The article discusses the results of a study on low-strain thermo-mechanical (one-step and iterative) processing to enhance Σ3n boundaries in a Ti-modified austenitic stainless steel (alloy D9). Solution annealed (SA) specimens were subjected to 10% thickness reduction by rolling followed by annealing at 1173, 1223, and 1273 K for 0.5, 1, and 2 h. Anomalous grain growth with moderate increase in Σ3n boundaries was observed after annealing at 1,173 K for 0.5 to 2 h. Prolific multiple twinning with minimum deviation of Σ3 and Σ9 boundaries from ideal orientation was achieved after annealing at 1,273 K for 0.5 to 2 h. A significant disruption in random boundary connectivity was obtained in these conditions due to the presence of large number of Σ3-Σ3-Σ9/Σ3-Σ9-Σ27 triple junctions. Iterative processing (up to 4 cycles) employing 10% thickness reduction followed by annealing at 1,273 K for 0.5 h revealed fluctuations in the evolution of Σ3 boundaries. The Σ3 fraction increased after 2nd and 4th iteration and there is a drop after 3rd iteration. This was attributed to the increased driving force for grain boundary migration due to dislocation pile-up at twin boundaries during earlier iterations. A two step iterative processing comprising of 10% deformation followed by annealing at 1,273 K for 0.5 h is the recommended thermo-mechanical processing to achieve enhanced fraction of Σ3n boundaries (~73%) in alloy D9.
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The twin boundaries that were excluded in the calculation of average grain size were Σ3 and Σ9 boundaries within a tolerance of 5°.
The hardness was measured with a FIE VM 50 Vickers hardness tester with 5 kg load for 15 s dwell time.
The color-scale-coded KAM map is plotted on the basis of first-neighbor kernel parameter with a maximum misorientation angle of 5°.
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The authors are thankful to Dr. Satyam Suwas, Department of Materials Engineering, IISc Bangalore, India, for extending the rolling mill facility during this study. Thanks are also due to Mr. Nilesh Gurao, Department of Materials Engineering, IISc Bangalore, for his help during rolling. One of the authors (VSS) thanks the Indo-US Science and Technology Forum (IUSSTF) for a sabbatical research fellowship during the period this article was finalized.
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Mandal, S., Bhaduri, A.K. & Subramanya Sarma, V. One-step and iterative thermo-mechanical treatments to enhance Σ3n boundaries in a Ti-modified austenitic stainless steel. J Mater Sci 46, 275–284 (2011). https://doi.org/10.1007/s10853-010-4982-6
- Twin Boundary
- Triple Junction
- Solution Anneal
- Electron Back Scatter Diffraction
- Coincidence Site Lattice Boundary