Dislocation free whiskers

As mentioned in the introduction, it is this discrepancy between the theoretical strength of bulk crystals and their actual strength which led to the concept that real crystals contain imperfections making them weak. The particular imperfection invented because it had the desired properties to account for the discrepancy was the dislocation or linear crystalline defect. It has been possible to grow dislocation-free, small-diameter whisker crystals and these indeed have the theoretical strength. >... 

Whisker. Tiny, whisker-like fiber (a few mm long, a few p.m in diameter) that is a single crystal and almost free of dislocations. Note that this term involves a material requirement. The small size and crystalline perfection make whiskers extremely strong, approaching the theoretical strength. 

The concentration of dislocations is conventionally defined as the number of dislocation lines that intersects a unit area. Carefully prepared crystals may contain 10 dislocation lines per square centimeter, and some bulk crystals and crystal whiskers have been prepared free, or nearly free, of all dislocations after plastic deformation the concentration of dislocations increases tremendously, being as high as 10 to 10" dislocations per square centimeter for some heavily deformed metals. (We should not use cm" units so beware.)... 

The theoretical tensile stress values just quoted are expected in a diainond fibre free of dislocations, cracks and other defects, usually a diamond whisker, Unfortunately there are no experimental data available for whiskers with diamond structure. Compressive strengths of the order of the theoretical tensile strength were observed in diamond anvils used to obtain ultra-high pressures (Wilks and Wilks, 1991), but in other applications diamonds fail at much lower loads due to the above-mentioned defects. 

Metals exhibit the maximum stress only in whisker form because they permit dislocation glide at low stresses, and whiskers are almost free of dislocations, in bcc metals, improved potential models will lead to a better understanding of the ideal strength than has so far been gained from either the Orowan-Polanyi approach or the use of the Morse potential predictions, for example, of the fracture stress for a-Fe whiskers in the (111) direction using the Orowan-Polanyi equation are 46 GPa, where the maximum tensile stress obtained by Brenner was 13.1 GPa (Brenner, 1956), at an elongation close to 0.05 (see paper by Kiinzi, this volume). 

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