Correlation between defect structure and mechanical properties of nanocrystalline materials

Abstract

The defect-related mechanical properties of nanomaterials are overviewed. The infl uence of small grain size on plastic deformation mechanisms, strength and ductility is discussed. The smaller the grain size in face-centred cubic (fcc) metals, the higher the activity of twinning at the expense of dislocation glide, while hexagonal close-packed (hcp) materials behave contrarily. The relationship between the dislocation structure and the yield strength is studied in detail. Above the grain size of ∼20 nm, the decrease of grain size is accompanied by an increase of yield strength and a decrease of ductility. The loss of ductility can be moderated by the incorporation of coarse grains into the nanocrystalline matrix. A combination of large strength with good ductility can be achieved with nanograins containing high density of twin boundaries. Below the grain size of ∼20 nm, the yield strength is found to decrease with the reduction of grain size, or remains unchanged with the experimental error. The possible explanations of this phenomenon are discussed in detail.

Dislocation versus grain boundary strengthening in SPD processed metals: Non-causal relation between grain size and strength of deformed polycrystals

“…The quantitative analysis described below uses the volume-averaged dislocation and grain boundary (GB) creation model outlined in [4,12,15]. Prior to considering that specific model in Section 2.2 we will first consider the generation of defects (incl.…”

“…To derive the correlation between dislocation and grain boundary densities in a quantitative way we employ the model for the analysis of volume-averaged defect and grain boundary creation described in [4,12,15]. This model has proved to be accurate in predicting the grain size and hardness of pure metals [4,12,15], and also correlates well with measured dislocation densities in a range of metals [4]. The model considers that the total cumulative dislocation linelength generated during the deformation processing, or annihilated within the grain [20], i.e.…”

“…The model considers that the total cumulative dislocation linelength generated during the deformation processing, or annihilated within the grain [20], i.e. [4]:…”

“…where fan is the (temperature and material dependent) annihilation fraction [4], Lig is the total dislocation linelength of dislocations stored in the grain and Lgb is the total dislocation linelength of dislocations that have moved to grain boundaries and have become part of the GB (they are 'subsumed' in the grain boundaries [12,15]). To provide a more transparent analysis we will here consider that the volume of the GB with associated dislocations is negligible as compared to the total material volume (i.e.…”

“…Severe plastic deformation (SPD) has attracted wide attention as a means of improving properties of metals and alloys, and especially improvements in strength have been targeted [1, ,2,3]. Although grain refinement is often mentioned as being the main factor in strengthening of metals processed by SPD, several more detailed analyses which incorporate strengthening models have indicated that dislocation hardening is the main factor determining the strengthening of many SPD processed pure and commercially pure metals [4,5,6,7,8,9,10,11,12], provided SPD and any post-SPD treatment are carried out at temperatures at which recovery is suppressed. Where dislocation density measurements (from diffraction line broadening) are available, this data also tend to indicate that dislocation strengthening is dominant through the well know equation describing the increment of critical resolved shear stress of grains due to dislocations, d [13,14]:…”