![Fig. 2. (a) Tensile true stress–true strain curves for three different as-deposited C 100 nm, 35 nm and 15 nm, respectively); for comparison, tensile stress–strain cu same electrodeposition solution are also included [25,76,78]. (Figure taken fro (where k is the average lamellar spacing determined from statistic TEM ob comparison, H–P plots for the mc Cu [58] (straight line and open circles) and taken from Ref. [77].)](/figures/fig-2-a-tensile-true-stress-true-strain-curves-for-three-2bcgg8c9.webp)
![Fig. 9. (a) HRTEM micrograph of an nc Ni grain after tensile testing at liquid nitrogen temperature. A twin boundary is indicated by an arrow. (b) High magnification of the white-framed region A in (a) showing the details of the twin. (c) High-magnification view of the white-framed region B in (a) showing the presence of several full dislocations near the twin boundary and grain boundary. The Burgers circuits were drawn to identify Burgers vectors: b1 = 1/2[011] and b2 ¼ 1=2½10 1 . (Figure reprinted with permission from Applied Physics Letters 2006;88(23):231911. Copyright 2006, American Institute of Physics.)](/figures/fig-9-a-hrtem-micrograph-of-an-nc-ni-grain-after-tensile-14unphvs.webp)
![Fig. 13. Simulated hardness vs. time [93], compared with the measurements of Zhang et al. [85]. The simulations were performed using a wide range for tf, i.e. 2 · 103 s 6 tf 6 108 s. (Figure taken from Ref. [93].)](/figures/fig-13-simulated-hardness-vs-time-93-compared-with-the-3vt50wfx.webp)
![Fig. 3. Tensile elongation to failure of various nanocrystalline metals (d 6 100 nm) in the literature [16,25,54,97–105] plotted against their yield strength. Note that most of the nanometals show a rather low ductility (610%). The data of nc Zn samples prepared by means of mechanical attrition [106] were not included in this figure, because Zn has a low melting point, and room temperature is at 0.42 of its melting temperature. It therefore may not be appropriate to compare it with the other materials in terms of deformation behavior at room temperature.](/figures/fig-3-tensile-elongation-to-failure-of-various-1ym5wdn3.webp)
![Fig. 14. (a) TEM observation of an nt-Cu-fine sample after tensile test showing dislocation pile ups along TBs and curved TBs; a region within a few nanometers of a TB is often heavily influenced by the dislocations along the TBs. (b) HRTEM of an nt-Cu-fine sample, showing partial dislocation disassociation, dislocation pile up and TB bending. (c) Schematic drawing of the proposed twin boundary affected zone (TBAZ) model, where TBAZ refers to the region adjoining the twin boundaries where the crystalline lattice is elastically strained. (d) A simple strain-based damage/failure criterion. The model assumes that each TB can accommodate dmax amount of slip deformation per unit length, i.e. cmax = ndmax, where n is the number of TBs per unit length (i.e. twin density). It is clear that local ductility is directly proportional to the twin density with this assumption. (Figure taken from Ref. [78].)](/figures/fig-14-a-tem-observation-of-an-nt-cu-fine-sample-after-3tf0lj4r.webp)
![Fig. 5. (a) Engineering stress–strain curves of an electrodeposited nanocrystalline Ni with an average grain size of 40 nm, obtained from tensile tests at different strain rates. (Figure taken from Ref. [103].) (b) Plot to determine strain rate sensitivity using Eq. (2), from stress change rate data obtained in a stress relaxation test at room temperature, for nc Ni (average grain size 30 nm). The strain-rate sensitivity is obtained from the slope of the linear fit, m = 0.02. (Figure taken from Ref. [120].)](/figures/fig-5-a-engineering-stress-strain-curves-of-an-1zbmmkkz.webp)
Q2. What is the effect of the twin spacing on the nt-Cu sample?
With increasing twin density, or decreasing twin lamellar spacing, the strength of the nano-twinned Cu (nt-Cu) sample increases gradually.
Q3. Why is the aggregate’s strength closer to d 1/2 than to d 1?
It may well be that, due to the dominance of the larger grains in the overall deformation, the aggregate’s strength scales closer to d 1/2 rather than to the d 1 as characteristic of nanoscale models for dislocation emission.
Q4. What is the main disadvantage of gas condensation and mechanical milling?
Both gas condensation and mechanical milling are capable of producing material with grain sizes below 100 nm; nevertheless, the principal disadvantage is that residual porosity may still remain as a compaction step is needed to reach the bulk form.
Q5. What is the sensitivity of Cu with a high density of coherent twin boundaries?
An increased strain-rate sensitivity was also observed in ultrafine grained Cu with a high density of coherent twin boundaries (CTBs).
Q6. What are the oldest preparation methods of nanostructured metals and alloys?
The oldest preparation methods of nanostructured metals and alloys are IGC [33,34] and ball milling [35], but soon after their introduction several drawbacks were noted.
Q7. What is the way to improve the fracture toughness of nanostructured materials?
These rather simple considerations suggest that the fracture toughness of nanostructured material would be improved by both a low elastic modulus and a high critical stress for fracture implying also a need for high hardness.
Q8. What is the mechanism of crack growth in coarser grained materials?
The model suggested that predominantly crystallographic and stage The authorcrack growth result in microstructurally tortuous crack paths in coarser grained materials.
Q9. What is the relationship between the grain size and the yield strength of polycrystalline metals?
The yield strength of polycrystalline metals is generally observed to increase as the grain size decreases according to the empirical Hall–Petch (H–P) relationship [51,52]:ry ¼ r0 þ Kdd 1=2 ð1Þ where d is the grain diameter, ry is the yield strength, and r0 and Kd are material dependent constants.