Utjecaj uvjeta grijanja na strukturnu jednolikost i lomno naprezanje aluminijske pjene

In this paper, the possibility to create uniform porous structure for non- symmetric precursor heating was investigated together with changes in collapse stress and their reproducibility. The statistic relative standard deviation (RSD) of pore size was used to determine the structural uniformity. It was shown that proper adjustment of the foaming parameters leads to formation of the uniform foam structure. The compression testing showed that standard deviation of collapse stress was higher for uniform structure than those obtained for non- uniform structure.U ovome radu istražena je mogućnost stvaranja jednolike porozne strukture nesimetričnim zagrijavanjem prekursora, te su isto tako istražene promjene lomnog naprezanja i njihova ponovljivost. Statistička relativna standardna devijacija (RSD) veličine pora koristila se za određivanje strukturne jednolikosti. Pokazano je da pravilan odabir parametara upjenjavanja dovodi do nastajanja jednolike strukture. Tlačna proba je pokazala da je vrijednost standardne devijacije lomnog naprezanja veća za jednoličnu strukturu od onih ostvarenih za nejednoliku strukturu

Effect of multi-pass friction stir processing on textural evolution and grain boundary structure of Al-Fe3O4 system

A mixture of pre-milled Fe 3O 4 and Al powder was added to the surface of an aluminum alloy 1050 substrate to obtain hybrid surface nanocomposites using friction stir processing. In situ nano-sized products were formed by the exothermic reaction of Al and Fe 3O 4. The reaction is triggered by hot working characteristics of the process. The microstructure and crystallographic microtexture transition and grain boundaries evolution of the fabricated nanocomposite were investigated using optical microscopy, X-ray diffraction, field emission scanning electron microscopy, and electron backscattered diffraction analyses. It is illustrated that matrix means grain size decreased in the specimens, which is processed without and with the introduction of the powder mixture to ∼8 and 2 μm, respectively. In addition, high angle grain boundaries showed marked increasing that demonstrates the happening of dynamic restoration phenomenon in the aluminum matrix. Moreover, the fraction of low ςCSL boundaries showed increasing (remarkably in the presence of hard particles); these boundaries play the main role in dynamic recrystallization. The incorporation of nano-sized products such as Al 13Fe 4 and Al 2O 3 in the dynamically recrystallized aluminum matrix produced a pre-dominantly Cube Twin texture component induced by the stirring function of the rotating tool. As a result, the effect of nano-sized products is constrained

Effects of graphene nano-platelets (GNPs) on the microstructural characteristics and textural development of an Al-Mg alloy during friction-stir processing

The final publication is available at Elsevier via http:/dx.doi.org/10.1016/j.surfcoat.2017.12.045 © 2018. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/The aim of this research is to characterize the unique microstructural features of Al-matrix nanocomposites reinforced by graphene nano-platelets (GNPs), fabricated by multi-pass friction-stir processing (FSP). During this process, secondary phase GNPs were dispersed within the stir zone (SZ) of an AA5052 alloy matrix, with a homogenous distribution achieved after five cumulative passes. The microstructural characteristics and crystallographic textures of different regions in the FSPed nanocomposite, i.e., base metal (BM), heat affected zone (HAZ), thermo-mechanical affected zone (TMAZ), and SZ, were evaluated using electron back scattering diffraction (EBSD) and transmission electron microscopy (TEM) analyses. The annealed BM consisted of a nearly random crystal orientation distribution with an average grain size of 10.7μm. The SZ exhibited equiaxed recrystallized grains with a mean size of 2μm and a high fraction of high-angle grain boundaries (HAGBs) caused by a discontinuous dynamic recrystallization (DDRX) enhanced by pinning of grain boundaries by GNPs. The sub-grains and grain structure modification within the HAZ and TMAZ regions are governed by dislocation annihilation and reorganization in the grain interiors/within grains which convert low-angle to high-angle grain boundaries via dynamic recovery (DRV). The FSP process and incorporation of GNPs produced a pre-dominantly {100} cube texture component in the SZ induced by the stirring action of the rotating tool and hindering effect of nano-platelets. Although, a very strong {112} simple shear texture was found in the HAZ and TMAZ regions governed by additional heating and deformation imposed by the tool shoulder. These grain structure and texture features lead to a hardness and tensile strength increases of about 55% and 220%, respectively.Slovak Foundation VEGA [grant 2_0158_16, and by grant APVV-14-0936

Effect of initial grain size on microstructure and mechanical properties of in situ hybrid aluminium nanocomposites fabricated by friction stir processing

Friction stir processing (FSP) offers a unique opportunity to tailor the microstructure and improve the mechanical properties due to the combination of extensive strains, high temperatures, and high-strain rates inherent to the process. Reactive friction stir processing was carried out in order to produce in situ Al/(Al13Fe4 + Al2O3) hybrid nanocomposites on wrought/as-annealed (673 K) AA1050 substrate. The active mixture of pre-ball milled Fe2O3 + Al powder was introduced into the stir zone by pre-placing it on the substrate. Microstructural characterisation showed that the Al13Fe4 and Al2O3 formed as the reaction products in a matrix of the dynamically restored aluminium matrix. The aluminium matrix means grain size was found to decrease markedly to 3.4 and 2 μm from ~55 μm and 40–50 μm after FSP using wrought and as-annealed substrates employing electron backscattered diffraction detectors, respectively. In addition, tensile testing results were indicative that the fabricated surface nanocomposite on the as-annealed substrate offered a greater ultimate tensile strength (~160 MPa) and hardness (73 HV) than those (146 MPa, and 60 HV) of the nanocomposite formed on the wrought substrate

Unconventional order/disorder behaviour in Al–Co–Cu–Fe–Ni multi-principal element alloys after casting and annealing

The effect of Cu concentration on the order/disorder behaviour of the AlCoCuxFeNi (x = 0.6 to 3.0) multi-principal element alloys was investigated. BCC and/or FCC phases were observed in the microstructures of the alloys after casting and annealing at 1050 ◦C followed by slow cooling. Interesting is that the alloys form ordered structures after casting and disordered structures after annealing and slow cooling, while the opposite would be expected. The ordering in the as-cast state is explained by the strong affinity of Al to transition metals, which results in the formation of supercell structures having sublattices occupied by certain elements only. Disordering after annealing has two reasons. Either the phase is composed of nearly pure element (Cu) and is disordered by default or it is composed of randomly distributed nano-segregated regions within a single phase resulting in a uniform distribution of all elements in the sublattices and therefore appearing to be macroscopically disordered. The reason for the formation of such nano-segregated areas might reside in the reduction of Gibbs free energy due to the annealing by the interplay between enthalpy and entropy

Depth-sensing thermal stability of accumulative fold-forged nanostructured materials

Accumulative fold-forging (AFF) as a newly developed severe plastic deformation (SPD) process based on the repetitive fold-forging steps is implemented for the production of the layered UFG (~200 nm) AA8006 alloy and AA8006-B4C nanocomposite (~35 nm, 10 vol%) materials from the initial AA8006 alloy foil. The remarkably refined grains and nanoparticles can control metallic materials' mechanical properties, including the strength, strain rate dependency, and thermal stability behavior. In this context, nano-grains' local mechanical response during nanoindentation can vary considerably depending on the testing temperature, and this has yet to be discussed. In this research, after materials characterization of produced nanostructured materials according to the AFF route, the relating depth-sensing thermal stability of them assessed by conducting the nanoindentation testing at different temperatures in the range of 300–523 K. Depth sensing softening behavior is elaborated to identify the low-temperature thermal stability of processed materials. The results enunciated the occurrence of thermal softening by refining the grain structure. However, introducing the reinforcing nanoparticles lead to a pinning action that stabilized the grain boundaries.The authors would like to acknowledge the Natural Sciences and Engineering Research Council of Canada (NSERC). FIB-TEM microscopy was performed at the Canadian Centre for Electron Microscopy at McMaster University, supported by NSERC and the Canadian Foundation for Innovation. The first author wants to thank Slovak Academy Information Agency (SAIA) for supporting the scholarship. This work was supported by the Slovak Research and Development Agency by grant APVV-18-0508 is gratefully acknowledged

Global urban environmental change drives adaptation in white clover

Urbanization transforms environments in ways that alter biological evolution. We examined whether urban environmental change drives parallel evolution by sampling 110,019 white clover plants from 6169 populations in 160 cities globally. Plants were assayed for a Mendelian antiherbivore defense that also affects tolerance to abiotic stressors. Urban-rural gradients were associated with the evolution of clines in defense in 47% of cities throughout the world. Variation in the strength of clines was explained by environmental changes in drought stress and vegetation cover that varied among cities. Sequencing 2074 genomes from 26 cities revealed that the evolution of urban-rural clines was best explained by adaptive evolution, but the degree of parallel adaptation varied among cities. Our results demonstrate that urbanization leads to adaptation at a global scale