Effects of Deformation Mode and Strain Level on Grain Boundary Character Distribution of 304 Austenitic Stainless Steel
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I.
INTRODUCTION
IT is well known that microstructure influences the performance of materials significantly. Grain boundary, as one of the most significant microstructural features, often exerts a strong influence on many kinds of properties of polycrystalline materials. Many studies have shown that many material properties such as resistance to intergranular corrosion,[1–3] radiation damage,[4,5] cracking,[6] precipitation[7,8] and weldability[9,10] strongly rely on the structure and character of grain boundaries. In the early 1980s, Watanabe[11] proposed the concept of ‘grain boundary design and control’, which was later evolved into ‘grain boundary engineering’ (GBE). Over the last three decades of development, GBE has become a promising approach to improve the grain boundary related properties in face-centered cubic metals with low to medium stacking fault energy, such as copper alloys,[12] Ni-based alloys,[13,14] Pb-based alloys[15,16] and austenitic stainless steels[17–19] by increasing the fraction of low-R coincidence site lattice
WEN FENG, SEN YANG, and YINBIAO YAN are with the School of Materials Science and Engineering, Nanjing University of Science and Technology, No. 200 Xiaolingwei, Nanjing 210094, China. Contact e-mail: [email protected] Manuscript submitted September 17, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
(CSL) boundaries through suitable thermomechanical processing (TMP). Although the sole increase in the fraction of low-R CSL boundaries is insufficient for the enhanced properties because the CSL model cannot describe the grain boundary plane of the grain boundary structure, the CSL model is widely used for convenience.[2,14,19] Besides the absolutely high fraction of low-R CSL boundaries, many studies[20–22] have suggested that the connectivity of random boundary network is a more reasonable predictor of resistance in intergranular degradation. Up to now, two TMP methods (namely strain annealing and strain recrystallization) have been commonly adopted to increase the fraction of low-R CSL boundaries and optimize grain boundary character distribution (GBCD). Strain annealing involves low levels of strain in the range of 2 to 7 pct followed by annealing treatment, which results in minimizing interfacial energy through a recovery process. Strain annealing method activates small adjustments of grain boundaries toward lower energy configurations. Shimada et al.[19] optimized the GBCD in commercial 304 austenitic stainless steel through introducing cold rolling reduction of 5 pct, followed by annealing at 1200 K (927 C) for 72 hours. Lee et al.[23] utilized one step of 6 pct tensile strain followed by annealing at 1173 K (900 C) for 10 minutes and increased the fraction of low-R CSL boundaries from 36.5 to 74.7 pct. In comparison, strain recrystallization is characterized by sequentially introducing low
to medium strain levels in the range of 5 to 30 pct, followed by annealing for short duration at relatively high temperature. Strain recrystallization method induces the formation of new
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