Gergely Farkas

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.

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.

Hardening and Softening Processes in an AJ51 Magnesium Alloy Reinforced with Short Saffil Fibres

AJ51 alloy (5Al-0.6Sr balance Mg in wt.%) reinforced with short δ-Al2O3 fibres (Saffil®) has been deformed in compression at temperatures between 23 and 300 °C. The yield stress and the maximum stress decrease with increasing testing temperature. The influence of reinforcement on both characteristic stresses falls down with increasing temperature. Various strengthening terms were taken into account. Individual strengthening terms were calculated and compared with experimental results. Acoustic emission signal recorded during plastic deformation of the matrix alloy and composite was used to elucidate details of the plastic deformation in the matrix alloy and composite. Residual thermal stresses, generated due to a great difference between the thermal expansion coefficients of the alloy and ceramic reinforcement, in the composite matrix at different strain levels were estimated using neutron diffraction method.