Abstract
The microstructure and microhardness evolution of a Ti/TiB and Ti-15(wt.%)Mo/TiB metal-matrix composites (MMC) during high-pressure torsion (HPT) at 400 °C was studied. The composites were fabricated by spark plasma sintering of either hcp α-Ti with 10 wt.% of TiB2 or Ti with 13.5 wt.% of Mo (resulted in a bcc β-Ti matrix) and of 10 wt.% TiB2 powders mixtures at 1000 or 1200 °C, respectively. An increase in the dislocation density, a considerable decrease in the grain size in both hcp or bcc Ti matrix, and shortening of TiB whiskers were observed in the microstructures of the composites during HPT. After five revolutions, a nanostructure with a (sub) grain size of ~30 or ~55 nm was produced in Ti or Ti-15Mo matrix, respectively. The microhardness increased with strain from 452 HV in the initial state to 520 HV after 5 revolutions for Ti/TiB MMC and from 575 to 730 HV for Ti-15Mo/TiB MMC. The contributions of various hardening mechanisms to the composites were evaluated.
Highlights
One of the well-known ways to increase strength of metallic materials is associated with a significant microstructure refinement through severe plastic deformation (SPD) [1, 2]
SPD of such metal-matrix composites (MMCs) induces the simultaneous operation of several hardening mechanisms [6]
Initial microstructure X-ray diffraction (XRD) showed that Ti/TiB MMC consisted of hcp α-Ti (78.6 vol.%), TiB2 with a hexagonal lattice (2.4%), and TiB with an orthorhombic lattice (19%)
Summary
One of the well-known ways to increase strength of metallic materials is associated with a significant microstructure refinement through severe plastic deformation (SPD) [1, 2]. Due to a considerable decrease in grain size, commercially pure titanium, for example, becomes strong enough for production of surgical implants [3]. Another promising approach to increase strength of titanium without losing its biocompatibility or high corrosion properties can be associated with embedding of ceramic reinforcements, like TiB, in titanium matrix [4, 5]. Some additional hardening mechanisms (solid solution hardening or strengthening due to the formation of ω-phase particles or twins) may be involved as a result of the transition of the titanium matrix from the hcp α-phase to the bcc β-phase. This transition can be attained by alloying of the titanium matrix with Mo, which is a strong beta-stabilizing element
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