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Solid-state modification of SiC nanoparticles in a ball-mill is a known method for deagglomeration of as-received nanoparticles in a bed of metallic carriers that will also aid their better injection and releasing into the melt. The melt temperature and condition of the melt is very important for a better interaction of composite powders including ceramic nanoparticles and metallic carrier with the melt. In this study, aluminum A356 matrix nanocomposites were produced by liquid-state stir casting at 715 °C followed by twin-screw rheo-extrusion (TSRE) in the semi-solid state to enhance the produced composite properties. For this purpose, SiC nanocomposite superstructures were prepared using ball-milling of SiC nanopowders with micron-sized titanium and nickel (separately) as carrier agents. Also, hot-rolling (HR) process was applied on liquid-state stir cast nanocomposites to compare the mechanical properties of rolled sheets with TSRE-processed bars. The experimental results indicated that stir-cast samples before TSRE and HR consisted various types of defects with poor dispersion of nanoparticles in some areas. However, more improved dispersions for the nanoparticles were obtained after TSRE and HR and rolled sheets had considerable improved mechanical properties compared with TSRE bars. This latter improvement was determined to be mainly due to extensive fragmentation and dispersion of eutectic silicon, Ti-rich, and Ni-rich intermetallic phases.

High-entropy alloys (HEAs) are among multi-component alloys with attractive microstructures and mechanical properties. In this study, molecular dynamics simulation was used to determine the tensile behaviour of glassy Al x CrCoFeCuNi HEAs from 300 ° K to 1300 ° K . For this purpose, this alloy with variations in chemical concentration of aluminum were heated and then cooled at a high cooling rate of 21 . 1 × 10 12 K / s . Results from radial distribution functions (RDF) and common neighbor analysis (CNA) indicated that no crystalline structures were formed for these glassy alloys. The deformation behaviour and mechanisms of the glassy alloys at room and high temperatures and various strain rates were investigated and reported. The tensile test results showed that the yield stress decreased markedly as temperature was increased for all the alloys. The alloys exhibited superplastic behaviour for all test conditions. More importantly, by increasing the molar ratio of aluminum from 0.5 to 3.0, the yield stress and elastic modulus decreased considerably. Also, the yield stress increased with increasing the strain rate for all samples with different aluminum concentrations. Free volume content of the alloys as well as shear banding were evaluated for these alloys to aid explanation of these results.

In this study, in order to understand the nanostructure of FeCrNi steels in the laser powder bed fusion (LPBF) process, directional solidification was simulated using large-scale molecular dynamics simulation (LSMDS). For this purpose an atomic box with a dimension of 70 × 40 × 60 ( n m 3 ) including random dispersion of Fe, Cr and Ni was created. Then, two different fixed temperatures were considered for the left and right side of the box during cooling from the liquid molten state. For evaluation of the uniformity in mechanical properties, uniaxial tensile tests were performed in the parallel and perpendicular directions. Extensive twinning induced plasticity (TWIP) occurred along side Shockley partial dislocations (DLs) evolution in both directions, while different ultimate tensile strengths (UTS) were obtained as a sign of non-uniform tensile behavior. Different plastic deformation mechanisms at the nano-scale including stacking faults (SFs) interaction with each other/grain boundaries (GBs)/twin boundaries (TBs), formation of defective coherent twins (DCTs), dynamic Hall-Petch, shear stress gradient (back stress), and a new mechanism for dynamic recrystallization at room temperature are discussed in detail.

In order to properly model and control the semi-solid processing of metallic alloys, their thixotropic behaviour requires proper characterisation. In particular, the effects of shear rate, shear time, temperature and rest time on the rheology of such slurries needs to be understood and quantified. A purpose-built high temperature Searle rheometer was used to determine the rheological behaviour of aluminium alloy slurries at shear rates from 3.1 to 124.8 s −1 , periods of shear of up to 60 min for each shear rate, and periods of rest (no stirring) of up to 60 min for Al–4wt.%Si and A356 alloys. Continuous cooling rheometry was used to determine the coherency point of the alloys. Isothermal fractions solid of 0.36 for Al–4%Si and 0.33 for A356 were investigated. Isothermal tests were used to follow the temporal evolution of viscosity, which was found to be significantly different for both alloys, particularly at low shear rates. Steady state viscosity values that were determined over a range of shear rates indicated severe pseudoplastic behaviour, as measured by a viscosity–shear rate power law index less than −1. This work confirms that this finding is an actual rheological feature and not an artefact of a particular measurement device. A study using shear rate jumps determined the isostructural behaviour of the alloys by discounting equipment inertial effects. It is shown that peak stress or apparent viscosity is a better indicator of slurry thixotropy than the hysteresis loop from shear ramping experiments, and the work also shows that the effect of agglomeration on fluidity is an important parameter to measure as it has consequences for thixoforming.

A comprehensive study was carried out to establish the effects of controlled stirring during solidification on the microstructure and mechanical properties of aluminium alloys, in comparison to conventionally gravity chill cast material. A novel device comprising a grooved reaction bonded silicon nitride rod rotating in a tube-like crucible was used to process aluminium alloys in the mushy state. The stir casting device was specially designed to also enable rheometric study of the alloys in this condition. A factorial design of experiments was used to determine the effect of the process variables shear rate ( γ ), shear time (ts), and volume fraction solid during shear (fs) on microstructure and both static and dynamic mechanical properties of the stir cast alloy. Investigation of the microstructure consisted of computer-aided image analysis of the primary phase morphology. A more globular primary phase was achieved at low values of fs, but this was not the optimum morphology for mechanical properties. In all cases, improved mechanical properties and reduced porosity were obtained in the stir cast condition in comparison with conventional casting and in comparison with previous work on stir casting. Comparison with alloy commercially rheocast via electromagnetic stirring, however, showed that the latter had superior mechanical properties. It is proposed that the mechanical stir casting process be considered as an alternative to gravity die casting in cases where very simple and thick walled shapes are required.