Growth mechanism of the tubular TiB crystals in situ formed in the coatings laser-borided on Ti–6Al–4V alloy

Coatings containing borides were fabricated on titanium alloy by laser boriding treatment. SEM images indicate that some rod-like TiB crystals with a hollow core (filled with α-Ti before etched) are formed in the coatings. The formation of rod-like hollow TiB is attributed to the following aspects: first, because the chemical composition in some areas in the laser melt pool is hypoeutectic and in some other areas is hypereutectic due to the boron atoms dissolved in the melt distributing unevenly during rapid solidification, β-Ti firstly crystallizes as primary phases in the hypoeutectic areas of the melt pool and acts as a heterogeneous nucleus for TiB to crystallize on it, and second, because the bond between B–B is stronger than that between Ti–B and between Ti–Ti. TiB grows along the B–B zig-zag chain as well as normal to the direction of the B–B zig-zag chain to form a tubular structure with a β-Ti core in it.