Reversible dislocation motion under contact loading in LiNbO 3 single crystal
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The room temperature deformation behavior of a LiNbO3 single crystal loaded along [0001] was studied by spherical nanoindentation. The threefold symmetry of the indentation marks was attributed to the formation of (101¯2) twins that reorient the basal planes to allow for basal slip, which is manifested by the formation of fully reversible, hysteretic loops upon cyclic loading. The differences in energy dissipation, threshold stresses, and loop shapes for the three different radii tips are attributed to the different sized twins that form. The results are consistent with our model for the formation of incipient kink bands within the twins. I. INTRODUCTION
Lithium niobate—LiNbO3—has attracted a lot of attention because of its important nonlinear optical properties for applications in electro-optic, acousto-optic, and optical storage devices.1 Precise knowledge of the mechanical deformation behavior is a prerequisite for successful manufacturing and operation of these devices. Despite the importance, very little is available on the elastic-plastic transition and dislocation movement during contact deformation in LiNbO3. Earlier studies on LiNbO3 single crystals have been limited to mostly uniaxial compression2 and Vickers microhardness.3 When single crystals are loaded in uniaxial compression at temperatures >1150 °C, (101¯2)[1¯011] twins form.2 Within these twins, basal slip is observed. More recently, Park et al.4 confirmed the existence of this twin system. Subhadra et al.3 reported a Vickers microhardness of ∼6.3 GPa, at 2 N load, for a single crystal loaded along [0001] and showed that, with increasing load, the hardness dropped. The phenomenon of fully reversible dislocation motion is not very widespread. Recently, there have been some reports of reversible plastic deformation in Au and Si, nanospheres.5,6 For example, Gerberich et al.6 presented evidence and proposed a model for small reversible plastic deformation in Si nanospheres. The model— based on the backstress of dislocation pileups— successfully explained such strains during their contact loading experiments. More recent work by the same group,7 however, suggests that phase transitions may in fact be the origin of these observations. Herein we also a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0150 1334
J. Mater. Res., Vol. 23, No. 5, May 2008
suggest that reversible dislocation motion, but not in the configuration of pileups, is responsible for the reversible strain measured. Recently it was postulated that most solids with c/a ratios >1.5 belong to the same class of solids that we labeled kinking nonlinear elastic (KNE).8–10 The signature of KNE solids is the formation of fully reversible, hysteretic stress–strain loops on repeat loadings.8,11 This full reversibility is attributed to the formation of incipient kink bands, (IKBs), that comprise multiple parallel dislocation loops [top inset in Fig. 1(a)] that either annihilate or shrink when the load is removed.11 In other words, solids that are
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