Creep mechanisms in particle strengthened α-Titanium–Ti2Co alloys

Quasi-eutectoid Ti–12Co–5Al and Ti–10Co–4Al alloys consist of an α-titanium solid solution matrix, a fine dispersion of intermetallic Ti2Co particles—topologically closed packed CF96 structure, prototype: Ti2Ni—in large volume fractions of about 30 and 22% and Ti3Al (α2) precipitates. These alloys exhibit high flow stress of ≈900 MPa, high elastic stiffness of ≈130 GPa and sufficient fracture toughness KIC equal to 21–35 MPa m−0.5 at room temperature. Because of the effective particle reinforcement, these alloys show good creep properties up to 600°C. The microstructure of the α-Ti/Ti2Co alloys is thermally stable and not sensitive to surface embrittlement due to α-case formation by oxygen penetration in contrast to high temperature resistant near α-titanium alloys. The creep properties of both alloys have been investigated in the temperature range of 500–700°C and in the strain rate range of 10−7–10−4 s−1. High threshold stresses and stress exponents n=8–10 and an activation energy for creep of ≈300 kJ mol−1 at temperatures up to 550°C are attributed to the strong dispersion strengthening effect caused by the high volume fraction of intermetallics. At 600 and 650°C, creep is controlled by dislocation climb (solid solution class II behavior) in the α-titanium matrix and the intermetallic Ti2Co phase (n=4–5). The activation energy of 305–410 kJ mol−1 is in between the values for self diffusion of titanium in the α-Ti(Al) solid solution and for chemical diffusion of Ti and Co in Ti2Co.