Solidification paths and reinforcement morphologies in melt-processed (TiB + TiC)/Ti In Situ Composites

Abstract

A novel in situ process was developed to produce titanium matrix composites reinforced with TiB and TiC of different mole ratios in which traditional ingot metallurgy plus self-propagation hightemperature synthesis (SHS) reactions between Ti and B₄C, graphite powder were used. Microstructures of (TiB+TiC)/Ti in situ composites were comprehensively characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM). Solidification paths were investigated using a differential scanning calorimeter (DSC). Results show that there is an apparent difference in morphologies of reinforcements. The reinforcements nucleate and grow from the melt in a way of dissolution precipitation. The different morphologies are related to their solidification paths and the particular crystal structure of the reinforcement. TiB grows along the [010] direction and forms short-fiber shape due to its B27 structure, whereas TiC with NaCl type structure grows in a dendritic, equiaxed, or near-equiaxed shape. The DSC results and analysis of the phase diagram yield three stages for the solidification paths of in situ synthesized titanium matrix composites: (1) primary phase, (2) monovariant binary eutectic, and (3) invariant ternary eutectic. The addition of graphite adjusts the solidification paths and forms more dendritic primary TiC. The addition of aluminum does not change the solidification paths. However, the reinforcements grow finer and lead to equiaxed or near-equiaxed TiC morphologies. The following consistent crystallographic relationships between TiB and titanium were observed by HRTEM, i.e., [010]_TiB//[$01\bar 10$]_Ti, (100)_TiB//($\bar 2110$)_Ti, (001)_TiB//(0002)_Ti, ($10\bar 1$)_TiB//($4\overline {22} 1$)_Ti and [001]_TiB//[$01\bar 10$]_Ti, ($0\bar 10$)_TiB//($\bar 2110$)_Ti, (200)_TiB//(0002)_Ti. The formation of the preceding crystallographic relationships is related to the growth mechanism of TiB. It also helps to minimize the lattice strain at the interfaces between TiB and the titanium matrix.