Study on thermal stability and biocompatibility of bimodal microstructure in Cr–Mn–N austenitic stainless steel

Heterostructured austenitic stainless steels (ASS) are becoming a significant research area because of their outstanding mechanical properties and considerable potential for various applications. However, the thermal stability of heterogeneous microstructures, particularly the bimodal microstructure, has received limited attention and lacks systematic investigation. Additionally, there is a scarcity of reports regarding the biocompatibility of bimodal microstructures. Herein, the thermal stability and biocompatibility of the bimodal microstructure prepared by cold rolling and annealing in Cr-Mn-N series ASS (without and with Nb microalloying) are studied. The findings demonstrate that the bimodal microstructures of ASS are formed after annealing at 700 °C, 800 °C, and short-time annealing at 900 °C. Moreover, the addition of Nb significantly enhances the thermal stability of the bimodal microstructure and maintains the bimodal feature up to 1000 °C. The thermal stability of bimodal microstructures depends on the competition in coarse and fine grains growth. The good thermal stability of Nb(C, N) at high temperatures leads to consistently higher pinning force within the fine-grained zone. As a result, the growth of fine grains lags behind that of coarse grains, which leads to the persistence of bimodal microstructure at higher temperatures. The bimodal microstructure of ASS demonstrates superior biocompatibility, attributed to its ability to promote higher cell viability, exhibit stronger fibronectin intensity, and facilitate a wider fibronectin expression network in osteoblasts. These characteristics make the bimodal ASS more favorable for osteoblast attachment and proliferation compared to its coarse-grained counterpart. This study significantly enhances our understanding of the thermal stability and cellular functionality of bimodal ASS, highlighting its potential for various biomedical applications.