Sangbong Yi

Texture Weakening Effects in Ce-Containing Mg Alloys

The purpose of the present study is to elucidate the cause of a texture transition observed in hot-rolled Mg-Ce alloys, with increasing Ce content. More dilute compositions show a predominance of basal 〈a〉 dislocations, while the more concentrated alloys are shown to contain significant populations of 〈c+a〉 and non-basal 〈a〉 dislocations in the as-rolled and the recovery-annealed conditions. Like Mg-Nd, the grain size in Mg-Ce decreases with increasing Ce. Unlike Mg-Nd, the second-phase particles observed are of the equilibrium phase, and the equilibrium solid solubility at the hot working temperature corresponds to the composition above which weak textures form. The significances of both non-basal slip and Zener pinning as contributors to texture randomization are discussed.

In situ analysis of the tensile deformation mechanisms in extruded Mg-1Mn-1Nd (wt%)

An extruded Mg–1Mn–1Nd (wt%) (MN11) alloy was tested in tension in an SEM at temperatures of 323 K (50°C), 423 K (150°C), and 523 K (250°C) to analyse the local deformation mechanisms through in situ observations. Electron backscatter diffraction was performed before and after the deformation. It was found that the tensile strength decreased with increasing temperature, and the relative activity of different twinning and slip systems was quantified. At 323 K (50°C), extension twinning, basal, prismatic ⟨a⟩, and pyramidal ⟨c + a⟩ slip were active. Much less extension twinning was observed at 423 K (150°C), while basal slip and prismatic ⟨a⟩ slip were dominant and presented similar activities. At 523 K (250°C), twinning was not observed, and basal slip controlled the deformation.

Twining and Slip Activity in Magnesium Single Crystal

Uniaxial μ-compression tests have been performed on single crystal Mg with a compression direction, an orientation unfavorable for basal slip. Results show that the early stages of deformation proceed via both twinning and dislocation plasticity. Twinning leads to a reorientation of the crystal favorable for basal slip, typically with the aligned with the compression direction. At a critical strain a large strain burst occurs, and is associated with both rapid propagation of the twin and the activation of basal slip within the twin. Such a mechanistic picture of the deformation behavior is revealed through SEM, EBSD and TEM characterization of the deformation structures.

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.

The Microstructure and Texture Development During Twin Roll Casting and Rolling of Magnesium Alloy AZ31

Twin-roll casting of magnesium alloys is seen as a promising processing route to enable a further development of sheet properties. In this regard the influence of melt temperature and rolling speed on the microstructure and texture development of magnesium strips is investigated for alloy AZ31. Microstructure and through-thickness texture measurements are used to reveal effects of grain growth during solidification and deformation of the strip. For this purpose synchrotron radiation at the PETRA III storage ring of DESY in Germany has been used. Twin-roll casting with varied processing parameters leads to microstructures ranging from directionally solidified grains to deformed microstructures with a profound fraction of recrystallized grains. Specifically, sheets rolled from different strips exhibit an alignment of their Mg basal planes in the sheet plane with certain significance depending on grain size and grain orientation effects.

Texture Transition in Steel ST37K, in situ Measurement at High Temperatures Using High-Energy X-rays

High energy X-rays are well known due to there high penetration power particular in materials testing devices. For diffraction experiments high energy X-rays with more than 50 keV can be obtained at storage rings or using a tungsten X-ray tube. According to the high penetration power, these beamlines offer a very high photon flux and an excellent brilliance. That means measurements can be carried out fast. As an example, the complete texture measurement at one position of a steel shaft with 34 mm in diameter has taken 45 minutes nondestructively. On the other hand the high photon flux allows to measure foils or thin wires down to 50–100 μm. These new and fast options make it possible to measure in situ textures under tension, compression and at high temperatures. We have used 100 keV X-ray to measure the texture transition as well as the phase transition in a steel sample. The experiments were done at the high energy beamline BW5 (Hasylab at Desy/Hamburg). 100 keV X-rays have a wavelength of 0.1240 A which means due to the Bragg’s law very low scattering angles. Using a MAR345 image plate detector one obtains a set of complete Debye-Scherrer cones in a 2θ-range of 7° in about 1 sec. At room temperature we found 100% ferrite. During heating up till the austenite region we were able to investigate the thermal expansion and the texture relation between ferrite and austenite, which follows in our case the Kurdjumov-Sachs model. Furthermore, the program package MAUD offers the possibility to follow the phase transition, so that the composition at all temperatures can be documented during heating. It has to be pointed out that the texture influence on the quantitative phase analysis can be included by MAUD, so that even for strong crystallographic textures the relation ferrite/austenite can be given very well.

The Deformation Behavior, Microstructure and Mechanical Properties of Cast and Extruded Mg-1Mn-xNd (wt%) at Temperatures between 50°C and 250°C

The tensile deformation behavior of as-cast and cast-then-extruded Mg-1Mn-1Nd(wt%) and Mg-1Mn-0.3Nd(wt%) alloys was studied by performing in-situ tests inside a SEM. A slip/twin trace analysis technique was used to identify the distribution of the deformation systems. Cast-then-extruded Mg-1Mn-1Nd(wt%) exhibited superior elevated-temperature strength retention compared to cast-then-extruded Mg-1Mn-0.3Nd(wt%). Basal slip and extension twinning were observed in the as-cast Mg-1Mn-1Nd(wt%) and Mg-1Mn-0.4Nd(wt%) alloys deformed at 50°C. In cast-then-extruded Mg-1Mn-1Nd(wt%), basal slip, prismatic slip, and pyramidal slip were active at all temperatures. In cast-then-extruded Mg-1Mn-0.3Nd(wt%), at lower temperatures, twinning dominated the deformation and no non-basal slip activity was observed. The extent of twinning decreased with increasing temperature and basal slip was the major deformation mode at 150–250°C in both cast-then-extruded materials. The estimated CRSS ratio of extension twinning with respect to basal slip in Mg-1Mn-1Nd(wt%) was close to unity, suggesting that the addition of Nd results in an increase in the CRSS of basal slip.

Deformation and Recrystallization Mechanisms and Their Influence on the Microstructure Development of Rare Earth Containing Magnesium Sheets

Many studies have shown that textures with less distinct alignment of basal planes and the related improvement of formability are found in alloys that contain rare-earth (RE) elements and zinc. However, the effect of the combination of these additional elements on the texture modification has not been yet clearly understood. In this work, sheet samples from Mg–Zn–RE alloys rolled at 400 °C were used for in situ synchrotron X-rays diffraction measurements under tensile loading at different temperatures, in order to track the development of diffraction profiles and textures during deformation. In Mg–Zn–RE alloys, a significantly retardation of recovery and dynamic recrystallization during the high temperature deformation is observed in comparison to the RE-free Mg–Zn alloy. The differences in the active deformation mechanisms as well as the dynamic recrystallization mechanisms are reviewed with respect to the texture alteration. For discussion of the impact of different mechanisms, EBSD observations reveal the microstructure development.

Development of Magnesium Sheets

Innovative semi-finished products require tailoring mechanical properties and improving formability. Due to science-based alloy design as well as optimization of processing parameters (process-property relationship), it is now possible to control microstructures, phase distributions and texture development of magnesium sheets. A combination of new technologies and innovative alloys could help to alleviate the strong textures formed in semi-finished Mg products. The presentation will show Magnesium sheet development via twin roll casting technology and the how the subsequent warm rolling process influences the properties of the final magnesium sheet metal. Further optimisation of process parameters of twin roll casting and of the rolling process in combination with alloy design lead to a microstructure showing promising mechanical properties like high formability und high strength.

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.