Planar Defect Nucleation and Annihilation Mechanisms in Nanocontact Plasticity of Metal Surfaces

Abstract

The incipient contact plasticity of metallic surfaces involves nucleation of crystalline defects. The present molecular dynamics simulations and nanoindentation experiments demonstrate that the current notion of nanocontact plasticity in fcc metals does not apply to high-strength bcc metals. We show that nanocontact plasticity in Ta-a model bcc metal-is triggered by thermal and loading-rate dependent (dynamic) nucleation of planar defects such as twins and unique {011} stacking fault bands. Nucleation of different planar defects depending on surface orientation leads to distinct signatures (pop ins) in the nanoindentation curves. Nanoscale plasticity is then ruled by an outstanding dynamical mechanism governing twin annihilation and subsequent emission of linear defects (full dislocations). While this investigation concerns Ta crystals, the present are landmark findings for other model bcc metals.

Incipient plasticity of single-crystal tantalum as a function of temperature and orientation

“…In a previous publication, a By comparing the location of the nucleated defects in (001) and (011) indentations, one can establish a clear correlation between the locations of the maximum shear stresses for the most critical {011} <0-11> and {112} <11-1> slip systems and those where SF and twins nucleate, respectively [10]. This shows that defect nucleation is indeed driven by shear.…”

“…Surprisingly, all activation volumes vary from around 0.9 b 3 for (111) to 0.2 b 3 for (110) and (100), and were found to be almost independent of temperature and strain rate. These values are significantly lower than the 10 b 3 measured by Sato et al revealed the highly compressive stresses encountered in indentation are creating shear strains that can only be accommodated by other mechanisms such as the SFs and twins [10]. As a consequence the activation volume determined by indentation in a BCCmaterial cannot be compared to the values obtained from classical strain-rate jump tests as it describes a very specific and complex deformation scenario.…”

“…The serrated flow behavior as found at 200°C is an evolution of the defect networks due to the specific quasi-elastic reloading at a given orientation [10].…”

“…The transition from elastic to the incipient plastic regime measured through nanoindentation experiments of different material's surfaces has been a topic of high interest in the past years [1][2][3][4][5][6][7][8][9][10][11][12][13]. The strength of this technique lies in its ability to probe ultra-small and therefore virtually "defect-free" sample volumes.…”

Atomistic Studies of Nanoindentation—A Review of Recent Advances

“…Upon indentation on a (111) surface loop generation was associated with shear stresses in their atomistic indenter. Considering tantalum as a model bcc material, Alcalá and co-workers [31] have shown that nanocontact plasticity occurs by the nucleation and propagation of twin and stacking fault bands driven by a combination of shear stresses and pressure. They suggested that dislocations appear after a thermally assisted twin annihilation and mentioned that this mechanism is common to other bcc metals.…”