Microstructure and mechanical properties of hetero-designed Ti-25Nb-25Zr alloy fabricated by powder metallurgy route

The harmonic structured (HS) materials have a heterogeneous microstructure consisting of bimodal grain size together with a controlled and specific topological distribution of ultra-fine grains (Shell) and coarse-grains (Core) areas. They have excellent strength combined with good ductility due to their unique heterogeneous three-dimensionally (3D) gradient microstructure, the two properties being rather an antagonist from the classical metallurgy point of view. In the present study, HS Ti-25Nb-25Zr alloy (mass %), compacts were successfully fabricated by a powder metallurgy method consisting of controlled mechanical milling (MM) of pre-alloyed Ti-25Nb-25Zr (TNZ) powder, followed by Spark Plasma Sintering. The MM leads to the severe plastic deformation at the powder particle surface. As a result, bimodal grains, with ultra-fine grains at the particle surface, and coarse-grains at the powder core, was achieved. After sintering of MM powder, the TNZ, compacts with HS was achieved. The HS TNZ exhibited higher strength together with acceptable ductility as compared to the homogeneous microstructured TNZ alloy fabricated by SPS of as-received TNZ powder. The systematic characterization was done using Scanning Electron Microscope (SEM) equipped with a backscattered detector (BSE), Electron Back Scatter Diffraction (EBSD) and Energy Dispersive X-ray spectroscopy (EDX), XRD, and tensile testing. It is shown that different powder conditions led to significantly different microstructures. Also, it was observed that the high ductility and low strength was achieved for the compact prepared from as-received powder whereas a good combination of strength and ductility was achieved for the specimen prepared from MM of as-received powder.