The sintering, sintered microstructure and mechanical properties of Ti-Fe-Si alloys

A systematic study has been conducted of the sintering, sintered microstructure and tensile properties of a range of lower cost Ti-Fe-Si alloys, including Ti-3Fe-(0-4)Si, Ti-(3-6)Fe-0.5Si, and Ti-(3-6)Fe-1Si (in wt pct throughout). Small additions of Si (a parts per thousand currency sign1 pct) noticeably improve the as-sintered tensile properties of Ti-3Fe alloy, including the ductility, with fine titanium silicides (Ti5Si3) being dispersed in both the alpha and beta phases. Conversely, additions of > 1 pct Si produce coarse and/or networked Ti5Si3 silicides along the grain boundaries leading to predominantly intergranular fracture and, hence, poor ductility, although the tensile strength continues to increase because of the reinforcement by Ti5Si3. Increasing the Fe content in the Ti-xFe-0.5/1.0Si alloys above 3 pct markedly increases the average grain size and changes the morphology of the alpha-phase phase to much thinner and more acicular laths. Consequently, the ductility drops to < 1 pct. Si reacts exothermically with Fe to form Fe-Si compounds prior to the complete diffusion of the Fe into the Ti matrix during heating. The heat thus released in conjunction with the continuous external heat input melts the silicides leading to transient liquid formation, which improves the densification during heating. No Ti-TiFe eutectoid was observed in the as-sintered Ti-Fe-Si alloys. The optimum PM Ti-Fe-Si compositions are determined to be Ti-3Fe-(0.5-1.0)Si.