Kristián Máthis

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.

In situ investigation of deformation mechanisms in magnesium-based metal matrix composites

We studied the effect of short fibers on the mechanical properties of a magnesium alloy. In particular, deformation mechanisms in a Mg-Al-Sr alloy reinforced with short alumina fibers were studied in situ using neutron diffraction and acoustic emission methods. The fibers’ plane orientation with respect to the loading axis was found to be a key parameter, which influences the acting deformation processes, such as twinning or dislocation slip. Furthermore, the twinning activity was much more significant in samples with parallel fiber plane orientation, which was confirmed by both acoustic emission and electron backscattering diffraction results. Neutron diffraction was also used to assist in analyzing the acoustic emission and electron backscattering diffraction results. The simultaneous application of the two in situ methods, neutron diffraction and acoustic emission, was found to be beneficial for obtaining complementary datasets about the twinning and dislocation slip in the magnesium alloys and composites used in this study.

The Effect of Matrix Composition on the Deformation and Failure Mechanisms in Metal Matrix Syntactic Foams during Compression

The influence of the matrix material on the deformation and failure mechanisms in metal matrix syntactic foams was investigated in this study. Samples with commercially pure Al (Al) and Al-12 wt % Si (AlSi12) eutectic aluminum matrix, reinforced by hollow ceramic spheres, were compressed at room temperature. Concurrently, the acoustic emission response and the strain field development on the surface were monitored in-situ. The results indicate that the plastic deformation of the cell walls is the governing mechanism in the early stage of straining for both types of foams. At large stresses, deformation bands form both in the Al and AlSi12 foam. In Al foam, cell walls collapse in a large volume. In contrast, the AlSi12 foam is more brittle; therefore, the fracture of precipitates and the crushing of the matrix take place within a distinctive deformation band, along with an occurrence of a significant stress drop. The onset stress of ceramic sphere failure was shown to be not influenced by the matrix material. The in-situ methods provided complementary data which further support these results.

Exploring Plastic Deformation of Metallic Materials by the Acoustic Emission Technique

In determination of direct correlation between material properties and parameters of testing environment, the non-destructive in-situ methods play an important role. Nondestructive experimental techniques facilitate correlation of material parameters to those of the testing environment in real time. Among the tools emerging in the last decades, the acoustic emission (AE) technique belongs to the most powerful and reliable ones. The AE method allows to determine, which external parameters are critical for behaviour of the investigated material. Thus the results of AE measurements could give an integral hint for further post mortem investigations (e.g. scanning or transmission electron microscopy), in which the material properties are evaluated just after the exposure of the specimen to the external environment.

Microstructural evolution of equal-channel angular pressed interstitial-free steel

Abstract Equal-channel angular pressing (ECAP) belongs to advanced technologies for improving mechanical properties of materials. In the present work, the influence of the number of ECAP passes using route BC on both grain size and mechanical properties of interstitial-free steel was investigated by means of transmission electron microscopy, electron back-scattering diffraction, and microhardness testing. It was found that the grain size decreases with the increasing number of passes. At the same time, an increase in microhardness was observed. The evolution of microstructure with increasing strain imposed through ECAP, in particular the process of grain fragmentation and the formation of high-angle boundaries were also examined.

Plastic Properties of a Mg-Al-Ca Alloy Reinforced with Short Saffil Fibers

AX41 magnesium alloy was reinforced with short Saffil fibers using squeeze cast technology. Samples of the composite were deformed in compression at elevated temperatures. The work hardening rate as a function of the flow stress in the matrix was investigated. A model taking into account two hardening and two softening processes was used for analyzing of experimental curves. Parameters of the model follow different temperature dependences. Possible hardening and softening processes are discussed.

Neutron Diffraction and Acoustic Emission Study of Mg-Al-Sr Alloy Reinforced with Short Saffil® Fibers Deformed in Compression

Neutron diffraction method has been applied in the ex-situ investigation of the residual stresses in Mg-5wt.%Al-1 wt.%Sr (AJ51) magnesium alloy reinforced with short Saffil® fibers deformed in compression at room temperature. The residual stresses were measured in the axial and radial directions with respect to the load direction. It is shown that in the initial state the tensile stress is present in the matrix phase. The in-situ acoustic emission measurements were performed with the aim to reveal the main deformation mechanisms operating in the particular stages of the plastic deformation. Ex-situ neutron diffractions experiments showed that the tensile axial residual stress in the matrix increases with increasing plastic deformation while the radial residual stress decreases. In situ acoustic emission measurements indicate that the main deformation mechanisms are twinning and glide of bigger dislocation ensembles in the early stages of the compressive deformation while the fibers breakage was observed in the vicinity of the maximum stress.

Investigation of the Microstructure Evolution and Deformation Mechanisms of a Mg-Zn-Zr-RE Twin-Roll-Cast Magnesium Sheet by In-Situ Experimental Techniques

Twin roll casting (TRC), with a relatively fast solidification rate, is an excellent production method with promising potential for producing wrought semi or final Mg alloy products that can often suffer from poor formability. We investigate in this study the effect of the TRC method and the subsequent heat treatment on the microstructure and deformation mechanisms in Mg-Zn-Zr-Nd alloy deformed at room temperature using the in-situ neutron diffraction and acoustic emission techniques and ex-situ texture measurement and microscopy, respectively. Although a higher work hardening is observed in the rolling direction due to the more intensive -type dislocation activity, the difference in the mechanical properties of the specimens deformed in the RD and TD directions is small in the as-rolled condition. An additional heat treatment results in recrystallization and significant anisotropy in the deformation. Due to the easier activation of the extension twinning in the TD given by texture, the yield stress in the TD is approximately 40% lower than that in the RD.

Effect of Extrusion Ratio on Microstructure and Resulting Mechanical Properties of Mg Alloys with LPSO Phase

The WZ21 (Mg + 1.8 wt% Y + 0.7 wt% Zn) magnesium alloy having an addition of 0.5 wt% of CaO was extruded with different extrusion ratios (4:1, 10:1, 18:1) at 350 °C. In all alloys, a long-period stacking-ordered (LPSO) phase composed of Zn and Y is formed. The microstructure was analyzed by electron backscatter diffraction (EBSD) mapping. The WZ21 alloy after extrusion with the extrusion ratio of 4:1 contains large grains. The fraction of recrystallized grains increases with increasing extrusion ratio. All samples have basal planes oriented parallel to the extrusion direction (ED) and this texture weaken with increasing extrusion ratio. Mechanical properties of the samples were investigated during compression along ED at room temperature and at a constant strain rate of 10−3 s−1. Concurrently, with the deformation tests, the acoustic emission (AE) response of the specimens was recorded. The maximum of the AE count rate in all cases corresponds to the macroscopic yield point.

Investigation of Twinning Activity in Magnesium Using Advanced In Situ Methods

The high-resolution neutron diffraction and acoustic emission (AE) techniques have been used for in-situ investigation of deformation twinning and microstructure evolution in cast polycrystalline magnesium. The combination of these two techniques results in obtaining complementary information about the twinning mechanism and evolution of the dislocation structure during the straining. The dependence of the mechanisms of the plastic deformation on loading mode is discussed in detail. The microscopy investigations revealed a difference in twin number and size after tension and compression, respectively.