The evolution of microstructure in a laser clad TiB–Ti composite coating

The microstructure of a TiB/Ti composite coating, obtained by laser cladding a Ti–6Al–4V substrate with a Ti/TiB2 powder mixture, was scrutinized using transmission electron microscopy. TiB showed three different morphologies: fine needles (200 nm diameter, 15 µm length); plates (thickness 1 µm, short length 3 µm and long length 15 µm); and coarse needles (diameter 3 µm and length of 50 µm). All TiB is composed of both the stable B27 and the metastable Bf crystal structures. Intimate mixing of B27 and Bf is possible because B27(200) planes fit excellently on Bf(110) or (11¯0) planes (with B27[010]//Bf[001]) that easily leads to stacking disorder. Aspects of disorder are quantitatively analyzed using high-resolution transmission electron microscopy (HRTEM). Throughout the fine needles, extensive stacking disorder occurs. In the plates a core of B27 (with relatively low stacking-fault (SF) density) is present, with faulted Bf on the outer surfaces that has a rough but fully faceted interface with the Ti matrix. The coarse needles consist predominantly of nearly defect-free B27. It is noteworthy about the coarse needles that they have a large core composed of Ti. On a much finer scale Ti is also dispersed in the plates and fine needles. It will be shown that the differences in microstructure among the three types of TiB morphologies provide important clues about the evolution of the TiB under the conditions of rapid growth.