Study Of TiC/(Ti-Al+ZrO₂) Composites

Titanium carbide has been extensively applied in cutting tools,die tools owing to their high melting point and Young's modulus,excellent high-temperature strength, thermal stability,hardness and wear-resisting property.But the metals,Co and Ni, which are used as binders in cemented carbide have lower melting points and thermal strength.The application,especially the high temperature application of TiC is greatly limited.With the invention of intermetallics which have high melting points and excellent thermal strength,cermets with TiC matrix got a new kind of binder.Ti-Al intermetallics exhibit some excellent properties such as low density,high specific strength and high melting temperature and environmental resistance at elevated temperature,which makes it a suitable binder.ZrO₂ has high melting point and good chemical stability and its Mohs hardness is 8-9.As it's phase transformations occur, there are volume effect and shearing strain,which can toughen many composites. According to the philosophy mentioned above,the application of Ti-Al intermetallics and ZrO₂ as reinforcement in TiC matrix with the aim of preparing one new TiC/(Ti-Al+ZrO₂)composite is conducted.We systematically study the optimization of components,preparation process,mechanical properties,microstructure and strengthening mechanism of this composite.The mechanical alloying of Ti/Al powder mixture was studied firstly.According to the analyzing of planetary high-energy ball mill's kinematics,we preliminary learned about the deforming of powder in ball-milling process.Then the milled powder is heat-treated after its microstructure and morphology are detected and observed.The results showed that structure and morphology of milled Ti/Al powder mixture will greatly change,and supersaturated solid solution,amorphous phases and intermetallics with lots of defects and stress will form in consequence during ball milling.The powder milled for 10 h will completely transform into intermetallics after 30 min heat treatment at 600℃in vacuum.The sintering process,adding type of Ti-Al and the content of ZrO₂ are studied by a series of experimental.At last,it was confirmed that the adding type of Ti-Al intermetallics is by directly blending the untreated milled powder with TiC and ZrO₂ powder,and then heating at 600℃for 30 min in sintering process.The optimal content of ZrO₂ is 15 wt%(mass fraction),and the sintering progress have been established as that the sintering temperature is 1550℃,holding temperature at 600℃,1100℃and 1550℃for 30min,heating rate is 20℃/min,pressure is 30 MPa.Composite samples with different dosages of Ti₃Al or TiAl as binder were sintered.According to their mechanical properties,we can find out the best binder and its suitable dosage.The results showed that with the increasing of intermetallics, the sintered samples tend to have more compact structure and improved properties. But when the dosages of intermetallics exceed a certain extent the properties of composite will deteriorate,and the best dosage of Ti₃Al and TiAl is 20 wt%and 25 wt%respectively.As a whole,the best content of composite is 25 wt%TiAl,15 wt% ZrO₂ 60 wt%TiC with respective bending strength 653.5 MPa,fracture toughness 7.11 MPa.m^1/2and hardness HRA 92.0.A series of TiC/(Ti₃Al+Al₂O₃)composites have been prepared as contrast.Though they have good bending strength,there are many pores and lamellar structures in them,which made them have poorer fracture toughness,6.12 MPa.m^1/2.The sintering mechanisms in sintering process of composite were analyzed, which conclude liquid phase sintering,hot-pressing sintering and vacuum sintering mechanisms.Samples respectively without ZrO₂ and Ti-Al are prepared to study their effects on the sintering process.According to the SEM figures of fracture morphology, we can find out that the sample without TiAl have not densified only with very few grown grains,while the sample without ZrO₂ are quite compact with fine and uniform grains.So we concluded that Ti-Al intermetallics play an important role in sintering process. The composites with same content,TiAl 25 wt%,ZrO₂ 15 wt%,TiC 60 wt%,are sintered for different time(5,15,30,60 min).According to their microstructures and mechanical properties,we can find out that sintering process of this kind of composites is similar to that of cemented carbide,in which the liquid phase sintering is the main process,and the second-arranged and dissolution-precipitation mechanism play an important role.TiC grains undergo particular transition from 15 to 30 rain.The sintering statuses of composites with different Ti-Al dosage are analyzed base on their properties and fracture morphologies.It was confirmed that the quantity of liquid play an most important role,and the corresponding best dosages of Ti₃Al and TiAl are 20 wt%and 25wt%respectively.And when the dosages increase sequentially,the sintering process will deteriorate.Further analyze showed that the effect of Ti liquid is more significant and its dosage is the key factor for sintering.When sintering temperature was increased to 1600℃,the composite overburned with abnormity grown grains and metal losing.Lots of porosities occurred around the grown TiC grains,which made the combination between TiC grains very friability and the structures,properties deteriorate.In order to ulteriorly know about the sintering rules of the composite,a small quantity of carbon was added.The varieties of sintering and properties showed that with the addition of carbon,the sintering process greatly changed with the second-arrange and dissolution-precipitation mechanism restrained.But when the dosage of carbon is 2 wt%,the mixture melt liquor of Ti and Al has a lower viscosity and large capillary force which may promote sintering and the refinement of TiC grains.With such content,the composite has quite excellent chemical properties,bending strength 712.6 MPa,fracture toughness 9.11 MPa.m^1/2,which are higher than the composites without carbon.According to SEM and TEM photos,the microstructures of TiC/(Ti-Al+ZrO₂) composites were observed,and the phase distribution is discussed to fred out the toughening mechanism in these composites.It was discovered that with the addition of Ti-Al and ZrO₂ the microstructures of composites will greatly change and become more compatible,which may improve the properties.The effects of Ti-Al intermetallics exhibit in followed aspect.First,Ti-Al promote sintering and refine the grains;secondly,it have better plasticity which may toughening composite by crack-bridging;in addition Ti-Al grains locate between TiC grains and strengthen their combination,which may improve the shock-resistance property and increase the possibility of transgranular fracture.Some refined ZrO₂ particles were restricted on grain boundaries to form intergranular structure,and other particles were incorporated by grown TiC or Ti-Al grains to form intragranular structure.In a word,by its phase transformation characteristic,ZrO₂ operate in grains or at grain boundaries to toughen the composites cooperating with Ti-Al.Based on the results mentioned above,the sintering mechanism,the improvement of microstructures and properties of TiC/(Ti-Al+ZrO₂)composites are investigated,which may provide an experimental and theoretical foundation for the practical application.The best bending strength and fracture toughness of TiC/(Ti-Al+ZrO₂)composites are improved to712.6 MPa and 9.11 MPa.m^1/2.