Microstructure transformation from lamellar to equiaxed microduplex through equal-channel angular pressing in an Al-33 p
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NTRODUCTION
MICROSTRUCTURE transformation driven by strain or mechanical deformation is an alternative to the dominant role of the thermally activated microstructure transformation in physical metallurgy. It is a common experience that the plastic deformation of a preannealed specimen may accelerate spheroidization of pearlite during subsequent annealing.[1] Plastic strain, in certain cases, plays a more important or even dominant role in the processing of a lamellar structure into an equiaxed, fine-grained duplex structure, which has been a successful practice for the processing of microduplex superplastic materials in the development of superlasticity.[2–5] The role of plastic strain on the structural evolution of lamellae, or any other kinds of structures, is strongly dependent on strain intensity and strain path. In the basic frame of continuum mechanics, there are two essential forms of strain: shear strain and normal strain (tensile or compression).[6] Conventional metalworking processes like rolling, forging, and drawing could be considered as processes having mainly the normal strain modes. As to the structural evolution of fully lamellar pearlite steel wire during cold drawing, the essential normal strain produces a progressive alignment of lamellae along the drawing axis to make a fibrous microstructure, a reduction of lamellar interspacing, and a thinning of cementite lamellae. Cementite plates are thinned and, finally, necked down into small fragments due to the increasing strain of drawing.[7] A direct proportionality exists between the average lamellar interspacing and the diameter of asdrawn wire.[8–11]
On the other hand, the essential shear strain can be achieved through equal-channel angular pressing (ECAP), which has attracted growing interest recently as a potential process for the preparation of ultrafine, or even nanostructured, materials.[12,13,14] The ECAP technique produces a strong and nearly ideal simple shear strain when the bulkmaterial samples are pressed through two channels of equal cross section intersecting at an angle of , and the pressings can be repeated until the required level of plastic strain is attained. Various ingot-metallurgy materials were processed to achieve high-strain-rate superplasticity through ECAP,[15] among which is the Zn-22 pct Al eutectoid alloy.[16] Although extensive works have been reported on the structural evolution and mechanical-property change of lamellae materials of eutectoid pearlite during cold-drawing deformation up to a large strain,[8–11] there are fewer reports on lamellar deformation behavior in simple-shear deformation. In addition, an understanding of the lamellar-structure transformation during ECAP will be very helpful. It is, thus, interesting and valuable to apply the simpleshear strain of ECAP to the processing of a lamellar structure, such as Al-Cu eutectic lamellar structure, to obtain an equiaxed ultrafine duplex alloy. This article presents a detailed investigation into the transformation of a lamellar structure into a fully equia
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