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Y6.10.1

Micromechanical Properties of He-Implanted Ni J. A. Knapp, D. M. Follstaedt, and S. M. Myers Sandia National Laboratories, Albuquerque, NM 87185-1056 ABSTRACT Detailed finite-element modeling of nanoindentation data is used to obtain the micromechanical properties of Ni implanted with ~5 at.% He to a depth of 600-700 nm. Properties of He-containing metals have implications for studies of radiation damage and for fundamental issues of dislocation pinning. Cross-section TEM shows the implantation produces a highly damaged layer containing a fine dispersion of He bubbles with diameters of ~1 nm or smaller, with some evidence for interconnection between bubbles. Nanoindentation of the Ni(He) layers gave a fairly hard, stiff response to depths of 100-120 nm, beyond which the layer failed. By modeling the layer as an isotropic, elastic-plastic solid with the Mises yield criterion, the Ni(He) is shown to have a hardness nearly 7 times that of untreated Ni. However, unlike other treatments that we have used to produce very hard Ni-based layers, the Ni(He) layer fails at relatively modest shear stress levels. INTRODUCTION The mechanical properties of metals containing dispersions of either small voids or gascontaining bubbles is of fundamental interest for understanding the effects of nanometer-sized inclusions on strength. For applications, inclusions of He gas are of particular interest in the nuclear power industry, as well as for neutron tubes, which are neutron-generating devices that involve the use of tritide films. The tritium slowly decays into He4, which can either be released from the film, leading to vacuum problems in a sealed tube, or, if retained, form high pressure bubbles and eventually fracture the film. Not only are the conditions leading to fracture of interest, but the mechanical properties of the layer before fracture is of fundamental importance. Here we use Ni(He) as a model system to study such effects. We use nanoindentation combined with finite-element modeling (FEM) to examine the mechanical properties of a Ni specimen containing high-pressure He bubbles. FEM has proven to be a valuable tool for interpreting nanoindentation measurements obtained from samples such as thin films or ion-implanted layers, allowing the mechanical properties of the films to be separated from those of the underlying substrates. We have used this technique in a wide variety of thin-film studies, and full details of the method are given elsewhere.[1,2] The specimen was prepared by ion implantation and the microstructure was examined using Transmission Electron Microscopy (TEM) and Secondary Electron Microscopy (SEM). IMPLANTATION AND MICROSCOPY In order to produce a uniform layer of He-containing Ni in the near-surface region of the well-annealed, pure Ni specimen, a series of He implants were performed at room temperature, with the energies and fluences chosen through TRIM simulations [3], as shown in Fig. 1. As seen in the figure, the simulations predict a uniform region of ~5.25 at.% He extending to

Y6.10.2