Ti2AlNb‒based alloys, so-called ordered orthorhombic (O (Cmcm)) phase alloys, have a chemical composition of Ti ‒ (18 ‒ 30) Al ‒ (12.5 ‒ 30) Nb (mole fraction, %). Since the discovery of orthorhombic (O) phase, Ti2AlNb‒based alloys have received special attention as a promising candidate for advanced aerospace and automotive application due to their high specific strength, excellent creep and oxidation resistance at elevated temperatures as well as good workability.
Up to now, many studies on the fabrication of Ti2AlNb-based alloys by casting, rolling and forging have been conducted, in which the major focus was set on the elimination of disadvantages such as microstructure segregation and inhomogeneity. It is because the thermodynamic properties of Ti, Al and Nb elements including a melting point, a density, a diffusion coefficient, etc. are very different.
Recently, powder metallurgy (PM) method such as spark plasma sintering (SPS) has been applied to the fabrication of Ti2AlNb‒based alloys, which made it possible to obtain fine and homogeneous microstructure. Several endeavors have been devoted to preparing PM Ti2AlNb‒based alloys from pre‒alloyed powder and elemental powders by vacuum hot pressing, and from pre‒alloyed powder by hot isostatic pressing (HIP). However, these as‒sintered compacts showed the microstructure with coarse grain size (>40μm), due to higher sintering temperature and longer sintering time. Therefore, the strength of these Ti2AlNb‒based alloys was not so high.
In recent years, several studies on the fabrication of high strength and ductility titanium alloys with nanostructured (NS) and ultrafine grained (UFG) microstructure have been reported. One process to achieve this microstructure consists of the preparation of NS or UFG powder by high energy ball milling (HEBM) and the consolidation of the powder into dense compacts by PM methods such as HIP and SPS. For instance, using the high energy ball-milled powder as a starting material, an ultrafine grained Ti‒6Al‒4V with high mechanical properties can be obtained.
In order to produce a high strength and ductility Ti‒22Al‒25Nb alloy from pre alloyed powder, Sim Kyong Ho, a researcher at the Faculty of Materials Science and Technology, has introduced HEBM with subsequent SPS. The process is as follows. Firstly, the Ti‒22Al‒25Nb pre‒alloyed powder with a weight of 75g was put in a stainless steel vial with bearing steel balls. HEBM was carried out in a high-energy planetary ball mill (QM-3SP4) under a high purity argon atmosphere for 20h at a rotation speed of 300 rpm. The HEBMed powder was directly loaded into a high-strength graphite mould, and subsequently consolidated by a LHPD250 SPS apparatus (FCT Co. Ldt., Germany) at SPS temperatures of 950℃ for SPS time of 10 min. After sintering, the sintered compact was cooled to the room temperature (RT) in the furnace. Finally, a sintered compact, with dimensions of φ50 × 15 mm, was obtained.
The Ti‒22Al‒25Nb alloy sintered at 950℃ for 10 min under the pressure of 50 MPa from the 20 h‒HEBMed powder showed the microstructure with a large amount of UFG/NS O-phase. The yield strength, tensile strength and elongation to failure were 1 092 MPa, 1 105 MPa and 9.4%, respectively. Compared with the referenced Ti‒22Al‒25Nb alloys, the Ti‒22Al‒25Nb alloy fabricated by HEBM and subsequent SPS exhibited higher strength and good ductility at the room temperature and high temperature. This indicates that HEBM with subsequent SPS is feasible for PM Ti‒22Al‒25Nb alloy with enhanced tensile properties.
More information about this is found in his paper “Fabrication of a high strength and ductility Ti‒22Al‒25Nb alloy from high energy ball-milled powder by spark plasma sintering” published in the SCI Journal “Journal of Alloys and Compounds”.
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