Combined grain size, strain rate and loading condition effects on mechanical behavior of nanocrystalline Cu under high strain rates

Molecular dynamics simulations of nanocrystalline Cu with average grain sizes of 3.1 nm, 6.2 nm, 12.4 nm and 18.6 nm under uniaxial strain and stress tension at strain rates of 10^8 s^-1, 10^9 S^-1 and 10^10 s^-1 are performed to study the combined grain size, strain rate and loading condition effects on mechanical properties. It is found that the strength of nanocrystalline Cu increases as grain size increases regardless of loading condition. Both the strength and ductility of nanocrystalline Cu increase with strain rate except that there is no monotonic relation between the strength and strain rate for specimens under uni- axial strain loading. Moreover, the strength and ductility of specimens under uniaxial strain loading are lower than those under uniaxial stress loading. The nucleation of voids at grain boundaries and their subsequent growth characterize the failure of specimens under uniaxial strain loading, while grain boundary sliding and necking dominate the failure of specimens under uniaxial stress loading. The rate dependent strength is mainly caused by the dynamic wave effect that limits dislocation motion, while combined twinning and slipping mechanism makes the material more ductile at higher strain rates.

Competing failure mechanisms of thin metal films on polymer substrates under tension

The ductility of thin metal films on polymer substrates reported in recent experiments has a huge disparity,ranging from less than 1 % up to more than 50 %.To reveal the underpinning origins for such a large variation,this paper reports a systematic computational study of two competing failure mechanisms:metal film necking and grain boundary cracking.The quantitative results suggest that strong grain boundaries and metal/polymer interfacial adhesion are keys to achieve high ductility of polymer-supported metal films.

Influence of microstructure on the corrosion resistance of Fe–44Ni thin films

An Fe–44Ni nanocrystalline（NC） alloy thin film was prepared through electrodeposition. The relation between the microstructure and corrosion behavior of the NC film was investigated using electrochemical methods and chemical analysis approaches. The results show that the NC film is composed of a face-centered cubic phase（γ-（Fe,Ni）） and a body-centered cubic phase（α-（Fe,Ni）） when it is annealed at temperatures less than 400℃. The corrosion resistance increases with the increase in grain size, and the corresponding corrosion process is controlled by oxygen reduction. The NC films annealed at 500℃ and 600℃ do not exhibit the same pattern, although their grain sizes are considerably large. This result is attributed to the existence of an anodic phase, Fe0.947Ni0.054, in these films. Under this condition, the related corrosion process is synthetically controlled by anodic dissolution and depolarization.

A Galvanostatic Modeling for Preparation of Electrodeposited Nanocrystalline Coatings by Control of Current Density

The correlation between the grain size of electrodeposited coatings and the current densities was modeled by considering galvanostatic conditions. In order to test the model by experimental results, nanocrystalline （NC） nickel samples were deposited at different current densities using a Watts bath. The grain size of the deposits was evaluated by X-ray diffraction （XRD） technique. Model predictions were validated by finding a curve being the best-fit to the experimental results which were gathered from literature for different NC coatings in addition to those data measured in this research for NC nickel coatings. According to our model, the variation of grain size with the reciprocal of the current density follows a power law. A good agreement between the experimental results and model predictions was observed which indicated that the derived analytical model is applicable for producting the nanocrystalline electrodeposits with the desired grain size by controling current density.

A novel Hf-Si-O wire drawing phenomenon after ablation of SiC_(nws)/ HfC-SiC coating on C/C composites

A SiC_(nws)/HfC-SiC coating was fabricated by a two-step chemical vapor deposition.After introducing SiC nanowires,the fracture toughness of HfC-SiC coating increases by 228%.After ablation under oxyacetylene torch for 60s,a bone-like HfO_(2) reticular structure was formed in the central ablation area.Interestingly,a novel Hf-Si-O wire-drawing phenomenon was happened in the transition area of ablated SiC_(nws)/HfC-SiC coating,providing a new toughness mechanism under ablation environment.Moreover,HfO_(2) precipitated from Hf-Si-O glass wires to generate nanocrystalline grains during ablation,which explains the formation of bone-like HfO_(2) reticular structure observed from central ablation area.