Abstract
Although extensive simulations and experimental investigations have been carried out, the plastic deformation mechanism of body-centered-cubic (BCC) metals is still unclear. With our home-made device, the in situ tensile tests of single crystal tantalum (Ta) nanoplates with a lateral dimension of ∼200 nm in width and ∼100 nm in thickness were conducted inside a transmission electron microscope. We discovered an unusual ambient temperature (below ∼60°C) ultra-large elongation which could be as large as 63% on Ta nanoplates. The in situ observations revealed that the continuous and homogeneous dislocation nucleation and fast dislocation escape lead to the ultra-large elongation in BCC Ta nanoplates. Besides commonly believed screw dislocations, a large amount of mixed dislocation with b=12<111> were also found during the tensile loading, indicating the dislocation process can be significantly influenced by the small sizes of BCC metals. These results provide basic understanding of plastic deformation in BCC metallic nanomaterials.
Highlights
We discovered an unusual ambient temperature ultra-large elongation which could be as large as 63% on Ta nanoplates
For small sized metals with the face-centered-cubic (FCC) structure, it is well established that their plastic deformation is governed by dislocations escaping more quickly than they can multiply, leaving the sample free of dislocations.[8,9,20,21]
For body-centered-cubic (BCC) structured metals, it is believed that their plastic deformations are controlled by screw dislocations at low temperature, even for sub-micro sized single crystals.[25,26,27,28,29,30,31]
Summary
The plastic deformation mechanism of small sized single crystals has attracted intensive attention recently because of their excellent mechanical properties[1,2,3,4,5,6] and unusual deformation behaviors.[7,8,9,10,11,12,13] It has been found that the fundamental dislocation mechanisms that initiate and sustain plastic flow and fracture in nano-sized materials are considerably different from their bulk counterparts.[14,15,16,17,18,19] For small sized metals with the face-centered-cubic (FCC) structure, it is well established that their plastic deformation is governed by dislocations escaping more quickly than they can multiply, leaving the sample free of dislocations (termed as “dislocation starvation”).[8,9,20,21] The dislocation starvation always leads the small sized FCC metals to exhibit both high strength and high ductility.[22,23,24]. For body-centered-cubic (BCC) structured metals, it is believed that their plastic deformations are controlled by screw dislocations at low temperature, even for sub-micro sized single crystals.[25,26,27,28,29,30,31] The low mobility and the self-propagation of screw dislocations[32] would make them pile up and entangle, and in turn restrict their further movements, resulting in poor plastic elongation capability of BCC metals.[33,34,35,36] In the case of tantalum (Ta), bulk Ta exhibits ∼20% homogeneous plastic elongation at room temperature.[37] Based on previous in situ and ex situ investigations,[28,34,38,39,40] the as-mentioned screw dislocation controlled deformation mechanism is valid even for the sub-micro-sized BCC structured single crystal metals When their sizes are down to ∼200 nm, whether the proposed dislocation mechanism mentioned above is valid is unclear because of very few direct evidence.[41].
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