Zs. Kovács

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.

Localized deformation bands in Portevin–LeChâtelier plastic instabilities at a constant stress rate

Abstract Portevin–Le Châtelier (PLC) plastic instabilities were examined in constant loading rate tensile tests. Irregular instability steps were observed in an intermediate stress rate region on the stress versus strain curves. The irregular steps are caused by the repeated activation of localized high deformation bands. The activation of the localized bands shows some regularity. The origin of the irregular instability steps goes back to the initialization of the first band at the onset point of the instabilities. It is argued that the appearance of irregular instability steps at low strain rates is related to the decrease of the depth of the dynamic strain aging toward small strains.

Effect of local ordering on the decomposition of an amorphous Cu–Zr–Ti alloy

We present free-energy calculations based on the random solution model. These calculations support the fact that the Cu-based solid solution observed to form during decomposition in amorphous Cu60Zr30Ti10 alloy is an ordered metastable L12 Cu3(Zr,Ti) phase which can nucleate due to its small unit cell and high mixing preference of Zr with Cu atoms.

Portevin–Le Châtelier type plastic instabilities in depth sensing macro-indentation ☆

Abstract Depth-sensing indentation at constant loading rate was applied to investigate the Portevin–Le Châtelier effect. Comparison of the different testing methods was achieved using representative strain and stress quantities. The depth of the dynamic strain ageing shows a good coincidence for Vickers and conical indentations, and for the large strain region of the tension tests. Tests with cylindrical indenters and tension experiments show a transient and a stationary behaviour for small and for large strains, respectively. The transient part can be interpreted by taking into account the free-flight time of dislocations in the Kubin–Estrin model.

Effect of Grain Boundaries on PLC Plastic Instabilities

The effect of single grain boundaries on the Portevin Le Châtelier (PLC) effect was investigated by depth sensing Vickers indentation. The experiments were carried out in load control. Independently appearing PLC bands were observed in the plastic volume and they were attributed to the hindered dislocation motion through the grain boundary. The hardening effect of the grain boundary and the release of the piled-up dislocations were also observed. This transient effect is manifested in a dynamic over-shooting in the hardness decrease, which is the consequence of the PLC effect as it is reproduced also by a phenomenological numerical model.

High pressure torsion of Cu-based metallic glasses

Cu-Zr-Ti metallic glass was subjected to high pressure torsion applying different revolution times (180s, 120s, 60s). Both deformation and deformation rate dependent microstructural and thermal properties were characterized by scanning electron microscopy, X-ray diffraction and calorimetry, respectively. In order to estimate the temperature rise in the metallic glass during high pressure torsion, quasi three-dimensional heat conduction equation with a source term was considered. Solutions indicate that the saturation temperature strongly depends on the revolution time, i. e. on the deformation rate.

Nanocrystallization in Al85Ce8Ni5Co2 amorphous alloy induced by heat treatment and severe plastic deformation

Al85Ce8Ni5Co2 metallic glass ribbon was devitrified by heat treatments just below its glass transition temperature and by severe plastic deformation applying ball milling and high pressure torsion. Evolution of the microstructure, thermal stability and further thermal history were compared for the different devitrification processes. Based on this mapping of phase selection processes, microstructural changes can be predicted for high temperature and/or high stress compaction which are often required for application of this light weight alloy.