Employing molecular dynamics to shed light on the microstructural origins of the Taylor-Quinney coefficient

Abstract

The Taylor-Quinney coefficient (TQC) or the cold-work conversion factor is the fraction of external work that is converted into heat during the plastic deformation of a material. Since it was first proposed almost a century ago, its value was measured experimentally and was found to span over a wide range of values between 0.1 and 1, and it is argued to depend on strain and strain rate. Despite, only little is known on how the microstructure corresponds to the value of TQC, and in particular how dislocations and grain boundaries correspond to energy storage during plastic deformation. We employ molecular dynamics simulations to study the work-to-heat conversion during plastic deformation in single crystal and nanograined samples of aluminum, copper, iron, and tantalum at extremely high strain rates. We found that if dislocation glide in the drag-controlled regime is the only deformation mechanism of plastic deformation, all external work is converted into heat as in the case of single crystals with dislocation dipoles. On the other hand, in nanograined samples we found that some energy is stored in- or released from the crystal as a result of an evolution in the morphology and distribution of grain boundaries. We also emphasize the important distinction between the integral and differential measurements of TQC, showing that the differential measure can become even greater than 1 if the strain increment results in annealing of the defects from the microstructure. Based on the atomistic observations, we propose that grain boundary evolution mechanisms can be responsible for energy storage in polycrystalline material at experimental strain rates.