Prediction of internal stresses during growth of first- and second-generation twins in Mg and Mg alloys

The present work is dedicated to the development of a mean-field continuum mechanics method capable of predicting internal stresses in parent and twin phases during first- and second-generation twinning. For that purpose, a generalized Tanaka-Mori scheme in heterogeneous elastic media with plastic incompatibilities is developed. The work is applied to the case of first- and second-generation twinning in hexagonal close packed magnesium. In the case of first-generation twinning, the model is capable of predicting the trends in the development of back-stresses within the twin domain. A parametric study is performed to explain the roles of grain and twin shape and of relative volume twin volume fractions on the magnitude and directions of the back-stresses. In addition, applying the methodology to the case of second-generation twinning allows identification, in exact agreement with experimental observations, of the most likely second-generation twin variants to grow in a primary twin domain.     

Pseudoelastic behaviour of cast magnesium AZ91 alloy under cyclic loading–unloading

Large stress–strain hysteresis loops are observed under cyclic loading after a small plastic prestrain. Loops have been observed in sand-cast material in a variety of tempers under tension or compression, and in high-pressure die-cast material with different cross-section thickness tested in tension. The loops are first observed after a nucleation strain of between 0.001 and 0.01% and grow to a maximum width after 1–2% plastic strain, becoming slightly narrower afterwards. When fully developed, the loops add a large (0.3–0.45%) pseudoelastic strain to the elastic strain, effectively decreasing the elastic modulus of the alloy by up to 70%. In sand-cast material of a given temper, the effects tend to be more pronounced in compression than in tension. Further, the effect is slightly larger in die-cast or aged sand-cast as compared to as-cast sand-cast material. The phenomenon is discussed in terms of the partial reversal of twins upon unloading.     

Microstructural Evolution and Biodegradation Response of Mg-2Zn-0.SNd Alloy During Tensile and Compressive Deformation

The effect of deformation behavior on the in vitro corrosion rate of Mg-2Zn-0.5Nd alloy was investigated experimentally after uniaxial tensile and compressive stress.The microstructure and texture were characterized using electron backscattered diffraction and X-ray diffraction,while potentiodynamic polarization and immersion tests were used to investigate the cor-rosion response after deformation.The result reveals that applied compressive stress has more dominant effect on the corro-sion rate of Mg-2Zn-0.5Nd alloy as compared to tensile stress.Both tensile and compressive strains introduce dislocation slip and deformation twins in the alloy,thereby accelerating the corrosion rate due to the increased stress corrosion related to dislocation slips and deformation twins.The { 10(1)2} tension twinning and prismatic slip were the major contributors to tensile deformation while basal slip,and { 10(1)2} tension twin were obtainable during compressive deformation.The twinning activity after deformation increases with the plastic strain and this correlates with the degradation rate.     

Microstructural Evolution and Biodegradation Response of Mg–2Zn–0.5Nd Alloy During Tensile and Compressive Deformation

The effect of deformation behavior on the in vitro corrosion rate of Mg–2Zn–0.5 Nd alloy was investigated experimentally after uniaxial tensile and compressive stress. The microstructure and texture were characterized using electron backscattered diffraction and X-ray diffraction, while potentiodynamic polarization and immersion tests were used to investigate the corrosion response after deformation. The result reveals that applied compressive stress has more dominant effect on the corrosion rate of Mg–2 Zn–0.5 Nd alloy as compared to tensile stress. Both tensile and compressive strains introduce dislocation slip and deformation twins in the alloy, thereby accelerating the corrosion rate due to the increased stress corrosion related to dislocation slips and deformation twins. The {10ī2} tension twinning and prismatic slip were the major contributors to tensile deformation while basal slip, and {10ī2} tension twin were obtainable during compressive deformation. The twinning activity after deformation increases with the plastic strain and this correlates with the degradation rate.     

Deformation processes in ZE41A cast magnesium alloy investigated by the acoustic emission technique

The acoustic emission (AE) technique is a very useful tool to detect the microplastic yielding occurring during macroscopic deformation. Cast ZE14A magnesium alloy was deformed in tension at temperatures between 20 and 350 °C and at a constant strain rate of 0.05 s⁻¹. Measurements of the AE during testing are presented and related to the microstructure of the sample material. AE count rates increase with increasing temperature from room temperature to a maximum at 330 °C. Above 330 °C temperatures count rate decrease. This behaviour is discussed with a view to the role of heat treatment, twinning and deformation mechanism.     

Twinning behavior of a strongly basal textured AZ31 Mg alloy during warm rolling

The twinning behavior of a strongly basal textured magnesium alloy AZ31 plate during warm rolling to a reduction of 9% has been investigated. Analysis of the twin types and twin variants was carried out based on orientation data collected using electron backscatter diffraction. Double twins (corresponding to contraction twinning followed by extension twinning) were the most commonly observed twin type, with more than 70% of the grains examined containing this type of twin. The double twin variant with a 38° misorientation to the original grain orientation was the most frequently observed, with the 30° variant being the next most frequently observed. The twinning variant selection was analyzed using both a modified Schmid factor for rolling and by use of normal strain maps, used to show the compatibility between the normal strains due to twinning and the applied rolling deformation. It was found that the 38° and 30° misorientation double twin variants provide normal strains that are similarly compatible with the applied deformation, with a slight advantage for the 30° variant. Overall, the results show that for rolling deformation external strain accommodation, rather than self-strain accommodation, is the dominant factor in determining the variant types of the double twins formed during warm rolling.     

Yield point elongation due to twinning in a magnesium alloy

Fine-grained magnesium alloy AZ31 displays a yield elongation when deformed such that yielding occurs by twinning. That is, following yielding there is a plateau in the stress-strain curve. The present paper presents a microstructural analysis of the twins in deformed samples. A major aim is to explain the yield elongation and in particular why it decreases and eventually disappears with increasing grain size. It is shown that during the Lüders yield elongation twins initiate twinning events in neighbouring grains and in this manner twinning spreads its way progressively over the sample. This occurs at a twinning frequency of approximately one twin per grain. A criterion for the presence of a Lüders strain is developed based on twin transfer across boundaries. It is shown that higher Lüders strains and stresses are expected for finer grain sizes. The key to understanding the effect is that it arises from the condition for Lüders band propagation whereby the twins on the Lüders band front must stimulate, on average, one twin each within the fresh material ahead of the front, at a constant value of applied stress. An important part of the derivation followed here is that at the higher stresses seen in fine-grained samples, the twin aspect ratio is larger and consequently the strain at the grain level corresponding to a single twinning event is higher in finer-grained samples.     

Twinning, dynamic recovery and recrystallization in the hot rolling process of twin-roll cast AZ31B alloy

Hot-rolling experiments with a reduction from 10% to 60% in single pass were conducted on AZ31B twin-roll cast sheets. Optical microscope (OM), electron backscattered diffraction (EBSD) tests and transmission electron microscope (TEM) were used to investigate twinning and DRV/DRX behaviors at different stage in the hot-rolling process. Two types of twinning occurred in the initial stage of hot-rolling process. DRV and discontinuous recrystallization dominated at moderate strain while continuous DRX took place homogeneously throughout original grains at the largest strains.     

Atomistic simulation of the structural evolution in magnesium single crystal under c-axis tension

Molecular dynamics simulation is applied to investigate the microstructure evolution of magnesium single crystals under c-axis extension at different temperatures. At low temperatures, both {10 ˉ 12} and {10 ˉ 11} twins are observed. At elevated temperatures, {10 ˉ 11} twining decreases quickly with increasing temperature, while the amount of {10 ˉ 12} twins increases. The {10 ˉ 12} twin is found to be the main deformation mechanism under the c-axis tension in the magnesium single crystal. Meanwhile, shear ...     

镁合金手机壳的温热液压成形实验及模拟研究

通过对镁合金手机壳进行温热液压成形实验及有限元模拟,分析了液压加载与冲头运动的不同匹配关系对成形结果的影响,以及成形过程中缺陷产生的原因,并确定了合适的成形工艺参数,即在170℃,控制冲头速度在0.2mm/s~1mm/s,液压载荷不低于5MPa,能一次完成成形底部带斜面,并具有较小圆角半径的镁合金手机壳。液压载荷应在坯料的拉深高度小于3mm时快速的施加,否则,在成形件的底部一平面和斜面与棱边交界处将形成难以消除的堆积缺陷。模拟结果与实验吻合良好。研究表明,温热液压成形可以实现常温或低温液压成形难以实现的复杂镁合金零件成形。     

Improving the sheared edge in the blanking of commercial AZ31 sheet through texture modification

Commercial rolled magnesium sheets of alloys AZ31 (Mg–3 mass%Al–1 mass%Zn) and ZE10 (Mg–1 mass% Zn–<1 mass% Rare Earths) in O-temper condition were used for blanking experiments near room temperature. A serrated fracture surface can be observed in case of AZ31 but not in case of ZE10. During the shearing process of the AZ31 sheet, many micro cracks parallel to the sheet plane are generated in the shearing zone. These micro cracks lead to the formation of loose particles during the shearing operation, which interfere with further processing of the part and incur additional costs by increasing the scrap rate.It is found that the strong basal texture of this alloy is an important reason for the generation of such serrated cracks. In this paper a new method of selective texture modification is described to locally change the mechanical properties of the AZ31 sheet. Subsequent shearing experiments show a significant change in the material behavior, especially regarding the direction of crack propagation, which leads to a better shearing performance. The commonly observed serrated crack does not occur any more after this local treatment and the sheared edge is clearly improved.