Reversible dislocation motion and microcracking in plastically anisotropic solids under cyclic spherical nanoindentation
- PDF / 334,215 Bytes
- 4 Pages / 612 x 792 pts (letter) Page_size
- 34 Downloads / 140 Views
esearch Letters
Reversible dislocation motion and microcracking in plastically anisotropic solids under cyclic spherical nanoindentation B. Anasori and M.W. Barsoum, Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104 Address all correspondence to B. Anasori at [email protected] (Received 7 May 2013; accepted 11 September 2013)
Abstract Recently, fully reversible dislocation motion was postulated to result in hysteretic nanoindentation load–displacement loops in plastically anisotropic solids. Since microcracking can also result in hysteretic loops, here we define a new parameter, reversible displacement (RD) that can differentiate between the two. For C-plane LiTaO3 surfaces and five other plastically anisotropic solids, the RD values either increase initially or remain constant with cycling. In contradistinction, for glass and A-plane ZnO surfaces, where energy dissipation is presumably due to microcracking or irreversible dislocation pileups, respectively, the RD values decreased continually with cycling.
Over the past half dozen years we have shown that many plastically anisotropic materials, in which dislocations are confined to two-dimensions, outline fully and spontaneously reversible hysteretic stress–strain loops upon cyclic loading. We postulated that the micromechanism for this phenomenon is the formation and annihilation of multiple, co-axial parallel dislocation loops, whose shape guarantees that they only remain open when a load is applied; unloading results in their shrinkage and/or their annihilation.[1–4] Among others, we have shown that a large number of seemingly unrelated solids such as C-plane ZnO,[5] BaTiO3,[6] sapphire,[7] LiNbO3,[8] LiTaO3,[9] Mg, Co, Ti, Zn,[10,11] graphite,[12] mica,[2,13] and the MAX phases,[3] trace fully reversible hysteretic stress–strain loops upon cyclic loading. Working mostly with single crystals, we showed that when one of the aforementioned materials is indented with a spherical indenter, a linear elastic regime is usually followed, in most cases, by pop-in events, that in some cases are massive, followed, after a few cycles, by fully and spontaneously reversible hysteretic load–displacement curves.[13] During the pop-in events, the strain energy released results in the creation of a multitude of nanodomains and, in the case of LiNbO3 and LiTaO3, twins.[8,9] When the nanoindenter is reloaded into the same location, we postulated that coaxial dislocation loops nucleate within the domains formed during the pop-ins. The reversible motion, of these dislocations in turn, is believed to result in the energy dissipated per unit volume per cycle, Wd. The presence of microdomains—that play the role of grain boundaries in polycrystalline solids—that do not allow the parallel dislocation loops to dissociate into mobile dislocation walls (MDWs) is thus essential for their reversible behavior.[5,7–9]
Previously, we referred to these co-axial, parallel dislocation loops as incipient kink bands (IKBs). Materials that dissipated e
Data Loading...
