Nguyen Q. Chinh

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.

Occurrence of plastic instabilities in dynamic microhardness testing

Plastic instabilities were observed to appear during dynamic ultramicrohardness testing of a solid solution Al–3.3 wt.% Mg alloy. The tests were carried out at room temperature with a Vickers hardness indenter in a computer-controlled dynamic ultramicrohardness testing machine. During the tests the applied load was increased from 0 to 2000 mN at constant loading rate. The instabilities appear as characteristic steps in the continuously recorded load-indentation depth curves. The physical basis for the occurrence of the instabilities is the interaction between moving dislocations and solute atoms, a phenomenon termed in the literature as serrated yielding, jerky flow, or Portevin-Le Châtelier effect. The instabilities start at a critical load, F c , in the depth-load curve. Varying the loading rate, μ, by two orders of magnitude F c was found to increase linearly with the loading rate.

Grain Size Dependence of the Work Hardening Process in Al99.99

The work hardening properties of 99.99% purity aluminum were investigated by tensile tests at RT on samples with average grain sizes between 40 and 300 μm. It is shown that the stress-strain curves can only be described by two different functions valid at moderate and at higher strains, respectively. New constitutive equations due to these two different stages are derived on the basis of a new phenomenological model, and their validity is proved by experimental results. It is shown that the work hardening parameters depend strongly on the grain size in the first stage, while the rate of hardening in the second stage is independent of the grain size. The constitutive equations derived are used to analyze the grain size dependence of the flow stress as a function of the strain.

Indentation test for the investigation of high-temperature plasticity of materials

High-temperature behaviour of materials has been investigated by indentation creep methods. Materials investigated belong to three typical groups of materials: glasses, pure metals and alloys. Indentation tests were performed with flat-ended cylindrical and hemispherical punches. It is shown that the hemispherical punch is also proper for the determination of the high-temperature creep characteristics of the materials investigated. It is also proved that there is a unique correspondence between the results obtained by the two indentation methods.

Indentation tests for the investigation of the plasticity of glasses

Various types of indentation tests are used to investigate the plasticity of glassy materials. It is shown that indentation creep tests performed with both flat ended cylindrical and hemispherical indenters are suitable for viscosity measurements in the viscosity range of 108–1011Pa·s. It is also shown that the activation energy of the flow process can be evaluated from the results of indentation measurements.

Threshold stress in dispersionally strengthened superplastic Al alloys

It is important for practical applications that some commercial alloys with stabilized finegrained structure should exhibit superplastic behaviour at high temperatures. In this paper the results of impression creep tests conducted on AlMgZn alloys are reported and the strain rate sensitivity and activation enthalpy were determined. The mechanical behaviour of the alloys as a function of the strain rate sensitivity can be divided into three regions. At low and high stresses the strain rate sensitivity parameter is low and the deformation process is not superplastic. Superplastic deformation takes place only at intermediate stresses. The microstructural interpretation of these processes involves, in general, the change of the micromechanisms controlling the different deformation processes. It was determined that by the supposition of a threshold stress depending strongly on temperature, the two regions due to low and intermediate stresses of the deformation can be described by the same constitutive equation.

The effect of grain boundary sliding and strain rate sensitivity on the ductility of ultrafine-grained materials

Most ultrafine-grained (UFG) materials produced by severe plastic deformation (SPD) exibit only limited ductility which is correlated with the low strain rate sensitivity (SRS) of these materials. Recently, it was demonstrated that SPD is capable of increasing the room temperature ductility of aluminum-based alloys attaining elongations up to 150%, together with relatively high strain rate sensitivity. In the present work, additional results and discussions are presented on the effect of grain boundary sliding (GBS) and SRS on the ductility of some UFG metals and alloys. The characteristics of constitutive equations describing the steady-state deformation process are quantitatively analyzed for a better understanding of the effects of grain boundaries and strain rate sensitivity.

Effect of Grain Size on Tensile Stress and Ductility in Al99.99

The effect of recrystallized grain size on the tensile stress and ductility of 99.99% purity aluminium was investigated at room temperature. It was proved that the grain size dependence of flow stress follows a modified Hall-Petch equation with coefficients depending linearly on e 1/2 up to the stability limit. The uniform strain can also be described by a linear dependence on d -1/2 according to which the uniform elongation increases with increasing grain size. The post-uniform elongation changes inversely to that of the uniform one accompanied by the decrease of the strain rate sensitivity.

Superplasticity of AlMgSi alloys

Commercial AlMgSi alloy sheets produced by thermomechanical treatment are found to be superplastic between 500 and 570°C at strain rates of 10−5–10−3−1 The strain rate sensitivity,m, is about 0.4. It was found that the highly alloyed sample contains pre-existing cavities in higher volume fraction than the alloy of lower concentration. An exponential growth of cavity volume fraction was found during superplastic deformation which is characteristic of plasticity controlled cavitation. The growth rate of the cavity volume fraction can be decreased by applying back pressure.

Stability of Ultrafine-Grained Microstructure in Fcc Metals Processed by Severe Plastic Deformation

The thermal stability of ultrafine-grained (UFG) microstructure in face centered cubic metals processed by severe plastic deformation (SPD) was studied. The influence of the SPD procedure on the stability was investigated for Cu samples processed by Equal-Channel Angular Pressing (ECAP), High-Pressure Torsion (HPT), Multi-Directional Forging and Twist Extrusion at room temperature (RT). It is found that HPT results in the lowest thermal stability due to the very high dislocation density. Furthermore, the effect of the low stacking fault energy of Ag on the stability is also investigated. It is revealed that the UFG microstructure produced in Ag by ECAP is recovered and recrystallized during storage at room temperature. The driving force for this unusual recovery and recrystallization is the high dislocation density developed during ECAP due to the high degree of dislocation dissociation caused by the very low stacking fault energy of Ag.