Z. Trojanová

Hardening and softening in deformed magnesium alloys

Abstract The deformation behaviour of three commercial magnesium alloys AZ91, AS21 and AE42 has been investigated in a wide temperature range. Specimens were deformed in tension and in compression in the temperature range of 300–573 K at constant but various strain rates. The form of the stress–strain curves is very sensitive to the test temperature and the strain rate. The deformation behaviour of the specimens can be attributed to the occurrence of hardening and softening during straining. In order to identify hardening and softening processes, the stress dependence of the strain-hardening coefficient was evaluated. Different models describing hardening and softening were used to analyse the observed behaviour. The model proposed by Lukac and Balik can describe the experimental data in the temperature range of 373–473 K. The analysis shows that conservative slip of dislocations is the main recovery process. The yield stress analysis reveals an asymmetry of values obtained in tensile and compression tests.

Stability of micro structure in magnesium reinforced by nanoscaled alumina particles

Abstract Changes in the microstructure of Mg with a grain size about 1 μm and Mg reinforced by 1 vol.% Al 2 O 3 nanoscaled particles due to thermal treatment are investigated by internal friction measurements. The internal damping was measured on bending beams at frequencies from 220 to 230 Hz. It is shown that the value of the logarithmic decrement is influenced by the specimen preparation as well as the by thermal treatment of specimens. Results are discussed on the basis of existing models. It is shown that the internal friction measurements can be used for non-destructive investigations of changes in the microstructure.

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.

Discontinuouslow temperature deformation of ZrSn alloys

Abstract Mechanical and microstructure properties have been investigated for ZrSn alloys with 0.74, 2.85, 4.27 and 6.19 wt% Sn deformed at 4.2, 77 and 300 K. The tin content plays an important role in the deformation behaviour of the alloys. Serrated yielding has been observed during deformation of ZrSn alloy samples with higher tin content if they are deformed at 4.2 K. Twinning has been found to be the microstructure characteristic for ZnSn alloy deformed at 4.2 K. A correlation between twins and stress drops has been discussed. It follows from that that the origin of the stress drops at low temperatures may be of a mechanical nature.

Non-destructive characterisation of microstructure evolution in Mg based metal matrix composites submitted to thermal cycling

Acoustic emission (AE), damping and dilatometry techniques have been developed to study microstructural changes due to thermal stresses in commercially pure Mg-alumina metal matrix composites submitted to thermal cycling. AE shows that the thermal stresses are accommodated by dislocation generation and motion in certain temperature ranges whose fashion is dependent on the upper temperature of cycling and the amount of reinforcement. An increment in dislocation density connected with a redistribution of solute atoms on dislocation lines is revealed after the thermal cycling by damping measurements. The thermal stresses are also characterised by the residual deformation measurements carried out by the dilatometry. For a constant upper temperature of cycling the microstructure does not develop distinctly with increasing number of cycles.

Unstable low temperature deformation in a Cu-2 Be alloy

Tensile tests in liquid helium are reported for various strain rates. As an example for an estimate of the local heat rise in the sample (governing the instability) from mechanical measurements, the 0.2% flow stress is analyzed in detail. An explanation is given for the occurence of characteristic patterns (instead of serrations) at high strain rates.

MECHANICAL PROPERTIES OF Mg AND Mg BASE ALLOY COMPOSITES

It is now well established that the strength and stiffness of a material may be improved significantly by introducing either a continuous or a discontinuous reinforcement into the matrix. Strengthening mechanisms of MMCs consist of physical and mechanical mechanisms. The physical strengthening mechanisms are contributed by the difference of thermal expansion coefficients (CTE) of constituents and interfacial physical effects, reflected by dense dislocations, precipitates and finer grain size. Mechanical mechanisms are composed by local stress division, stress coordination, strain constraint in the matrix and the effect of interphase residual stresses.

Thermal Cycling Effects in Magnesium Composites

This paper describes the influence of thermal cycling on changes in the microstructure of Mg composites determined by non-destructive methods. Thermal cycling was performed between room and an upper temperature up to 450°C. The dimensional changes of composite specimens and the acoustic emission activity measured during thermal cycling indicate the creation and motion of dislocations. The strain dependence of the logarithmic decrement measured after thermal cycling can be explained assuming that the microstructure of composites changes due to the creation of new dislocations during thermal cycling.

Elastic constants of the alloys Cd1−xZn x (x<0.3%) and their temperature dependence

The temperature dependence of elastic constants of Cd1−xZnx alloys, havingx=0.021, 0·042 and 0·233 at.%, has been measured in the temperature range from 4·2 to 300 K by the pulse-echo-overlap ultrasonic method. The adiabatic compressibilities and the Debye temperatures have been calculated. All elastic constants with the exception ofc44 increase slightly with the growing concentration of Zn.