Klaudia Horváth

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.

Thermo-Mechanical Treatment of Extruded Mg–1Zn Alloy: Cluster Analysis of AE Signals

The proper thermo-mechanical treatment can improve mechanical properties of extruded Mg alloys through a solute segregation and precipitation along twin boundaries. The effect of heat treatment on mobility of twin boundaries with respect to applied loading direction was studied in extruded Mg–1Zn alloy using the acoustic emission (AE) technique. The adaptive sequential k-means clustering (ASKC) was applied to analyze the AE data in order to determine the dominant deformation mechanism in a given time period. The AE energy, median frequency and the number of elements in individual AE clusters are the main parameters of presented clustering analysis. Active deformation mechanisms are discussed with respect to mutual orientation of grains and loading direction.

Mechanical Properties of Thermo-Mechanically Treated Extruded Mg–Zn-Based Alloys

Mechanical properties of extruded Mg alloys are significantly influenced by the activation of extension twins during compression along the extrusion direction (ED) because of a strong texture with basal planes oriented parallel to ED. At the same time, the heat treatment is also supposed to tune mechanical properties via strengthening or softening mechanism. The influence of heat treatment on the mechanical behavior of Mg–Zn-based alloys with an addition of Ca and Nd in as-extruded state and after pre-compression (i.e. partly twinned microstructure) is discussed in the paper. Difference in distribution of precipitates for two materials after applying heat treatment at 200 °C for 16 h was observed. Isothermal ageing of pre-strained samples leads to strengthening in ZN10 alloy and softening in ZX10 alloy.

Acoustic Emission Study of High Temperature Deformation of Mg–Zn–Y Alloys with LPSO Phase

Magnesium alloys with different content of zinc (Zn) and yttrium (Y) were extruded at an extrusion ratio of 18:1 at 350 °C. The alloying elements in both Mg alloys formed a long period stacking ordered (LPSO) phase, which during the extrusion process was elongated along the extrusion direction (ED). The magnesium matrix has bimodal character composed by fine dynamically recrystallized (DRX-ed) grains and initial coarse grains elongated along ED. Compression tests with concurrent acoustic emission (AE) measurements were performed along ED at 200, 300, and 400 °C. The deformation mechanisms and the mechanical properties at 200 °C are very similar to those obtained at ambient temperatures, i.e. in the alloy with low volume fraction of the LPSO phase (<10%) twinning controls the yielding, while in the alloy with high volume fraction of the LPSO phase (around 35%) dislocation slip and kink formation are dominant. At 300 °C the reinforcing effect of the LPSO phase is reduced and at 400 °C it is not effective anymore.

In Situ Investigation of Deformation Mechanisms in Mg–Zn–Y Magnesium Alloy with LPSO Phase by Diffraction Methods and Acoustic Emission

The influence of the LPSO (long-period stacking ordered) phase orientation and the microstructure of the magnesium matrix on the deformation mechanisms of Mg–Zn–Y magnesium alloy has been investigated by diffraction methods and acoustic emission (AE) measurements. The adaptive sequential k-means analysis (ASK) method, offering identification of the dominant deformation process (basal, non-basal slip, twinning, kinking) in a given time period, has been used for AE data evaluation. The results indicate that the kinking mechanism, twinning and activation of non-basal slip exhibit a significant dependence on the initial texture and the orientation of the LPSO phase with respect to the loading axis.

Characterization of the Acoustic Emission Response and Mechanical Properties of Mg Alloy with LPSO Phase

Mechanical properties of extruded WZ72 magnesium alloy with long-period stacking ordered (LPSO) phase were investigated during compression loading at room temperature and at a constant strain rate of 10-3 s-1. The samples of (8 x 4 x 4) mm3 were compressed along three directions with respect to the lamellar LPSO-phase: parallel (ED), perpendicular (TD) and under 45°. Concurrently with the deformation tests, the acoustic emission (AE) response of the specimens was recorded. The AE measurements revealed that both the twinning activity and the kinking of the LPSO phase significantly depend on the orientation of LPSO phase. The highest strength was observed for the sample which was compressed parallel to the LPSO phase (extrusion direction). The highest AE activity was also measured in this sample.