Microstructural Evolution in Multiphase NiAl-2.5Ta-7.5Cr Alloy during Annealing at Different Temperatures

The microstructure evolution of NiAl-2.5Ta-7.5Cr alloy during annealing at different temperatures was investigated. It is found that the microstructure of the as-cast alloy consists of coarse Laves phase Ta(CrNiAl)2 with C14 structure at grain boundaries and Cr-rich particles with sizes in the range of 400-500 nm in the NiAl matrix. The Ta and Cr concentrations in the NiAl are ∼0.6% and ∼2.5% (at%), respectively. When the alloy is annealed at the temperature above 1000 °C, fine rod-like Laves phases with C15 structure begin to precipitate dispersively in the NiAl. After annealing at 1200 °C for 2 h, the volume fraction of the particles increases, and in the mean time, the Ta concentration in NiAl matrix reduces to ∼0.2at%. As the annealing temperature further increases to 1400 °C, the Laves phase precipitates dissolve in the NiAl matrix completely. Therefore, the precipitation of Laves phase in NiAl during the annealing in the temperature range of 1000∼1300 °C may be attributed to the diffusing followed by the supersaturation of Ta in NiAl after the solidification.