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

School of Materials Engineering, Shanghai University of Engineering Science, Shanghai 201620, P.R. China
e-mail: jacob_lijun@sina.com

Received: 30 October 2015
Accepted: 2 March 2016

Abstract

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 °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 HV_0.2 (0 h) 933.2 HV_0.2 (2 h), 903.8 HV_0.2 (4 h), 890.4 HV_0.2 (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).