Deformation-Induced Grain Refinement and Amorphization in Ti-10V-2Fe-3Al Alloy
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RE plastic deformation (SPD)-induced grain refinement of metals and alloys has attracted an intense research interest during the last decades.[1–12] It is widely accepted that deformation-induced grain refinement mechanism of metals and alloys depends strongly on their crystal structure and stacking fault energy (SFE).[1] In body-centered cubic (bcc) metals with high SFE, such as Fe,[2] the dislocation walls and cells are formed to accumulate strain. Well-developed sub-boundaries divide the original coarse grains. In face-centered cubic (fcc) metals with medium SFE, such as Cu,[3] equiaxed dislocation cells are the typical dislocation configuration. With progression of plastic deformation, twins and sub-boundaries having small misorientation are produced and accumulated to form grain boundaries with high misorientation. In hexagonal close-packed metals and alloys with medium SFE, such as AZ91D Mg alloy,[4] deformation twinning was observed to divide the original coarse grains resulting in the formation of dense, fine twin platelets. Subsequent dislocation slip subdivided these twin platelets into subgrains. Therefore, the grain refinement is induced by dislocation slip WEI CHEN, Postdoctor, QIAOYAN SUN, Associated Professor, LIN XIAO and JUN SUN, Professors, are with the State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, P.R. China. Contact e-mail: lxiao@mail. xjtu.edu.cn Manuscript submitted January 7, 2010. Article published online September 7, 2011 316—VOLUME 43A, JANUARY 2012
and/or twinning in metals and alloys with medium or high SFE. In metastable alloys with low SFE, stress-induced phase transformation can occur and play an important role in refining grain size during SPD. It was reported that nanocrystalline grains were produced more easily in a Ti-25.4Nb-7.1Ta-1.2In alloy, where stress-induced phase transformation can occur, than in a Ti-23.7Nb4.7Ta-4.8Cr alloy without stress-induced phase transformation.[5, 6] Recently, a remarkably refined grain size of 412 nm was observed in a Ti-24.6Nb-5Zr-3Sn alloy with stress-induced phase transformation resulting from equal-channel angular pressing (ECAP) for four passes. In contrast, less grain refinement was acquired in the Ti-19.6Nb-4.5Ta-2.4Cr alloy where stress-induced phase transformation did not occur.[7] Therefore, stressinduced phase transformation could be expected to promote grain refinement of metals and alloys during plastic deformation. In metastable materials, large plastic deformations could induce a transformation from crystalline to amorphous as massive crystal atoms shift into metastable positions and reach a nonequilibrium state, where free energy is even higher than that of the amorphous phase.[13] Hao et al.[14] observed the amorphous formation in the metastable Ti-24Nb-4Zr-7.9Sn alloy subjected to compression deformation of 50 pct. Koike et al.[15] reported a crystal-to-amorphous transformation in a NiTi alloy, which was cold rolled with a thickness reduction of 60 pct. They suggested that the amorphizat
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