Nanostructured Composites: Ti-Base Alloys
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Metastable materials with microstructural features, i.e., particles or grain sizes, layer thicknesses, or domain sizes, in the nanometer range (typically less than 100 nm at least in one dimension) have unique features, such as an appreciable fraction of their atoms in defect environments at the grain–interface boundaries, leading to a significant impact on their mechanical, physical, and chemical properties, which significantly differ from those of their conventional coarse-grained counterparts. Normally, nanostructured materials (NsM) with an average grain size of 5 nm have about 50% of their atoms within the first two nearest-neighbor planes of the grain boundaries, in which distinct atomic displacements from the normal lattice sites occur. Obviously, this has a strong impact on the properties of the material.
In 1959, Feynman at Caltech, U.S.A., has proposed the idea of obtaining an enormously greater range of possible properties of condensed matter by control over the structure in the range of extremely small dimensions (nanometer scale). Later (1982–1985), Gleiter was the first to realize a method for the production of such materials with nanometer-sized grains (crystallites). These materials have been categorized by Gleiter as follows: 1) the dimensions/size of the materials/devices itself are on a nanometer scale; 2) bulk materials/devices in which a thin surface region has a nanometer-sized microstructure; or 3) building blocks (grains) of a bulk solid having a nanometer length scale. The technological applications of the above mentioned materials are 1) catalysts and semiconductor devices and 2) surfaces with enhanced corrosion resistance, hardness, wear resistance or protective coatings, and quantum dots. The third category of materials with homogeneous nanometer-sized building blocks is often called “NsM” or nanophase materials, nanocrystalline materials, or supermolecular solids. These metastable materials can be synthesized in both metallic and nonmetallic/polymeric systems. The attempts on controlled processing, microstructural, mechanical, and specific property characterization to find the applicability of such NsM of either single or multiple phases with layered, rod-shaped, or equiaxed crystallites with/without tailored grain boundary composition or homogeneous dispersion in a second phase matrix are a part of the emerging and rapidly growing field of nanotechnology.