Revisiting the intra-granular dislocation extension model for flow stress in nanocrystalline metals

Abstract

In our previous work (R.J. Asaro, P. Krysl and B. Kad, Philos. Mag. Lett. 83 (2003) p.733; P. Gu, B. Kad and M. Dao, Scr. Mater. 62 (2010) p.361), the intra-granular partial dislocation extension model was shown to be consistent with the experimental data of flow stress in nanocrystalline FCC materials. However, since the averaged extension was taken for a nonuniform loop, the model predicted small dislocation extension across the grain. In this article, extending our previous work, we reformulate the intra-granular partial dislocation model for FCC nanocrystalline materials using a more realistic loop. The flow stress obtained from the reformulated model shows good agreement with experimental data for various nanocrystalline FCC materials and expectedly large dislocation extension across the grain. In the second portion of this article, the mechanistic model for partial dislocation extension is extended to develop an intra-granular perfect dislocation extension model. The perfect dislocation model is examined by comparing its prediction of flow stress with experimental data of nanocrystalline Fe. Additionally, activation volume and strain-rate sensitivity are discussed within the mechanistic model in the light of available experimental data on nanocrystalline Fe.

Six decades of the Hall–Petch effect – a survey of grain-size strengthening studies on pure metals

“…Measurements showing a plateau or even a decrease in the dislocation density at ~100 nm suggest that this transition in the dislocation behavior may also contribute to a lowering of the Hall-Petch slope at such grain sizes. 236,237 In light of these results, it seems that the strength of nanocrystalline metals is determined by the stress required to nucleate, propagate, or reabsorb individual dislocations, and this motivated the dislocation-nucleation models of the Hall-Petch breakdown developed by Asaro and co-workers, [238][239][240][241] which are relevant in the range of grain sizes down to perhaps ~10 nm.…”

Strengthening at nanoscaled coherent twin boundary in f.c.c. metals

“…Figures 3 and 4 show the prediction of inter-twin flow stress for nt-Al and nt-Ni against the inverse of square root of twin thickness. Also plotted in the two figures are the flow stress data for nc-Al and nc-Ni [10,31]. The inter-twin flow stress of nt-Cu discussed in [11] is plotted in Figure 5 together with experimental data for nt-Cu [2] against the inverse of square root of twin thickness.…”

A model for 〈c+a〉 dislocation transmission across nano-spaced parallel basal stacking faults in a HCP alloy

“…The left end and right end of the loop are pined to the grain boundary where the 〈c + a〉 dislocation is emitted into the first nanospace. From above, the maximum size of the loop within a nanospace is d= sin h. An activation methodology similar to that in the step (1) appears in the recent study of dislocation transmission across twin boundary in nano-twinned FCC metals [18,19]; a loop growth scheme similar to that used in the step (2) was employed in the study of intra-granular dislocation emitted from grain boundaries of nano-grained FCC metals [20][21][22][23].…”