<B> In-Situ</B> Formation of Nitride Particle Reinforced Titanium Aluminide by Reactive Plasma Arc Melting Process

Abstract

A reactive plasma arc melting process using a mixture of Ar and N2 as the carrier gas for titanium and aluminum powders has been applied to synthesize nitride particle reinforced Ti–34 mass%Al intermetallic matrix composites (IMCs). This technique successfully allowed to produce in-situ nitride particle dispersed IMCs and the volume fractions of nitride increased from 6 vol% to 54 vol% with increasing mixing ratios of N2 gas from 10 vol% to 100 vol%. In the IMCs both rod-like Ti2AlN and coarse two-phase nitride particles consisting of the core of TiN and the outer shell of Ti2AlN were formed in the matrix of a full lamellar structure or a lamellar containing small amount of equiaxed γ phase structure. The IMCs had significantly fine grains of which size was about 1/4 that of the unreinforced Ti–34 mass%Al. The Rockwell hardness of the IMCs increased abruptly from 36.5 to 48.4 HRC with increasing volume fraction of nitride. Unlike the hardness, the tensile strength of the IMCs had a maximum value of 507 MPa, which was approximately 170 MPa higher than that of the unreinforced Ti–34 mass%Al, at 13 vol% nitride, beyond which the strength decreased. The strengthening is derived from complex reinforcing effects of both direct strengthening effects of nitride particles due to the interaction of dislocations with the particles and grain refinement. The degradation in tensile strength at higher volume fractions of nitride is considered to be attributed to higher population of clustered coarse two-phase nitride particles, which can act as crack initiation and propagation sites. As for the strength at elevated temperatures, the IMC with 13 vol% nitride had higher tensile strengths than the unreinforced Ti–34 mass%Al by 100 MPa at 1173 K and 61 MPa at 1273 K.