P. Lukáč

Strain hardening behaviour and the Taylor factor of pure magnesium

Taylor orientation factors for strain hardening in textured and random polycrystals of magnesium were derived from the ratio of the strain hardening rates of polycrystals to that of single crystals deforming by equivalent polyslip. For polycrystals with textures that inhibit basal and prismatic slip while favouring pyramidal polyslip, the Taylor factor is estimated to be between 2.1 and 2.5, increasing to about 4.5 for randomly textured polycrystals. The micromechanics of strain hardening in polycrystals are discussed.

Investigating the Portevin-Le Châtelier effect by the acoustic emission and laser extensometry techniques

Abstract The Portevin–Le Châtelier (PLC) effect is a spectacular effect of dynamic strain aging in many alloys deformed in certain intervals of strain rates and temperatures. The main feature of the PLC effect is a negative strain rate sensitivity of stress, which is linked with stress fluctuations, a macroscopic spatio-temporal localization of plastic deformation (nucleation and propagation of deformation bands) and an intense acoustic emission. Recent theoretical studies have pointed out that cooperative dislocation motion is a necessary condition for plastic instabilities to occur under conditions of negative strain rate sensitivity. In this work the potential of acoustic emission and laser extensometry to monitor in situ cooperative dislocation motion due to PLC effect is reviewed and examined experimentally in an Al–1.5 wt.% Mg alloy. At the conditions of testing, the alloy exhibits a Luders phenomenon followed by the C- and/or B-type of the PLC effect. The results indicate that different dislocation processes are responsible for the Luders phenomenon, the nucleation of PLC bands and the propagation of PLC bands.

Strain hardening due to {10 12} twinning in pure magnesium

The effect of grain segmentation by mechanical twinning on the strain-hardening behaviour of textured and random polycrystals is assessed using the Kocks-Mecking method of analysis. Profuse twinning in the first 6–8% strain in textured polycrystals has relatively small strengthening effects despite the large volume fraction of grains undergoing twinning. This is due to the small value of the Hall–Petch constant in textured polycrystals. For random polycrystals, the Hall–Petch constant is much larger but the overall hardening effect is reduced due to the small volume fraction of grains undergoing twinning. Additional hardening effects due to the twinning crystallographic transformation on dislocation mobility are deemed small in cast polycrystals due to their low dislocation density, but may be more important in textured polycrystals with higher dislocation densities. Grain size-independent storage of dislocations accounts for the strain hardening at large strains.

Hardening and softening in deformed magnesium alloys

Abstract The deformation behaviour of three commercial magnesium alloys AZ91, AS21 and AE42 has been investigated in a wide temperature range. Specimens were deformed in tension and in compression in the temperature range of 300–573 K at constant but various strain rates. The form of the stress–strain curves is very sensitive to the test temperature and the strain rate. The deformation behaviour of the specimens can be attributed to the occurrence of hardening and softening during straining. In order to identify hardening and softening processes, the stress dependence of the strain-hardening coefficient was evaluated. Different models describing hardening and softening were used to analyse the observed behaviour. The model proposed by Lukac and Balik can describe the experimental data in the temperature range of 373–473 K. The analysis shows that conservative slip of dislocations is the main recovery process. The yield stress analysis reveals an asymmetry of values obtained in tensile and compression tests.

Inverse critical strains for jerky flow in Al-Mg alloys

The influence of the initial precipitation microstructure, as produced by various treatments, has been investigated in the past years in several Al alloys. However, the results obtained so far have been inconclusive or contradictory. The working hypothesis to be tested in the present work is that the initial dislocation density governs the critical strain behavior. For this purpose, the influence of prestrains performed in different conditions is examined and discussed in simple terms.

Mechanisms of serrated flow in aluminium alloys with precipitates investigated by acoustic emission

By measuring the acoustic emission, it is possible to distinguish between deformation which is controlled by normal dislocation motion through a crystal lattice, and deformation controlled by the shearing of coherent particles. These results from alloys with different precipitate structures can be correlated with the appearance of serrated flow: when shearing is pronounced, serrated flow is minute. The claim that shearing is directly responsible for serrated flow is not supported by these findings.

Modelling of strain hardening and its relation to the onset of Portevin-Le Chatelier effect in Al-Mg alloys

Abstract Deformation behaviour of two pure aluminium-magnesium polycrystalline alloys with actual concentrations of 2.57% Mg and 4.8% Mg, respectively, was investigated at room temperature. Tensile tests with constant cross-head speed were performed for the initial strain rates from the interval 6.8 × 10 −7 s −1 ≤ ge 0 ≤ 4.3 × 10 −2 s −1 . The critical strains for the onset of the Portevin-Le Châtelier (PLC) effect at low strain rates were measured as a function of strain rate. Experimental data are fitted according to the model proposed by Kubin and Estrin. In addition to this, the work hardening behaviour of both alloys is analyzed with respect to the model equations proposed by Malygin and by Balik and Lukac. On this basis a modification of the evolution equations of the mobile and immobile dislocation densities, which are used in the model of Kubin and Estrin, is suggested.

Hardening and softening during plastic deformation of hexagonal metals

The influence of temperature on the deformation behaviour of single crystals and polycrystals is investigated. The temperature dependences of the critical resolved shear stress, work hardening rates, the stress at the onset of stage C., the yield stress and maximum stress are reported. Possible deformation mechanisms concerning hardening and softening during plastic deformation of hexagonal metals are discussed.

Study of relaxation of residual internal stress in Mg composites by internal friction

Abstract When a metal matrix composite is subjected to temperature changes, thermal stresses arise at the interfaces between the matrix and the reinforcement as a result of the considerable mismatch between the thermal expansion coefficients of the matrix and the reinforcement. Even moderate temperature changes can produce thermal stresses that exceed the matrix yield stress. Consequently, new dislocations are generated at the interfaces causing thermal fatigue (microstructural changes, matrix plastic deformation and irreversible shape changes). The microstructural changes in the matrix can be detected by damping and stress relaxation measurements. The logarithmic decrement and modulus defect were measured after thermal treatment at temperatures between room temperature and temperatures up to 400°C. The strain dependence of the logarithmic decrement can be divided into two regions. The values of the logarithmic decrement are also influenced by heat treatment and foreign atoms in the matrix. The results can be explained assuming that straining and thermomechanical treatment produce changes in the microstructure of the composites. The stress relaxation curves were analysed and the modulus defect, relaxation strength and activation energy for dislocation motion were estimated. It is very difficult to draw conclusions on the mechanisms responsible for the stress relaxation but part of the relaxation strength is due to reversible dislocation movement.

Temperature and concentration dependence of the critical resolved shear stress of cadmium-zinc alloy single crystals

The increase of the critical resolved shear stress of cadmium single crystals by additions of zinc has been investigated in the temperature range 77 to 295 K. The temperature dependence of the critical resolved shear stress can be divided into two temperature regions. At all temperatures the critical resolved shear stress was found to increase withc2/3 wherec is the atomic concentration of zinc as solute. The concentration dependence of the plateau stress is explained according to the theory of Labusch [5].