Effect of scan line spacing on texture, phase and nanohardness of TiAl/TiB2 metal matrix composites fabricated by selective laser melting

Abstract

The TiAl/TiB2 metal matrix composites (MMCs) are fabricated by selective laser melting (SLM), and the effects of scan line spacing on the grains feature, crystallographic texture, phases evolution mechanism and nanohardness have been systematic studied. With the increase of scan line spacing, relatively coarse grains with average grain size of 4.9 ± 0.2 μm and strong (0001) orientation gradually transforms into refined grains with average grain size of 4.2 ± 0.1 μm and dominated (112¯1) and (011¯1) orientations during the SLM process. The SLM-processed TiAl/TiB2 MMCs samples are dominated by high-angle grain boundaries (HAGBs) and α2 phase, with the increase of scan line spacing from 80 μm to 140 μm, both the volume fraction of HAGBs and α2 phase decrease. During the SLM process, the phases evolution mechanism of TiAl/TiB2 MMCs can be expressed as: (200)β first transformed to (112¯0)α2, (202¯0)α2 and (110)γ, then the TiB2, residual B2 and incompletely transformed γ phases randomly distribute in the α2 matrix, and the orientation relationship of the α2, γ, B2 and TiB2 phases can be concluded as: (112¯0)α2//(110)B2//(110)γ//(110)TiB2. Besides, there are some TiB phase in the TiAl/TiB2 MMCs which is induced by the Ti + TiB2 → 2TiB reaction during SLM. The TiAl/TiB2 MMCs samples show an ultra-high nanohardness ranging from 8.6 ± 0.4 GPa to 9.4 ± 0.5 GPa with the increase of scan line spacing, which is superior than that of the TiB2 reinforced TiAl alloy counterpart (6.7 ± 0.4 GPa) fabricated by traditional roll bonding.