Analysis of Strain Rate Sensitivity of Ultrafine-Grained AA1050 by Stress Relaxation Test
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UCTION
DURING the last decade, it has been well illuminated that Severe Plastic Deformation (SPD) is an effective way to achieve ultrafine-grained (UFG)[1,2] and bulk nanostructured[3,4] metals. Among various SPD processes, equal channel angular pressing (ECAP),[5] accumulative roll bonding, (ARB)[6] and high-pressure torsion (HPT)[7] have been the most investigated processes on bars, sheets, and disk-shaped metals, respectively. As it can be applied by means of a conventional rolling facility, ARB overcomes limitations of the two other processes, i.e., low productivity of ECAP and small work-piece size of HPT.[1] Along with increased strength, unfortunately, low ductility and elongation are reported as inevitable consequences of ARB.[8–10] While tension test is performed almost in all studies to define mechanical properties of the ARBed sheets, the low elongation has been an obstacle for a complete flow stress analysis. In order to overcome this drawback of tension test, a test is required through which UFG MOHAMMAD SADEGH MOHEBBI, Ph.D. Student, is with the Department of Materials Science and Engineering, Sharif University of Technology, Azadi Ave., P.O. Box 11155-9466, Tehran, Iran, and also with the Korea Institute of Industrial Technology, Songdo-dong, Yeonsu-gu, Incheon, Republic of Korea. Contact e-mail: msmohebi@ mehr.sharif.edu ABBAS AKBARZADEH, Associate Professor, is with the Department of Materials Science and Engineering, Sharif University of Technology. BONG HWAN KIM, Ph.D. Student, and SHAE-KWANG KIM, Principal Researcher, are with the Korea Institute of Industrial Technology. Manuscript submitted May 10, 2014. Article published online August 29, 2014 5442—VOLUME 45A, NOVEMBER 2014
material could undertake high deformation without fracture. At the same time, such a test must be applicable on geometry of thin sheets. The authors have shown in the previously published paper[11] that plane strain compression (PSC) test is a good alternative in this case. Strain rate sensitivity exponent (m-value) (which is simply called as strain rate sensitivity, SRS) is a very important parameter in study of mechanical properties of the nanostructured and UFGed metals. This parameter is connected to the viscous nature of thermally activated plastic deformation and superplasticity.[12] Strain rate jump test is the most widely used method of m-value valuation.[13] However, this approach would only give the value of SRS at a single strain. Moreover, as the obtained value is linked to a jump between two strain rates, it is not possible, or at least is not accurate, to find it as a function of strain rate. Another technique for evaluation of the SRS is stress relaxation.[14] The stress relaxation test is a popular method for characterization of viscous and creep properties of the metals. When a tension or compression test is interrupted by stopping the cross heads, the sample continues to be deformed plastically in expense of reduction of the elastic strains in the machine elements as well as in the sample. This leads to a redu
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