Strain-rate sensitivity of hardness of nanocrystalline Ni 75at.% Al 25at.% alloy film
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J.B. Pethica Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, United Kingdom
H.P. Ng Laboratoire des Matériaux et du Génie Physique, Ecole Nationale Supe´rieure de Physique de Grenoble-Institut National Polytechnique de Grenoble (ENSPG-INPG), UM 5628, BP 46, 38402, St. Martin d’Hères, France (Received 1 September 2002; accepted 30 October 2002)
Room-temperature indentation experiments carried out on nanocrystalline Ni75at.%Al25at.% alloy films with a range of grain sizes revealed that the strain-rate sensitivity of hardness is nearly zero and that the hardness increases as grain size decreases. The strain-rate insensitivity of hardness indicates that the room-temperature strength of these alloy films is dominated by an athermal, strain-rate-insensitive component. The hardness of the films was found to be in the range of 2.4 to 3.3 GPa, depending on grain size.
I. INTRODUCTION
When the grain size is reduced to the order of a few nanometers, it is often believed that dislocation activities will be suppressed.1–4 The need to understand the deformation mechanisms of nanocrystalline materials under load therefore opens up a new direction of research. In particular, it has been found that when the grain size falls below a very small critical value, the Hall–Petch slope is nearly zero with little increase in strength on decreasing grain size, or even the inverse Hall–Petch behavior will occur, in which the strength actually decreases with decreasing grain size.5 Recent experiments on nanocrystalline copper also indicated the existence of a thermal and an athermal component of the flow strength.6 The thermal component was found to have a strain-rate dependence of unity, i.e., strain rate is proportional to stress. This indicates a diffusive, Coble-creep-like mechanism.7 When the deformation temperature is too low for diffusion to occur efficiently, the deformation mechanism is largely unknown. In this work, we performed room-temperature indentation experiments on a series of nanocrystalline Ni75at.%Al25at.% alloy films to measure the strain-rate sensitivity and grain-size dependence of the strength (hardness) of this type of materials. We selected depth-sensing indentation as a means of deforming the material because it is a technique in which the deformation condition can be well-controlled with minimal requirements on the sample geometry. Theoretical 382
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J. Mater. Res., Vol. 18, No. 2, Feb 2003 Downloaded: 14 Mar 2015
self-similarity and steady state of the deformation field can be achieved by using a load schedule in which the indentation load rate is proportional to the load P, i.e., P˙ = kP or P = Poekt ,
(1)
where k is the desired constant strain rate, and Po a small initial load.8,9 We used a type of Ni75at.%Al25at.% thin film sputter-deposited on nickel substrate as prototype nanocrystalline materials. The film thickness was about 3 m, and the indentation depths were typically shallower than 0.5 m, so in most cases the effect of the substrate was expe
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