Abstract
Higher strength levels, achieved for dimensionally-smaller micro- and nano-scale materials or material components, such as MEMS devices, are an important enabler of a broad range of present-day engineering devices and structures. Beyond such applications, there is an important effort to understand the dislocation mechanics basis for obtaining such improved strength properties. Four particular examples related to these issues are described in the present report: (1) a compilation of nano-indentation hardness measurements made on silicon crystals spanning nano- to micro-scale testing; (2) stress–strain measurements made on iron and steel materials at micro- to nano-crystal (grain size) dimensions; (3) assessment of small dislocation pile-ups relating to Griffith-type fracture stress vs. crack-size calculations for cleavage fracturing of α-iron; and (4) description of thermally-dependent strain rate sensitivities for grain size strengthening and weakening for macro- to micro- to nano-polycrystalline copper and nickel materials.
Highlights
To underscore the relevance of the current topic, a number of example engineering-based references are provided for micro- and nano-crystal mechanical property measurements and the applications of them [1,2,3,4,5]
The topics relate to crystal size effects in manufacturing; strength measurements relating to MEMS applications; micro-forming of foils; nano-metric machining defects; and advanced machine tool manufacturing
Such observations involving extremely small dimensionally-dependent mechanical property measurements have followed a pioneering report by Brenner [6] of greatly-enhanced strength levels being achieved for smaller diameter “whisker” materials
Summary
To underscore the relevance of the current topic, a number of example engineering-based references are provided for micro- and nano-crystal mechanical property measurements and the applications of them [1,2,3,4,5]. These points that illustrate a significant “pop-in” behavior at initial plastic yield have been shifted on the abscissa scale for clarity. The transition occurs when the dislocation pile-up length is sufficiently reduced such that only one dislocation loop, n = 1.0, is able to be produced within a restricted slip plane length to overcome the nano-polycrystal grain boundary resistance On such a restricted dislocation number basis, the dislocation model description for an H–P dependence connects with the described strength dependence of whiskers, nano-wires, and micro-pillars on their specimen diameters. Pile-up stress intensity; an expanded version of compiled results is reported in reference [26]
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