Effect of tantalum addition on precipitate coarsening kinetics and mechanical properties of the γ′-strengthened chemically complex alloys

Abstract

The application of ${\gamma }^{\prime }$${\gamma }^{\prime }$-strengthened chemically complex alloys (CCAs) at high temperature critically depends on the thermal stability of ${\gamma }^{\prime }$${\gamma }^{\prime }$ precipitates. In this paper, the effects of Ta on the microstructural evolution, the coarsening behavior of ${\gamma }^{\prime }$${\gamma }^{\prime }$ precipitates, and the mechanical properties of the alloys were systematically studied. Ta addition resulted in an increase in the volume fraction of the precipitate, an increase in the number density, and a decrease in the average diameter. And the strong partitioning into the ${\gamma }^{\prime }$${\gamma }^{\prime }$ phase of Ta prompted the increase of the $\gamma /{\gamma }^{\prime }$$\gamma /{\gamma }^{\prime }$ lattice misfit, leading to a spheroidal-to-cuboidal precipitate morphological transition. It was also demonstrated that Ta could effectively decrease the coarsening rate of ${\gamma }^{\prime }$${\gamma }^{\prime }$ precipitates, which was primarily attributed to the reduction of effective diffusion coefficient of alloys. Through a combination of diffusion-couple experiments and first-principles calculations, the reduction of the interdiffusion coefficients among all the principal elements, with Co as the rate-limiting element, was due to the enhanced bonding between Co atoms and neighboring atoms induced by Ta addition. Moreover, incorporating Ta significantly enhanced the mechanical properties of the alloys, achieving yield and ultimate tensile strengths of 855 MPa and 1255 MPa, respectively, after aging. Importantly, the underlying ${\gamma }^{\prime }$${\gamma }^{\prime }$ coarsening kinetics and corresponding mechanical response in Ta-containing alloys were also analyzed and discussed. Our present findings will give valuable insights into future design of ${\gamma }^{\prime }$${\gamma }^{\prime }$-strengthened CCAs for high-temperatures structural applications.