Yield of In x Ga 1-x As Superlattices Under Bending and Nanoindentation
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Yield of InxGa1-xAs superlattices under bending and nanoindentation Stephen J. Lloyd1, Ken M.Y. P’ng2 Andy J. Bushby2, David J. Dunstan2 and William J. Clegg1 1
Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB2 3QZ, UK 2 Centre for Materials Research, Queen Mary, University of London, London E1 4NS, UK ABSTRACT A series of InxGa1-xAs superlattices grown on InP substrates with differing coherency strains have been deformed by bending at 500 ºC and by nanoindentation at room temperature. The deformation was characterised by transmission electron microscopy through examination of thin sections machined in a focused ion beam microscope. The bent samples sheared along {111} planes, and the most highly strained samples partially relaxed through the formation of misfit dislocations. Under indentation the majority of the plastic strain in the multilayers is accommodated by twinning whereas no twins were observed under indents in the InP substrate. The overall dimensions of the plastic zone increased linearly with indent load. INTRODUCTION The hardness of multilayered materials can vary as a function of bilayer thickness for bilayers periods of nanometre dimensions. Models used to explain this behaviour usually concentrate on the barrier that the layering can provide to dislocation flow [1]. The influence of coherency strains on the hardness is not fully understood. In this paper we examine the deformation of a series of InxGa1-xAs superlattices in which the coherency strain can be continuously varied through control of the In content. Since these structures can be grown with a low defect density, and the elastic and plastic properties of the component layers vary little with In content, they are a model system to investigate the effects of coherency strain alone on yielding. In addition, the layered structure provides internal markers to chart the plastic flow of material. Previous work has shown that these superlattices display a remarkable resistance to strain relaxation at temperatures up to nine-tenths of their melting point [2]. Under nanoindentation at room temperature the yield stress of the superlattices decreases with increasing strain modulation in the layers [3], while under 3-point bending at 500 ºC recent experiments show that there is a small increase in the yield stress with increasing strain modulation [4]. Here we examine these contrasting behaviours using transmission electron microscopy (TEM) to image the local deformation. The electron transparent sections required for TEM were machined using a focused ion beam (FIB) microscope in which a beam of Ga ions sputters material and the secondary electrons emitted are used to image the sample. FIB milling allows large areas of uniform thickness to be machined which is useful to examine the deformation of the uniformly loaded bent specimens. Uniquely, sections can also be made from specific sites of localised deformation such as nanoindents. The ability of the FIB to section in this way has allowed the various deform
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