Nearly dense Ti–6Al–4V/TiB composites manufactured via hydrogen assisted BEPM

Abstract

Titanium matrix composites reinforced with in situ formed titanium boride whiskers have long aroused significant interest for advanced applications in fields such as aerospace, biomedicine, and armaments. However, processing approaches dedicated to fabricating these composites have usually been limited by the cost-performance dilemma, thereby limiting commercial success. Blended elemental powder metallurgy (BEPM) has historically been the most economical route to produce titanium-based composites. At the same time, the need to reduce undue sinter porosities has imposed complicated and expensive extra thermomechanical steps in BEPM manufacturing. In the present study, nearly dense Ti–6Al–4V-based composites reinforced with in situ synthesized titanium monoborides (TiB) are prepared by simple press-and-sinter hydrogen-assisted BEPM without hot deformation or hot pressing using TiH2, TiB2, and master alloy (Al–V) powder blends as starting material. Vacuum sintering of compacted powder blends results in the formation of a dehydrogenated Ti–6Al–4V matrix with excessive porosity and unevenly distributed partially reacted TiB2 particles. Such an inappropriate pre-sintered microstructure can be completely transformed into low-porosity uniform Ti–6Al–4V/TiB composites with tailored grains by using hydrogenation and milling of pre-sintered material into hydrogenated pre-alloyed powders and, finally, by using these powders in a second press-and-sinter processing step. The useful influence of hydrogen as a temporary alloying element on microstructure formation is discussed. The densification of hydrogenated powder compacts upon vacuum heating, and the hydrogen emission from the material is studied via dilatometric tests. The evolution of microstructure and phase composition during processing steps was investigated by scanning electron microscopy and x-ray diffraction. Compressive tests were used to evaluate the mechanical properties of materials produced after the first and second sintering. The results show that hydrogen-assisted BEPM can be a cost-effective route for in situ fabrication of Ti–6Al–4V/TiB composites with reliable mechanical properties.