Formation mechanism of (TiNb)C surface reinforced layer by in-situ solid-state carbonization reaction

To investigate the formation mechanism of the (TiNb)C in Ti-Nb system during the surficial carburization, in-situ solid-state carbonization reaction was adopted to fabricate (TiNb)C surface reinforcement layer (SRL) in TiNb alloy. The experimental results have been employed to research a relationship between (TiNb)C formation mechanism and carbon diffusion mechanism. The research results confirmed that the carbon diffusion flux mainly governs the SRL formation mechanism and changes the one-step carbide formation, i.e., TiNb →C (TiNb)CI, into two-step process, i.e., TiNb →C (TiNb)2C →C (TiNb)CII + Nb. The outer layer is formed by the one-step reaction, where the micro-sized particles comply with the Baker-Nutting orientation relationship and have higher average hardness of 33.10 GPa. However, the inner layer is formed by the two-step process, which consists of submicro-sized particles presented a random orientation relationship, and the average hardness of inner layer is 30.97 GPa. Moreover, the X-ray diffraction analysis and EDS results indicate that the Ti/(Ti + Nb) ratios of Nb-rich and Ti-rich particles in the outer layer are ~62.26% and >73.41%, respectively. Whilst, all the particles in the inner layer are Ti-rich and the Ti/(Ti + Nb) ratios range from 81.62% to 83.48%.