Effect of the heat treatment time on microstructure and fracture toughness of laser-clad TiB2-TiC pseudoeutectic reinforced TiNi/Ti2Ni intermetallic matrix composite coating

A TiNi/Ti2Ni intermetallic matrix composite coating reinforced by TiB2 and TiC pseudoeutectic structures was prepared on a Ti6Al4V substrate by laser cladding using Ni based and B4C powder as the cladding material. The coatings were heat treated for different durations (2, 4, and 6 h) at 600 degrees C. The microstructures of the heat-treated and untreated coatings were then characterized by X-ray diffraction, scanning electron microscopy, and energy dispersive spectrometry. The microhardness and fracture toughness of the coatings were also evaluated. Results reveal that the coating without heat treatment was mainly composed of TiNi/Ti2Ni as the matrix and TiB2/TiC as the reinforcements. TiB2 and TiC belong to the pseudoeutectic microstructures, in which a large number of fine columnar/spherical TiC particles are uniformly distributed within the coarse willow-shaped TiB2 particles. Many fine needle-shaped Ti2Ni particles were precipitated within the initial TiNi grains in the coatings with heat treatment. With the extension in heat treatment time, more Ti2Ni particles were precipitated. Furthermore, the average microhardness of the coatings was decreased: 941.3 HV0.2 (0 h) 933.2 HV0.2 (2 h), 903.8 HV0.2 (4 h), 890.4 HV0.2 (6 h), whereas the average fracture toughness presented an opposite trend: 4.18 MPa.m(1/2) (0 h), 4.26 MPa.m(1/2) (2 h), 4.37 MPa.m(1/2) (4 h), 4.90 MPa.m(1/2) (6 h).