Microstructure and hardness of as-cast in situ TiB short fibre reinforced Ti-6Al matrix composites

XD(TM) technique has been successfully used to prepare TiB short fibre reinforced Ti-6Al matrix composites. Macrostructure and microstructure have been observed by optical microscopy and SEM in order to study the influence of cooling rate on the morphology, size and distribution of TiB. Due to the cooling rate, there exist three kinds of macrostructure: fine grain zone, columnar grain zone and coarse equiaxed grain zone, corresponding to the cooling rate of 100-500 K/s, 20-50 K/s and less than 10 K/s respectively. In the fine grain zone, TiB distributes randomly in matrix with main rod morphology with similar to3 mum in width and similar to50 mum in length. In the columnar and coarse grain zone, a colony structure was observed in which TiB distributes with a special orientation direction with matrix. A lamellar TiB with up to 50 mum width and 200 mum length was also formed. It was indicated that the decreasing of the cooling rate changes the morphology of TiB from rod to lamellar shape, and markedly increase the length and aspect ration of TiB, from similar to50 mum to similar to200 mum and from about similar to15 to similar to200, respectively. TEM results show that the rod TiB has a hexagonal cross section. Vickers hardness testing shows a little reinforcement geometry dependence, but the average hardness of 484 MPa is much higher than that of unreinforced matrix alloy. (C) 2002 Kluwer Academic Publishers.