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The puzzling anomalous slip of dislocations in bcc metals is finally solved !

par Guy Molénat - publié le , mis à jour le

We have deformed niobium samples at -170°C in a transmission electron microscope, and observed in real time the corresponding microscopic mechanisms. Some extremely fast dislocation movements correspond to the “anomalous slip” first reported 50 years ago, but never elucidated until now. Atomistic simulations are in good agreement with observations and allow us to understand the frequency of occurrence of this phenomenon in various body-centered cubic metals.

Crystal strength and plastic flow are controlled by the motion and interaction of dislocations, the line defects carrying atomic shear increments. While, in most crystals, deformation develops in the crystallographic planes where the glide force acting on dislocations is maximum, plasticity in body-centred cubic (BCC) metals is more complex. Slip systems where the resolved shear stress is not the highest can dominate at low temperature, leading to the so-called anomalous slip, first observed in 1969, but never elucidated at the microscopic scale.

Image of in situ electron microscopy at -170°C, showing mobile dislocations during the plastic deformation of niobium 

Using in situ tensile tests at -170°C in a transmission electron microscope, we can observe the motion of dislocations, in niobium samples and in real time, and determine all their different properties. Here we show that anomalous slip arises from the high mobility of multi-junctions, i.e. junctions between more than two dislocations (4 dislocations in the present case), which glide at a velocity several orders of magnitude larger than single dislocations. These multi-junctions result from the interaction of a simple binary junction with a gliding dislocation.

Although elasticity theory predicts that these binary junctions should be unstable in crystals with a weak elastic anisotropy like tungsten, both experiments and atomistic simulations reveal that such junctions can be created under dynamic conditions, in agreement with the existence of anomalous slip in almost all BCC metals, including tungsten.

Publication :
Anomalous slip in body-centred cubic metals
Daniel Caillard, Baptiste Bienvenu & Emmanuel Clouet
Nature, vol 609, 29 September 2022

Contact :
Daniel Caillard, daniel.caillard[at]cemes.fr