Novel Nanostructured Metal and Ceramic Composites
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Novel Nanostructured Metal and Ceramic Composites J. Narayan, Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695-7916. Abstract We have designed a unique synthesis procedure to create nonomaterials of uniform grain size and control the chemistry of interfaces between the grains. The metastability of nanocrystalline materials is a major challenge which can be addressed by controlling the chemistry of interfaces. The hardness of these films having a uniform size was measured as a function of grain size using a nanoindentation technique. It was found that hardness increased with decreasing grain size in accordance with Hall-Petch model. However, below a critical grain size we observed a decrease or softening with a further decrease in grain size. These observations in metals and ceramics are modeled in view of intragrain deformation (Hall-Petch regime) and intergrain deformation ( grain boundary shear/sliding )in the softening regime. Since we can change the alloying of interfacial region, we can address the metastability as a function of temperature, which is crucial from applications viewpoint of these materials.
Introduction Nanocrystalline (nc) materials with grain size 1-100 nm range exhibit interesting physical and mechanical properties (1-6). These properties include enhanced yield strength and hardness, ductility and toughness, electrical resistivity, specific heat, coefficient of thermal expansion, and superior magnetic properties compared to their coarse-grained counterparts. The mechanical properties (yield strength and hardness) of nc materials increase with decreasing grain size, which is described at least qualitatively by the Hall-Petch relationship (7-9). It also has been observed that this correlation holds true up to a certain grain size below which softening is observed with decreasing grain size (a reverse Hall-Petch effect) (4-6,9). The reduction in elastic modulus has been attributed to increased porosity in nc materials and/or to enhanced volume fraction of grain boundary and triple junction regions, which represent weaker regions of the material (10-11). The properties of nc materials in general and mechanical properties in particular have been the subject of many controversies specifically related to enhanced hardness and reduced modulus. The situation has been compounded due to unavailability of nc samples, which are free from porosity and other artifacts. Schiotz et al. have recently used computer simulation to predict yield and flow stresses as a function of grain size (12). They showed a softening or decreasing yield and flow stresses with grain size below 7 nm. According to this model, in the 3 to 7 nm grain size range, plastic deformation occurred due to a larger number of small “sliding” events of atomic planes at the grain boundaries, and the plastic deformation or dislocation activity within the grains was rather small. Thus the softening is attributed to the large fraction of (less strong with more separation) atoms at the grai
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