Static Softening in a Ni-30Fe Austenitic Model Alloy After Hot Deformation: Microstructure and Texture Evolution
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TRODUCTION
STATIC recrystallization (SRX) is one of the most common and important restoration mechanisms taking place during thermomechanical processing of steels.[1] This process involves the nucleation and growth of dislocation-free grains in the deformed matrix, which simultaneously undergoes recovery/dislocation annihilation, during the post-deformation annealing. SRX has attracted a significant attention among different research groups around the world, largely focusing on the mechanical response, kinetics, and grain size changes as a function of thermomechanical parameters e.g. References 2 through 4. However, for the case of steels in the austenite state, there is currently a limited understanding of the evolution of the dislocation substructure characteristics of the deformed (statically recovered) matrix, which coexists with recrystallized grains and is progressively consumed during SRX process. In addition, more information is required regarding the crystallographic texture evolution for both the deformed austenite matrix and SRX grains during the post-deformation annealing. This is largely for the reason that the phase transformation occurring HOSSEIN BELADI, PAVEL CIZEK, ADAM S. TAYLOR, and PETER D. HODGSON are with the Institute for Frontier Materials, Deakin University, Geelong, VIC 3216, Australia. Contact e-mail: [email protected] GREGORY S. ROHRER is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890. Manuscript submitted May 30, 2016. Article published online November 29, 2016 METALLURGICAL AND MATERIALS TRANSACTIONS A
in steels on cooling prevents the direct investigation of the austenite microstructure and texture behavior during deformation and post-deformation annealing process. This has led to the design of model austenitic alloys (e.g., Ni-30Fe alloy), which do not transform on cooling and have similar stacking fault energy to low-carbon low-alloy steels in the high-temperature austenite regime.[5–8] The latter ensures similar deformation behavior to the steel at high deformation temperatures while the former enables us to study the hot deformed austenite microstructure at ambient temperature. The austenitic model alloys have been widely used to study the deformed microstructure obtained under different thermomechanical processing conditions, e.g. References 6 through 14. These studies have revealed that the deformation of austenite, in the hot working temperature range employed for steel, largely leads to the formation of arrays of extended parallel planar dislocation walls bounding so called ‘‘microbands’’ (MBs) within a majority of grain interiors. These MB arrays are typically characterized by systematically alternating misorientations across consecutive extended boundaries.[12–17] It is of interest to elucidate how such unique self-screening dislocation arrangements impact on the dislocation annihilation processes during the post-deformation annealing. It has been demonstrated that the MB characteristics (i.e., misorie
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