Dynamic Recrystallization Behavior of a Coarse-Grained Mg-2Zn-2Nd Magnesium Alloy
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namic recrystallization (DRX) is an effective method for grain refinement as well as for the randomization of deformation textures.[1–5] Three kinds of DRX mechanism have been reported in magnesium alloys[6–8]: (i) discontinuous DRX (DDRX), which involves nucleation and growth, (ii) continuous DRX (CDRX), where new grain boundaries gradually form from low-angle grain boundaries without nucleation, and (iii) twinning DRX (TDRX), which occurs when new grains form in twins and at twin–twin intersections. Several types of preferential sites for the nucleation of DRX grains have been reported in magnesium alloys, two of which involve shear band nucleation (SBN) and particle-stimulated nucleation (PSN).[9,10] Here we examine the effect of strain rate in promoting or preventing the operation of each of the above DRX mechanisms. In this work, a coarse-grained Mg-2Zn-2Nd material was deformed at 673 K (400 °C) at the strain rates of 0.1, 0.01, and 0.001/s. At a true strain of 0.3, new DRX grains were observed at all three strain rates. DDRX,
TONG WANG, JOHN J. JONAS, and STEPHEN YUE are with the Department of Mining and Materials Engineering, McGill University, 3610 University St., Montreal, QC H3A 2B2, Canada. Contact e-mail: [email protected] Manuscript submitted July 12, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
CDRX, and TDRX were all observed in the 0.1/s sample. In the 0.01/s sample, DDRX grains formed at grain boundaries at moderate strains. On further straining, CDRX took place in the DDRX grains. Finally, in the 0.001/s sample, only CDRX took place. Cylindrical samples with diameters of 6 mm and heights of 9 mm were machined from a Mg-2Zn-2Nd material. Before machining, the continuously cast ingots were homogenized at 723 K (450 °C) for 2 hours. This procedure led to an initial grain size of about 500 lm. The chemical composition of the alloy is provided in Table I. Compression tests were performed at 673 K (400 °C) at the strain rates of 0.1, 0.01, and 0.001/s up to the true strains of 0.2 and 0.3. The deformed samples were quenched in less than 2 seconds after testing. For microstructural observation, the deformed samples were cut along the compression direction. The longitudinal surfaces were ground with 600, 800, and 1200 grit SiC papers and then mechanically polished with 3- and 1-lm diamond paste as well as with colloidal silica. To remove the effects of mechanical polishing, the samples were then electropolished using a 10 pct Nital solution at 253 K (20 °C) for 2 seconds. The deformation microstructures and microtextures were analyzed by electron backscattered diffraction (EBSD) methods on an SU3500 scanning electron microscopy (SEM) interfaced with an HKL system. For all the EBSD scans, the samples were placed with their compression directions along the X-axis. A backscattered electron (BSE) image of the initial microstructure is shown in Figure 1(a). Appreciable amounts of two types of intermetallic phases were found in the matrix in three morphologies: dendritic, coarse, and fine particles. The
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