Role of Twinning on Dynamic Recrystallization and Microstructure During Moderate to High Strain Rate Hot Deformation of
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THE szoundness of components and their in-service performance largely depend on the thermomechanical processing adopted for their fabrication. During thermomechanical processing, the material undergoes shape and microstructural changes depending on the processing history. Therefore, attention should be directed toward the optimization of the process parameters such as temperature, strain, and strain rate to achieve a defectfree component with the desired microstructure. In low stacking fault energy (SFE) fcc alloys such as austenitic stainless steels, dynamic recrystallization (DRX) occurs readily during hot working. This is because dynamic recovery is sluggish (as climb and cross-slip are inhibited) and the driving force for recrystallization is maintained.[1] The phenomenon of DRX is technologically important, because it softens metals during hot deformation and reduces the hot working loads.[2] Further, it can lead to significant refinement of the microstructure, which improves the mechanical properties and formability of the materials.[3,4] The microstructure control through DRX requires detailed knowledge of microstructural evolution as a function of process parameters (i.e., strain, SUMANTRA MANDAL, Scientific Officer – E, and A.K. BHADURI, Associate Director, are with the Materials Technology Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India. Contact e-mail: [email protected] V. SUBRAMANYA SARMA, Associate Professor, is with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600036, India. Manuscript submitted April 28, 2011. Article published online February 14, 2012 2056—VOLUME 43A, JUNE 2012
strain rate, and temperature). Special emphasis needs to be given on understanding the nucleation mechanisms during DRX. The influence of strain rate on DRX as a function of temperature and strain in the low strain rate domain (105 to 1 s1) was studied in detail in different materials.[5–7] It was observed that the fraction of recrystallized grains increased with the decrease in strain rate in the ranges 103 to 1 s1 in Superalloy 718.[5] While studying the DRX behavior in Mg alloy, Beer and Barnett[6] also observed that the size of the DRX grains and the percentage of DRX increased with a decrease in strain rate in the range of 0.01 to 1 s1. Miura et al.[7] observed that the nucleation of DRX grains became more significant (i.e., DRX becomes easier) in the Cu alloy bicrystal with the decrease in strain rate in the range of 4 9 105 to 4 9 103 s1. The acceleration of DRX at lower strain rates is attributed to the decrease in the critical dislocation density and consequent lowering of the critical strain required for the occurrence of DRX. In addition, the lower strain rate provided longer times for nucleation and growth of DRX grains. Although the influence of strain rate on DRX in the lower strain rate domain seems to be well established, the influence of strain rate on DRX at higher strain rates (‡1 s1) is not clear. This coul
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