Origin of nucleation and growth of extension twins in grains unsuitably oriented for twinning during deformation of Mg-1%Al

A large number of anomalous extension twins,with low or even negative twinning Schmid factors,were found to nucleate and grow in a strongly textured Mg-1Al alloy during tensile deformation along the extruded direction.The deformation mechanisms responsible for this behaviour were investigated through in-situ electron back-scattered diffraction,grain reference orientation deviation,and slip trace-modified lattice rotation.It was found that anomalous extension twins nucleated mainly at the onset of plastic deformation at or near grain boundary triple junctions.They were associated with the severe strain incompatibility between neighbour grains as a result from the differentbasal slip-induced lattice rotations.Moreover,the anomalous twins were able to grow with the applied strain due to the continuous activation ofbasal slip in different neighbour grains,which enhanced the strain incompatibility.These results reveal the complexity of the deformation mechanisms in Mg alloys at the local level when deformed along hard orientations.

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.

Effect of twinning on flow stress during initial stage of hot compression of twin roll cast Mg-5.51Zn-0.49Zr alloy

A Thermecmastor-Z hot deformation simulator,optical microscopy,XRD and TEM were employed to characterize the flow stress behavior and microstructure of twin roll cast ZK60 magnesium alloy during initial stage of hot compression at elevated temperature of 300 ℃ and 400 ℃ and a given strain rate of 10-2s-1.The results suggest that flow stress drop during initial stage of hot compression at 300℃,generally led by dynamic recrystallization,is attributed to twinning,correspondingly to dynamic recrystallization as deformation temperature is raised to 400 ℃.

变形镁合金中的织构及其优化设计

针对变形镁合金存在的典型织构以及织构优化设计方面的研究工作和进展进行综合评述。镁合金由于基面滑移和{1 012}孪生是最容易开动的变形模式,在变形镁合金中容易形成挤压丝织构及轧制板织构。通过引入剪切变形,改变成型过程中外加应力的取向,能够有效地改变变形镁合金的织构,同时通过添加微量稀土元素Nd、Ce和Y等,能够明显弱化或随机化变形镁合金织构。织构随机化后的镁稀土合金具有较好的强韧性,合金的变形各向异性得以改善。添加稀土元素后会改变稀土元素与Mg原子间的键能,改变稀土元素周围Mg-Mg原子之间的结合能等,增加非基面滑移的可能性,减弱基面滑移及{1 012}孪生所占的比率,有效地弱化镁合金的织构。

镁合金轧制板材低温变形行为与微观机制

研究AZ31镁合金轧制板材在低温(室温至423K)拉伸条件下的力学性能,并通过高温金相显微镜的拉伸附件观察动态微观组织的变化过程。结果发现:镁合金的塑性随温度的升高和应变速率及负载的降低而增加,表现出一定的应变速率敏感性。常温下的主要变形机制为{1011}压缩孪生,其形核和长大速度很快,但同时也存在少量的基面滑移,不同类型的孪晶在晶粒中彼此相交;而随着温度的升高,a+c非基面滑移的临界剪切应力迅速下降,大量非基面滑移系被激发,板材塑性性能明显改善;当应变速率较高时(423K,应变速率10?2/s),断裂方式为有一定塑性特征的解理形貌;随着应变速率的降低(423K,应变速率10?3/s),断裂方式逐渐变为塑性断裂。

歧口凹陷沿岸带新生代构造特征及其形成机制

歧口凹陷沿岸带是一条分隔歧口凹陷陆上和海域不同构造体系的NNE—NE的复杂构造变形带。在其两侧,盆地结构分别表现为半地堑和地堑式。在平面上,自南向北歧口凹陷沿岸带可以分为三个具有明显不同的新生代构造变形组合特征的区段:南部的斜列式走滑变形带,体现为一系列阶梯状展布的张扭断层,同时伴有岩浆活动和滑塌浊积体发育;中部的宽缓挠曲变形带,构造样式简单,地层只有微弱变形,但仍然伴随强烈的火山活动;北部的复杂断裂褶皱带是受基底走滑断层影响而形成的古近纪重力滑脱断层和新近纪花状构造的有机结合体。总体上,沿岸带复杂的构造组合特征是受到深部向北隐没的新生代右旋基底走滑断裂活动的控制,是渤海湾盆地内兰聊断裂通过歧口凹陷时的构造表现。歧口凹陷沿岸带在不同区段的构造变形特征,也造成了不同区段的油气成藏模式的差异性。

AZ31镁合金蠕变初期的变形特征

通过蠕变曲线的测定和TEM的形貌观察,研究了AZ31镁合金在蠕变初期的变形特征及组织演化规律.结果表明,蠕 变初期的变形特征是:大量形变产生的<A>位错在合金的基面和非基面滑移, <A+c>位错在锥面滑移.其中<a>位错通过位错 分解反应可由一非基面交滑移至另一非基面.随蠕变进行,高密度的形变位错发生动态回复,可进一步束集形成位错胞和位错墙. 蠕变初始阶段,在应力的作用下,适当取向的晶体发生孪生,并作为一种附加的变形机制而改善合金的韧性.

AZ31镁合金轧制板材各向异性行为的晶体塑性研究

室温下,镁合金的主要变形机制是滑移和孪生相互竞争。为了从介观尺度准确描述这种变形机制,晶体塑性本构关系需要考虑滑移和孪生的耦合作用。基于滑移-孪生耦合的晶体塑性本构关系,本文采用代表体积单元法建立了多晶模型,并对AZ31镁合金沿RD和TD方向进行拉伸模拟分析。结果表明:沿RD方向的变形机制为基面滑移、柱面滑移、锥面滑移,沿TD方向为柱面滑移和锥面滑移。沿不同方向基面滑移对变形的贡献不同,造成AZ31镁合金轧制薄板沿RD方向的屈服强度小于TD方向,表现出强烈的力学性能各向异性。

两步升温热轧工艺对AZ31镁合金薄板各向异性及成形性能影响

采用两步梯度升温轧制工艺对AZ31镁合金板材进行轧制,探究轧制过程对镁合金板材显微组织织演化、各向异性及成形性能的影响。第I步轧制在300℃开展,其每道次压下量为15%;第II步轧制在550℃进行,其每道次压下量为40%。经过共4道次轧制后,最终获得厚度为1 mm的镁合金薄板。结果显示,第I步低温轧制过程不更换轧制方向时,试样中生成大量剪切带;而更换轧制方向时,组织内部主要为孪晶和再结晶晶粒。随着II步轧制温度的升高,由于动态再结晶急剧激发,剪切带数量及尺寸逐渐减小,晶粒明显细化。根据IGMA分析得出,非基面滑移,特别是棱柱面滑移活动增强。轧制退火后的AZ31镁合金薄板的力学性能得到提高,各向异性减小,冲压成形性能得到明显改善。

考虑土体强度不均匀性时宽窄基坑坑底隆起稳定研究

在基坑坑底隆起稳定性分析中,准确的破坏模式及滑动面是确定安全系数的基础。然而,基坑坑底隆起破坏模式种类繁多且形式各异。传统的经典基坑坑底隆起稳定分析以及规范分析方法在破坏模式及滑动面的选取上均存在差异和局限性。基于不连续布局优化法,首先研究了土体强度不均匀性对不同宽度基坑坑底隆起稳定性的影响。结果表明,软土层厚度较大且土体强度沿深度逐渐增长时,宽基坑坑底隆起滑动面并不覆盖整个坑底,即相当于软土层下部存在一硬土层,限制了滑动面向下延伸。进一步,分析了围护结构长度及抗弯强度对破坏模式及安全系数的影响,并提出以滑动面触碰围护结构来区分宽窄基坑。在宽基坑中,当围护结构嵌固深度较小时,失稳破坏滑动面并不通过桩底,此时应用绕最下道支撑的圆弧滑动模式时应适当修正滑动面半径。对于窄基坑,应用圆弧滑动模式时需修正坑内滑动面,以考虑窄基坑的空间效应对安全系数的提高作用。

{101^(-)2}twinning nucleation in magnesium assisted by alternative sweeping of partial dislocations via an intermediate precursor

Molecular dynamic simulation and transmission electron microscopy(TEM)characterization was employed to investigate the{1012}twinning mechanism in magnesium.A partial dislocation assisted twinning nucleation mechanism was proposed based on simulation results,in which the twin lattice was reconstructed from parental matrix by two-step sweeping of partial dislocations on different close packed planes from matrix and the subsequently formed twin precursor respectively.A{101^(-)2}twin precursor was observed adjacent to matrix/twin interface by a spherical aberration corrected TEM,which indicated the hexagonal-close-packed(hep)matrix-quasi face-centered cubic(fee)twinning precursorsh.c.p twin transformation sequence during{101^(-)2}twinning process.

Inhibiting effect of I-phase formation on the plastic instability of the duplex structured Mg-8Li-6Zn-1.2Y(in wt.%)alloy

In this work,the tensile behaviors of Mg-8%Li and Mg-8%Li-6%Zn-1.2%Y alloys at ambient temperature were investigated and compared.It revealed that the plastic instability of Mg-8%Li alloy was quite remarkable and the variation range of serrated flowing stress can reach5 MPa.For the Mg-8%Li-6%Zn-1.2%Y alloy,the formation of I-phase(Mg_(3)Zn_(6)Y)can simultaneously enhance the tensile strength and eliminate the plastic instability phenomenon,whilst its ductility was degraded.The in-situ tensile tests revealed that for the Mg-8%Li alloy,the severity and number of slip traces present in bothα-Mg andβ-Li matrix phases increased remarkably with the applied tensile strain.However,slip traces were quite fine inβ-Li matrix phase and could be clearly observed when the applied tensile strain exceeded 18%.Due to the incompatibility of plastic deformation occurred in two matrix phases,the induced strain concentration atα-Mg/β-Li interfaces caused their subsequent cracking.For the Mg-8%-6%Zn-1.2%Y alloy,the I-phase distributed atα-Mg/β-Li interfaces suppressed the plastic deformation ofα-Mg matrix phase and the tensile strain was dominated by theβ-Li matrix phase,resulting in the disappearance of plastic instability.Moreover,the plastic strain would preferentially concentrate at I-phase/β-Li interfaces and subsequently induced the cracking of I-phase.

A quantitative microplasticity-based approach to rationalize the poor strengthening response of polycrystalline Mg alloys

This work aims to understand the inefficiency of nanoprecipitates to strengthen a weakly textured,polycrystalline Mg-Gd-Y-Zr alloy.An experimental micromechanical approach consisting on micropillar compression combined with analytical electron microscopy is put in place to analyze the effect of nanoprecipitation on soft and hard basal slip and twinning in individual grains with different orientations.This study shows that,in grains that are favorably oriented for basal slip(“soft”basal slip),aging leads to extreme localization due to the ability of basal dislocations to shear the nanoparticles,resulting overall in the softening of basal systems.Additionally,in grains in which the c-axis is almost perpendicular to the compression axis,prismatic slip dominates deformation in the solid solution state and nanoprecipitation favors twinning due to the concomitant lattice solute depletion.Finally,in grains oriented with their c-axis making an angle of about 5-7°with respect to the compression axis,which deform mainly by“hard”basal slip,precipitation leads to the strengthening of basal systems in the absence of obvious localization.This work reveals that the poor hardening response of the polycrystalline alloy is related to the capability of basal dislocations to shear the nanoparticles,in the absence of Orowan looping events,and to the associated basal slip localization.

不同沟底摩阻力工况下泥石流动力学研究

本文以暴雨诱发的裕东路泥石流为例,通过泥石流动力学分析软件2d-DMM建立了摩擦模型、Voellmy模型两种沟槽底部摩阻模型,对该次泥石流的速度、运动距离等特性进行了分析和比较。通过对泥石流特定点的运动速度与观测速度的比较、模拟运动距离与观测运动距离的比较,得出香港典型泥石流动力学分析的相关摩阻模型,并提出泥石流支护防治建议。

基坑抗隆起稳定分析的现状与进展

基坑抗隆起稳定性验算对基坑支护设计的成败有着重要的意义。经过合理地推导圆弧滑动模式的基坑抗隆起分析方法,给出了该模式下正确的计算公式。针对目前基坑工程实践中常用的两种基坑抗隆起分析模式作了比较全面和深入的探讨。通过对影响抗隆起稳定分析的各种因素的分析和探讨,总结了两种计算模式的适用范围;同时通过对比也揭示了目前已有的圆弧滑动模式的抗隆计算公式以及本文计算公式的优劣。Terzaghi(1943)所提出黏土基坑不排水条件下的抗隆起分析模式是抗隆起稳定分析的经典模式,直到目前仍在应用,由于Terzaghi(1943)采用的是极限平衡理论,而极限平衡理论是不严格的理论,采用笔者新近应用成功的多块体上限方法对Terzaghi公式作了评价。

高温终轧工艺对初始织构不同的AZ31镁合金板材组织的影响

以初始织构不同的AZ31镁合金板材为研究对象进行了高温终轧和最终退火,结合转靶X射线衍射仪和电子背散射衍射分析手段分析了不同板材基面织构的变化情况以及滑移和孪生对形变织构的影响规律。结果表明,高温终轧后,两种板材的宏观织构存在着单峰和倾斜的双峰基面织构区别;最终退火后,两种板材的织构强度都得到弱化,但弱化程度存在差异,并且再结晶织构在一定程度上遗传了形变织构的特征。对于初始织构强度较高的板材,高温终轧时锥面滑移活性增强,柱面滑移受到抑制,从而表现出带状变形集中区域,以此来弥补由于位错滑移受限引起的变形能力不足,此时形变织构的变化主要受到滑移和孪生的共同作用的影响。对于初始织构强度较弱的板材,高温终轧时非基面滑移活性得到增强,滑移和孪生相互竞争,孪生活动受到了明显抑制,此时形变织构的变化主要受到滑移的影响。

非金属溶质C、H、O和N对α型钛合金基面滑移的影响

采用基于密度泛函理论的第一性原理方法,研究了非金属溶质C、H、O和N对α型Ti合金(0001)[11 20]和(0001)[10 10]基面滑移的影响。研究结果表明,非金属原子使(0001)[10 10]基面滑移的广义层错能降低。其中,不稳定层错能γus的降低将导致该方向滑移的运动性能增强,而内禀层错能的降低将有利于I2层错的形成。对于(0001)[11 20]基面滑移,除H原子外,C、N和O原子都使Ti合金的广义层错能升高,这将使基面在[11 20]方向上滑移的能垒增加,促使该方向上的全位错向其他方向分解。通过进一步对Ti合金的Bader电荷、差分电荷密度分布和局域态密度的计算,从微观上分析了非金属溶质原子对Ti合金基面滑移的影响。电子结构的研究表明,非金属原子X(X=C、H、O和N)与其邻近的Ti原子之间形成强度稳定的TiX共价键,其对滑移过程起着重要的作用。

探讨基坑抗隆起稳定性验算

本文探讨新颁布规范中基坑抗隆起稳定性验算的方法和规定。

Plastic deformation mechanisms of AZ31 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 and 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.

二连盆地巴音都兰凹陷反转构造及其对油气成藏的影响

重点探讨了对二连盆地巴音都兰凹陷反转构造的类型识别、成因机制及油气成藏的影响。综合分析认为,从腾二段沉积期开始,巴音都兰凹陷结构格局开始发生根本性转变,中洼由原来盆地的沉降中心演变成了包楞构造带低凸起和巴一号构造带次级沉降中心,并使整个巴音都兰凹陷的沉降中心分别向北洼和南洼的中部迁移。走滑牵引型反转构造经历了由走滑伸展断层牵引非强硬盖层形成反转背斜褶皱和背斜脊部断裂-塌落形成负花状构造两个演化阶段,是较长时间持续应力作用的结果;而基底隆升型反转构造是深部异常体上涌导致的局部构造隆升现象,为突发性的短时事件。基底隆升型反转构造对于提高古地温梯度和降低生油门限等方面具有非常重要的影响,而走滑牵引型反转构造对于储层改善、圈闭形成以及油气运移和浅部聚集成藏等方面具有重要影响。本文研究对于正确认识二连盆地乃至我国东部盆地群在早白垩世发育的反转构造的成因类型、动力机制及对油气成藏的影响等方面具有重要的借鉴意义。