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.     

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.     

Predicting twinning stress in fcc metals: Linking twin-energy pathways to twin nucleation

Deformation twinning is observed in numerous engineering and naturally occurring materials. However, a fundamental law for critical twinning stress has not yet emerged. We resolve this long-standing issue by integrating twin-energy pathways obtained via ab initio density functional theory with heterogeneous, dislocation-based twin nucleation models. Through a hierarchical theory, we establish an analytical expression that quantitatively predicts the critical twinning stress in face-centered cubic metals without any empiricism at any length scale. Our theory predicts a monotonic relation between the unstable twin stacking fault energy and twin nucleation stress revealing the physics of twinning.     

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.     

Temperature Effects on the Microstructures of Mg-Gd-Y Alloy Processed by Multi-direction Impact Forging

A high strain rate multi-directional impact forging(MDIF) was applied to a solutionized Mg-Gd-Y-Zr alloy in the temperature range of 350-500℃.Results demonstrate that the dominant deformation mode is twinning at a temperature below 400℃,whereas at a medium temperature of 450℃ considerable continuous dynamic recrystallization was promoted by{10-12} extension twins.At a higher temperature of 500℃,twinning activation was suppressed.New DRX grains were observed but their sizes were much bigger than those resulting from the MDIFed 50 passes at 450℃,which are ascribed to the larger grain boundary mobility and atomic diffusion at 500℃.Moreover,a non-basal weak texture was gained afterward MDIF at each temperature,which is credited to the MDIF process and the minor strain applied in each pass.     

Phase-field analysis of fracture-induced twinning in single crystals

Deformation twinning at the tip of a straight crack or notch is analyzed using a phase-field method that seeks equilibrium twin morphologies via direct minimization of a free energy functional. For isotropic solids, the tendency to twin under mode I or mode II loading is found to depend weakly on Poisson’s ratio and elastic nonlinearity and strongly on surface energy and twinning shear (i.e. eigenstrain). Depending on the coherent twin boundary energy, anisotropy of surface energy is important for mode I loading but less so for mode II. Model predictions for several single crystals are in agreement with experimental observations. Calcite demonstrates a preference for mode I cleavage crack extension over crack tip twinning. Magnesium shows a likelihood for tensile twinning from a pre-existing crack on the basal plane. In sapphire, a preference for rhombohedral twins over basal twins is apparent, with the latter thinner in shape than the former.     

The role of twinning during dynamic recrystallization in alloy 800H

Twinning plays an important role for nucleation and growth of dynamic recrystallization (DRX) grains in alloy 800 H. The locally varying twin frequency is shown to be quantitatively related to lattice rotations during deformation. Evidence for repeated nucleation of DRX was found.     

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.     

Atomistic simulation of tension deformation behavior in magnesium single crystal

The deformation behavior in magnesium single crystal under c-axis tension is investigated in a temperature range between 250 K and 570 K by molecular dynamics simulations. At a low temperature, twinning and shear bands are found to be the main deformation mechanisms. In particular, the {1012} tension twins with the reorientation angle of about 90° are observed in the simulations. The mechanisms of {1012} twinning are illustrated by the simulated motion of atoms. Moreover, grain nucleation and growth are found to be accompanied with the {1012} twinning. At temperatures above 450 K, the twin frequency decreases with increasing temperature. The {1012} extension twin almost disappears at the temperature of 570 K. The non-basal slip plays an important role on the tensile deformation in magnesium single crystal at high temperatures.     

Microstructural stability during cyclic loading of multilayer copper/copper samples with nanoscale twinning

The response to cyclic deformation has been studied for Cu/Cu multilayer material consisting of columns of closely spaced, parallel nanotwins. The fatigue life under stress-controlled cycling is greatly improved over that of coarse-grained Cu. Nanotwinning provides significant strengthening, which is unchanged by fatigue or severe compression. Observations by focused ion beam microscopy showed that the microstructure is quite stable under deformation. Localized deformation from indentation produced shear bands and apparently some loss of nanotwinned area but no decrease in hardness.     

Reorientation and stress relaxation due to twinning: Modeling and experimental characterization for Mg

A study of the mechanical response of Mg AZ31 when deformed under twinning dominated conditions is presented. In addition to the well-known rapid texture variation, neutron diffraction measurements reveal a ‘sense-reversal’ of the internal stress in the twinned grains. The latter is characterized experimentally and an elasto-plastic polycrystal model is extended in order to account for twin domain reorientation and associated stress relaxation. It is concluded that the texture variation due to twinning is sufficient to explain the observed macroscopic stress–strain response. However, the evolution of internal stresses in diffracting subsets of grains is complex and more challenging to explain. It seems to be strongly controlled by the order in which slip and twinning are activated, the stress relaxation associated with twin propagation, and neighbor constraint effects.     

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.     

强基面织构AZ31合金压缩过程中的孪晶对行为

挤压态AZ31合金在室温下沿挤压方向进行压缩变形,合金中产生大量的拉伸孪晶。综合分析了孪晶对的斯密特因子(SF)和应变兼容因子(m_f),其中孪晶对包括相连的孪晶对和非相连的孪晶对。结果表明:相连的孪晶对优先在取向差约为25°的相邻晶粒的晶界上形核。大约88%的相连孪晶对具有很高的斯密特因子,大约76%的相连孪晶对具有很高的应变兼容因子。低斯密特因子的孪晶对的发生能够通过高应变兼容因子进行解释。大约23%的非相连孪晶对的应变兼容因子接近于0。     

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 ...