Characterisation of high rate plasticity in the uniaxial deformation of high purity copper at elevated temperatures

Abstract

In uni-axial compression at strain rates above 10 4 s −1 , FCC metals exhibit a rapid increase in strength. Mechanisms proposed to be responsible for this transition can be broadly split into two categories; that mobile dislocation velocities become limited by quasi-viscous scattering from phonons, or that some change occurs in the evolution of the materials dislocation structure. The relative contribution of each mechanism is difficult to identify, in part due to a scarcity of experimental measurements in varying deformation conditions. In this paper, we perform uni-axial compression experiments that reach rates between 10 4 and 10 5 s −1 , at temperatures between 300 and 600K. Analysis of the data at 0.1 strain shows both the absolute and relative levels of thermal softening increase with strain rate, an anomalous result in comparison to both existing models and measurements below the transition.

Time limited self-organised criticality in the high rate deformation of face centred cubic metals

“…Importantly, repeat experiments at raised temperatures ( Supplementary Fig. 2) show the mechanical threshold, at a fixed post-transition rate, decreases with increasing temperature, thus excluding phonon drag as a potential origin of increased work hardening in the studied regime 7,30 . At higher rates (>10 6 s −1 ) or temperatures close to melt [30][31][32] , drag will likely become a dominant factor, further disrupting any remaining SOC 23,33 .…”

“…Given these results, the relationship between SOC avalanche dynamics and strain rate can be examined around the strength transition. Strain limitation reduced adiabatic heating during high rate loading to approximately 10 K 7,28 . Importantly, repeat experiments at raised temperatures ( Supplementary Fig.…”

“…In the more precise form (equation 8 in the supplementary methods), when the minimum permitted avalanche size passes through the size of a single dislocation, physically corresponding to none of the metal being slip eligible, the rate of work hardening diverges. In a real material the main mode of deformation will change to other mechanisms such as twinning, which is expected at higher strains, higher strain rates, or lower temperatures for the current grain size 7,54 , or nucleation limited processes expected at substantially higher stresses 55 . However, the current theory could be used to predict the point at which such changes should occur.…”

“…Despite originally being observed in 1984 2 , the physical nature of the transition remains controversial [3][4][5] . Studies of the effect have been limited by a lack of experimental data above the transition which can separate instantaneous strength contributions 2,[5][6][7] (e.g., due to viscous effects or phonon drag) from structural strength contributions 1,5,8 (e.g., due to changes in dislocation structure). In this paper we experimentally demonstrate the transition to be of a structural nature.…”

Development and Validation of a Hopkinson Bar for Hazardous Materials

“…It is possible to perform a correction to account for pressure wave distortion if the temperature gradient in the bars is known [21,22], but this is often neglected, as it is in this work. Other researchers have used pressure bars made from materials with elastic properties that are relatively constant with respect to temperature, such as Inconel 718 [23]. Using the in-situ approach the testing temperature is limited due to the thermal softening of the pressure bars.…”