A critical examination of the paradox of strength and ductility in ultrafine-grained metals
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Megumi Kawasaki Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, South Korea; and Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA
Terence G. Langdon Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California, Los Angeles, CA 90089-1453, USA; and Materials Research Group, Faculty of Engineering and the Environment, University of Southampton, Southampton SO17 1BJ, UK (Received 21 May 2014; accepted 22 August 2014)
The paradox of strength and ductility is now well established and denotes the difficulty of simultaneously achieving both high strength and high ductility. This paradox was critically examined using a cast Al–7%Si alloy processed by high-pressure torsion (HPT) for up to 10 turns at a temperature of either 298 or 445 K. This processing reduces the grain size to a minimum of ;0.4 lm and also decreases the average size of the Si particles. The results show that samples processed to high numbers of HPT turns exhibit both high strength and high ductility when tested at relatively low strain rates and the strain rate sensitivity under these conditions is ;0.14 which suggests that flow occurs by some limited grain boundary sliding and crystallographic slip. The results are also displayed on the traditional diagram for strength and ductility and they demonstrate the potential for achieving high strength and high ductility by increasing the number of turns in HPT.
I. INTRODUCTION
The processing of metals through the application of severe plastic deformation (SPD) has now become a major tool for producing bulk solids with ultrafine grains (UFG), where these grains are within the submicrometer or even the nanometer range.1,2 Several different SPD processing techniques are available3 but most attention to date has concentrated on the two procedures of equal-channel angular pressing (ECAP)4 and high-pressure torsion (HPT)5 and in practice processing by HPT is especially advantageous because, by comparison with ECAP, it produces materials having smaller grain sizes and higher fractions of high-angle boundaries.6–8 The primary advantage of producing UFG materials is that, as a consequence of the Hall–Petch relationship in which the yield stress varies inversely with the square root of the grain size,9,10 the strength of the metal is increased when the grain size is reduced. But conversely the overall ductilities or elongations to failure are generally very small in UFG metals, thereby leading to a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.272 2534
J. Mater. Res., Vol. 29, No. 21, Nov 14, 2014
http://journals.cambridge.org
Downloaded: 30 Mar 2015
the maxim that “materials may be strong or ductile, but rarely both at once”.11,12 The loss in ductility at small grain sizes is due to the low rate of strain hardening and the low strain rate sensitivity, m. If the rate of strain hardening is
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