Study On Preparation Of High Performance Alloys And Cermets By Ultrafast High-temperature Sintering (UHS)

High-temperature alloys and electrode materials are widely used in aerospace and high-temperature electrolytic fields by their excellent properties.The microstructure homogeneity of the materials plays a decisive role in their macroscopic properties.Compared with ingot metallurgy,powder metallurgy has more advantages in suppressing macroscopic segregation and improving tissue homogeneity.However,the traditional powder metallurgy sintering method has problems such as a long preparation cycle,high production cost,and poor mechanical properties.The new ultrafast high-temperature sintering（UHS）technology has many characteristics different from the traditional high temperature heat treatment technology in terms of sintering molding,synthesis and preparation of materials,such as inhibiting grain growth,promoting densification process and optimizing the comprehensive properties of sintered materials.A key factor contributing to the above characteristics is the high sintering temperature（3000 K）and high heating and cooling rate（10⁴ K/min）of UHS.The large gradient thermal field can affect the diffusion kinetics of materials during reaction synthesis and densification processes,thereby affecting the microstructure and macroscopic properties of materials.In order to verify these speculations,Ti Al alloys（with significant differences in diffusion coefficients between the two phases）,Ni-based cermet（with low homogeneity）,and oxide-doped cermet（with unclear strengthening mechanisms）,which are representative of powder metallurgical sintering,were selected for this study.The characteristics of the process,properties,and tissue structure of several typical materials sintered by UHS were investigated to reveal the mechanism of the effect of rapid temperature rise on their microstructure,and the main findings are as follows:（1）The TiAl alloy with the targetγ-TiAl phase andα₂-Ti₃Al phase was synthesized in10s by UHS sintering technology using single-phase elemental powder as raw material densities of the alloy were further increased to over 90%by UHS secondary sintering.The densification and phase transformation of the alloy was completed in only 10 s at 1300°C by modulating the morphology of the raw material and improving the sintering process,resulting in a Ti Al alloy with 98.86%densities and 582 MPa microhardness.UHS’s ultra-fast heating rate and ultra-high temperature inhibited surface diffusion at low temperatures during solid phase sintering.They promoted the bulk and grain boundary diffusion of powder particles at high temperatures,thus effectively promoting the densification process of the sintered body.（2）17Ni/（10NiO-NiFe₂O₄）cermet with a penetrating honeycomb structured metallic phase are prepared in a few tens of seconds by UHS sintering.The ultra-fast heating rate of UHS promotes the evolution of the liquid phase sintering（LPS）to a non-equilibrium state,controlling the metal phase segregation and filling the particle gap uniformly.The UHS technique effectively forms metal ceramics with a penetrating honeycomb structure without increasing the metal phase composition.Experimental results confirm that UHS sintered cermet have a 15-fold increase in electrical conductivity,high density,and good mechanical properties compared to other pressureless sintering methods with low heating rates.In addition,the UHS short sintering process inhibits the over-agglomeration of metal phases and effectively improves the corrosion resistance of the anode.（3）UHS sintering technique successfully prepared different oxide-doped cermet inert anode materials.The rare earth oxide Ce O₂ is solidly dissolved in the ceramic matrix.In this process,it increases the lattice defects by replacing Fe³⁺and Ni²⁺in the Ni Fe₂O₄ matrix,increasing the sintering driving force and accelerating the densification process of the cermet during UHS sintering.Moreover,Ce O₂ promotes the formation and homogenization of the honeycomb metallic Ni phase,significantly improving the cermet’s overall performance.