Stress field interaction during propagation of adjacent tensile twinning nuclei in magnesium

The shear stress field proximity to the twinning and prismatic/basal interfaces of {1012} tensile twins is shown by molecular dynamics simulations.The stress field interacts and influences the twinning growth mode in the subsequent deformation process,which is simulated by changing the relative positions of the nuclei.An asymmetrical growth mode appears,in which the growth of one twin is predominant over the other when they are oriented at 45° to each other.This growth mode is sensitive to the simulation temperature and strain rate and can be attributed to the interaction of the stress field proximity to the prismatic/basal interfaces and twinning planes.     

Microstructure and Texture Evolution During the Direct Extrusion and Bending–Shear Deformation of AZ31 Magnesium Alloy

AZ31 alloys were extruded by direct extrusion and bending–shear deformation(DEBS). The microstructure characteristic and texture evolution of DEBSed AZ31 sheets were investigated by electron backscattered diffraction(EBSD). It is found that DEBS technique could effectively refine grains and weaken texture. Besides, we also investigate how twinning affects dynamic recrystallization during hot extrusion. {10–12} extension twins can offer nucleation sites and enough energy to trigger dynamic recrystallization. Moreover, the character of direct extrusion and bending–shear die can lead to the activation of non-basal slip system and further dramatically weaken the basal texture of the microstructure with many preactivated basal slip systems.     

Molecular Dynamics Simulations of the Orientation Effect on the Initial Plastic Deformation of Magnesium Single Crystals

Molecular dynamics simulation is employed to study the tension and compression deformation behaviors of magnesium single crystals with different orientations.The angle between the loading axis and the basal a direction ranges from 0° to 90°.The simulation results show that the initial defects usually nucleate at free surfaces,but the initial plastic deformation and the subsequent microstructural evolutions are various due to different loading directions.The tension simulations exhibit the deformation mechanisms of twinning,slip,crystallographic reorientation and basal/prismatic transformation.The twinning,crystallographic reorientation and basal/prismatic transformation can only appear in the crystal model loaded along or near the a-axis or c-axis.For the compression simulations,the basal,prismatic and pyramidal slips are responsible for the initial plasticity,and no twinning is observed.Moreover,the plastic deformation models affect the yield strengths for the samples with different orientations.The maximum yield stresses for the samples loaded along the c-axis or a-axis are much higher than those loaded in other directions.     

Twinning during hot compression of ZK30 ＋ 0.3Yb magnesium alloy

The twinning process of ZK30＋0.3Yb magnesium alloy was studied. The results show that twinning occurs at the initial stage of deformation, and decreases during further deformation. The original grain is fragmented after small straining. It is investigated that the twinning boundary activates the occurrence of the non-basal slip system due to the stress concentration at the vicinity of twin boundary introduced by the dislocation pile-ups at the vicinity of twinning boundary. The rearrangement of dislocation after dislocation climb introduces new grain boundary. Simultaneously, twinning occurs to form “polygonization” due to the stress concentration relaxation, and the “polygonization” will transform into low angle boundary to refine the original grain under the shear stress with further straining.     

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 ...     

Plastic deformation mechanisms ofAZ31 magnesium alloy under high strain rate compression

The twinning and slip activities of AZ31 magnesium alloy sheet at a strain rate of 1 200 s^-1 were investigated. Dynamically mechanical properties of various oriented samples were measured using Split Hopkinson Pressure Bar（SHPB）. Optical microscope observations reveal that the dominant deformation mechanism is twinning for 90° oriented sample, and is slip for 45° and 0° oriented samples. TEM analysis for samples at a strain of 0.3% shows that the main deformation mechanisms for 90°, 45° and 0° oriented sample are {1012} 〈 1011 〉 and {1011} 〈 1012 〉 twinning, basal slip and non basal slip, respectively. The main features of the true stress--true strain curves can be explained based on deformation mechanism analysis.     

Compression Deformation Mechanisms at the Nanoscale in Magnesium Single Crystal

The dominant deformation mode at low temperatures for magnesium and its alloys is generally regarded to be twinning because of the hcp crystal structure. More recently, the phenomenon of a "loss" of the twins has been reported in microcompression experiments of the magnesium single crystals. Molecular dynamics simulation of compression deformation shows that the pyramidal slip dominates compression behavior at the nanoscale. No compression twins are observed at different temperatures at different loadings and boundary conditions. This is explained by the analyses, that is, the {102} and {101} twins can be activated under c-axis tension, while compression twins will not occur when the c/a ratio of the hcp metal is below √3. Our theoretical and simulation results are consistent with recent microcompression experiments of the magnesium (0001) single crystals.     

Dynamic Recrystallization and Texture Evolution of GW94 Mg Alloy During Multi-and Unidirectional Impact Forging

Multi-and unidirectional impact forgings were successfully applied to a(GW94) Mg–RE alloy.The microstructure and texture evolution were investigated systematically.The obtained results indicated that during unidirectional impact forging,a bimodal chain deform microstructure was sustained till last forging pass,whereas {10–12} extension twins-assisted continuous dynamic recrystallization took place during the multidirectional impact forging(MDIF).The coalescence and intersection of {10–12} extension twins during MDIF efficiently refined the original coarse grains and led to an almost recrystallized homogeneous microstructure.The texture analysis demonstrated that unidirectional impact forging yielded out the strong basal texture; however,MDIF resulted in non-basal texture,which was attributed to the cooperative effects of continuous DRX,twinning,and MDIF itself during the deformation process.     

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.     

Microstructure and Mechanical Properties of Mg–3Al–Zn Magnesium Alloy Sheet by Hot Shear Spinning

Hot shear spinning experiments with Mg–3.0 Al–1.0 Zn–0.5 Mn(AZ31 B, wt%) magnesium alloy sheets were conducted at various temperatures, spindle speeds and feed ratios to investigate the effects of these processing parameters on the microstructure, crystallographic texture and mechanical properties. The AZ31 B sheet displayed good shear formability at temperatures from 473 to 673 K, spindle speeds from 300 to 600 rev/min and feed ratios from 0.1 to 0.5 mm/rev. During the dynamic recrystallization process, the grain size and texture were affected by the deformation temperature of the hot shear spinning process. Each of the spun sheets presented a strong basal texture, and the c-axis of most of the grains was parallel to the normal direction. The optimal hot shear spinning parameters were determined to be a temperature of 473 K, a spindle speed of 300 rev/min and a feed ratio of 0.1 mm/rev. The yield strength, ultimate tensile strength and elongation in the rolled direction reached 221 MPa, 288 MPa and 14.1%, and those in the transverse direction reached 205 MPa, 280 MPa and 12.4%, respectively. The improved strength and decreased mechanical anisotropy resulted from the fine grain size and strong basal texture.     

Relationships between deformation mechanisms and initial textures in polycrystalline magnesium alloys AZ31

1　INTRODUCTIONMagnesiumisthelightestmetallicstructuralma terialwithhighspecificstrengthandisusedinauto motive ,electronicsandaerospacefield[1,2 ] .Howev er ,magnesiumoftenshowspoorformabilityatroomtemperatureduetoitshexagonalstructurewithlessindependentslipsystems ,whichlimitsitsuse .Dif ferentslipsystemshavebeendetectedinmagnesiumsuchasbasalslipof { 0 0 0 1}〈112 0〉 ,prismslipof{ 10 10 }〈112 0〉[3,4 ] andpyramidslip (includingthea typeof {hkil}〈112 0〉andthea +ctypeof {hkil}〈112 3〉[…     

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.     

Effect of microstructural inhomogeneities on the evolution of texture during rolling and recrystallization of copper

Microstructural inhomogeneities in polycrystalline copper was intentionally varied by the deformation at the liquid nitrogen temperature and the rolling without lubrication. The effect of different rolling conditions on the evolution of microstructure and texture during rolling and subsequent recrystallization was studied by means of X-ray texture measurement and microstructural observations. Applying cold rolling at a low temperature led to the formation of deformation twins in grains at the Cu-orientation, and shear bands developed when cold rolling was performed without lubrication. The evolution of the normal rolling texture was suppressed by deformation twinning. Shear bands in deformed structure resulted in a very weak cube-recrystallization texture.     

Mechanism of formation of shear bands upon cold deformation of a commercial Fe-3% Si alloy

A model of the formation of shear bands upon cold deformation of {111}〈112〉 crystals of a commercial Fe-3%Si alloy is proposed. The model suggests a two-stage mechanism for the formation of a shear band and its fine structure. At the first stage, there occurs an anomalous twinning in the {114}〈221〉 system; at the second stage, an almost complete secondary twinning of the band in two standard {112}〈111〉 systems occurs. As a result of these transformations, the shear band consists of regions with an almost cube-on-edge orientation {11 11 1}〈1 1 22〉 and an “octahedral” {111}〈112〉 orientation, which is symmetrical with respect to the initial {111}〈112〉 orientation. A crystallographic mechanism responsible for an anomalous twinning in the {114}〈221〉 system by the slip of (8a/18)〈221〉 partial dislocations is proposed. It is supposed that the shear bands observed consist of groups of different numbers of shear microbands.     

Orientation factor analysis of deformation mechanisms under special processing techniques in AZ31 magnesium alloys

Experiments show that special processing techniques such as asymmetrical rolling(ASR),equal channel angular pressing(ECAP) and equal channel angular rolling(ECAR) can weaken the basal texture of the magnesium alloys and therefore improve their plasticity.However,the deformation mechanisms related are different.In this paper,we determine the deformation mechanisms activated during ASR,ECAP and ECAR by calculation of orientation factors.Analysis shows that during ASR the shear stress σ13 on the rolling plane ...     

热轧过程中AZ31镁合金的组织及织构演变

对AZ31镁合金铸轧板进行单道次热轧实验,利用光学显微镜、X射线和透射电镜对热轧过程中微观组织和织构的演变规律进行研究。结果表明:AZ31镁合金铸轧板具有较强的基面织构,当热轧变形量较小时,孪生是主要的变形机制;当热轧变形量较大时,位错滑移成为主要的变形机制;10%热轧态中出现的透镜状的{1012}宽孪晶使基面织构明显减弱;20%热轧过程中则出现{1012}、{1011}-{1012}两种不同形貌的孪晶;当变形量大于20%时,位错滑移大量开动,基面织构也显著增强,并在随后的退火过程形成细小均匀的再结晶组织。     

Role of shear band in texture control of Al-Mg alloys

Shear bands formed in Al-Mg alloys during cold rolling are nucleated on grain boundaries. Their mechanism of formation is different from that already proposed in the case of single crystals of the same alloy. Since recrystallized grains of non-cube orientations are nucleated on these shear bands during annealing, the development of cube recrystallization texture can be strongly suppressed by enhancing shear banding during cold rolling. Control of shear band thus provides a new fundamental technological tool to improve drawability of Al alloy sheets. In a microscopic scale, deformation in grain boundary regions also plays here a very important role, as in the case of the formation of {111}uvw recrystallization textures in low carbon steel sheets.     

Exploring Plastic Deformation Mechanism of Multilayered Cu/Ti Composites by Using Molecular Dynamics ModelingEI北大核心CSCD

Multilayered metallic composites have attracted great interest because of their excellent characteristics. In recent years, the mechanical behavior of Cu/Ti composites is described in terms of macroscopic or mesoscopic scales, but the micromechanism regarding dislocation slip, twinning and shear banding at heterogeneous interfaces remains unclear. In this work, the molecular dynamics method is used to study the uniaxial tensile and plane strain compression deformation of the Cu/Ti multilayered composites with characteristic initial crystal orientations. The simulation results show that under the tensile load, dislocations are preferentially nucleated at the heterogeneous interface between Cu and Ti, and then slip along {111} plane within the Cu layers. The corresponding mechanism is confined layer slip. With the multiplication of dislocations, dislocations interact with each other, and intrinsic stacking faults and deformation twins are formed in Cu layers. However, no dislocation slip or twinning is activated within the Ti layers at this stage of deformation. As the load increases, the stress concentration at the Cu/Ti interface leads to the fracture of the composites. For the composites under plane strain compression, the stress concentration at the Cu/Ti interface triggers the formation of shear bands in the Ti layer, and there are only very limited dislocations within the shear bands and their adjacent area. With the increase of applied strain, the common action of various deformation mechanisms causes the grains to rotate, and the disorder degree of complex atoms increases. In addition, the micro-plastic deformation mechanism and mechanical properties of Cu/Ti complex with different initial orientations and strain rates are significantly different. The results reveal the microscopic deformation mechanism of the laminated composites containing hcp metals.     

等通道球形转角挤压过程中工业纯铝的微观组织演变与力学性能

突破传统ECAP变形全过程通道等截面思路,提出一种耦合剪切应变和正应变于一体的新型等通道球形转角挤压(equal channel angular extrusion with spherical cavity,ECAE-SC)工艺。在自行研制的模具上对工业纯铝进行室温单道次ECAE-SC挤压实验,采用OM、EBSD和TEM等技术手段,研究了ECAE-SC变形过程中工业纯铝微观组织的演变规律,并测试了变形后试样的显微硬度。结果表明,在ECAE-SC工艺剧烈简单剪切变形诱导下,工业纯铝仅需1道次挤压变形即可获得等轴、细小、均匀的超细晶组织,平均晶粒尺寸约为400 nm;工业纯铝室温ECAE-SC变形以位错滑移为主并伴有不完全连续动态再结晶,其微观组织经历了剪切带→位错胞→小角度亚晶→大角度等轴晶粒等动态演化过程。1道次ECAE-SC变形后,工业纯铝组织以{110}001高斯织构为主,同时存在部分{111}112铜型织构;材料显微硬度值大幅提升,由初始289.4 MPa提高到565.3 MPa,增幅高达95.33%,且分布均匀性良好。     

Microstructure Evolution and Hardness of an Ultra-High Strength Cu-Ni-Si Alloy During Thermo-mechanical Processing

Microstructure evolution and hardness changes of an ultra-high strength Cu-Ni-Si alloy during thermo-mechanical processing have been investigated. For hot-compressive deformation specimens, dynamic recrystallization preferentially appeared on deformation bands. As deformation temperature increased from 750 to 900 °C, elongated grains with the Cubic texture {001} 〈100〉 were substituted by recrystallized grains with Copper texture {112} 〈111〉. For the samples having undergone cold rolling followed by annealing, static recrystallization preferentially occurred in the deformation bands, and then complete recrystallization occurred. Goss, Cubic, and Brass textures remained after annealing at 600 and 700 °C for 1 h; R texture {111} 〈211〉 and recrystallization texture {001} 〈100〉 were formed in samples annealed at 800 and 900 °C for 1 h, respectively. For samples processed under multi-directional forging at cryogenic temperature, the hardness was increased as a result of work hardening and grain refinement strengthening. These were attributed to the formation of equiaxed sub-grain structures and a high dislocation density.