超高重力场燃烧合成TiC-TiB2的组织变质与断裂行为

摘要

采用超高重力场燃烧合成工艺，并从500 g 到2500 g每间隔500 g 依次增大超重力场加速度，制备系列TiC-TiB2凝固陶瓷。经XRD、FESEM和EDS分析，发现陶瓷显微组织均由片晶的TiB2基体相、不规则的TiC第二相及少量的Al2O3夹杂与Cr基金属相组成。增大超重力场加速度，反应熔体内部各组份之间的对流(Stokes)加强，可加快Al2O3液滴的上浮与分离，促进TiC-TiB2-Me液相成分均匀化，使陶瓷显微组织得以细化，且当超重力场加速度超过2000 g 时，出现TiB2片晶厚度小于1μm 的超细晶组织，同时随陶瓷基体上Al2O3夹杂量降低、TiB2片晶异常长大弱化，陶瓷组织均匀性提高。经FESEM断口形貌与裂纹扩展观察，发现TiB2基体相的裂纹桥接与拔出，并耦合晶间 Cr 基延性相增韧构成陶瓷的复合增韧机制，且随超重力场加速度增大，陶瓷的致密性与组织均质性得以提升，不仅促进TiB2基体相裂纹桥接与拔出，而且可增大Cr基延性对陶瓷增韧的贡献，使得陶瓷弯曲强度与断裂韧性分别同时达到最大值(975±16) MPa和(16.8±1.2) MPa·m1/2。%By increasing the acceleration of high gravity field from 500 g to 2 500 g at intervals of 500 g, a series of solidified TiC-TiB2 were prepared by combustion synthesis in high gravity field. XRD, FESEM and EDS results show that the ceramics are comprised of TiB2 platelets as the primary phases, irregular TiC grains as the second phases, a few of Al2O3 inclusions and Cr-based metallic phases. Because of the enhanced Stokes flow in mixed melt with increasing acceleration in high gravity field, Al2O3 droplets are promoted to float up and separate from TiC-TiB2-Me liquid while constitutional distribution became more and more uniform in TiC-TiB2-Me liquid, resulting in not only the sharply-reduced Al2O3 inclusions in the solidified ceramic but also the achievement in refinement of the solidified microstructure. As the high-gravity acceleration exceeds 2 000 g, ultrafine-grained microstructure is obtained with TiB2 platelets in thickness smaller than 1 μm, resulting in the improvement of microstructure homogeneity. FESEM fractographs and FESEM images of crack propagation paths show that the procedure of toughening ceramic is achieved by coupling crack bridging and subsequent pullout by TiB2 platelets with ductile-phase-induced toughening by Cr-based intercrystalline phases. With increasing high-gravity acceleration, densification and homogeneity of the ceramics increase which not only promotes crack bridging and pullout of TiB2 matrix, but also increases the contributions of Cr-based ductile phases to toughening ceramic. The maximum values of flexural strength and fracture toughness ((975±16) MPa and (16.8±1.2) MPa·m1/2, respectively) of the solidified TiC-TiB2 composite are obtained in current experiment.