Jaroslav Balík

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.

Physical aspects of plastic instabilities

Serrated yielding is observed in Al-Mg and Al-Zn alloys if they are deformed at suitable strain rate at room temperature. To describe the physical origin of plastic instabilities, the critical strain at which the instabilities begin is investigated as a function of strain rate. This paper describes how a combination of tensile testing with various experimental techniques, such as acoustic emission and resistivity measurements, and with a theoretical model, could help to elucidate the fundamental mechanisms.

The onset of Portevin–LeChatelier instabilities in tensile testing☆

Abstract The critical strains for the onset or disappearance of the Portevin–LeChatelier effect are discussed based on the ‘intrinsic’ approach of the elementary strain. To obtain the parameters of dynamic strain ageing (DSA), B-type serrations are analysed considering the accommodation effect. Comparing the model relations with an experimental map of critical strains for Al–3Mg and with transient behaviour after stress relaxations, it is suggested that trapping of dislocations plays a significant role during intensive DSA.

Strain-hardening behaviour of AZ31 magnesium alloys

Abstract The in-plane tensile behaviour of rolled sheets of the magnesium alloy AZ31 was investigated in both an H24 state and an aged state. Whereas the very high initial strain-hardening rate decreases monotonically with strain for the H24 samples, the annealed structure exhibits a striking increase in the strain-hardening rate. To explain this phenomenon, the occurrence of sessile dislocations associated with a secondary (pyramidal) slip is proposed. The absence of increasing hardening for the H24 state is supposed to be related to higher yield stresses (predeformations) within the grains of the polycrystalline aggregate.

Modelling of work hardening

This paper presents simulation of the work hardening curves of metals deformed at a constant temperature and strain rate under conditions of dynamic recrystallization (DRX). A work hardening model has been developed based on the evolution of the dislocation structure during deformation. A statistical approach is used to predict the microstructure evolution which is consistent with DRX. The model provides a description of the stress-strain curves for some specified combinations of temperature and strain rate.

Dislocation glide and multiple slip

Abstract The deformation processes controlled by dislocation glide are investigated for the multiple-slip regime. On the basis of the evolution law for sessile dislocation segment density the concept of a course variable is introduced. The proposal is made that in the domain of glide-controlled processes the work-hardening analysis should be done using the relative strain e rather than the true strain ln(l + e).

Deformation Behavior of ZE10 Magnesium Alloy Sheet

Rolled Mg-Zn-Rare earth sheet after annealing was deformed at room temperature with the tensile axis oriented in both the rolling (RD) and the transversal direction (TD). The deformation behavior of the sheet has been investigated by acoustic emission (AE) and electron backscattered diffraction (EBSD) measurements, where the AE signal analysis correlates the microstructure and the stress-strain curves to the active deformation mechanisms. X-ray diffraction was applied to receive orientation images in order to characterize the deformation texture before and after the test.

Crack Growth Anomalies in Base Steel P91 and in HAZ

The growth of cracks in base steel P91 of 9Cr-1Mo class and in intercritical layer of HAZ is measured under creep coditions. For long term tests, a material degradation was detected consisting in an increase of crack growth rate and in a decrease of crack initiation time. An attempt is made to connect these effects with drop in ductility during thermomechanicalexposition.