Takanori Mimoto

Strengthening behaviour and mechanisms of extruded powder metallurgy pure Ti materials reinforced with ubiquitous light elements

Ubiquitous light elements (ULEs) like oxygen (O), nitrogen (N), carbon (C) and hydrogen (H) can be the key additive elements to achieve the high specific strength and cost-effectiveness of powder metallurgy (PM) titanium (Ti) materials. They have high potential to be employed as promising useful reinforcements of conventional expensive alloying elements like vanadium (V) and Niobium (Nb). In this study, two ULEs, oxygen and hydrogen, were induced into pure Ti matrix via the PM route, and then the processed Ti powders were consolidated into the extruded Ti–O–H materials. The additive ULEs’ contribution to the microstructures and strengthening behaviour of Ti wrought materials was investigated in detail. The extruded Ti–0.97O–0.11H specimen exhibited an excellent combination of ultimate tensile strength (UTS) of 1158 MPa and tensile elongation of 23.9%, which were superior to those of Ti–6Al–4V alloy (UTS: 1047 MPa, elongation: 16.6%).

Influence of Carbon Reinforcements on the Mechanical Properties of Ti Composites via Powder Metallurgy and Hot Extrusion

Ti metal matrix composites (Ti–MMCs) reinforced by vapor grown carbon nanofiber (VGCF) and graphite particle (Gr) were prepared via powder metallurgy and hot extrusion. Ti with 0~0.4wt% VGCF/Gr mixture powders were consolidated by using spark plasma sintering (SPS) at 800 °C. Hot extrusion was then performed at 1000 °C with an extrusion ratio of 37:1. Microstructures and mechanical properties of the as-extruded Ti composites were investigated. Tensile strength of Ti–VGCF/Gr composites was steadily augmented when additions of VGCF/Gr were increased from 0.1 to 0.4 wt%. YS and UTS were increased 40.2% and 11.4% for Ti–0.4wt%VGCF as compared to pure Ti, while those values were 30.5% and 2.1% for Ti–0.4wt%Gr. The strengthening mechanism including grain refinement, carbon solid solution strengthening and dispersion hardening of TiC/carbon was discussed in detail.

The Development of a Ti-6Al-4V Alloy via Oxygen Solid Solution Strengthening for Aerospace and Defense Applications

The high cost of titanium has historically prevented widespread use in military ground vehicles. Two strategies to make this material more cost effective and viable are to reduce the cost of titanium armors or to improve the ballistic performance of titanium and reduce the amount of material required. This paper investigates the latter strategy. Mixtures of titanium powders and TiO2 particles were employed as starting materials and consolidated by spark plasma sintering (SPS) and hot extrusion. The content of TiO2 particles was 0~1.5% of the mass mixture. Solidification of oxygen atoms (from TiO2 particles) into Ti matrix occurred at 1073K for 1800 seconds in a vacuum. Tensile testing showed that Tensile Strength (TS) and Yield Strength (YS) increased in proportion to TiO2 content but elongation decreased slightly with increased TiO2 content. Extruded pure Ti powder material with 1.5% TiO2 particles produced 1040 MPa TS, 902 MPa YS and 25.1% elongation when tested. When using Ti-6Al-4V (Ti-64) alloy powders with 0.5% TiO2 particles, the final extruded Ti-64 powder bars with oxygen solid solution showed 1226 MPa TS and 22.7% elongation. Initial ballistic evaluation showed the Ti-64 powder bars with 0.5% TiO2 particles yielded a marked improvement over the conventionally rolled Ti-64 alloy plate.

Effect of vapor grown carbon fiber content on microstructure and tensile properties of Ti64/TiC composite fabricated by powder metallurgy method

Ti-6%Al-4%V (Ti64) alloy powder coated with vapor grown carbon fibers was consolidated to fabricate titanium matrix composites by spark plasma sintering, followed by hot extrusion process. Four compositions of the additive vapor grown carbon fibers were 0.1, 0.2, 0.3 and 0.4 wt. %. The microstructure was changed from the full lamellar of monolithic Ti64 alloy to the bimodal structure by addition of vapor grown carbon fibers to Ti64 alloy. The changing of microstructure was attributed by an α-stabilize effect of carbon (vapor grown carbon fiber). Almost all the vapor grown carbon fibers reacted with Ti and result in formation of Ti6C3.75 carbide phases detected by x-ray diffractometer and energy dispersive spectrometer. Hardness was improved by effect of solid solution strengthening of carbon in the Ti matrix and reached the maximum value of 495.8 Hv. 0.2%YS and UTS were significantly increased by addition of 0.1 wt. % vapor grown carbon fiber. However, they were not much improved when more vapor grown carbon fiber content was applied. Ductility of the sample was affected by a change in microstructure when 0.1 wt. % of vapor grown carbon fibers was added because the bimodal structure shows higher ductility compared to the full lamellar structure. The carbide precipitation in TMCs was not much contributed in improving of strength and decreased a ductility of material.

Tribological Properties of Titanium Plate Coated with Carbon Nanotubes

This study investigated the tribological property and wear behavior of pure titanium (Ti) plate coated with un-bundled multi-walled carbon nanotubes (MWCNTs). The network-structured MWCNT films were formed on Ti substrate, and their tribological properties were examined by the ball-on-disk wear test equipment under dry sliding condition. SUS304 stainless steel ball was used as a counterpart material in this test. The mean friction coefficient of the Ti plate coated with MWCNTs was remarkably lower and stable compared to the as-received pure Ti plate without any coating films. SEM-EDS analysis showed the network-structured MWCNT films obviously remained after wear test for 3.6 ks in sliding and no seizure phenomena with the SUS304 ball. The above excellent tribological performance was due to CNTs self-lubricant, their bearing effects and the strong metallurgical bonding between Ti plate and MWCNT films by annealing.