High ductility of ultrafine-grained steel via phase transformation
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H. Chooa) Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996; and Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Y.H. Zhao Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
X-L. Wang Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
Y.T. Zhu Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545
Y.D. Wang Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996
J. Almer Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439
P.K. Liaw Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996
J.E. Jin and Y.K. Leeb) Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Korea (Received 26 September 2007; accepted 27 December 2007)
There is often a tradeoff between strength and ductility, and the low ductility of ultrafine-grained (UFG) materials has been a major obstacle to their practical structural applications despite their high strength. In this study, we have achieved a ∼40% tensile ductility while increasing the yield strength of FeCrNiMn steel by an order of magnitude via grain refinement and deformation-induced martensitic phase transformation. The strain-rate effect on the inhomogeneous deformation behavior and phase transformation was studied in detail.
I. INTRODUCTION
In conventional metals and alloys, high strength is often accompanied by low ductility, and vice versa1; rarely can both be achieved at the same time. Unfortunately, this also applies to ultrafine-grained (UFG) materials. Many studies in recent years have shown that the tensile ductility became very low when the grain size was reduced to nanocrystalline (grain size d < 100 nm) and Address all correspondence to these authors: a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2008.0213 1578 J. Mater. Res., Vol. 23, No. 6, Jun 2008 http://journals.cambridge.org Downloaded: 13 Mar 2015
UFG (grain size d < 1 m) scale, while the strength was significantly increased. A variety of techniques were developed to solve the problem, but only a few of them were reported to successfully increase the tensile ductility to 20%.2–6 It is well known that the tensile ductility is related to the work-hardening capacity; i.e., a high and sustainable work hardening is the prerequisite for a high tensile ductility.7 Previous studies on low-carbon UFG steels worked on enhancing work hardening in UFG materials include methods such as bimodal grain size distribution, precipitation hardening, etc.8–13 However, each of these approaches is either limited to a certain material system © 2008 Materials Research Society IP address: 132.174.255.116
S. Cheng et al.: High ductility of ultrafine-grained steel via phase transformation
or the ductility is still not fully r
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