Regulation Of In-situ Reaction Degree Of Ti-B₄C With Hifh B₄C Content And Its Effect On Mechanical Properties Of Composites

Titanium boride and titanium carbide ceramics are considered as high temperature structural ceramics with great potential due to their have good mechanical properties,which have a wide range of application prospects in cutting tools,wear-resistant parts and other fields.Ti-B₄C system can generate TiB,TiB2 and TiC through in-situ reaction,the preparation of titanium matrix composites and ceramic matrix composites based on this system is one of the current research focuses.In this paper,It proposed to control the heat release of in-situ reaction by adjusting the content and size of B₄C.Firstly,the composition and reactant sizes of Ti-B₄C system were designed through thermodynamic calculation.The reliability of self-propagating adiabatic temperature criterion was verified was mastered based on the displacement curve and microstructure of sintering process.Secondly,the effects of different reaction conditions on the microstructure and phase composition of the composites were studied,and the formation competition and reaction mechanism of TiB and TiB2 in Ti-B₄C system were discussed.Finally,under the control of reaction degree,the composite materials with different reactant contents and sizes were prepared and the hardness,compressive strength and impact toughness were evaluated.The influences of reaction degree and product characteristics on the mechanical properties of the composite materials were clarified.The main conclusions of this paper are as follows:（1）The relationship between the adiabatic temperature of Ti-B₄C system and the content and scale of B₄C was established based on thermodynamic calculations.The applicability of the adiabatic criterion（Tad≥1800K）in determining whether the Ti-B₄C system undergoes selfpropagation was verified through experimental research.When B₄C size attained 0.5 μm and the content exceeded 9.85 wt.%,the system will undergo self-propagating reaction;when the content of B₄C was 18.75wt.%,the particle size of the reactant B₄C should be appropriately increased,the self-propagating reaction of the system can be suppressed.（2）When the content of Ti exceeded in the system（B₄C content<18.75wt.%）,the composite was composed of α-Ti,TiC and TiB three phases regardless of whether the self-propagating reaction occurred,TiB presented thick strip or irregular flocculent in the solid-state reaction samples.TiB presented clustered whisker or thick strip while in the self-propagating reaction sample,but TiC was uniformly dispersed in the sample in the form of fine particles.Meanwhile,the excess of metal Ti made the composite materials have the good compactness.（3）In the absence of excessive Ti in the system（B₄C content=18.75wt.%）,the composites consisted of TiB,TiC,and a little TiB2,regardless of whether the self-propagating reaction occured.In the sample of solidity reaction,the matrix was completely composed by TiB,TiC particles grow up and connected with each other to agglomerate in it.In the sample of selfpropagating reaction,the micro structure presented solidification characteristics,and the TiB phase transformation became continuous and dense.Coarse TiC particles were distributed and a small amount of TiB2 particles were wrapped in the matrix.Solid diffusion samples exhibited uniform,fine sintered pores and self-propagating reaction samples exhibited dense structures in other regions except for local regions where existed millimeter scale shrinkage pores.（4）The competitive growth of TiB and TiB2 in the Ti-B₄C system were mainly related to the content of B₄C in the system,but the reaction path was related to whether the system undergoes self-propagating reactions.In a solid-state reaction,the reaction path is the B and C atoms generated after the decomposition of B₄C diffused into Ti,and the reaction products were merely TiB and TiC.Under the self-propagating reaction,high temperatures promoted the complete melting of the system.TiC first precipitated in a low content system,and then eutectic reactions occured to generate TiB and α-Ti,which eventually merged and grew at high temperatures.However,TiC and TiB2 precipitated firstly from the high-temperature melt at high B₄C content,which followed by peritectic reaction of Ti melt wrapping TiB2 to generate TiB.When excessive B₄C was presented,the residual TiB2 phase exceeded in TiB.（5）In the Ti-B₄C system composites without self-propagating reaction,the greater the impact toughness.The lower the compressive strength and hardness of the composites with the same B₄C content.When the particle size of B₄C is the same,the impact toughness of the composite decreases and the compressive strength and hardness increase with the increase of B₄C content.The compressive strength of 0.5μmB₄C composite with 6wt.%was 2100MPa.