Effect of sintering temperature on microstructure and mechanical properties of Inconel 718 superalloy prepared by micro-FAST

Micro-forming combined with field-activated sintering technology, named micro-FAST, is a novel method to efficiently fabricate metallic or nonmetallic materials with excellent mechanical properties and fine microstructure. In this research, the Inconel 718 superalloy powders after ball milling were sintered via micro-FAST at three temperature conditions of 950 °C, 1000 °C and 1050 °C, respectively. The microstructural evolution and macroscopical mechanical properties of the sintered samples were researched. The results showed that the local Joule heat generated by the alternating electric current significantly promotes the formation and growth of the sintering necks, intensifies the volume diffusion of the substance, and improves the densification of the samples. In addition, the relative density of the sintered samples is all greater than 98%. During the micro-FAST, low sintering temperature promotes the grain refinement of the samples, and high sintering temperature enhances the grain boundaries migration. The average grain size of the sintered samples increased from 2.03 μm at 950 °C to 17.8 μm at 1050 °C, and the proportion of twins rises from 36.6% to 47%. Furthermore, the proportion of high-angle grain boundaries (HAGBs) is greater than 94% under the three sintering temperatures. The yield strength and hardness of the sintered sample are 554 MPa, and 397 Hv at 950 °C, respectively, presenting the most desirable mechanical properties. Compared with mechanical properties of the sample sintered by spark plasma sintering (SPS), the samples achieved by micro-FAST present superior strength and plasticity.