Daria Drozdenko

UH 3 -based ferromagnets: New look at an old material

UH3 is the first discovered material with ferromagnetism based purely on the 5f electronic states, known for more than half century. Although the U metal is Pauli paramagnet, the reduced 5f-5f overlap in compounds allows for moment formation and ordering, typically if the U-U spacing exceeds the Hill limit, i.e. about 340 pm. The stable form of UH3, known as β-UH3, has rather high TC ≈ 170 K. Such high value is rather unusual, considering dU-U = 331 pm. Properties of metastable α-UH3 with dU-U = 360 pm could be never well established. Using the fact that α-UH3 is in fact bcc U with interstitials filled by H, we attempted to synthesize α-UH3 starting from the γ-U alloys, with the bcc structure retained to room temperature by doping combined with ultrafast cooling. While up to 15% Zr a contamination by β-UH3 was obtained, 20% Zr yielded single phase α-UH3. The TC value remains high and very similar to β-UH3. One can see an increase up to 187 K for 15% Zr, followed by a weak decrease. Magnetic moments remain close to 1 μB/U atom. An insight is provided by ab-initio calculations, revealing a a charge transfer towards H-1s states, depopulating the U-6d and 7s states, leaving almost pure 5f character around the Fermi level. The 5f magnetism exhibits a high coercivity (μ0Hc up to 5.5 T) and large spontaneous volume magnetostriction of 3.2*10-3. Even higher increase of TC, reaching up to 203 K, can be achieved in analogous Mo stabilized hydrides, which yield an amorphous structure. The compounds represent, together with known hydrides of U6Fe and U6Co, a new group of robust 5f ferromagnets with small dU-U but high TC. Although common hydrides are fine powders, some of the new hydrides described as (UH3)(1-x)T x (T = Zr or Mo) remain monolithic, which allows to study transport and thermodynamic properties.

Effect of Thermo-mechanical Treatment of Extruded Z1 Mg Alloy on Resulting Mechanical Properties

The formability and mechanical properties of Mg alloys are strongly influenced by a formation and growth of twins. The contribution of twinning to plastic deformation can be modified by initial texture, introducing solute segregation and precipitation at the twin boundaries. The interaction of solute atoms and precipitates with grain and twin boundaries during thermo-mechanical treatment and their effect on mechanical properties will be discussed in term of acoustic emission (AE). An AE signal, recorded during deformation tests, can provide information about active deformation mechanisms during plastic deformation with respect to the microstructure and texture as well as to solute segregation and precipitates along dislocations, grain and twin boundaries. The microstructure development of the extruded Z1 Mg alloy prior and after pre-treatment as well as after subsequent loading will be investigated by scanning electron microscopy (SEM) including electron backscattered diffraction (EBSD) technique.

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.

In Situ Neutron Diffraction and Acoustic Emission During the Biaxial Loading of AZ31 Alloy

The evolution of twinning in randomly textured magnesium alloy and rolled AZ31 alloy during biaxial mechanical tests has been monitored using concurrent application of acoustic emission and neutron diffraction methods. The influence of the loading path on twinning is discussed in detail. It is shown that the twinning is strongly sensitive to the load path.

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.

An Acoustic Emission Study of Deformation Behavior of Wrought Mg Alloys

The influence of the pre-compression level on subsequent tensile deformation behavior has been investigated for two extruded Mg alloys with a different grain size distribution. The Mg–Zn–R are earth alloy has homogeneous microstructure, while the Mg–Al–Zn alloy exhibits bimodal microstructure. Deformation tests were performed at room temperature and at a constant strain rate of 10−3 s−1. Three pre-compression stress levels were chosen to receive microstructure containing a low number of twins, partially and fully twinned grains, respectively. The concurrent acoustic emission (AE) measurement provides real time information about collective dislocation motion and twin nucleation. Active deformation mechanisms during tensile loading are discussed in term of the AE response.

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.