Dislocation Configurations Around Nanoindentations in Reconstructed Au(001)
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Dislocation Configurations Around Nanoindentations in Reconstructed Au(001) O. Rodríguez de la Fuente, M.A. González and J.M. Rojo Departamento de Física de Materiales, Universidad Complutense, 28040 Madrid, Spain ABSTRACT Nanoindentations resulting from STM tungsten tip contacts with the 5x20 reconstructed surface of a Au(001) crystal around the indentation point have been studied by Scanning Tunnel Microscopy (STM). A novel defect structure is recognised: a row of hillocks extending along more than a hundred nm along the directions stemming from the indentation point. These hillocks have about 7 nm of side and 0.06 nm of height. With the help of simulation models we identify individual hillocks as dislocation configurations consisting of two stacking fault ribbons encompassed by Shockley partials having a stair-rod dislocation as a ridge. It is proposed that hillocks are generated by plastic flow, from an initially nucleated V-shaped dislocation loop intersecting the surface. INTRODUCTION Nanoindentation techniques provide information about mechanical properties and deformation mechanisms of materials and thin films at a very small scale. Simulations have recently been used to investigate the detailed mechanisms for plastic deformation at an atomic level in the very close proximity of the indentation point [1-4]. Though progress has been made [5-8], precise experimental information about single dislocation nucleation events (discrete plastic events) during nanoindentation is still lacking. In this work we focus on incipient plasticity and characterise some very initial stages of plastic deformation of a Au(001) surface. We show that plastic displacement parallel to the surface gives rise to dissociated dislocation loops intersecting the surface. EXPERIMENTAL DETAILS Experiments were carried out at room temperature with a scanning tunnel microscope (STM) in ultra-high vacuum. The Au(001) sample is cleaned following the standard procedure of low-energy ion bombardment and high temperature annealing. In this way a clean (as monitored by Auger spectroscopy) and defect-free surface is obtained. Images prior to nanoindentation shows wide terraces with a typical width of 100 nm [9]. The characteristic 5x20 hexagonal reconstruction of the Au(001) surface is present. Atoms in the last layer adopt an hexagonal symmetry. The lack of registry between this hexagonal layer and the second one, which remains square, results in a corrugated surface in the form of straight ridges parallel to the direction. Nanoindentations are performed with the STM tungsten tip, pushing it gently towards the surface and retracting it afterwards. With the same tip an STM image is obtained to analyse the defects.
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Figure 1. (Top) STM image of a hillock defect. The image size is 24x24 nm. (Bottom) Dislocation configuration proposed for the defect. Double splitting of an initial loop with Burgers vector BA parallel to the surface (left) gives rise to four Shockley partial dislocations, one stair-rod dislocation and two stacking faults (right)
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