Cyclic deformation behavior of high-purity titanium single crystals: Part II. Microstructure and mechanism
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I.
INTRODUCTION
IT is widely recognized that mechanical twinning can become an important mode of deformation in hexagonal close-packed (hcp) metals. One major conclusion of a recent review is that, in hcp metals with reasonably high surface energies, the greater the number of operative twin modes, the larger the overall ductility will be.[1] Experimentally, it is observed that deformation twinning can effectively strengthen a material under some circumstances and weaken it under others. This complexity results mainly from the intricate inter-relationship between slip, twinning, and cracking of hcp metals. The presence of more than one twinning system operative in either compression or tension is the main reason that titanium exhibits extensive ductility.[2] Titanium is known to have a high stacking-fault energy[3,4] and, thus, wavy slip behavior, which might also be expected to enhance the ductility of a specimen deformed at room temperature. Fatigue cracks have been observed to follow slip planes or twin/matrix interfaces.[5,6] Mechanical twinning appears to play a distinct role in the fatigue behavior of titanium. Twins of the {1121} type, formed during cyclic loading, exhibited permanent fatigue damage at the twin/matrix interface.[7] The {1012} twins introduced by prior deformation have also been shown to provide sites for preferential damage upon subsequent cyclic loading.[5] Ward-Close and Beevers[8] studied the high-cycle fatigue crack growth characteristics of coarse-grained titanium. Three distinct types of fracture morphology were identified: cleavagelike facets on the basal planes (0002), striations on planes normal to (0002), and furrows in the [0001] direction. X. TAN, Postdoctoral Fellow, formerly with the Research Institute for Strength of Metals, Xi’an Jiaotong University, is with the Hemispheric Center for Environmental Technology, Florida International University, Miami, FL 33199. H. GUO, Graduate Student, formerly with the Research Institute for Strength of Metals, Xi’an Jiaotong University, is with the Department of Metallurgy and Materials Engineering, Ecole Polytechnique, Montreal, PQ, Canada. H. GU, Professor, is with the Research Institute for Strength of Metals, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China. C. LAIRD, Professor, is with the Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA 19104. N.D.H. MUNROE, Associate Professor, is with the Hemispheric Center for Environmental Technology, Florida International University. Manuscript submitted April 23, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
The dislocation structures produced after cyclically deforming commercial-purity titanium to large cycle numbers resemble those produced in other metals. The observation of dipolar walls suggests that screw dislocation mobilities are sufficient to permit cooperative glide across dipolar walls. In addition, edge dislocations are concluded to bow out of walls.[9] At high plastic strain amplitudes, cyclic deformation is contro
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