Microstructure of second-phase particles in Ti-5Al-4Sn-2Zr-1Mo-0.25Si-1Nd alloy

Abstract

The microstructure of second-phase particles in the Ti-55 alloy was studied by scanning electron microscopy, transmission electron microscopy (TEM), and highresolution electron microscopy (HREM) observations. The second-phase particles in the conventional ingot-cast Ti-55 alloy of 1 to 15 m in diameter and uniform distribution in matrix were observed, where the majority of these particles are elliptical. The mean free path between the particles is about 46 m, and the volume fraction (pct) is 2.35. The second-phase particles typically contain Nd, Sn, and O in substantial amounts, and the content of Nd is the largest in the three elements. The elements Ti, Al, Zr, Mo and Si are depleted in the particles. The second-phase particle consists of either a dark or bright matrix and some small dark blocks dispersed within the matrix. Dark blocks match SnO (orthorhombic, a ϭ 0.500 nm, b ϭ 0.572 nm, and c ϭ 1.120 nm), and the matrix consists of a nanocrystalline phase with a stoichiometric Nd 3 Sn structure having a space group of Pm3m and lattice parameter of a ϭ 0.344 nm. The grain size of the nanocrystalline Nd 3 Sn phase is about 3 to 15 nm. The melting range of the second-phase particle is estimated to be 1042 ЊC to 1600 ЊC. The microstructure of the second-phase particles in the quenched Ti-55 alloy was also studied. Fine and uniform dispersoids (6 to 15 nm in diameter) were observed in the as-quenched state. Some lenslike particles occur at the grain boundaries, other elliptical particles appear within the grains, and some particles within the grains form rows which are parallel to the advancing liquid-solid interface. After annealing at 980 ЊC (1 to 10 hours), of the as-quenched Ti-55 alloy, coarse particles are 17 to 42 nm in average diameter, and the growth of the particles is very slow. The dispersoids in the asannealed Ti-55 alloy are identified as nanocrystalline Nd 5 Sn (orthorhombic, Pnmn, a ϭ 0.814 nm, b ϭ 1.732 nm, and c ϭ 0.814 nm) intermetallic compound, and the interface between the Nd 5 Sn 4 phase and the matrix is a typical high-angle grain boundary.

Effects of Sc addition on the microstructure and tensile properties of Ti–6.6Al–5.5Sn–1.8Zr alloy

“…Hence, similar with the results reported in Ref. [15], it can be inferred that the particles in the studied alloys belong to the oxides surrounded by Sc 5 Sn 3 compounds. It should be mentioned that oxygen enrichment is not found in some regions contained high levels of Sn and Sc, which may be attributed to the oxides are wrapped by the Sc 5 Sn 3 compounds so that the electrons cannot penetrate into these oxides.…”

“…Moreover, the high temperature yield strength of Tie48Al alloys can be improved by a minor Sc addition [22]. The improvement of high temperature strength is attributed to the formation of fine plate-like Ti 3 (Al, Sc) precipitates [22], however it is also pointed out that the Sceoxide rather than Ti 3 (Al, Sc) precipitates is formed in the TieAleSc alloys [21,23], which is consistent with the results in other REs-containing Ti alloys [14,15,24]. Therefore, it is still unclear how the Sc addition affects the microstructure and therefore improves the mechanical properties of the titanium alloys.…”

“…The REs addition in the Ti alloy will result in the formation of REcontaining oxides [15,24]. It is reported that the (Ti, Al) 3 Sc and Sc- containing oxides were found in the Ti alloys [20,22].…”

“…Although oxide was not detected by XRD, we think that the particles in the TiSc38 alloy are a mixture of Sc 5 Sn 3 and oxides. Li et al [15] reported that the NdeSn and SnO phases are formed in the Ti-55 (Tie5Ale4Sne2Zre1Moe0.25Sie1Nd) alloy. As seen from the elemental mappings of Sn, Sc and O, the oxygen-enriched regions are mainly distributed in the interior of the particles, while both Sn and Sc elements has similar distributions and shows high contrast outward the oxygen riched position.…”

Microstructures Evolution and Mechanical Properties of a Powder Metallurgical Titanium Alloy with Yttrium Addition

“…Also, oxides can be produced by casting processes, which involve a solid-liquid diffusion and reaction. In this process, reactants (rare earth elements) react with oxygen in the melt to form in situ oxides [2,12,13]. A blended elemental powder metallurgy approach is another promising route for fabrication of titanium alloys dispersed with oxides, due to its flexibility in addition of rare earth elements [14,15].…”