Study of phase states and oxidation of B2-structure based Al–Ni–Ru–M alloys

Abstract

Phase states and oxidation resistance of B2-structure based Al–Ni–Ru–M alloys have been investigated on equilibrium alloys and by the diffusion couple approach. The isothermal cross-sections of the Al–Ni–Ru phase diagram have been partially defined in a range of Al<70 at.% at 900 and 1100 °C. The existence of a miscibility gap between AlRu (β1) and AlNi (β2) phases in the Al–Ni–Ru ternary system has been clearly established. The following phase equilibria have been experimentally identified: β1-AlNi 3, β1–β2, β2-Al 2Ru, Al 2Ru–AlNi 3–Al 13Ru 4 at 900 °C and β1–β2, β2-Al 2Ru at 1100 °C. The high temperature properties of B2-structure based Al–Ni–Ru–M alloys are presented. All the studied samples display high melting points up to more than 2100 °C. Oxidation of Al–Ni–Ru–M B2 alloys in laboratory air has been found to follow parabolic law at 950 and 1050 °C and it is slower than that of a commercial MCrAlY alloy. There is no remarkable difference of oxidation mechanisms of the B2 structure based Al–Ni–Ru–M alloys at 950 and 1050 °C. Protective α-Al 2O 3 scale develops already after 100–150 h of oxidation. The protective α-Al 2O 3 scale (i) is mono-phased (α-Al 2O 3), (ii) is dense, (iii) does not contain remarkable amount of Ni and Ru and (iv) has good adhesion to bulk alloy. A dense continuous Ru underlayer forms between the α-Al 2O 3 film and the bulk alloy for compositions with Ru>25 at.%. The Ru layer can contain considerable amounts of Cr or Ir and exhibits good adhesion to both the scale and the bulk alloy. Physical properties of the B2 structure based Al–Ni–Ru–M alloys make them good candidates for oxidation resistant coatings in different high temperature industrial applications.