ИСТИНА

The development of new heat-resistant alloys based on cobalt is one of the urgent tasks, the solution of which is necessary for the further exploration of Space. To obtain alloys with a given microstructure, it is extremely important to understand the mechanism by which the formation of nanoparticles of strengthening phases occurs. Previously, a mechanism was proposed to describe the decomposition of a supersaturated fcc cobalt solid solution in the ternary Co-Ta-Re system, which was presented. It consists of the formation of phase ε'-Co3Ta (prototype Mg₃Cd) nanoparticles through the Guinier-Preston zone stage with the participation of Suzuki atmospheres. A similar mechanism is realized during the aging of four-component Co-Ti-Ta-Re alloys based on fcc cobalt with a titanium content of up to ~5 at.%. With an increase in the titanium content to 8 at.%, the evolution of the Guinier-Preston zones leads to the formation of nanoparticles of the strengthening phase γ'-Co3(Ti,Ta), which transforms into ε'- Co3(Ti,Ta) upon cooling. The aim of this work was to find out the reasons for such differences in the aging mechanisms of alloys with different titanium and rhenium contents. The alloys were prepared in an electric arc furnace in an argon atmosphere, heat treatment was carried out in evacuated quartz ampoules in electric resistance furnaces. Heat treatment was carried out in three stages: homogenization at 1375 K (100 h), aging at 1200 K (24 h) and tempering at 1030 K (24 h). The methods of scanning electron microscopy, EDX, X-ray phase analysis, transmission electron microscopy and electron diffraction were used in this work. It was established that the direct formation of phase nanoparticles from the Guinier-Preston zones was characteristic of the concentration region of the γCo-phase, which was in equilibrium with the Laves phase λ' (prototype MgNi2). Preliminary formation of phase γ'-Co3(Ti,Ta) particles is characteristic of concentrations in the equilibrium region. Increasing the rhenium content to results close to the limit of its solubility in the gamma phase in an alloy with 8 at.% titanium initially leads to the formation of nanoparticles γ'-Co3(Ti,Ta) without the preliminary formation of Guinier-Preston zones. The presence of submicron particles of another phase in a homogenized alloy can catalyze the preferential formation of phases γ'-Co3(Ti,Ta) or ε'- Co3(Ti,Ta) during aging. Thus, the design of the new cobalt alloys can be modified by minor adjustments to the elemental and phase composition, as well as heat treatment parameters.