Non-merohedrally Twinned Crystal Structure of the Co-crystal Ethyl 6-(2-5-(Ethoxycarbonyl)-pyridin-2-yl-1,2-dihydroxyethyl)pyridine-3-carb-oxylateethyl 6-(2-5-(Ethoxycarbonyl)pyridin-2-yl-2-hydroxyacetyl)pyridine-3-carboxylate(0.69/0.31)

The two component molecules of the co-crystal 0.69（C18H2ON206）.0.31（C18H18N2O6） lie on a center-of-inversion that exists midway along the ethylene chain connecting the two aromatic rings. The two molecules are superimposed. The crystal is also a non-morohedral twin with a minor 37.7（2）% component. The refinement of this twinned and disordered crystal structure is detailed. Crystal data： C18H19.38N2O6, monoclinic, P21/c, a = 17.1687（8）, b = 5.4389（2）, c = 9.3261（4） A, b = 95.270（5）° and V= 867.18（6） A3 at -173 ℃.

Investigation of mechanical properties of twin gold crystal nanowires under uniaxial load by molecular dynamics method

Twin gold crystal nanowires, whose loading direction is parallel to the twin boundary orientation, are simulated.We calculate the nanowires under tensile or compressive loads, different length nanowires, and different twin boundary nanowires respectively. The Young modulus of nanowires under compressive load is about twice that under tensile load.The compressive properties of twin gold nanowires are superior to their tensile properties. For different length nanowires,there is a critical value of length with respect to the mechanical properties. When the length of nanowire is greater than the critical value, its mechanical properties are sensitive to length. The twin boundary spacing hardly affects the mechanical properties.

Three-dimensional crystal plasticity and HR-EBSD analysis of the local stress-strain fields induced during twin propagation and thickening in magnesium alloys

Present work focuses on analysis of the stress and strain fields inside and around the individual {10–12} twin in magnesium alloy. The 3D crystal plasticity model represents twin as an ellipsoidal inclusion surrounded by the matrix. Five different twin thicknesses and three different lateral twin lengths are used for stress/strain analysis. The simulations are complemented with experimental observations using high-resolution electron backscattered diffraction. The simulations and experiments show a similar distribution of the shear stress and the spatial activity of individual slip systems(basal, prismatic, pyramidal). Plasticity induced inside the twin is dominantly caused by the prismatic dislocations slip and does not influence twin back stress which is identical to pure elastic twin. The twin with larger lateral dimension requires lower equilibrium stress which suggests anisotropic twin propagation and increased thickness of such twins. The lateral twin propagation is mostly influenced by prismatic and pyramidal slip in the twin vicinity. The twin thickness can reach a maximal level that is driven by the critical resolved shear stress values for dislocation slip with the significant influence of basal slip.

Influence of temperature on twinning dominated pop-ins during nanoindentation of a magnesium single crystal

The present paper examines the temperature sensitivity of tensile twinning in a magnesium single crystal during nanoindentation of the prismatic plane. High temperature indentations from 25 ℃ to 250 ℃ were employed on a well polished magnesium single crystal {10-10}plane. For a indentation curve displaying a pop-in, a single twin was seen on the sample surface using Atomic Force Microscopy(AFM)imaging. For indentations that produced no pop-in, no twinning was observed. We thus conclude the pop-in arises from a twinning event in the present case. With increasing temperature, the mean pop-in load(measured from 200 repeat indentations of each testing temperature)drops markedly. This is interpreted by the thermal activation of nucleation of lattice dislocations, which immediately trigger a twinning event.Thermal activation analysis yields activation energies that are consistent with this idea. With increasing temperature the pop-ins became deeper and the twins, after further indentation, showed more growth. It is likely that non-basal slip is activated in the stress concentrations that arise during twinning and the thermal activation of this slip accounts for the observed temperature effects. It is concluded that in interpreting the temperature sensitivity of twinning stresses, any associated lattice dislocation activity must be considered.

Effects of temperature on critical resolved shear stresses of slip and twining in Mg single crystal via experimental and crystal plasticity modeling

Magnesium(Mg)single crystal specimens with three different orientations were prepared and tested from room temperature to 733 K in order to systematically evaluate effects of temperature on the critical resolved shear stress(CRSS)of slips and twinning in Mg single crystals.The duplex non-basal slip took place in the temperature range from 613 to 733 K when the single crystal samples were stretched along the<0110>direction.In contrast,the single basal slip and prismatic slip were mainly activated in the temperature range from RT to 733 K when the tensile directions were inclined at an angle of 45°with the basal and the prismatic plane,respectively.Viscoplastic self-consistent(VPSC)crystal modeling simulations with genetic algorithm code(GA-code)were carried out to obtain the best fitted CRSSs of major deformation modes,such as basal slip,prismatic slip,pyramidalⅡ,{1012}tensile twinning and{1011}compressive twinning when duplex slips accommodated deformation.Additionally,CRSSs of the basal and the prismatic slip were derived using the Schmid factor(SF)criterion when the single slip mainly accommodated deformation.From the CRSSs of major deformation modes obtained by the VPSC simulations and the SF calculations,the CRSSs for basal slip and{1012}tensile twinning were found to show a weak temperature dependence,whereas those for prismatic,slip and{1011}compressive twinning exhibited a strong temperature dependence.From the comparison of previous results,VPSC-GA modeling was proved to be an effective method to obtain the CRSSs of various deformation modes of Mg and its alloys.

Molecular and Crystal Structure of Hexacobalt Carbonyl Cluster Containg Twin Co_3S(CO)_7 Fragments Linked by a Novel Phosphido-Thiolato Ligand as Backbone

Reaction of dimers of (R) thionophosphine sulfide (R = P - C6H4OMe,SMe ) with Co, (CO), yields the novel hexacobalt cluster [Co'(μ3, -S)2, (CO) 14 (μ4 -μ4- and the trinuclear clusterPSCH,)J 2' In 1, tw0 Co3S(C0)7 units linked by a novel phosphido-thio1ato bifunc-tiona1 bridging p'-SPR ligand as backbone.

Analysis of deformation mechanisms in magnesium single crystals using a dedicated four-point bending tester

In this study,we explored the deformation mechanisms of Mg single crystals using a combination of scanning electron microscopy and electron backscattered diffraction in conjunction with a dedicated four-point bending tester.We prepared two single-crystal samples,oriented along the<1120>and<1010>directions,to assess the mechanisms of deformation when the initial basal slip was suppressed.In the<1120>sample,the primary{1012}twin(T1)was confirmed along the<1120>direction of the sample on the compression side with an increase in bending stress.In the<1010>sample,T1 and the secondary twin(T2)were confirmed to be along the<1120>direction,with an orientation of±60°with respect to the bending stress direction,and their direction matched with(0001)in T1 and T2.This result implies that crystallographically,the basal slip occurs readily.In addition,the<1010>sample showed the double twin in T1 on the compression side and the tertiary twin along the<1010>direction on the tension side.These results demonstrated that the maximum bending stress and displacement changed significantly under the bend loading because the deformation mechanisms were different for these single crystals.Therefore,the correlation between bending behavior and twin orientation was determined,which would be helpful for optimizing the bending properties of Mg-based materials.

Crystal anisotropy of AZ31 magnesium alloy under uniaxial tension and compression

To investigate the deformation twinning and the plastic anisotropy of the hexagonal-close-packed（HCP） single crystal, the crystal plastic constitutive model including slip and twinning deformation was established with finite element method based on crystal plasticity theory. The model was verified by test data. Newton-Raphson iteration method was developed with the stress components directly as the basic variables of iteration. The plastic deformation behavior of single crystal AZ31 alloy was analyzed numerically under monotonic tension and compression, respectively, in four different strain paths（i.e. along 〈2110〉, 〈 0110〉, 〈0001〉 and 〈0111〉） with this model. The stress-strain curves were obtained in the above paths. The numerical calculation results show that this crystal model is feasible to predict the activity of slip/twinning system and to describe the number of active twin variants, the types of dominant twin variants and twin intersection. Due to the polar nature of mechanical twinning in inelastic deformation of the material, the plastic behavior of the single crystal material is demonstrated to be notably anisotropic and high asymmetry.

Simulation of mechanical behavior of AZ31 magnesium alloy during twin-dominated large plastic deformation

Experiments and visco-plastic self-consistent (VPSC) simulations were used to quantify the amount of twinning and the relationship to stress?strain behavior in a textured Mg?3Al?1Zn plate. Two different compression directions were utilized to favor{1012} extension or{1011} compression twinning.{1012} twins nucleate at the beginning of plastic deformation and grow to consume the parent grains completely. During compression along the normal direction,{1011} twinning and{1011}?{1012} double twinning start at strain of 0.05, and the number of twins increases until rupture, above strain of 0.15.{1011} and{1011}?{1012} twinning also occur during compression along the transverse direction, start at strain of 0.06 and then multiply in grains totally reoriented by{1012} twins. Using suitable parameters, the VPSC model can accurately predict the occurrence of extension, compression and double-twinning as well as the flow stresses and deformed textures. According to VPSC simulations, twinning and slip have the same latent hardening parameters.

Embryo-to-lamella transition of grain boundary twins in magnesium

A combined experimental and computational analysis is performed to investigate the less commonly studied embryo-to-lamella transition of deformation twins in magnesium. This work aims to understand the structural variables controlling the embryo-to-lamella transition from grain boundaries. Statistical analysis of hundreds of early-stage twins in the lightly deformed microstructure reveals a prevailing wedge shape,with a much thicker base along the grain boundary(GB) where they originate and a thinner tip terminating in the crystal. The analysis also shows that the GB base is super thick and identifies a minimum GB twin thickness among all early-stage twins that is about one micron. A crystal plasticity-based full-field model is employed to calculate the driving forces to migrate the boundary of a three-dimensional GB twin embryo. The stress analysis, considering a full range of embryo shapes and neighboring grain orientations, indicate that the twin embryo is most likely going to form a wedge shape when it first propagates. The calculations predict that the thickness of the embryo at the GB needs to be significantly larger than its length into the crystal in order to propagate into the crystal. The analysis finds that the more aligned the twin embryo variant is with basal slip in the neighboring grain, the thinner the twin embryo needed for propagation.

EFFECTSOFLAMELLARBOUNDARIESONCREEPBEHAVIOROFPSTCRYSTALSOFTiAlALLOYS

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Crystal Structure of Ni_(50)Mn_(29)Ga_(21)Tb_(1.2) Magnetic Shape Memory Alloy at Room Temperature

Martensitic transformations, martensitic structures and substructures of Ni50Mn29Ga21Tb1.2 shape memory alloy were studied by DTA, X-ray diffraction and electron diffraction. The results show that the mainly phase at room temperature has body-centred tetragonal structure with the lattice parameters ： a = b = 0.60 ran, c = 0.5546 nm; and has body-centred monoclinic with lattice parameters： a =0.616 nm, b =0.581 nm, c = 0.553 nm, β = 90.8° in some tiny area. The substructures of Ni50Mn29Ga21Tb1.2 at room temperature are twin.

Local hardening and asymmetric twin growth by twin-twin interactions in a Mg alloy

In this study,the role of twin-twin interactions on the distributions of local defects(e.g.,dislocations)and stress fields in a magnesium alloy is investigated.A co-zone(1012)-(1012)tensile twin junction in a deformed Mg-3wt.%Y alloy is analyzed using transmission electron microscopy(TEM).The results show that the morphology of the impinging(1012)twin is asymmetric,and the non-interacting boundary of the recipient(1012)twin is irregular.Detailed analysis of TEM images reveals that type-II pyramidal[1213](1212)dislocations concentrate in the vicinity of the twin-twin junction site.The same<c+a>dislocations are also observed inside the interacting twin domains along with a few <a> dislocations.The<c+a>dislocations emanating from the impinging(1012)twin boundary have edge character and are extended with faults parallel to the basal plane.In contrast,the<c+a>dislocations connected to the recipient(1012)twin are predominantly screw orientation and compact.Elasto-viscoplastic fast Fourier transform based crystal plasticity calculations are performed to rationalize the observed twin morphology and local dislocation distribution.The model calculations suggest that the local stress fields generated at the junction site where the two twins meet are responsible for the experimentally observed concentration of<c+a>dislocations.The calculated stress fields are asymmetric with respect to the junction site,explaining the observed asymmetric morphology of the impinging twin.Overall,these findings show strong effects of twin-twin interactions on the distribution of dislocations as well as the evolution of the twinned microstructure and as such,can help advance understanding of twinning in Mg alloys and their effect on mechanical behavior.

Temperature-dependent constitutive modeling of a magnesium alloy ZEK100 sheet using crystal plasticity models combined with in situ high-energy X-ray diffraction experiment

A multiscale crystal plasticity model accounting for temperature-dependent mechanical behaviors without introducing a larger number of unknown parameters was developed.The model was implemented in elastic-plastic self-consistent(EPSC)and crystal plasticity finite element(CPFE)frameworks for grain-scale simulations.A computationally efficient EPSC model was first employed to estimate the critical resolved shear stress and hardening parameters of the slip and twin systems available in a hexagonal close-packed magnesium alloy,ZEK100.The constitutive parameters were thereafter refined using the CPFE.The crystal plasticity frameworks incorporated with the temperature-dependent constitutive model were used to predict stress–strain curves in macroscale and lattice strains in microscale at different testing temperatures up to 200℃.In particular,the predictions by the crystal plasticity models were compared with the measured lattice strain data at the elevated temperatures by in situ high-energy X-ray diffraction,for the first time.The comparison in the multiscale improved the fidelity of the developed temperature-dependent constitutive model and validated the assumption with regard to the temperature dependency of available slip and twin systems in the magnesium alloy.Finally,this work provides a time-efficient and precise modeling scheme for magnesium alloys at elevated temperatures.

拉伸情形下孔洞缺陷对单晶镁裂纹扩展影响的分子动力学模拟

采用分子动力学方法对预设裂纹和孔洞的单晶镁模型进行了沿C轴拉伸加载的数值模拟,研究在镁晶体中不同位置的孔洞缺陷对裂纹扩展的影响。模拟结果分析表明,在相同特征尺寸下,孔洞会影响单晶镁模型的屈服强度,不同孔洞位置的模型屈服强度不同,其中当孔洞位于裂纹[101ˉ0]方向时,模型的屈服强度最低。孔洞能在一定程度上缓解裂纹在该方向上的应力集中,影响裂纹的孪晶扩展速率。裂纹处靠近孔洞侧的孪晶生长速度相对较慢,其中孔洞位于裂纹[101ˉ0]方向时,孪晶生长速度最慢。孔洞对裂纹的扩展方向影响较小,一般情况下裂纹与孔洞合并后沿拉伸方向扩展,整体呈现比较对称的锥面扩展,但孔洞位于裂纹[0001]方向时,裂纹并未与孔洞出现明显的联合现象。

不同应变速率下TWIP钢力学性能及微观组织演变

以Fe-Mn-C-Al系TWIP钢为研究对象,将试验钢通过退火热处理后在室温下进行拉伸,然后通过XRD、OM以及SEM等试验手段,研究了在不同应变速率下TWIP钢的力学性能、微观组织演变以及断口形貌变化规律。结果表明,不同应变速率拉伸后试验钢为单一奥氏体相,应变速率在3.33×10^(-4)~3.33×10^(-1)s^(-1)拉伸下,试验钢出现了再结晶以及二次孪晶,抗拉强度和伸长率在较低应变速率下处于较高水平,晶粒强化效果明显;随着应变速率的增大再结晶和二次孪晶的数量逐渐减少并消失,从而导致了抗拉强度和伸长率的减小。随着应变速率的增大,断口微观形貌由平整变得高低起伏,韧窝尺寸变大并由浅变深,材料脆性断裂的趋势越来越明显。

Seeded growth of Ti-46Al-8Nb polysynthetically twinned crystals with an ultra-high elongation

Large size polysynthetically twinned crystals of Ti-46 Al-8 Nb alloy with a parallel lamellar microstructure were successfully prepared using a Ti-43 Al-3 Si seed by our new operation.A large amount of columnar B2 phase paralleling to the growth direction was found in the final lamellar microstructure.Higher growth rate(>30 mm/h)led to the failure of seeding process.Based on these results,a new mechanism is proposed to describe the seeding process of the hypo-peritectic Ti Al alloys.The peritecticαphase is suggested to directly nucleate from the melt,and then act as nucleus for transformedαphase in the subsequentβtoαtransformation.At the higher growth rate,the appearance ofβphase secondary dendrites and homogeneous nucleation lead to the failure of seeding process.High Nb addition leads to a large amount of residualβphase,and theseβdendrites finally evolve into B2 phase.The room temperature tensile elongation was measured to be 11.9-18.5%for Ti-46 Al-8 Nb PST crystals,which is the highest ever reported value for Ti Al based alloys.

Five-fold twinned Ir-alloyed Pt nanorods with high C1 pathway selectivity for ethanol electrooxidation

Developing efficient and robust electrocatalysts toward ethanol oxidation reaction(EOR)with high C1 pathway selectivity is critical for commercialization of direct ethanol fuel cells(DEFCs).Unfortunately,current most EOR electrocatalysts suffer from rapid activity degradation and poor C1 pathway selectivity for complete oxidation of ethanol.Herein,we report a novel electrocatalyst of five-fold twinned(FFT)Ir-alloyed Pt nanorods(NRs)toward EOR.Such FFT Pt-Ir NRs bounded by five(100)facets on the sides and ten(111)facets at two ends possess high percentage of(100)facets with tensile strain.Owing to the inherent characteristics of the FFT NR and Ir alloying,the as-prepared FFT Pt-Ir NRs display excellent alkaline EOR performance with a mass activity(MA)of 4.18 A·mgPt^(-1),a specific activity(SA)of 10.22 mA·cm^(-2),and a Faraday efficiency of 61.21%for the C1 pathway,which are 6.85,5.62,and 7.70 times higher than those of a commercial Pt black,respectively.Besides,our catalyst also exhibits robust durability.The large percentage of open tensile-strained(100)facets and Ir alloying significantly promote the cleavage of C-C bonds and facilitate oxidation of the poisonous intermediates,leading to the transformation of the dominant reaction pathway for EOR from C2 to C1 pathway,and effectively suppress the deactivation of the catalyst.

柱状晶生长取向对定向凝固Mg-Gd合金拉伸和压缩性能的影响

利用定向凝固技术,制备了生长取向为〈2110〉的柱状多晶Mg-6.3Gd合金和生长取向为〈3214〉的柱状多晶Mg-8.2Gd合金,并研究加载方式(拉伸、压缩)、柱状晶取向与力学性能之间的相关性。结果表明:拉伸载荷下,〈2110〉取向柱状晶的基面〈a〉滑移Schmid因子(Fs)低于0.051、柱状晶内启动{1011}压缩孪生协调应变,因此,Mg-6.3Gd合金屈服强度较高(为176 MPa),但伸长率仅8%;〈3214〉取向柱状晶的基面〈a〉滑移Fs高于0.46、柱状晶内启动{1012}拉伸孪生协调应变,因此,Mg-6.3Gd合金屈服强度较低(为57 MPa),但伸长率达到22%。压缩载荷下,两种取向合金启动{1121}和{1012}两种拉伸孪生协调应变,具有较高的形变硬化能力,抗压强度(分别为309 MPa和301 MPa)较屈服强度分别提高214 MPa和219 MPa,且具有较高的塑性。

SMAT处理AZ80镁合金组织演变与低周疲劳行为研究

用表面机械研磨处理(SMAT)挤压态AZ80镁合金,研究其组织与力学性能变化,用应变控制探讨其疲劳性能。结果表明:SMAT处理的AZ80变形镁合金由表面到心部形成了梯度结构组织,且变形层厚度随SMAT时间增加而增加,并伴有大量的孪晶产生。在SMAT处理6 min后,剧烈变形层厚度约为200μm。其表层至心部的显微硬度呈梯度分布。应变为0.0064时,SMAT处理后,AZ80镁合金的循环加工硬化率和疲劳寿命明显高于原始挤压态,这主要归因于梯度结构组织及变形产生的残余压应力。