Effect of High-Pressure Torsion on Structure and Microhardness of Ti/TiB Metal–Matrix Composite

Sergey Zherebtsov, Maxim Ozerov, Nikita Stepanov, Margarita Klimova, Yulia Ivanisenko
2017 Metals  
The microstructure and microhardness evolution of a Ti-15(wt.%)Mo/TiB metal-matrix composite (MMC) during high-pressure torsion (HPT) at 400 • C was studied. The composite was fabricated by spark plasma sintering of a Ti, Mo and TiB 2 powders mixture at 1200 • C. In the initial condition, the structure of the composite consisted mainly of body-centered cubic (bcc) Ti solid solution and TiB whiskers. An increase in dislocation density, a considerable decrease in a grain size in the bcc Ti
more » ... n the bcc Ti matrix, and breaking/rearrangement of the TiB whiskers were observed during HPT. The (sub)grain size in the bcc Ti matrix attained after 1 revolution was~75 nm and then gradually decreased to~55 nm after 5 revolutions. The TiB particle sizes after 5 revolutions was found to be 130-210 nm. The microhardness increased with strain from 575 HV in the initial state to 730 HV after 5 revolutions. Various hardening mechanisms' contributions in the Ti-15Mo/TiB were evaluated. high specific strength, excellent biocompatibility and low Yong's modulus. In addition, the strength and hardness of β titanium alloys with a (meta)stable bcc structure can be considerably enhanced by thermomechanical treatment [9] . Some mechanical properties of alloys can be modified substantially using severe plastic deformation (SPD) [10] due to refinement of their microstructure down to the nanoscale interval. Although different SPD methods was a subject of comprehensive investigations during recent decades [10] [11] [12] , the influence of severe deformation on the structure and properties of MMCs has been poorly studied yet [13, 14] . Meanwhile, the SPD of such types of materials is expected to refine the matrix microstructure, decrease the size of particles and improve the particle distribution; the operation of several strengthening mechanisms can result in a record level of strength and hardness in severely deformed MMCs [13] [14] [15] . Since the ductility of MMCs is usually not high enough, the most suitable SPD process seems to be high-pressure torsion (HPT) at elevated temperature, which was successfully used earlier for nanostructuring of various MMCs, including hcp Ti/TiB [14] . It should be mentioned, however, that examinations of MMC microstructure evolution during SPD were mainly carried out using composites with oxide or carbon-based equiaxial particles [12, 13] . Composites reinforced by fibers or elongated particles, like those formed in the Ti/TiB system [8, 14] , have been studied to a much lesser extent. Meanwhile (to the best of the authors' knowledge) the structure and mechanical properties of bcc-Ti based MMCs subjected to HPT has not been studied so far. In the present work, a Ti-15Mo/TiB composite was fabricated through SPS using a Ti-13.5(wt.%) Mo-10(wt.%) TiB 2 powder mixture at 1200 • C. The composite was then deformed using high-pressure torsion at 400 • C to different strain levels. The microstructure evolution of the composite was comprehensively studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM); microhardness was measured to evaluate the influence of HPT on its mechanical properties. Experimental Powders of commercial purity Ti (99.1% purity), Mo (99.95% purity) and TiB 2 (99.9% purity) were (Guangzhou Hongwu Material Technology Co., Ltd., Guangzhou, China) used for the sintering. The average sizes of the Ti, Mo and TiB 2 particles in the powders were 25, 20 and 4 µm, respectively. A powders mixture contained 86.5 wt.% Ti, 13.5 wt.% of Mo and 10 wt.% of TiB 2 was produced (to obtain a Ti-15wt.% Mo alloy with 17 vol.% of TiB) using a Retsch RS200 vibrating cup (RETSCH Technology, Haan, Germany) mill in ethanol for 1h; a milling rotation speed was 700rpm. Specimens measured Ø19 mm ×~20 mm height of Ti-15Mo/TiB MMC were produced by SPS at 1200 • C and 40 MPa under vacuum for 5 min on a Thermal Technology SPS10-3 set-up (Thermal Technology LLC, Santa Rosa, CA, USA). Disks 10 mm in diameter and 0.7 mm in thickness were strained by HPT in a Bridgman-type anvil unit equipped with a custom-built computer-controlled device (W. Klement GmbH, Lang, Austria) at 400 • C with a speed of 1 rpm and load of 6 GPa. The quantity of turns was 1, 3 or 5. Shear strain γ for the corresponding number of revolutions can be evaluated as [12] :
doi:10.3390/met7110507 fatcat:uzqnqyxgu5fujmrfijlxd6nqji