Microstructure And Mechanical Properties Of In-situ （TiB+La₂O₃）/TC4Titanium Matrix Composites

Titanium matrix composites (TMCs) have bright future due to theirsuperior mechanical properties than titanium alloy. TiB and La2O3reinforced titanium matrix composites have been extensively studied due totheir excellent high-temperature mechanical properties. However, themechanism of in-situ reaction and the formation mechanism of rare earthoxides reinforcements still needs further study. This paper focus on thein-situ (TiB+La2O3)/TC4titanium matrix composite and the effect of theaddition mode of reinforcements on microstructure and mechanicalproperties of TMCs has been studied, which provides theoretical supportfor the further research of in situ reaction and process optimization.In-situ (TiB+La2O3)/TC4composites were produced by vacuumconsumable arc melting, forging and heat treatment utilizing theself-propagation high-temperature synthesis reactions between Ti and LaB6or between Ti, B and La. Basketweave structure and short fibrous TiB isobtained in TMCs after forging and annealing while there are two kinds ofmorphology of La2O3: coarse short-fiber shape and fine dispersed particles.In Ti-B-La system, La powder have prior reaction with oxygen beforemelting due to its lower melting-point and higher chemical activity,therefore there are more La element forms primary-precipitation La2O3which exhibits high aspect ratio of short-fiber shape due to its A-M2O3structure. In contrast, in Ti-LaB6system there are more La elementprecipitates during the/β phase transition of titanium and forms moresecondly-precipitation La2O3. The size of reinforcements in TMCsincreases with the increase of the content of B and La. And the aspect ratioof La2O3increases with the increase of the content of La while the aspect ratio of TiB decreases with the increase of the content of B. The result ofthermodynamic calculation shows that the alloying element Al has apromotion effect on the in situ reaction in Ti-B-La system. Therefore, theaspect ratio of TiB and La2O3in Ti-B-La system is higher than that inTi-LaB6system so that abnormal coarse primary La2O3is rarely formed.The fracture mode at room and elevated temperatures are ductilefracture. With the increase of the content of reinforcements, the strength ofTMCs increases while the ductility decreases. The strength of TMCsdecreases when the temperature improves. And at500℃, the tensilestrength of TMCs decreases significantly. TMCs in Ti-B-La system exhibithigher tensile strength than that in Ti-LaB6system under the identicalcontent of reinforcement and the same temperature, because the higheraspect ratio La2O3has a better bearing effect at room temperature and hightemperature.