On the interplay between interstitial doping, grain size, and TRIP effect in a CrCoNi alloy

Abstract

After the popularization of medium/high entropy alloys, some CrCoNi alloys are gaining attention for their outstanding toughness and mechanical performance. The Cr- and Co-rich alloys, such as Cr40Co40Ni20, present a favorable combination of two mechanisms: the FCC to HCP strain-induced phase transformation (TRIP effect), which provides improved ductility and toughness; and a high strengthening potential via grain refining, which significantly enhances its mechanical strength. Furthermore, interstitial doping with C or N can strengthen the alloy by inducing precipitation and microstructural refinement. However, according to recent review papers, the influence of these interstitial elements on TRIP effect and stacking fault energy (SFE) is still controversial, and the related explanation is not completely convincing. The objective of this work is to evaluate the interplay between interstitial doping, grain refinement, and TRIP effect. Firstly, in situ synchrotron X-ray diffraction during tensile testing allowed us to observe and quantify the strain-induced transformation throughout the entire test. The results indicated that: in terms of stress, the critical stress for TRIP is proportional to the grain refinement strengthening; and in terms of strain, grain size has no obvious influence on TRIP effect. Then, the effect of C and N on phase equilibria, grain refinement capacity, and TRIP effect was evaluated. C-doping promoted the formation of M23C6 precipitates, which was highly effective in inducing grain refinement through the pinning mechanism, while N-doping did not promote the formation of nitrides, but also induced grain refinement to a lesser extent. Both C and N affected the extent of the TRIP effect by altering the SFE. The N addition disfavored the TRIP effect, while C addition did not show monotonic behavior, but higher C levels also disfavored TRIP effect. For these low SFE alloys, the martensitic HCP phase is transformed back into the FCC phase during heating (martensite reversion) by recrystallization of the FCC matrix, and not by a displacive or diffusional HCP to FCC transformation before recrystallization. These findings highlight the complex interplay of mechanisms governing the microstructure and deformation behavior of CrCoNi alloys.