Influence of molybdenum and rhenium alloying additives on NiAl–Cr–Co cast alloy structure and properties

Abstract

A centrifugal SHS casting technology was used to obtain NiAl–Cr–Co–(X) alloys where X = 2.5÷15.0 wt.% Mo and up to 1.5 wt% Re. The study covers the effect of modifying additives on the combustion process as well as the phase composition, structure, and properties of cast alloys. Alloying up to 15 % Mo and 1.5 % Re provided the highest improvement of properties in relation to the base alloy in terms of overall performance. Molybdenum formed a plastic matrix and improved strength properties to the following values: uniaxial compressive strength σucs = 1730±30 MPa, yield strength σys = 1560±30 MPa, plastic component of deformation εpd = 0.95 %, and annealing at t = 1250 °С improved them to: σucs = 1910±80 MPa, σys = 1650±80 MPa, εpd = 2.01 %. Rhenium modified the alloy structure and improved its properties to: σucs = 1800±30 MPa, σys = 1610±30 MPa, εpd = 1.10 %, and annealing further improved them to: σucs = 2260±30 MPa, σys = 1730±30 MPa, εpd = 6.15 %. The mechanical properties of the NiAl, (Ni,Cr,Co)3Mo3C, Ni3Al, (Cr, Mo) and MoRe2 phases, as well as the hypothetical Al(Re,Ni)3 phase, were determined by the nanoindentation method. According to the Guinier–Preston structural transformation, local softening upon annealing at t > 850 °С increases the proportion of plastic deformation during compression tests due to the lost coherence of the boundaries of nanosized plate-shaped Cr-based precipitates with a supersaturated solid solution. A hierarchical three-level structure of the NiAl–Cr– Co–15%Mo alloy was established: the first level is formed by β-NiAl dendritic grains with interlayers of molybdenum-containing phases (Ni,Co,Cr)3Mo3C and (Mo0.8Cr0.2)xBy with a cell size of up to 50 μm; the second one consists of strengthening submicron Cr(Mo) particles distributed along grain boundaries; the third one is coherent nanoprecipitates of Cr(Mo) (10–40 nm) in the body of β-NiAl dendrites. The cast alloy mechanical grinding techniques were used to obtain a precursor powder with an average particle size of Dav = 33.9 μm for subsequent spheroidization.