J. Lendvai

Decomposition processes in Al-Zn-Mg alloys

This review surveys the present knowledge of the mechanisms of decomposition processes in Al-Zn-Mg alloys. The various experimental observations connected with the formation and properties of the different second phase particles are described and critically analysed. The effect of different heat treatments, compositions, impurities, etc on the different stages of the decomposition process is discussed in detail.

Dislocations and grain size in ball-milled iron powder

The microstructure of ball-milled iron powder has been investigated by high resolution X-ray line profile analysis. Analysis of line breadths suggested that the contrast factors related to dislocations have to be taken into account in the Williamson-Hall procedure. This concluded to a modified Williamson-Hall plot which provided, in a straightforward manner, the grain size refinement of nanocrystals ball-milled for different periods of time. At the same time it has been shown that strain broadening, even in these nanoscale small-grain particles, is caused by the presence of dislocations. The line profiles were also studied by the method of Fourier analysis, which gave the absolute values of dislocation densities.

The mechanism of clustering in supersaturated solid solutions of A1-Mg2Si alloys☆

Abstract The early stages of the precipitation in the Al-Mg2Si system were investigated by isothermal resistivity measurements. The process of clustering was studied over a wide range of temperature (between − 50 and 200°C) after two different quenching methods. The continuous observation of the resistivity led to the investigation of the kinetics of the process near room temperature. The effect of preageing on the subsequent ageing was also studied by electrical resistivity and microhardness measurements. On the basis of the experimental results a new model is suggested for the process of clustering. The most important feature of the process seems to be the decisive role of Si.

Indentation creep in a short fibre-reinforced metal matrix composite

Abstract Creep properties of an unreinforced M124 (AlSi12CuMgNi) base alloy and an Al 2 0 3 (Saffil) fibre reinforced M124+s metal matrix composite (MMC) were investigated by indentation tests performed between 250 and 350°C. It has been found that the creep curve of the base alloy consists of two stages (transient and steady state), whereas the curve of the composite material contains a decelerating third stage as well. This creep behavior is correlated with the changes of the microstructure below the indenter during the deformation process. In the region of steady state creep the stress exponent and the activation energy was determined for both materials.

Correlation between resistivity increment and volume fraction of G.P. zones in an Al-3·2 wt % Zn-2·2 wt % Mg alloy

Abstract The correlation between resistivity increment caused by G.P. zones and their volume fraction has been investigated in an Al-3·2 wt % Zn-2·2 wt % Mg alloy. It is found that the resistivity increment is a linear function of the 2/3 power of the volume fraction. The results show that the resistivity increase is mainly determined by the total volume of zones and is not sensitive to their size distribution

A new method for hardness determination from depth sensing indentation tests

A new semiempirical formula is developed for the hardness determination of the materials from depth sensing indentation tests. The indentation works measured both during loading and unloading periods are used in the evaluation. The values of the Meyer hardness calculated in this way agree well with those obtained by conventional optical observation, where this latter is possible. While the new hardness formula characterizes well the behavior of the conventional hardness number even for the ideally elastic material, the mean contact pressure generally used in hardness determination differs significantly from the conventional hardness number when the ideally elastic limiting case is being approached.

Dislocation densities and stored energy after cold rolling of Al-Mg alloys: Investigations by resistivity and differential scanning calorimetry

The authors have shown that the heat released during heating samples of Al-2.5%Mg cold rolled at different strains stems from two contributions: one corresponds to the annihilation of defects during recrystallization and the other to desegregation of solute atoms from the core of dislocations. A low temperature endotherm peak is also observed and can be attributed to dissolution of Mg clusters formed in dislocation walls. The authors have shown that the hardness of the material can be described by a single internal variable which is the density of dislocations. In contrast to this, the energy stored during cold rolling can not be described by a single variable theory. As a consequence, the yield stress alone can not be sufficient to depict stored energy and thus recrystallization kinetics.

Nanocrystallization Studies of an Electroless Plated Ni-P Amorphous Alloy

Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) measurements were performed on an electroless plated Ni-P amorphous alloy to study the influence of different heat-treatments (linear heating and isothermal annealing). The phases formed after crystallization and the average grain size of the crystallization products were determined from XRD line broadening, and the heat evolved during the structural transformations were established from DSC measurements. A detailed study of the transformation products obtained along different heating routes was performed. From these studies, a scheme of the structural transformations and their energetics was constructed. The grain boundary energies in the different nanocrystalline states were also estimated.

Characterization of plastic instability steps occurring in depth-sensing indentation tests

Abstract Plastic instabilities in solid solution Al–Mg and supersaturated Al–Zn–Mg–(Cu) alloys were investigated by depth-sensing microhardness tests. Experimental results show that in the case of Al–Mg alloys the characteristics of the plastic instability depend only on the solute concentration. In the case of age-hardenable Al–Zn–Mg–(Cu) alloys, however, although plastic instabilities were observed during indentation following the quenching, the development of the instability steps is strongly influenced by Guinier–Preston zone formation in the early stage of natural aging. After a certain time, t i , of aging the steps disappear completely. The value of t i obtained for the Cu-containing alloy was found to be considerably longer than that obtained for the ternary Al–Zn–Mg alloy. This fact – together with the smaller initial rate of hardening in the Cu-containing alloy – indicates that the addition of Cu retards the early part of the decomposition process at room temperature in this alloy system.

On the decomposition behaviour of Al-4.5 at% Zn-2 to 3 at% Mg alloys during continuous heating

The decomposition processes taking place in the Al-4.5 at % Zn-2 to 3 at % Mg alloys were studied during continuous heating by means of electrical resistivity, XSAS and DSC measurements and by TEM investigations. It was found that the room temperature pre-ageing has no significant influence on the processes taking place above 230° C. Several temperature ranges were determined in which the decomposition of the solid solution and/or the transformation of the different particles of the second phases take place by different mechanisms.