Low-cost Powder Metallurgy Of Titanium Alloys

Blended Elemental P/M process (BE) is believed to be the most promising one for commercial uses of titanium alloys owing to the economical efficiency and the high degree of freedom in selection of alloy composition as well as microstructural design over the ingot metallurgy process and other powder metallurgy processes. The purpose of this study is to obtain full-density as-sintered titanium alloys with mean and homogeneous microstructure and low impurity content by very simple processes. In this article, effects of Fe, Mo and Nd elements on the sintering behavior, microstructure evolution and mechanical properties of titanium alloys were investigated by means of Expansion/contraction behaviors testing, Optical microscopy (OP), X-ray diffraction (XRD), Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM). The analyzing results are shown as follows:1) Addition of iron can bring about higher sinterability in titanium alloys. In the widmanstatten-like microstructure of sintered Ti-Fe alloys, the sizes of prior p phase and the a phase colonies increase slightly, the thickness of a lamellar decreases rapidly and the mechanical properties improve greatly with the increase of Fe content; Meantime, with the increase of sintering temperatures, the sizes of prior B phase and the a phase colonies and the thickness of a lamellar grow rapidly, the mechanical properties of sintered Ti-Fe alloys deteriorate sharply. All of these results can be attributed to B stabilizing effect and high diffusion rate of Fe element in titanium alloys.2) Addition of Mo refines the microstructures of Ti-Mo alloy due to the low diffusion rate of Mo, and raising sintering temperature can effectively increase the density without grain coarsening. Neglecting the density factor, the addition of Mo refines the microstructure, and improves the mechanical strength by Hall-Petch relationship.3) Addition of Nd content enhances the sinterability of titanium alloy, which is interpreted by assuming the formation of transient liquid phase caused by decomposing of NdAl2, NdAl3 and Nd3Al phase. Two kinds of Nd-rich dispersoids were formed in titanium matrix: some precipitated within the grain, they are ellipsoid and contain a nearly uniform structure; others existed in the grain boundary and their shapes are mainly irregular, which contain multiple-layer structure. The unambiguous structures of thosedispersoids cannot be determined in this article, but they can be referred to as Ti-Nd-O complexs. The forming mechanisms for the two kinds of dispersoids are predicted. Addition of Nd content improves the mechanical properties of titanium alloy, which is mainly attributed to higher sinterability and scavenging oxygen dissolved in the titanium alloy to achieve depuration and substantial retention of B phase.