SYNTHESIS OF Ti-TiB FUNCTIONALLY GRADED MATERIALS BY REACTION SINTERING

摘要

Titanium (Ti) and titanium boride (TiB) FGM tiles were fabricated in-situ by simultaneous TiB whisker formation and densification during the reaction sintering process. The macrostructure of the graded material is made of a β-Ti layer on one side with the composite layers of Ti-TiB mixture having increasing volume fraction of the TiB through the thickness. The FGM fabrication utilized a tri-modal powder mixture consisting of a-Ti powder, a master alloy of the β-stabilizing-element powders (Fe-Mo) and TiB_2. The structure and properties of both of these FGM tiles were characterized by X-ray diffraction, scanning electron microscopy, and microhardness measurements. Two different kinds of TiB whisker morphologies were observed in the FGMs. The Ti-rich layers were found to have large pristine TiB whiskers uniformly distributed in the Ti matrix. On the other hand, the TiB-rich layer was found to have a network of interconnected and relatively smaller TiB whiskers appearing as clusters. The layers of intermediate TiB volume fractions were found to consist of both morphologies of TiB. Vickers hardness level was found to increase from 420 kgf/mm~2 in the β-Ti layer to 1600 kgf/mm~2 in the TiB rich layer. The residual stresses retained in the graded layers after fabrication were also calculated based on a multilayer model. The aspects of microstructure, hardness and the residual stresses in the FGMs are discussed.