Last month, Materials Science and Engineering student Qingjie Li took home the Gold award in graduate student poster presentations at the TMS Annual Meeting. The poster, titled Surface Rebound of Relativistic Dislocations Directly and Efficiently Initiates Deformation Twinning, is based on Dr. Ma’s group’s recent research on high-speed dislocations.
Qingjie states the group found that dislocations often behave counter-intuitively when they move at speeds close to or exceeding the speed of sound. This causes them to “exhibit behaviors beyond current textbook description for ordinary dislocations”.
“The motivation of this work came from recent experimental work on nanoscale pristine metals. It has been reported that nanoscale pristine metals often yield by ‘instantaneous’ initiation of deformation twinning or structural ‘collapse’ during mechanical loading. The timescale on which these phenomena occurred is much less than 0.01 s, the typical time resolution of imaging techniques used in current transmission electron microscopy. The generation of a large number of dislocations within a very short time period suggests strongly correlated dislocation dynamics, i.e., dislocation activities are cooperative in both time and space. However, such strongly correlated dislocation dynamics have not been well understood. We noticed that the dislocations in these nanoscale pristine metals nucleate at ultra-high stress levels (on the order of 10-2G, where G is the shear modulus), so we looked into the behaviors of dislocations under such high driving forces”, Qingjie explained.
Dislocations are common defects in crystals, with a larger driving force leading to a faster dislocation. Depending on the magnitude of the dislocation speed, they can be characterized as ordinary dislocations, relativistic dislocations, transonic dislocations, or supersonic dislocations.
The kinetic energy carried by a relativistic dislocation is usually large and can lead to the generation of new defects during a reaction. Qingjie says that “the rebirth of new defects from excess kinetic energy is akin to the creation of elementary particles in high-energy physics, where once a certain energy threshold is reached, new charged particles could be created out of a vacuum. The relativistic dislocation mechanisms revealed in this work may also have relevance to high-stress or high-strain-rate deformation of crystals in general, where strongly overdriven dislocations interact with interfaces”.
