Abstract
The mechanical strengths of nano-scale individual crystal or nanopolycrystalline metals, and other dimensionally-related materials are increased by an order of magnitude or more as compared to those values measured at conventional crystal or polycrystal grain dimensions. An explanation for the result is attributed to the constraint provided at the surface of the crystals or, more importantly, at interfacial boundaries within or between crystals. The effect is most often described in terms either of two size dependencies: an inverse dependence on crystal size because of single dislocation behavior or, within a polycrystalline material, in terms of a reciprocal square root of grain size dependence, designated as a Hall-Petch relationship for the researchers first pointing to the effect for steel and who provided an enduring dislocation pile-up interpretation for the relationship. The current report provides an updated description of such strength properties for iron and steel materials, and describes applications of the relationship to a wider range of materials, including non-ferrous metals, nano-twinned, polyphase, and composite materials. At limiting small nm grain sizes, there is a generally minor strength reversal that is accompanied by an additional order-of-magnitude elevation of an increased strength dependence on deformation rate, thus giving an important emphasis to the strain rate sensitivity property of materials at nano-scale dimensions.
Highlights
The strong increase in the mechanical strength of materials at nano-scale dimensions is an important research activity involving tests on both individual crystals and polycrystalline samples.Such strength levels, that are achieved for individual nanoparticles, or as thin films, micropillars, or other polyhedral-shape crystals, frequently involve the employment of nano-testing systems, such as electron microscopic ‘in situ’ observations or nanoindentation hardness testing, and, perhaps most often, involves determining the strength properties of bulk materials composed of nano-scale crystal constituents
That are achieved for individual nanoparticles, or as thin films, micropillars, or other polyhedral-shape crystals, frequently involve the employment of nano-testing systems, such as electron microscopic ‘in situ’ observations or nanoindentation hardness testing, and, perhaps most often, involves determining the strength properties of bulk materials composed of nano-scale crystal constituents
The current measurements cover on a log/log basis the size dependence of strength based on the following effective length scales: the average grain diameter for conventional 0.15 wt % carbon mild steel and other decarburized [9], IF iron [10], and ball-milled iron [11] materials; the lamellar spacing for eutectoid iron-Fe3 C wire [12,13]; and, the subgrain boundary spacing for the topmost hypereutectoid wire material measurement [14], which strength level falls coincidentally at the previously estimated value of E/30
Summary
The strong increase in the mechanical strength of materials at nano-scale dimensions is an important research activity involving tests on both individual crystals and polycrystalline samples. Such strength levels, that are achieved for individual nanoparticles, or as thin films, micropillars, or other polyhedral-shape crystals, frequently involve the employment of nano-testing systems, such as electron microscopic ‘in situ’ observations or nanoindentation hardness testing, and, perhaps most often, involves determining the strength properties of bulk materials composed of nano-scale crystal constituents. The testing of individual nanocrystals often requires comparable-sized engineering test systems and special force and displacement measuring capabilities, as compared to conventional test systems being suitable for bulk materials containing nano-scaled constituents
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
