Microstructure Characteristics and Elevated Temperature Mechanical Properties of a B Contained β-solidified γ-TiAl Alloy

Abstract

The improved microstructure and enhanced elevated temperature mechanical properties of Ti-44Al-5Nb-(Mo, V, B) alloys were obtained by vacuum arc re-melting (VAR) and primary annealing heat treatment (HT) of 1 260 °C/6 h/Furnace cooling (FC). The phase transformation, microstructure evolution and tensile properties for as-cast and HTed alloys were investigated. Results indicate that three main phase transformation points are determined, T _eut=1 164.3 °C, T _{γ solv} = 1 268.3 °C and T _{β trans} = 1 382.8 °C. There are coarse lamellar colonies (300 µm in length) and neighbor reticular B2 and γ grain (3–5 µm) in as-cast alloy, while lamellar colonies are markedly refined and multi-oriented (20–50 µm) as well as the volume fraction and grain sizes of equiaxed γ and B2 phases (about 15 µm) significantly increase in as-HTed alloy. Phase transformations involving α+γ→α+γ+β/B2 and discontinuous γ coarsening contribute to the above characteristics. Borides (1–3 µm) act as nucleation sites for β _eutectic and produce massive β grains with different orientations, thus effectively refining the lamellar colonies and forming homogeneous multi-phase microstructure. Tensile curves show both the alloys exhibit suitable performance at 800 °C. As-cast alloy shows a higher ultimate tensile stress of 647 MPa, while a better total elongation of more than 41% is obtained for as-HTed alloy. The mechanical properties improvement is mainly attributed to fine, multi-oriented lamellar colonies, coordinated deformation of homogeneous multi-phase microstructure and borides within lamellar interface preventing crack propagation.

Keywords

TiAl alloy / phase transformation / heat treatment / boride / microstructure / mechanical properties

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