Microstructural characteristics of PTA-overlayed NbC on pure Ti

Abstract

This study uses the technique of PTA (plasma transferred arc) to overlay NbC reinforcing particles on the surface of commercially pure Ti in order to investigate the microstructural features of the overlayer and the interface between the overlayer and base metal by changing the overlaying current. The results indicate that the matrix phase of the overlayer was α-Ti containing about 10 at.% Nb and 1 at.% C. NbC and precipitated TiC produced by dissolved NbC reacted with Ti dispersed in the matrix. The microstructure of the cross-section of the overlayer (from surface to base metal), which was composed of α-Ti, can be separated into three layers: an upper overlayer with TiC, a middle overlayer with TiC and NbC, and a lower overlayer (interfacial layer and heat affected zone, HAZ). Due to solidification beginning at the interface and the effect of dilution, the TiC in the interfacial layer was finer. Owing to faster solidification under low-current conditions, the TiC particles were finer than under high-current conditions. Also, dendritic TiC under a low-current in the upper overlayer was also finer than under a high-current. Meanwhile, TiC precipitate that resulted from heterogeneous nucleation and Gibbs' free energy was also found around NbC. This NbC diffusion layer between TiC and NbC may have been βNb 2C phase.