Investigation of Effect of Preliminary Annealing on Superplasticity of Ultrafine-Grained Conductor Aluminum Alloys Al-0.5%Mg-Sc

Effect of preliminary precipitation of Al3Sc particles on the characteristics of superplastic conductor Al-0.5%Mg-X%Sc (X = 0.2, 0.3, 0.4, 0.5 wt.%) alloys with ultrafine-grained (UFG) microstructure has been studied. The precipitation of the Al3Sc particles took place during long-time annealing of the alloys at 300 °C. The preliminary annealing was shown to affect the superplasticity characteristics of the UFG Al-0.5%Mg-X%Sc alloys (the elongation to failure, yield stress, dynamic grain growth rate) weakly but to promote more intensive pore formation and to reduce the volume fraction of the recrystallized microstructure in the deformed and non-deformed parts of the aluminum alloy specimens. The dynamic grain growth was shown to go in the deformed specimen material nonuniformly–the maximum volume fraction of the recrystallized microstructure was observed in the regions of the localization of plastic deformation.

Micromechanisms of Deformation and Fracture in Porous L-PBF 316L Stainless Steel at Different Strain Rates

The process of an unstable plastic flow associated with the strain rate sensitivity of mechanical properties was studied in porous 316L austenitic steel samples manufactured by laser powder bed fusion (L-PBF). Different micromechanisms of deformation and fracture of porous samples dependent on strain rate were found. It was found that despite the porosity, the specimens showed high strength, which increased with the loading rate. Porosity led to lower ductility of the studied specimens, in comparison with literature data for low porous 316L L-PBF samples and resulted in de-localization of plastic deformation. With an increase in strain rate, nucleation of new pores was less pronounced, so that at the highest strain rate of 8, only pore coalescence was observed as the dominating microscopic mechanism of ductile fracture.

Extracting information from noisy data: strain mapping during dynamic in situ SEM experiments

Micromechanical testing techniques can reveal a variety of characteristics in materials that are otherwise impossible to address. However, unlike to macroscopic testing, these miniaturized experiments are more challenging to realize and analyze, as loading and boundary conditions can often not be controlled to the same extent as in standardized macroscopic tests. Hence, exploiting all possible information from such an experiment seems utmost desirable. In the present work, we utilize dynamic in situ microtensile testing of a nanocrystalline equiatomic CoCrFeMnNi high entropy alloy in conjunction with initial feature tracking to obtain a continuous two-dimensional strain field. This enables an evaluation of true stress–strain data as well as of the Poisson’s ratio and allows to study localization of plastic deformation for the specimen. We demonstrate that the presented image correlation method allows for an additional gain of information in these sophisticated experiments over commercial tools and can serve as a starting point to study deformation states exhibiting more complex strain fields. Graphic abstract

Localization of plastic deformation in stretching plates: effect of crystalline structure

The integration of the polycrystalline structure in the simulation of stretching plates, performed by the random generation of a grain aggregate and a set of lattice orientations, gives new insights into the phenomenon of plastic strain localization in the form of necking, albeit well predicted at the scale of continuum by instability analysis. A transition is displayed from initial grain scale heterogeneity, with some connection to crystal lattice orientation towards stress axes, to the onset of macroscopic localization patterns. Depending on the number of grains and on the stretching rate, it seems that a competition emerges between a “weakest link” process for which the final necks are located in places where initial deformation is high and an instable mode controlled process during which larger patterns emerge driving the location of the final necks. At high stretching rates, it seems that the second effect is enhanced with, accordingly, a pattern size almost insensitive to the grain size and a limited variability with the grain structure occurrence.

A study of plastic strain localization under static and dynamic loading using the StrainMaster non-invasive strain measurement system

Static and dynamic testing of specimens specially designed for studying the localization of plastic deformation in AMg6 and D16 alloys were performed on then electromechanical Testometric machine and split Hopkinson pressure bar using the StrainMaster system for noninvasive measurement of shape and deformation. Displacement and strain fields are plotted for special-shaped specimens of AMg6 and D16 alloys subjected to static deformation and dynamic loading. Comparison between the experimentally obtained strain fields and the results of numerical simulation made with account of the kinetics of microdefect accumulation in the examined material demonstrates good agreement to the accuracy of ~20%. The performed tests and their numerical simulation with consideration for the evolution of the defect material structure confirm the concept of the strain localization mechanism associated with the processes in the system of microdefects.

Failure Mechanisms of Alloys with a Bimodal Graine Size Distribution

A multi-scale computational approach was used for the investigation of a high strain rate deformation and fracture of magnesium and titanium alloys with a bimodal distribution of grain sizes under dynamic loading. The processes of inelastic deformation and damage of titanium alloys were investigated at the mesoscale level by the numerical simulation method. It was shown that localization of plastic deformation under tension at high strain rates depends on grain size distribution. The critical fracture stress of alloys depends on relative volumes of coarse grains in representative volume. Microcracks nucleation at quasi-static and dynamic loading is associated with strain localization in ultra-fine grained partial volumes. Microcracks arise in the vicinity of coarse and ultrafine grains boundaries. It is revealed that the occurrence of a bimodal grain size distributions causes increased ductility, but decreased tensile strength of UFG alloys. The increase in fine precipitation concentration results not only strengthening but also an increase in ductility of UFG alloys with bimodal grain size distribution.