Phase evolution and densification behavior of MIM418 superalloy utilizing master alloy approach

MIM418 superalloy was prepared by metal injection molding of master alloy powder and carbonyl Ni powder. Phase formation process and its evolution during heating process was followed by high temperature x-ray diffraction, and the relationship between phase transition, diffusion process and the densification behavior was discussed in detail. Microstructural characteristics of the samples during sintering process and the densification mechanism were investigated. Asynchronous diffusion of dissolvent elements, γʹ phase and carbides were observed. Severe outward diffusion of γʹ forming elements (Al) from the master alloy to the nickel powder leads to the precipitation of a large population of γʹ phase in the temperature range of 700-900 °C. The mass transfer of γ-partitioning elements (Cr, Mo) almost accomplished at 1100 °C. The phase transformation of carbides evolves from non-equilibrium MCʹ carbides in the master alloy powder to M23C6 carbides at 900-1100 °C, and finally to thermodynamically stable MC carbides in the homogenized sample. Accelerated diffusion and densification process were observed during transient liquid phase sintering process. The relatively density achieves 98.4% and ultimate strength (1047 MPa) is comparable with that of the prealloyed sample.