Effects of carbon addition on the microstructure and mechanical property of in-situ reinforced TiAl matrix composite powders produced by plasma rotating electrode process

Abstract

To investigate effects of carbon addition on microstructural characteristics of rapidly solidified TiAl alloys, Ti–43Al–6Nb-xC (x = 0,0.5,1.0 at.%) powders are produced by plasma rotating electrode process (PREP). Due to the greater viscosity of composite melts with carbides than those without carbides, the average sizes of carbon containing TiAl powders are much larger than theoretical average diameters. In addition, phase compositions of carbon-free TiAl powders usually consist of α phase and β phase, while α phase is predominant in carbon containing TiAl powders due to α-phase stabilizer. β→α phase transformation during the rapid solidification and Burgers relationships {110}β||(0001)α and <111>β||(11–20)α are illustrated in TiAl powders. Three types of microscopic structures are unfolded in TiAl composite powders: smooth structures, dendrite structures and equiaxed structures, which stem from various cooling rates. With carbon contents rising to 1.0 at.%, the average grain size in TiAl powders with the particle size range of 100∼150 μm is refined from 24.31 ± 0.38 μm to 11.32 ± 0.36 μm, moreover, the distribution ranges of particle sizes vary dramatically. Additionally, the micro-hardness of TiAl powders gradually upgrades as carbon contents ascend. The maximum micro-hardness of TiAl composite powders with 1.0 at.% carbon addition touches at 758.7 ± 15.4HV. Solid solution strengthening, refinement strengthening and second-phase strengthening are main sources for the ascent of micro-hardness.