru has the capability to increase the microstructural stability of other material systems [3]. In particular, Ru (2334 °C) has a superior melting point compared to Ni (1543 °C), making Ru-based alloys suitable for high temperature structural applications. Previously in Ru–Cr phase diagram, many structures such as Cr3Ru, Cr2Ru and Cr4Ru phases were found to exist experimentally in different temperature formations [4–6], while the narrow homogeneity range of 31.5 atm% Ru and 32–36 atm% Ru for A15 Cr3Ru and Cr2Ru (σ phase) have been identified by Venkatraman and Neumann [7] respectively. Recent studies in this class of alloys have projected phase stability in a several X-Ru (X = Mo, Ti, V, Hf, Ir, Os, Pt, Ta, Tc, Mn and Zn) binaries at low temperatures [8, 9]. Ruthenium alloys with platinum and palladium make extremely durable electrical contacts and resistors. Ruthenium thin films are used in hard disk drives and plasma display panels [8]. The addition of ruthenium improves the mechanical properties and corrosion resistance of titanium, platinum, palladium, gold, and nickel-based superalloys used in jet engine turbine blades [10]. Also, the addition of ruthenium in modern nickel superalloys inhibits the formation of topographical closed packing (TCP) phases, thereby extending their creep capability to higher temperatures [11–13].

In this chapter, the structural, magnetic, electronic and elastic properties of the A15 Ru-based alloys in the X3Ru (X = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu and Zn) crystal phase are investigated using first principles density functional theory calculations. All the structures proposed here in A15 phase are new except A15 Cr3Ru that exist experimentally [7] as stated above. Therefore more experimental research is needed specifically for these novel alloys studied herein. We determine the heats of formation, density and magnetic moments, these properties are very important in aerospace and spintronic applications. Stability study based on heats of formation can be used to identify suitable X3Ru material for high temperature structural application. The electronic properties such as band structures and density of states are useful to provide valuable information about a material's conducting characteristics at the Fermi energy level. Knowledge of the values of elastic constants (Cij) is crucial in describing the mechanical resistance in a crystal when external stresses are applied. From the Cij's, we can determine the bulk, shear and Young's modulus that provides information about the strength of the material. To gain deeper understanding of X3Ru alloys, we compute more properties such as anisotropic factor and Poisson's ratio. The computed properties are compared with the available theoretical and experimental results. The results found herein will pave way to recommend new metals in elevated temperature applications.
