Synthesize And Properties Of The （Ti,M）₂AlC/Al₂O₃Composites

The ternary layered compound Ti₂AlC possesses remarkable properties thatcombines many of the merits of both metals and ceramics, and has attractedincreasing attention due to its good electrical and thermal conductivity, lowdensity, ready machinability, excellent thermal shock resistance and hightemperature oxidation resistance. To date, unfortunately, its wide application isstill limited by its relatively low hardness and low strength, which originatedfrom the comparatively weak bonding between the Al layers and edge-sharedTi6C octahedral in the structure. At present, both composite strengthening andsolid-solution are effective approachs for improving fracture toughness andflexural strength of Ti₂AlC.Until now, the most common way to fabricate the Ti₂AlC based compositesis to mix the raw materials, and then hot pressing. However，the synthesizedproducts have defects, such as coarse grains and poor dispersion, which limitfurther increasing the properties. In this work, in-situ hot-pressing method withexothermic reaction of Al-V₂O₅or Al-Nb₂O₅in Ti-Al-C system, has been used tofabricate （Ti,V）₂AlC/Al₂O₃and （Ti,Nb）₂AlC/Al₂O₃composites, in which theAl₂O₃enforcing phase was in-situ formed by thermite reaction of Al-V₂O₅orAl-Nb₂O₅, at the same time, the in-situ formed M （V or Nb） atom substituted Tito form （Ti,M）₂AlC matrix phase. For comparison, fully dense Ti₂AlCmonolithic phase was also synthesized by hot pressing method using Ti, Al andC as starting materials. The reaction mechanisms and microstructures of the（Ti,M）₂AlC/Al₂O₃（M=V or Nb） composites were investigated in details by X-raydiffraction （XRD）, scanning electron microscopy （SEM） and energy dispersiveanalysis （EDS）. The effects of V₂O₅and Nb₂O₅addition on the microstructuresand properties were also studied. Furthermore, the fracture models, strengtheningand toughening mechanisms were analyzed. The main results are shown as follows:The results of the synthesis for Ti₂AlC compound show that the fully denseTi2AlC ceramic was synthesized at1350℃using the molar ratio of Ti:Al:C as2:1.1:0.7. The as fabricated Ti2AlC possesses a relative density of99%and agrain size of about1μm. The Vickers hardness （Hv）, flexural strength （σb） andfracture toughness （KIC） are6.79GPa,271MPa and4.08MPa·m1/2, respectively.The synthesis process of （Ti,V）₂AlC/Al₂O₃composite with the rawmaterials of Ti-Al-C-V2O5is as follows: When the hot pressing temperature was700℃, series of Ti-Al intermetallics （Ti3Al、TiAl、TiAl3） were formed byreaction of Ti and Al. When the temperature increased to900℃, Al₂O₃and Vwere formed by the thermite reaction between Al and V2O5, at the same time,TiC was also formed.（Ti,V）2AlC phase was formed as the temperature increasedto1100℃. Finally, when the temperature was increased to1350℃, high purity（Ti,V）2AlC/Al₂O₃composite was successfully synthesized, which was decreasedby200-300℃compared with the traditional synthesis methods. The grain sizeof the matrix was deceased by increasing the Al₂O₃content, and the more Al₂O₃distribution, the more uniform in the matrix. The Vickers hardness （Hv） andcompressive strength increased with the increasing of Al₂O₃content. But asAl₂O₃content was further increased over10wt%, the Al₂O₃particles becameagglomerate seriously. The flexural strength （σb） and fracture toughness （KIC）were obtained the maximum values of434.36MPa and5.68MPa m1/2at10wt%Al₂O₃content, which were increased by132%and166%, respectively,compared with those of the monolithic Ti2AlC. This improvement results from acomprehensive effect of solid solution and the second phase strengthening.The （Ti,Nb）₂AlC/Al₂O₃composites were also successfully synthesized fromelemental powder mixtures of Nb2O5, Ti, Al and carbon by using the samesynthesis technique. The reaction path and microstructure were similar to thoseof the （Ti,V）2AlC/Al₂O₃composite. Compared with the monolithic Ti₂AlC, theflexural strength and fracture toughness were enhanced by57%and9%,respectively, when the Al₂O₃was5wt%（7.69wt%Nb2O5）. However, incomparison with the （Ti,V）2AlC/Al₂O₃composites, the mechanical properties of（Ti,Nb）2AlC/Al₂O₃composites are still lower, and the reason would be furtherinvestigated in detail.