Effect of cryorolling on the microstructure and tensile properties of bulk nano-austenitic stainless steel Barna Roy, Rajesh Kumar 1 , Jayanta Das n Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, West Bengal 721302, India article info Article history: Received 21 January 2015 Received in revised form 18 February 2015 Accepted 19 February 2015 Available online 27 February 2015 Keywords: Electron microscopy X-ray diffraction Nanostructured materials 304L stainless steel Grain refinement Twinning abstract We report the synthesis of nanostructured austenitic AISI 304L stainless steel (SS) through cryorolling (CR) and reversion annealing in the temperature range of 700–800 1C. Severe CR at sub-zero temperature promotes twinning in γ-austenite, which transform into α'-martensite with lath thickness of 50–100 nm. Whereas, 50–300 nm size γ-grains recrystallize in nano-twinned α' through reversion annealing as confirmed by transmission electron microscopy (TEM) and electron back scattered diffraction (EBSD) imaging. The evolution of highly processable bulk nano-austenitic SS with bimodal grain size distribution on achieving high strength ( 1295 MPa), large tensile ductility ( 0.47), and true necking strain of 0.59, have been discussed. & 2015 Elsevier B.V. All rights reserved. 1. Introduction Austenitic SS is one of most attractive engineering alloys due to their good corrosion resistance and malleability [1]. However, the major drawback of austenitic SS is low yield strength about  100 MPa due to the presence of soft face centered cubic (fcc) γ-austenite phase [2,3]. The strength of the austenitic SS can be improved by grain refinement, solid solution strengthening and/or work-hardening [1]. Among all these mechanisms, the grain refine- ment has been achieved by adopting thermo-mechanically con- trolled processing (TMCP) treatment, which improves both strength up to 1200–1600 MPa and toughness of austenitic SS, simulta- neously [4]. On the other hand, severe plastic deformation techni- ques introduce large structural defects into the bulk specimen, and produce much fine grain structure (  300 nm) than that obtained by adopting TMCP (2–5 μm) [5]. As example, nano-/ultrafine- grained austenitic SS has been synthesized by multiple compression [6,7], hydrostatic extrusion [8], equal channel angular pressing (ECAP) [9–11], and high-pressure torsion (HPT) [12–15]. Grain refinement in austenitic SS is accelerated during severe plastic deformation (SPD) due to the formation of deformation twins in metastable γ-grains, which undergo stress-induced phase transition causing rapid microstructural refinement [16–20]. On the other hand, deformation twinning and stress induced martensitic transformation in γ can be influenced by the stacking fault energy (SFE) of SS [21]. The plastic deformation of γ occurs through dislocation glide in SS for SFE E45 mJ m 2 [21–24]. It has been reported that direct γ-α' trans- formation is promoted upon straining of SS with SFE below 18 mJ m 2 [17,21,25]. Whereas, deformation twinning has occurred in austenitic SS with SFE in the range of 18–45 mJ m 2 [22–24]. Olson et al. [22] and Sato et al. [26] have proposed two major phase transformation pathways in 304SS on the basis of SFE: (i) γ-austenite-ε-martensite -α'-martensite (SFEo18 mJ m 2 ), and (ii) γ-austenite-twinned austenite -α'-martensite (SFE 418 mJ m 2 ). The evolution of microstructure and its effect on the mechanical properties of austenitic SS upon cold rolling and subsequent annealing, have been studied by several researchers [27–29]. Ma et al. have reported that severe cold rolling in the range of 75–90% introduces 200–300 nm α'-lath in 304L SS, which finally recrystal- lized into 300 nm size γ-grain upon reversion annealing at 640 1C for 10 min, resulting an improvement in the yield strength from 120 MPa to 708 MPa without loss of macroscopic plasticity [27]. Kumar et al. have performed cyclic annealing at 900 1C for 45 s and 850 1C for 45 s of a 90% cold-rolled 304L SS, and have produced 300–2000 nm size γ-grains, which has shown 710 MPa yield strength and 36% tensile ductility [28]. Similarly, high yield strength of 2050 MPa with negligible plasticity (2.2%) have been achieved in a 98% cold rolled 304H SS with 50 nm size α'-subgrains. Whereas, annealing of the as-rolled specimen at 800 1C for 30 min produced 450 nm size γ-grain, which improved the ductility upto 21% with low yield strength of 670 MPa only [29]. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/msea Materials Science & Engineering A http://dx.doi.org/10.1016/j.msea.2015.02.050 0921-5093/& 2015 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ91 3222 283284; fax: þ91 3222 282280. E-mail address: j.das@metal.iitkgp.ernet.in (J. Das). 1 Present address: Defence Metallurgical Research Laboratory, Hyderabad 500058, India. Materials Science & Engineering A 631 (2015) 241–247