Phase formation and evolution during transient liquid phase sintering of MIM418 superalloy with master alloy addition

Abstract

Phase formation and evolution during transient liquid phase sintering of MIM418 superalloy fabricated via master alloy approach have been investigated through a combination of thermo-dynamic calculation and diffusion couple method. Diffusion couple experimental results reveal that solid phase diffusion of Al from master alloy to Ni particles leads the formation of NiAl phase at Ni/master alloy interface initially before the formation of liquid phase, which transforms to γʹ(Ni3Al) phase subsequently. Thermo-dynamic calculation results reveal that Cr-rich liquid phase forms in the master alloy powder much lower than sintering temperature. The melting point of the Cr-rich phase increases gradually with the dissolution of Ni particles, resulting in the vanishing of the liquid in the sinter system. The Cr-rich transient liquid phase spreads and wets Ni particles, then transforms to M23C6 carbide mainly on grain boundaries after solidification, while the MCʹ carbide in master alloy powder tightly limits the migration of MC carbide forming elements (Ti, Nb), and the resultant MC carbide is confined in the original master alloy region. The tensile and yield strength of MIM418 superalloy are 53% and 99% higher than that of the cast K418 alloy, and the reduced ductility is associated with the uneven distribution of carbides.