Microstructures and Hardness Properties for β-Phase Ti-24Nb-4Zr-7.9Sn Alloy Fabricated by Electron Beam Melting

Atomized, pre-alloyed Ti-24Nb-4Zr-7.9Sn (wt%) powder was used to fabricate solid, prototype components by electron beam melting (EBM). Vickers microindentation hardness values were observed to average 2 GPa for the precursor powder and 2.5 GPa for the solid, EBM-fabricated products. The powder and solid product microstructures were examined by optical and electron microscopy. X-ray diffraction analyses showed that they had bcc beta-phase microstructure. However, it was found by transmission electron microscopy that the EBM-fabricated product had plate morphology with space similar to 100-200 nm. Although the corresponding selected area diffraction patterns can be indexed by beta-phase plus alpha''-martensite with orthorhombic crystal structure, the dark-field analyses failed to observe the alpha''-martensite. Such phenomenon was also found in deformed gum metals and explained by stress-induced diffusion scattering due to phonon softening.