Preparing dense powder metallurgy superalloy via pressureless transient liquid phase sintering based on a composition-splitting strategy

A composition-splitting strategy for enabling the preparation of superalloys via pressureless transient liquid phase sintering was proposed. A low-melting component was split from the composition system of superalloys, and a high-melting component was designed to coordinate it into the general composition of superalloys. The low-melting component consisted of the common elements in superalloys (Ni, Cr, Co, Al, Ti, and Ta) with a melting point of 1215.1 °C. Powder metallurgy superalloys were prepared by pressureless transient liquid phase sintering the mixed powder compact of the low/high melting components under 1250 °C. The influence of the liquid phase content on sintering behavior and tensile properties was analyzed. The increased liquid phase content promoted filling the gaps between solid particles, resulting in a rapid rise in relative density. However, the excessive liquid phase content shifted the solidification mode from isothermal solidification liquid (L) → γ to cooling solidification L → γ-γ^′ eutectic. These mechanisms determined the optimum amount of liquid phase in the transient liquid phase sintering. A superalloy with a relative density of 98% and free of prior particle boundary defects was sintered with the optimum amount of liquid phase. It possessed a yield strength of 844 MPa, an ultimate tensile strength of 1248 MPa, and an elongation of 16.3%. A processing parameter D, defined as the ratio of actual to theoretical yield strength, was proposed to characterize the efficiency of the preparation process. The pressureless transient liquid phase sintering with the optimum amount of liquid phase yielded a D of 0.92, which was higher than that of traditional casting and forging, only slightly lower than that of hot isostatic pressing.