Formation laws for grain boundary layers of α-Ti phase in binary titanium-based alloys

Abstract

The microstructure of polycrystalline titanium alloys with added chromium (2, 4, and 5,5 wt.%), cobalt (2 and 4 wt.%) and copper (2 and 3 wt.%) was studied. A series of long isothermal annealing cycles (under vacuum) was performed for these materials within the temperature range from 600 to 850 °C. The annealing temperatures were in the α(Ti,Me) + β(Ti,Me) two-phase regions of the Ti–Cr, Ti–Co and Ti–Cu phase diagrams. The temperature dependence plots were built for the β(Ti,Me)/β(Ti,Me) portion of grain boundaries completely «wetted» by layers of the α(Ti, Me) second solid phase and the average contact angle. The results of microscopic investigation showed that the type and content of the second component in the alloy greatly affect formation of equilibrium grain boundary layers. We were the first to discover a non-monotonic temperature dependence of the portion of grain boundaries completely «wetted» by layers of the second solid phase in the absence of bulk «ferromagnetic–paramagnetic» phase transitions.