Daria Zhemchuzhnikova

Effect of Mg Content on High Strain Rate Superplasticity of Al-Mg-Sc-Zr Alloys Subjected to Equal-Channel Angular Pressing

Aluminium alloys with a chemical compositions of Al–5.8%Mg–0.52%Mn–0.2%Sc–0.07%Zr–0.16%Fe-0.1%Si and Al-5.4%Mg-0.34%Mn-0.2%Sc-0.07%Zr-0.07%Fe-0.02Si (in weight %), denoted as 1570 and 1570C, respectively, were processed by equal-channel angular pressing (ECAP) at 300°C up to strain ε~12. Extensive grain refinement provided the formation of fully recrystallized structure with the average grain sizes of 0.7 and 0.6 μm, respectively. Tensile tests were carried out in the temperature interval 200–550oC at strain rates ranging from 10-4 to 10-1 s-1. Very high tensile elongations (>1000%) were achieved in the both alloys at T350oC and strain rates higher than 10-3 s-1.

Correlation versus randomization of jerky flow in an AlMgScZr alloy using acoustic emission

Jerky flow in solids results from collective dynamics of dislocations which gives rise to serrated deformation curves and a complex evolution of the strain heterogeneity. A rich example of this phenomenon is the Portevin-Le Chatelier effect in alloys. The corresponding spatiotemporal patterns showed some universal features which provided a basis for a well-known phenomenological classification. Recent studies revealed peculiar features in both the stress serration sequences and the kinematics of deformation bands in Al-based alloys containing fine microstructure elements, such as nanosize precipitates and/or submicron grains. In the present work, jerky flow of an AlMgScZr alloy is studied using statistical analysis of stress serrations and the accompanying acoustic emission. As in the case of coarse-grained binary AlMg alloys, the amplitude distributions of acoustic events obey a power-law scaling which is usually considered as evidence of avalanchelike dynamics. However, the scaling exponents display specific dependences on the strain and strain rate for the investigated materials. The observed effects bear evidence to a competition between the phenomena of synchronization and randomization of dislocation avalanches, which may shed light on the mechanisms leading to a high variety of jerky flow patterns observed in applied alloys.

The Portevin-Le Chatelier Effect and Kinematics of Deformation Bands in an Al-Mg-Sc Alloy: Effect of Grain Size

Stress serration patterns and kinematics of deformation bands associated with the Portevin-Le Chatelier (PLC) effect were examined for an Al–6%Mg–0.35%Mn–0.2%Sc–0.08%Zr–0.07%Cr (in wt.%) alloy with two grain sizes: 22 μm and 0.7 μm. The fine-grained structure of the alloy was obtained using equal-channel angular pressing (ECAP) at 320°C up to a total strain of ~12. Tensile tests were carried out at room temperature and strain rate ranging from 10-5 s-1 to 10-2 s-1. In addition, high-frequency local extensometry technique was applied to monitor the evolution of the axial strain distribution during deformation. Depending on the strain rate, conventional A, B, C, or mixed types of serrations were observed on the stress-strain curves. These types of behavior usually correspond to different kinematics of the PLC bands, including band propagation and localization. However, the propagation regime was found to dominate in the investigated alloy in the entire strain-rate range. This unusual behavior of deformation bands and their features depending on the grain size are discussed.

Effect of Annealing on Structure and Mechanical Behavior of an AA2139 Alloy

An AA2139 alloy with a chemical composition of Al–4.35Cu-0.46%Mg–0.63Ag-0.36Mn–0.12Ti (in wt.%) and an initial grain size of about 155 μm was subjected to annealing at 430°C for 3 h followed by furnace cooling. This treatment resulted in the formation of a dispersion of coarse particles having essentially plate-like shape. The over-aged alloy exhibits lower flow stress and high ductility in comparison with initial material in the temperature interval 20-450°C. Examination of microstructural evolution during high-temperature deformation showed localization of plastic flow in vicinity of coarse particles. Over-aging leads to transition from ductile-brittle fracture to ductile and very homogeneous ductile fracture at room temperature.

Effect of Grain Size on Cryogenic Mechanical Properties of an Al-Mg-Sc Alloy

An aluminum alloy with a chemical composition of Al–6%Mg–0.35%Mn–0.2%Sc–0.08%Zr–0.07%Cr (in wt.) and an initial grain size of ∼22 μm was subjected to equal-channel angular pressing (ECAP) at 593 K up to a total strain of ~12. Extensive grain refinement provided the formation of fully recrystallized structure with an average grain size of ∼0.6 μm. The mechanical properties of the alloy in two different structural conditions were examined at temperatures ranging from 77 to 293 K. It was shown that ECAP highly enhanced the strength, ductility and fracture toughness of the material over the wide temperature interval. Positive effect of grain refinement tends to increase with decreasing temperature due to suppression of brittle intergranular fracture. At ambient temperature, the extensive grain refinement provides +65% increase in yield stress (YS) and ductility, concurrently. At 77 K, YS increase is + 77%, and the ductility increase is +113% owing to grain refinement. Effect of the grain size on fracture toughness at cryogenic temperatures is discussed.

Effect of Pre-Strain Rolling on Annealing Behavior of Friction-Stir Welded AA6061-T6 Aluminum Alloy

In this work, the effect of pre-strain cold rolling on thermal stability of friction-stir welded AA6061-T6 alloy was studied. The pre-strain rolling was found to be very effective in suppression of abnormal grain growth during standard post-weld T6 heat treatment. It was also shown that the efficiency of this approach essentially depends on rolling path and the rolling along welding direction was the most effective rolling schedule.

Friction-Stir Welding of Work-Hardened Al-Mg Alloy with Nanoscale Particles

In this work, feasibility of friction-stir welding (FSW) for joining of heavily deformed 5083 aluminum alloy was studied. To produce work hardening condition, the commercially available material was homogenized to precipitate strengthening particles and then subjected to equal-channel angular pressing (ECAP) at 300 °C to a true strain of ~12 via BC route and successive rolling at the same temperature to 80 pct. of thickness reduction. Despite the subsequent FSW resulted in significant microstructural changes in stir zone, joint efficiency was found to be 98 pct.

Crack Resistance of an Alloy of the Al – Cu – Mg – Ag System

Cyclic crack resistance of an aluminum alloy of the Al – Cu – Mg – Ag system with a high content of doping elements is studied. It is shown that the alloy possesses a high threshold value of the stress intensity factor for nucleation of fatigue cracks. However, the nonuniform distribution of second-phase particles in the material reduces the crack resistance. Special features and main differences of the relief of fatigue fracture surfaces of the alloy in various regions of the kinetic diagram are considered. Coefficients of the Paris’ equation for the region of linear fatigue-crack growth are determined.

Effect of Microstructure on Impact Energy of an Al-Mg-Sc Alloy

Analysis of the absorbed impact energy of an Al-Mg-Sc alloy after different thermo-mechanical processing routes was investigated between-196°C and 20°C. The material with a grain size of ∼ 22 μm in cast condition and with an average grain size of 0.7 μm produced by was produced by equal-channel angular pressing (ECAP) exhibits well-defined ductile-brittle transition in the temperature interval-60...-100°C, however, even at-196°C the value impact energy of fine-grained alloy is higher by a factor of 2 in comparison with coarse-grained state. The impact toughness of the hot rolled alloy linearly decreases with decreasing temperature. The influence of different microstructures on impact toughness and fracture behavior of alloy is discussed.

Effect of Rolling on Mechanical Properties and Fatigue Behavior of an Al-Mg-Sc-Zr Alloy

An aluminum alloy with a chemical composition of Al–6%Mg–0.35%Mn–0.2%Sc–0.08%Zr–0.07%Cr (in wt.) was rolled up to different reductions of 75, 88 and 95% at 360oC and at ambient temperature. The static mechanical properties and the high-cyclic fatigue (HCF) life were examined. It was shown that the hot rolling results in increased yield stress (YS) and ultimate tensile strength (UTS). However, ductility and fatigue limit of the hot rolled alloy and initial as-cast ingot are nearly the same. The combination of hot and cold rolling leads to significant improvement of tensile strength and fatigue resistance, while ductility tends to reduce with increasing the rolling reduction. The cold rolled alloy exhibits the endurance limit under fatigue conditions, while the alloy in the both as-cast and hot rolled conditions exhibits only fatigue strength. The effect of the deformation structure on the mechanical properties is discussed.