Development of Highly Machinable Ti Alloy with Exceptional Tensile Properties by Er Alloying Element Addition

Abstract

This study demonstrates that the addition of the rare earth element Erbium (Er) significantly enhances the machinability and tensile properties of titanium (Ti). Pure Ti alloys and Er-added Ti alloys with 0.5-1.1 wt.% Er content were prepared, and their microstructure, machinability, and tensile properties were compared. Two different types of Er secondary phase particles were identified in the microstructure: pure Er and Er-oxide. The amounts of these particles increased with higher Er content. The machinability of the Eradded Ti alloys was significantly improved due to the ability of Er secondary particles to cut machining chips or absorb heat from localized deformation within the Ti matrix. In addition, Er-added Ti alloys exhibited higher strength than pure Ti. The strength enhancement was attributed to grain refinement induced by the Er element. Er secondary phase particles reduced the β grain size during solidification, and they also served as preferential sites for α nucleation during the β → α phase transformation, resulting in a refined microstructure. In addition, the Er secondary phase contributed to the strength enhancement through the well-known precipitation strengthening mechanism. Although ductility decreased with higher Er content due to the increased amount of Er secondary phase particles, 0.5 wt.% Er-added Ti showed no such degradation; its ductility was comparable to that of pure Ti. Er-oxidation was expected to reduce oxygen content within the Ti matrix, enhancing intrinsic Ti ductility; this effect offset the adverse impact on ductility caused by the Er secondary phase particles. Above 0.5 wt.% Er, the adverse effects caused by the Er secondary phase particles overwhelmed the beneficial effect caused by the reduction in oxygen content. The present findings will contribute significantly to the development of highly machinable Ti alloys with superior tensile properties.