Materials Science & Engineering A 799 (2021) 140057 Available online 9 August 2020 0921-5093/© 2020 Published by Elsevier B.V. Short communication An anomalous effect of grain refnement on yield stress in friction stir processed lightweight steel B. Mirshekari a , A. Zarei-Hanzaki a, * , A. Barabi a, b , H.R. Abedi c, * , S.J. Lee d, * , H. Fujii e a Hot Deformation and Thermomechanical Processing Laboratory of High Performance Engineering Materials, School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran b Department of Mechanical Engineering, Polytechnique Montr´ eal, Montr´ eal, Qu´ ebec, Canada c School of Metallurgy & Materials Engineering, Iran University of Science and Technology (IUST), Tehran, Iran d Department of Advanced Materials Engineering, Korea Polytechnic University, Siheung-si, 15073, Republic of Korea e Joining and Welding Research Institute, Osaka University, 11-1, Mihogaoka, Ibaraki, Osaka, 567-0047, Japan A R T I C L E INFO Keywords: Friction stir processing Hall-petch effect Grain refnement Yielding behavior Transformation induced plasticity ABSTRACT Friction stir processing was utilized for enhancing mechanical properties of a lightweight steel through devel- opment of fne grain structures with high austenite stability. Interestingly, an anomalous effect of grain refnement on yield stress was found which was justifed considering the dominant strain accommodative mechanisms in the initial and processed microstructures. 1. Introduction It has been reported that yielding of duplex steels contains three stages [1–3]. In the frst stage, elastic loading leads to linear deformation of the bulk; in the second stage, initiation of yielding in the soft phase takes place and subsequently, in the third stage, the hard phase con- tributes in strain accommodation [3]. Along with the aforementioned stages, the magnitude of yield stress is assumed to be scaled inversely by grain refnement of each phase [2]. In this respect, Friction stir pro- cessing (FSP) has been widely used to form a fne grain structure with uniform distribution and superior mechanical properties [4,5]. This method contains a non-deformable rotating cylindrical pin which is plunged into the workpiece which creates a frictional force between the pin and the material and leads to both heating and severe deformation the workpiece. The mentioned deformation in high temperatures pro- cedures fne grain structures due to dynamic recrystallization (DRX) which improves the mechanical properties in an appreciable manner [6, 7]. However, over the last few years, an infrequent behavior has been observed. In this respect, Lee et al [5] conducted FSP on an austenitic TWIP steel and observed that the yielding behavior of processed samples violated the Hall-Petch relation. The mentioned behavior was related to the low-acquired critical stress for the formation of deformation twins on the onset of necking. Likewise, in our previous work, by conducting FSP on a duplex ferritic lightweight steel [8], it was observed that yielding behavior of as annealed sample was different with the pro- cessed samples and an unexpected deviation from Hall-Petch effect was observed which was associated with stability of austenite and an anomalous effect of grain refnement. Such behavior was mainly attributed to increment in austenite stability. Similar results were ob- tained in different processing procedures such as cold roll and annealing of duplex medium Mn steels. For instance, Gibbs et al. conducted cold rolled and annealing on a TRIP steel and reported that yielding in 650 ◦ C annealed sample is mainly attributed to transformation of austenite to deformational martensite while by changing the annealing temperature to 600 ◦ C, initiation of yielding shifted from austenite to ferrite leading to formation of Luders band [9]. This change in yielding phase was related to the different stability of austenite rather than hardness of constituent phases. According to the aforementioned studies, yielding behaviors are more complex than just three stages and microstructural characteristics have great impacts. Therefore, in the present paper, a purposeful microstructural examinations have been conducted in order to diagnose the infuence of microstructural characteristics of each sample on its yielding behavior comprised of yield stress and work hardening at the onset of yielding in processed and as annealed duplex ferritic light- weight steel. * Corresponding authors. E-mail addresses: zareih@ut.ac.ir (A. Zarei-Hanzaki), habedi@iust.ac.ir (H.R. Abedi), leesj@kpu.ac.kr (S.J. Lee). Contents lists available at ScienceDirect Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea https://doi.org/10.1016/j.msea.2020.140057 Received 20 May 2020; Received in revised form 1 August 2020; Accepted 3 August 2020