The Formation and Evolution of Defects in Nanocrystalline Fe During Indentation: The Role of Twins in Pop‐Ins

Abstract

Nanoindentation is a most common experimental tool used for obtaining information on the mechanical behavior of materials. This is done by qualitatively relating the occurrence of pop‐ins (in the load–displacement plots) to microstructural changes such as dislocation formation, fracture of surface oxides, or slip transmission. The present study takes a first approach in directly verifying the micro‐plasticity processes that give rise to such pop‐ins by performing molecular dynamics indentation simulations in BCC Fe‐nanocrystals with a Σ5 symmetric tilt boundary. The simulations allow to track the material behavior throughout the indentation process, and illustrate that each pop‐in is related to twin formation, twin growth, de‐twinning, dislocation nucleation and glide, or dislocation–grain boundary interactions. For the particular Σ5 boundary considered, it is found that the pop‐ins are most closely associated with twin formation. Although pop‐ins have been related to dislocation nucleation, a direct correlation between twinning and pop‐ins has not been shown before. Adding C segregants to the Fe sample, reduced the formation of twins after initial yielding, and allowed for dislocation activity to become the more dominant deformation mechanism.

Gradient Microstructure and Properties of Surface Mechanical Attrition–Treated AZ91D Alloy: An Effect of Colliding Balls Velocity

“…The interaction of twins and dislocations with grain boundaries results in pop‐in effects. [ 53 ] Dislocations’ annihilation or splitting, twins’ migration or growth, and de‐twining of the preexisting twins could also cause the pop‐ins. [ 53 ] The pop‐in phenomenon has been reported in various deformed alloys.…”

“…[ 53 ] Dislocations’ annihilation or splitting, twins’ migration or growth, and de‐twining of the preexisting twins could also cause the pop‐ins. [ 53 ] The pop‐in phenomenon has been reported in various deformed alloys. For instance, Hu et al [ 11 ] reported pop‐in events in the ultrafine‐grained AZ31 magnesium alloy.…”

Pop-In Phenomenon as a Fundamental Plasticity Probed by Nanoindentation Technique

“…The other twin and crack formation mechanisms are discussed in the transition from pure elastic to other behaviors. Twin formation upon plasticity initiation has been reported in sapphire [78], high-Mn steel [79], Ta [80], Mg-6Zn [81], and nanocrystalline (nc)-Fe [82]. Crack initiation can also be the dominant mechanism for the event, as shown mainly in brittle materials, including polycrystalline alumina [83], TiN-based thin-film coatings [84,85], Si and Ge [86,87], tungsten carbide [88], and various ceramics [89].…”

Defect‐Induced Effects in Nanomaterials

“…Kuhr and K.E. Aifantis took a first approach in directly verifying the micro‐plasticity processes that give rise to pop‐ins in the load‐displacement plots by performing molecular dynamics indentation simulations in bcc Fe‐nanocrystals with a Σ5 symmetric tilt boundary. T. Radchenko et al combined two theoretical approaches, grazing dynamical diffraction and efficient numerical simulations, for studying electronic properties in uniaxially strained graphene layers with impurity atoms.…”