Deformation Characteristics and Recrystallization Response of a 9310 Steel Alloy
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INTRODUCTION
NOVEL methods of grain size refinement have been given a great deal of attention in recent years by those looking to further enhance material properties. Ultra-fine grained (UFG) materials exhibit enhanced strength, wear resistance and superplastic behavior, without the substantial reduction in fracture toughness and ductility associated with many other methods of enhancing these properties. The benefits of UFG materials stem from a high fraction of grain boundaries, which serve to limit dislocation mobility and lead to strengthening according to the Hall-Petch relation.[1,2] These potential benefits have led many to investigate various methods of producing UFG materials. Well established methods include powder metallurgy techniques,[3] controlled crystallization of an initially amorphous material,[4] along with various physical, chemical and electrical deposition methods.[5–7] All of these techniques have certain drawbacks, including high production costs, difficulties in producing large-scale parts or achieving uniform grain structures at full density. To avoid these limitations, much research has been devoted to severe plastic deformation methods, which accomplish grain refinement via recrystallization under a combination of intense plastic strain and elevated temperature. Such methods are ideal for their ability to produce large-scale bulk UFG parts at lower relative costs. Commonly employed methods of DAVID SNYDER, Graduate Research Assistant, and SAMMY TIN, Associate Professor, are with the Illinois Institute of Technology, 10 W. 32nd St., Chicago, IL 60616. Contact e-mail: dsnyder2@hawk. iit.edu EDWARD Y. CHEN, President, and CHARLIE C. CHEN, Technical Advisor, are with the Transition45 Technologies, Inc., 1739 N. Case St., Orange, CA 92865. Manuscript submitted February 5, 2012. Article published online September 18, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
severe plastic deformation include equal-channel angular pressing,[8–10] accumulative roll-bonding,[11] friction stir processing,[12] high-pressure torsion[13,14] and multi-axial forging.[15,16] Substantial grain refinement has shown to be achievable by these methods, provided sufficiently high plastic strains are achieved. Deformation temperature is also a key factor, as it controls both the recrystallization and grain growth kinetics. Recent studies[17–23] have shown that severe plastic deformation processing at relatively low temperatures (below that typically used in conventional forming) can yield a fine recrystallized grain size with very little subsequent growth, due to the differences in low-temperature deformation kinetics found in many alloy systems. When deformed at elevated temperatures, metals can accommodate the induced strains by various dynamic mechanisms, including dynamic recovery and dynamic recrystallization. The method of accommodation that predominates depends on a number of factors related to diffusion and dislocation kinetics, such as temperature, crystal structure, and the presence of microstructural features that interact
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