INFLUENCE OF AL CONTENT ON MORPHOLOGY AND PROPERTIES OF PRIMARY (Ti,Nb)_2AlC PARTICLES IN CAST Ti-Al-Nb-Mo MATRIX IN-SITU COMPOSITES

摘要

The effect of Al content on morphology, chemical composition, nanohardness and elastic modulus of primary (Ti,Nb)2AlC particles was investigated in in-situ composites with nominal composition Ti-xAl-8Nb-1Mo-0.1B-3.7C (at%), where x ranged from 38 to 45 at%. The in-situ composites were prepared by vacuum induction melting in the graphite crucibles and consecutive tilt casting into a graphite mould. Microstructural analyses show that the microstructure of the composites consists of irregular shaped, plate-like and regular shaped primary carbide particles which are relatively homogeneously distributed in the multiphase intermetallic matrix. The particles consist of (Ti,Nb)_2AlC phase with small amount of (Ti,Nb)C phase in the cores of some coarse irregular shaped ones. The Al content has no effect on the measured volume fraction of the particles, but affects their morphology. The shape factor of the particles increases with increasing Al content. The size of the coarse irregular shaped particles decreases with increasing Al content. The solubility of Nb in (Ti,Nb)_2AlC phase is not affected by the Al content and reaches only (0.78 + 0.06) of the average Nb content in the composites. Mo dissolves predominantly in the matrix. The variation of the Al content in the composites has no significant effect on nanohardness and elastic modulus of (Ti,Nb)_2AlC particles.