Effect of thermomechanical processing on the grain boundary character distribution and mechanical properties of biomedical stainless steels

Microstructural engineering (ME) is employed to enhance the properties of austenitic stainless steels through thermomechanical processing (TMP). Despite its potential, the application of ME to biomedical stainless steels remains underexplored, representing a significant gap in the literature. This study investigates the influence of processing routes on the grain boundary character distribution (GBCD) and mechanical properties of two austenitic stainless steels for biomedical applications, ASTM F138 and ISO 5832-9. Both steels are derived from AISI 316L, with ASTM F138 being Cr and Ni-richer and free of additional alloying elements, while ISO 5832-9 contains higher levels of Cr, Mn, and N, along with Nb addition. The iterative processing route, characterized by smaller deformation steps and intermediate annealing, yielded the highest fractions of low-Σ coincidence site lattice (CSL) boundaries, reaching 75 % in ASTM F138 and 71 % in ISO 5832-9. The route with a single deformation step resulted in finer grains and higher strength, while the iterative route enhanced ductility through strain-induced boundary migration and annealing twin formation, with strength superior to the requisites prescribed by the standards. This route also led to grain growth and notable differences in CSL boundary fractions between edge and center regions. These findings emphasize that a high proportion of low-Σ boundaries can be achieved using a viable industrial processing route with conventional deformation and heat treatment conditions and underscored the critical influence of processing parameters, such as deformation magnitude and annealing steps, in tailoring the GBCD and mechanical properties of biomedical stainless steels.