Effect of mechanical alloying on sinterability and phase evolution in pressure-less sintered TiB_(2)-TiC ceramics

Phase relation and microstructure evolution in the pressure-less sintered TiB_(2)‒TiC ceramics preceded with mechanical alloying were systematically studied by a combination of SEM analysis.WC debris from milling balls promotes sintering by dissolving into the TiC phase to achieve dense microstructures at 1600℃.Variation of W solution in TiC grains exposes two types of core-rim structures,with no or more W in dark and white cores respectively but with common medium W in both rims.Diminishing whitecores reveal an exchange reaction between WC and TiC via mechanical alloying to form the Ti_(1-z)W_(z)C phase prior to sintering.The dark-cores inherit from the as-milled TiC power to further enable the reprecipitation of rims from a mixed liquid-phase,which facilitated also the anisotropic growth of TiB_(2) grains.The dark-cores grow persistently in the second-step at 2000℃ enabled by this liquid-phase,which coarsens the TiB2 grains too.With more alloyed phase,sintering was insufficient at 1500℃ with only the surface fluidity from the primary powders,and the second-step sintering increased the fluidity in the liquid-phase to fully densify the binary microstructure.Re-distribution of the alloyed W by two-step sintering rationalizes the evolution process of the binary microstructures and leads to better understanding of the mechanical behaviors.

Core-rim structure,bi-solubility and a hierarchical phase relationship in hot-pressed ZrB2-SiC-MC ceramics(M=Nb,Hf,Ta,W)

Core-rim structures were identified as a common feature in hot-pressed ZrB2-SiC-MC ceramics(M=Nb,Hf,Ta and W)by a combination of X-ray diffraction,scanning and transmission electron microscopies.Quantitative analyses associate them with the bi-solubility of M in ZrB2 phase,in which transition of solubility across the core/rim boundary is abrupted,signifying their creation via dissolution-reprecipitation process facilitated by transient liquid-phase.The cores were retained from starting powder after surface melting and the rims were grown from the liquid-phase to incorporate more solutes,leaving the residual liquid to turn into ZrC phase with higher solubility of M.We propose g-point scheme in the ZrB2-MB2 diagrams to combine the bi-solubility and the core-rim structures into an intra-phase relationship created by sintering,leading further to a hierarchical phase relationship.The temperature dependence of flexural strength in the ZrB2-SiC-MC ceramics varies with MC additions,which can be respectively strengthened by the strain energy created in the core-rim structures and metal segregation to grain boundaries.

Transient liquid-phase to guide multiphase evolution in reactive-hotpressed ZrB_(2)-SiC-ZrC ceramics

In reactive-hot-pressed ZrB_(2)-SiC-ZrC ceramics,ZrO_(2) was found to replace ZrC phase,hence leading to confusion in designing ultra-high-temperature ceramics(UHTCs).We employ high-precision X-ray diffraction and electron microscopies to reassess the phase behavior during entire reaction and densification and to reveal the evolution of multiphase relationship at different stages before reaching the final ZrB2-SiC-ZrO_(2) composition.Frozen from transient liquid-phase,bulk glassy phase of 15 vol% was found to be constituted of Zr-Si-B-C-O with stable Zr:O ratio,which starts as early as in the intermediate stage to suppress ZrC in favor of SiC nucleation.Inhomogeneity in phase relations and microstructures results from variation in local transient liquid-phase to develop SiC phase in various modes and rates.As inferred from the earlier report of phase formation,competing reactions for ZrC and ZrB_(2) phases in the initial stage below 1000℃ were mediated via Zr-O-B-C liquid phase.Such liquid phase was moderated by stable B-O components,as initiated from surface oxides of starting powders.This picture under a continuous mother liquid phase can unify the reactions and sintering into a collective meltingenucleationegrowth process,which enables and guides the evolution of multiphase relationship through several stages to reach final densification at relatively low temperature with the help of residual oxides.