Stored Energy in Nickel Cold Rolled to Large Strains, Measured by Calorimetry and Evaluated from the Microstructure
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INTRODUCTION
THE processing of metals by plastic deformation requires mechanical energy, of which only a small fraction is stored in the metal in the form of dislocations, point defects, high-angle boundaries, and, in some materials, twins.[1] The stored energy can be measured directly by calorimetry[1,2,3–11] or it can be estimated based on a microstructural characterization.[12–14] In recent studies, the correlation between the stored energy of the deformation and the characteristics of the deformed microstructure has been analyzed.[12,14] In these studies, it has been shown that the stored energy can be estimated by applying the Read–Shockley equation to the boundaries observed in the deformed microstructure. Note that, in this article, we use the term ‘‘dislocation boundaries’’ to refer to boundaries with a misorientation angle of less than 15 deg, and the term ‘‘high-angle boundaries’’ to denote all other boundaries in the microstructure (including the original grain boundaries and those boundaries with a misorientation angle >15 deg formed during deformation). For aluminum, it has been observed that there is a relationship between the stored energy and both the applied strain and the flow stress after deformation.[12] The topic of stored energy is of general importance in various T. KNUDSEN, Ph.D. Student and N. HANSEN, Doctor Techn., are with the Center for Fundamental Research: Metal Structures in Four Dimensions, Risø National Laboratory, Roskilde, Denmark. Contact e-mail: [email protected] W.Q. CAO, formerly Ph.D. Student with the Department of Materials Science and Engineering, Tsinghua University, Beijing, P.R. China, is currently Post Doctor, with the Department of Materials Engineering, Monash University, Monash, VIC 3800, Australia. A. GODFREY Professor, is with the Department of Materials Science and Engineering, Tsinghua University, Beijing, China. Q. LIU, Professor, is with the Department of Materials Science and Engineering, Chongqing University, Chongqing, P.R. China. Manuscript submitted May 15, 2007. Article published online January 3, 2008 430—VOLUME 39A, FEBRUARY 2008
materials science fields, including, for example, recovery, recrystallization, and crystal plasticity. The correct estimation of the stored energy in a deformed metal is, therefore, crucial when analyzing and modeling such phenomena. The objective of the present study is to supplement the microstructure-based method by measuring the stored energy directly by differential scanning calorimetry (DSC). This permits analysis of the question of whether other sources of stored energy exist in addition to the energy stored in the form of dislocations and high-angle boundaries. Studies with a similar objective have been carried out previously by combining calorimetry, dislocation density measurements by using transmission electron microscopy, and X-ray line broadening.[15,16] These earlier studies were focused on single crystals of copper deformed in tension to low and medium strains. In contrast, the present study concentrates on po
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