František Chmelík

Thermal stability of submicron grained copper and nickel

The features of structure and thermal stability of submicron grained copper and nickel processed by severe plastic deformation are considered. The results of studies by various techniques: transmission electron microscopy, X-ray diffraction, differential scanning calorimetry, electrical resistance and microhardness are presented. The investigations have shown that thermal stability of submicron grained materials is determined not only by a mean grain size but also by the density and distribution of the grain boundary dislocations. The relaxation of the grain boundary dislocations precedes the grain growth starting at 175°C and influences on thermal stability.

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.

Structure of Silicon Processed by Severe Plastic Deformation

Abstract The structural features of nanocrystalline (NC) Si processed by severe plastic deformation are considered. The results of studies by various techniques (transmission electron microscopy, X-ray diffraction, Raman scattering and photoluminescence) are presented. The investigations show that the structure of NC materials is characterized by both a small grain size and a specific defect structure of grain boundaries associated with a high level of elastic strains and significant microdistortions of the crystal lattice. The Raman spectrum reveals a peak shift of 2.5 cm −1 to lower frequencies, a peak broadening up to 14.2 cm −1 , an asymmetry of the peaks and an additional peak at frequencies from 480 to 500 cm −1 . A visible photoluminescence with a peak maximum at a wavelength of about 650 nm is observed.

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.

Thermal structure changes in copper and nickel processed by severe plastic deformation

Abstract The results of thermal structure evolution studies of ultrafine grained copper and nickel by transmission electron microscopy, X-ray diffraction, DSC, electrical resistance and microhardness are presented. The investigations have shown that the heat release, the recovery of electrical resistance, the relaxation of elastic strains precede the grain growth starting above 175 °C. Different techniques have shown that thermal stability of ultrafine grained structure is associated not only with grain growth but also with the relaxation of the grain boundary structure.

TEM, XRD and Raman scattering of germanium processed by severe deformation

Nanocrystalline (NC) Ge samples processed by severe plastic deformation under high pressure (6 GPa) were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman scattering. TEM studies of NC Ge revealed a mean grain size of about 24 nm and high angle misorientations of neighbouring grains. The XRD patterns showed decreasing peak intensities, peak broadening and a mean grain size of about 10.4 nm. The Raman spectrum revealed decreasing peak intensities, a peak shift to lower frequencies of 2 cm−1, peak broadening up to 14.0 cm−1, asymmetry of the peaks, additional peak at frequencies from 280 cm−1 to 300 cm−1 and a mean grain size of about 9 nm. A structural model for NC materials processed by severe plastic deformation is used to explain the results. This model deals with two grain areas: the interior with a perfect crystal lattice and the grain boundary (GB) defect structure associated with a high level of elastic strains and distortions of the crystal lattice.

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 Portevin-Le Châtelier effect in Al-2.92%Mg-0.38%Mn alloy and linear location of acoustic emission

Abstract The Portevin-Le Châtelier (PLC) effect (discontinuous plastic flow) in an Al-2.92%Mg-0.38%Mn alloy deformed at room temperature has been studied using linear location of acoustic emission (AE). Location distributions have been found to be essentially dependent on strain. At the initial stage of plastic deformation a significant AE activity was monitored along the entire gauge length, well tracing the propagation of deformation bands; subsequently, some AE activity remained in the vicinity of the sample heads only. The results are discussed in terms of the present knowledge of the PLC effect and AE and possible mechanisms responsible for the observed AE activity are suggested.

Characteristics of thermal cycling in a magnesium alloy composite

Acoustic emission (AE) and dilatometry were used to monitor the internal thermal stresses and the related structural changes and matrix plastic deformation induced in a magnesium-based composite during thermal cycling. It is shown that the AE response and the residual strain after cycling depend significantly on the upper temperature of cycling. The results are explained in a quantitative analysis using a model that correlates the internal thermal stresses with the change in temperature.

An evaluation of the creep characteristics of an AZ91 magnesium alloy composite using acoustic emission

Creep experiments were undertaken to determine the potential for using acoustic emission (AE) to monitor the high temperature deformation of an unreinforced AZ91 magnesium alloy and a similar alloy reinforced with short alumina fibers after a conventional T6 heat treatment. Samples of each material were tested to failure and the fracture surfaces of selected specimens were examined using scanning electron microscopy. The results show there is an improved creep resistance in the composite by comparison with the unreinforced alloy. It is demonstrated that the use of AE provides a sensitive procedure for monitoring the nature of the creep deformation.