Paradox of Strength and Ductility in Metals Processed Bysevere Plastic Deformation
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Y.T. Zhu and T.C. Lowe Materials Science and Technology Division, Los Alamos National Laboratory, New Mexico 87545 (Received 13 June 2001; accepted 26 October 2001)
It is well known that plastic deformation induced by conventional forming methods such as rolling, drawing or extrusion can significantly increase the strength of metals However, this increase is usually accompanied by a loss of ductility. For example, Fig. 1 shows that with increasing plastic deformation, the yield strength of Cu and Al monotonically increases while their elongation to failure (ductility) decreases. The same trend is also true for other metals and alloys. Here we report an extraordinary combination of high strength and high ductility produced in metals subject to severe plastic deformation (SPD). We believe that this unusual mechanical behavior is caused by the unique nanostructures generated by SPD processing. The combination of ultrafine grain size and high-density dislocations appears to enable deformation by new mechanisms. This work demonstrates the possibility of tailoring the microstructures of metals and alloys by SPD to obtain both high strength and high ductility. Materials with such desirable mechanical properties are very attractive for advanced structural applications.
In this work, we report on how inducing severe plastic deformation (SPD) by equal channel angular pressing (ECAP) and high pressure torsion (HPT)1–3 can produce both high strength and high ductility. Both ECAP and HPT can subject a metal work-piece to arbitrarily large shear strain under high pressure without changing the work-piece dimensions. Figure 2 shows schematics illustrating both methods. In ECAP, the work-piece is repeatedly pressed through the same die. For an ECAP die with an angle ⌽ ⳱ 90° [Fig. 2(a)], each processing pass introduces a shear strain of 2 (or a von-Mises strain of 1.15). An important merit of ECAP is its potential to be scaled-up for industrial applications.3 The HPT technique imposes large shear strain through friction between the disk-shaped sample and a rotating plunger. To date it has only been applied to produce thin samples (艋1 mm). The pressure imposed on the sample is over 2 GPa in both techniques. In this investigation, pure Cu (99.996%) was processed using ECAP with 90° clockwise rotations along the billet axis between consecutive passes,1 while pure Ti (99.98%) was processed using HPT. All processes were performed at room temperature. Strength and ductility were measured by uniaxial tensile tests performed using samples with gauge dimensions of 5 × 2 × 1 mm. Resulting engineering stress–strain curves J. Mater. Res., Vol. 17, No. 1, Jan 2002
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are shown in Fig. 3. Results for Cu tested at room temperature in its initial and three processed states are shown in Fig. 3(a). The initial coarse-grained Cu, with a grain size of about 30 m, has a low yield stress but exhibits significant strain hardening and a large elongation to failure. This behavior is typical of coarseg
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