A.V. Mikhaylovskaya

Effect of the solid-solution composition on the superplasticity characteristics of Al-Zn-Mg-Cu-Ni-Zr Alloys

The structure and superplasticity characteristics of the Al-Zn-Mg-Cu-Ni-Zr alloys with different amounts of Zn and Mg have been analyzed. It has been demonstrated that for the formation of a micrograined structure and superplasticity effects, a bimodal size distribution of particles is required, i.e., the presence of coarse eutectic particles of Al3Ni and dispersed Al3Zr particles, and also the presence of highly alloyed solid solution is obligatory. In the case of a low-alloy solid solution in the alloys of this system, the recrystallization is hampered, both at heating and at the initial stage of the alloy deformation. An increase in the amount of Zn and Mg in the aluminum solid solution stimulates the formation of finer grains, a decrease in the yield stress, an increase in the strain-rate sensitivity m, and a growth in the values of relative elongation.

High-strain-rate superplasticity of the Al–Zn–Mg–Cu alloys with Fe and Ni additions

During high-strain-rate superplastic deformation, superplasticity indices, and the microstructure of two Al–Zn–Mg–Cu–Zr alloys with additions of nickel and iron, which contain equal volume fractions of eutectic particles of Al3Ni or Al9FeNi, have been compared. It has been shown that the alloys exhibit superplasticity with 300–800% elongations at the strain rates of 1 × 10–2–1 × 10–1 s–1. The differences in the kinetics of alloy recrystallization in the course of heating and deformation at different temperatures and rates of the superplastic deformation, which are related to the various parameters of the particles of the eutectic phases, have been found. At strain rates higher than 4 × 10–2, in the alloy with Fe and Ni, a partially nonrecrystallized structure is retained up to material failure and, in the alloy with Ni, a completely recrystallized structure is formed at rates of up to 1 × 10–1 s–1.

Study of the structure and properties of a wrought Al–Mg–Mn aluminum alloy on a Gleeble 3800 simulator designed for physical modeling of thermomechanical processes

The mechanical behavior of aluminum alloy 5454, belonging to the system Al–Mg–Mn, is studied during hot plastic deformation on a Gleeble 3800 simulator designed for physical modeling of thermomechanical processes. The study was conducted under different temperature-rate conditions. Examination of the structure of the hot-deformed specimens showed that dynamic recrystallization is impeded in the alloy. The substructure obtained after deformation at 400°C consists mainly of equiaxed subgrains with dimensions of 1–2 μm, the exact size depending on the rate of deformation. A mathematical model that was constructed to relate the flow stress of the given alloy to the rate, temperature, and degree of deformation can be used for finite-element calculation of the properties of different sections of products of complex shape that are made of this alloy by various methods of hot plastic deformation.

Formation of microstructure and the superplasticity of Al–Mg-based alloys

The influence of a 3–10% content of magnesium in Al–Mg–Mn(Cr) alloys on the characteristics of the microstructure of sheet blanks and their superplasticity has been examined. It has been shown that the minimum size of grains and the best superplasticity are characteristic of the alloy that contains about 7% magnesium and is additionally alloyed simultaneously with manganese and chromium. An increase in the content of magnesium leads to the formation of conglomerates of particles of a chromium–manganese phase and, as a result, to a coarsening of the grain structure and a deterioration of superplasticity.

Study of the mechanisms of superplastic deformation in Al–Mg–Mn-based alloys

The contributions of grain boundary sliding and intragranular dislocation slip in the AMg4 alloy (analog AA5083) during superplastic deformation have been analyzed by analyzing the deformation-induced changes in the sample surface having marker grids patterned by ion beam etching. Optical, electron transmission, and scanning microscopy techniques, and electron backscatter diffraction were used to analyze the changes in dislocation, grain, and subgrain structures during superplastic deformation. It has been shown that dynamic polygonization develops during superplastic deformation. The contribution of diffusion creep is defined from the analysis of precipitation-free zones observed after deformation.

Structure of the aluminum alloy Al-Cu-Mg cryorolled to different strains

Methods of optical metallography, X-ray diffraction, and transmission and scanning electron microscopy were used to study changes in the structure of the aluminum alloy D16 (2024) caused by isothermal rolling at a temperature of liquid nitrogen. It has been established that the basic structural changes that take place in the material upon deformations to e ∼ 2.0 are due to the formation and evolution of the dislocation structure, which contains cells of nanometer size. With further straining to e ∼ 3.5, the processes of recovery and recrystallization become activated, which lead to the formation of a mixed grain-subgrain nanosized structure.

Analysis of softening alloys of the Al-Ni system containing particles of variable dispersity

Regularities of the deformation strengthening and softening of aluminum alloys containing second-phase Al3Ni particles 0.3 to 2.2 μm in size with a volume fraction from 0.03 to 0.1 are investigated during cold deformation and subsequent annealing at 0.6tm. It is shown that the largest hardness increment is observed for alloys with a maximal fraction of fine particles (d = 0.3 μm) after rolling deformation larger than 0.4. Fine particles prevent the development of crystallization upon true deformation up to 2.3, thereby effectively inhibiting softening. An increase in the particle size to 1.2–2.2 μm stimulates nucleation during recrystallization, substantially accelerating this process. For example, in order to ensure recrystallization uniformly over the entire sheet volume at d = 2.2 μm, cold deformation with ɛ = 0.4 is sufficient.

Effect of microadditions of magnesium and zinc in aluminum upon heating of cold-rolled sheets

Methods of structural investigations, measurements of hardness, temperature, time, and amplitude dependences of internal friction have been used to study processes of polygonization and recrystallization in Al-0.3 at % Mg and Al-0.3 at % Zn alloys. It has been established that the magnesium atoms more efficiently pin dislocations in the aluminum-based alloys as compared to zinc atoms. Therefore, in the alloy with zinc the process of polygonization occurs more intensely and the processes of softening upon recrystallization occur more slowly than those in the alloy with magnesium.

Fine-Grained Structure and Superplasticity of Al – Cu – Mg – Fe - Ni Alloys

The microstructure of Al – Cu – Mg – Fe – Ni alloys with Mn and Zr additions was analyzed by optical and scanning electron microscopy, internal friction, X-ray and calorimetric analysis in order to optimize technology of superplastic alloy preparation. It is shown that the S (Al2CuMg) phase precipitates during hot rolling and dissolves during annealing. This allows to create fine-grained recrystallized structure and to achieve elongation of 320 % at the strain rate of 1×10-3 s-1 during superplasticity testing. It is shown that annealing in saltpeter before superplastic deformation improves the superplastic behavior: at the constant strain rate of 4×10-3 s-1 elongation is 500 %.

Influence of Al3Ni crystallisation origin particles on hot deformation behaviour of aluminium based alloys

Abstract Binary Al–Ni, Al–Mg and ternary Al–Mg–Ni alloys containing various dispersions and volume fraction of second-phase particles of crystallisation origin were compressed in a temperature range of 200–500 °C and at strain rates of 0.1, 1, 10, 30 s−1 using the Gleeble 3800 thermomechanical simulator. Verification of axisymmetric compression tests was made by finite-element modelling. Constitutive models of hot deformation were constructed and effective activation energy of hot deformation was determined. It was found that the flow stress is lowered by decreasing the Al3Ni particle size in case of a low 0.03 volume fraction of particles in binary Al–Ni alloys. Intensive softening at large strains was achieved in the alloy with a 0.1 volume fraction of fine Al3Ni particles. Microstructure investigations confirmed that softening is a result of the dynamic restoration processes which were accelerated by fine particles. In contrast, the size of the particles had no influence on the flow stress of ternary Al–Mg–Ni alloy due to significant work hardening of the aluminium solid solution. Atoms of Mg in the aluminium solid solution significantly affect the deformation process and lead to the growth of the effective activation energy from 130–150 kJ/mol in the binary Al–Ni alloys to 170–190 kJ/mol in the ternary Al–Mg–Ni alloy.