Enhanced nanohardness and new insights into texture evolution and phase transformation of TiAl/TiB2 in-situ metal matrix composites prepared via selective laser melting

TiAl/TiB2 in-situ metal matrix composites (MMCs) with greatly enhanced nanohardness are prepared via selective laser melting (SLM) for the first time in this study. The effect of TiB2 reinforcement on the microstructural characteristics, texture evolution and phase transformation of TiAl-based alloy is investigated. The results show that with increasing the TiB2 content, the average grain size gradually decreases, and the crystallographic orientation transforms from a strong (0001) direction to (10 (1) over bar1) and (1121) directions. Meanwhile, TiB2 has a great effect on the texture of SLM-processed TiAl/TiB2 MMCs. With increasing the TiB2 content, more textured TiAl/TiB2 MMCs can be produced. The TiAl/TiB2 MMCs are dominated by alpha(2) phase and small amounts of gamma, B2, TiB2 and TiB phases are also detected. alpha(2) phase contains the most important texture components of prismatic fiber with {10 (1) over bar0} < 11<(2)over bar>0 > orientation, basal fiber with {0001}< 11<(2)over bar>0 > orientation and pyramidal fiber with {10 (1) over bar1}< 11<(2)over bar>0 > and {11 (2) over bar2} < 11<(2)over bar>3 > orientations. The TiB2 reinforcements are in the forms of the needlelike micro-TiB2 and irregular nano TiB2 particles in the TiAl-based alloy matrix, and the nano TiB2 particles are uniformly distributed with the size of 10 nm in length and 3-5 nm in width. The SLM-produced TiAl/TiB2 MMCs exhibit superior nanohardness of 10.57 +/- 0.53 GPa, which is much higher than those of the traditional roll bonding fabricated TiB2 reinforced TiAl-based alloy. The findings would be a valuable reference for fabricating TiAl/TiB2 MMCs parts with controlled grain features, crystallographic texture and phase composition, enhanced mechanical properties and complex structures by SLM. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.