Mechanical properties of Co3Ti polycrystals alloyed with various additions

Third elements including vanadium, tantalum, chromium, molybdenum, tungsten, iron, nickel, aluminium and germanium, were added to L1₂-type Co₃Ti polycrystals with a composition of 23 at % Ti. Tensile tests from 77 to 1273 K were carried out to investigate the mechanical properties of these alloys. Anomalous increases of the yield stress with increasing temperature were observed from 473 to 973 K (or 1100 K depending on the strain rates) for all tested alloys. It was also observed that the yield stress increased with decreasing temperature below 473 K and decreased with increasing temperature above 973 K (or 1100 K). The yield stresses above the peak temperature were very sensitive to the strain rate and grain size. Microstructural observation showed that gram-boundary sliding had a considerable contribution to the fall in yield stress at high temperatures. The elongation showed a maximum around 673 K and a minimum around 1073 K for all alloys. Microstructural and fractographic observation showed that most alloys tested suffered from hydrogen-related embrittlement at ambient temperatures and grain-boundary cavitation-related embrittlement around 1073 K. Addition of aluminium and iron was found to produce a significant improvement in hydrogen-related embrittlement at room temperature.