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When load exceeds the flow stress, materials will plastically deform under the interaction of several independent mechanisms, such as nucleation and propagation of dislocations, diffusional flow, and viscous flow. Representing the deformation of materials about the range of dominance of each of the mechanisms in a two-dimensional map has been pioneered by Frost and Ashby, which have already proven to be very useful for predicting new phenomena and for choosing material for specific applications. While constructing such a map to take account of both size and temperature effects is still challenging due to the lacking of systematic temperature-controlled nanomechanical testing.


Liuhas now reported inPhysical Review Letters that he shows a wayto construct size-temperature-deformation maps by superplastic nanomolding (SPNM) technique. This technique, developed by Liu in 2017, uses hard nanomolds to shape crystalline metals at temperatures around half the melting temperature and pressures of several hundred MPa (Nature Communications, 8, 14910, 2017). During SPNM, the cavity size (in the mold) is coupled into the materials flow and so a new dimension, the cavity size, can be introduced into the prevalent deformation maps. Based on this idea, the size-temperature-deformation maps for gold were firstly determined from experiments. It was found that thewell-known phenomenon “smaller is stronger”is significantly weakened as temperature increasing, which was attributed to the operating of competing diffusion mechanism at high temperatures. Besides, by varying the molding temperature, the transition from dislocation motion to diffusion dominated deformation as temperature increasing was also identified.


Finally, Liu showed that the constructed size-temperature-deformation maps are very powerful in guiding on controlled fabrication of high-surface area metal nanostructures with extreme small size (~ 5 nm) or with ultrahigh aspect-ratios (>100, Figure 1a). Most interestingly, it was found that Au nanorods arrays can be molded at temperature as low as -60ºC (Figure 1b), the lowest nanomolding temperature reported in crystalline solids!

Figure 1: (a) Ultra-long Bi nanowires arrays prepared by SPNM with Bi at 285 °C (∼1.02Tm). (b) Au nanorods arrays prepared by SPNM with Au at about −60 °C (∼0.16Tm). Credit: Ze Liu, Wuhan University.

 

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