The effect of alloy additions on the rod-plate transition in the eutectic NiAl−Cr

Alloys of the eutectic NiAl-34 at. pct Cr with additions of Mo, V, W, and Fe were directionally solidified to determine the effect of the addition element and solidification rate on structure. The directionally solidified NiAl?Cr consists of 〈100〉-oriented columnar grains with long chromium rods of round cross section. The addition of 0.6 at. pct Mo or 0.7 at. pct W or 1.3 at. pct V causes a transition from the round rod grains to 〈111〉-oriented grains with faceted rods and plates of Cr(Mo), Cr(W), or Cr(V). At these compositions, the lattice parameters of the chromium-rich phase and the NiAl matrix phase are nearly equal. Additions of iron produced only the round rod structure as the difference of the lattice parameters was increased. High temperature anneals changed the nonequilibrium faceted rods to rods with regular hexagonal cross sections. Coherency strains near zero mismatch may influence the morphology and may be responsible for the effect of mismatch on structure. Further additions of molybdenum, tungsten, and vanadium resulted in a gradual reduction of the number of faceted rods and finally, the formation of cells with radial plates. Higher solidification rates formed cells and dendrites. A model relating cell and dendrite formation to freezing range of the eutectic alloy was derived.