Influence of Molybdenum and Rhenium Dopants on the Structure and Properties of NiAl–Cr–Co Cast Alloy

Centrifugal SHS casting has been used for the production of NiAl–Cr–Co–X alloys, where X = 2.5–15.0 wt % Mo and up to 1.5 wt % Re. The influence of modifying additives on burning has been studied, as well as on the phase composition, structure, and properties of cast alloys. The addition of up to 15% Mo and 1.5% Re in terms of all properties makes the highest improvement of the properties with regard to basic alloy. Molybdenum, while forming the plastic matrix, improves strength properties up to the following levels: uniaxial compressive strength σ ucs = 1730 ± 30 MPa, yield strength σ ys = 1560 ± 30 MPa, and plastic deformation constituent ε pd = 0.95%; annealing at t = 1250°C improves them to σ ucs = 1910 ± 80 MPa, σ ys = 1650 ± 80 MPa, and ε pd = 2.01%. Rhenium modifies the alloy structure and improves its properties to σ u = 1800 ± 30 MPa, σ ys = 1610 ± 30 MPa, and ε pd = 1.10%, whereas annealing improves them to σ ucs = 2260 ± 30 MPa, σ ys = 1730 ± 30 MPa, and ε pd = 6.15%. Using the nanoindentation method, the mechanical properties of NiAl, (Ni,Cr,Co) 3 MO 3 C, Ni 3 Al, (Cr,Mo), and MoRe 2 phases, as well as of Al(Re, Ni) 3 hypothetic phase have been determined. It has been demonstrated that local softening upon annealing from t > 850°C improves the portion of plastic deformation upon compression tests, which is related with the loss of coherency of boundaries of nanosized disk deposits on the Cr base with an oversaturated solid solution similar to the Guinier–Preston structural transformation. The three-level hierarchical structure of the NiAl–Cr–Co–15% Mo alloy has been established: the first level is formed by β-NiAl dendritic grains with interlayers of (Ni,Co,Cr) 3 Mo 3 C and (Mo 0.8 Cr 0.2 ) x B y molybdenum containing phases with cell sizes up to 50 μm, the second level is comprised of Cr(Mo) strengthening submicron particles distributed along the grain boundaries, and the third level is comprised of coherent Cr(Mo) nano deposits (10–40 nm) in the body of β-NiAl dendrites. Using procedures of mechanical grinding of the cast alloy, a powdered precursor has been obtained with an average particle size of D avg = 33.9 μm for subsequent spheroidization.