双峰组织ZA21镁合金棒材的拉–压不对称性及变形机制研究

本文通过挤压及热处理分别制备了两种具有完全再结晶晶粒的ZA21镁合金棒材,分别为晶粒尺寸成大小两种状态分布的双峰组织以及晶粒尺寸均匀的均匀组织,对这两种棒材沿轴向分别进行了拉伸及压缩试验,旨在研究具有双峰组织和均匀组织特征的ZA21镁合金棒材的拉伸?压缩不对称性（拉?压不对称性）,并揭示相应变形机制。结果表明,拉伸和压缩状态下双峰组织的屈服强度分别为206.42和140.28 MPa,高于均匀组织在拉伸和压缩状态的屈服强度,分别为183.71和102.86 MPa。变形过程中,拉伸状态下的柱面滑移和拉伸孪生、压缩状态下的基面滑移和拉伸孪生主导了屈服行为,导致了轴向拉?压不对称性。然而,由于拉伸状态下细晶中基面滑移的激活以及压缩状态下细晶对拉伸孪生的抑制作用,相较于均匀组织较高的拉-压不对称性（0.56）,双峰组织的拉?压不对称性（0.68）明显降低。变形过程中出现了多种拉伸孪晶,孪晶变体的选择取决于母晶可能激活的六种变体的施密特因子。此外,通过改进的霍尔?佩奇公式,发现双峰组织对屈服的强化作用取决于平均晶粒尺寸以及粗晶和细晶的占比。     

退火纯钛板压缩力学性能的各向异性

沿退火纯钛板材轧向(RD 0°)、横向(RD 90°)以及轧制平面内与轧向成45°(RD 45°)等3个方向取圆柱形试样,采用Instron电子拉伸机和分离式Hopkinson压杆,进行准静态和动态压缩实验,获得不同应变率下的应力-应变曲线,计算3个方向的应变率强化效应.研究结果表明:退火纯钛板准静态和动态压缩力学性能均表现出明显的各向异性,其中RD 90°方向屈服强度最大,RD 45°方向次之,RD0°方向屈服强度最小;在较小的应变程度下流变应力也具有同样的规律.不同方向上的应变率强化效应也存在显著差异:RD0°方向最强,RD 45°方向次之,RD 90°方向最弱;基于纯钛{0001}〈11(2)0〉基面和{10(1)0} <1(2)10〉棱柱面滑移微观塑性变形机制,结合晶体塑性变形理论,考虑多晶板材晶体取向分布,定性解释了退火纯钛板压缩力学性能各向异性.     

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。     

挤压比对AZ31B镁合金组织和室温力学性能的影响

研究了不同挤压比对AZ31B镁合金显微组织?力学性能的影响?采用光学显微镜观察了挤压棒材显微组织,通过材料万能试验测试了拉伸和压缩性能,并配合扫描电镜观察了拉伸试样的断口处显微组织和形貌?结果表明,随着挤压比的增加,组织由部分动态再结晶向完全动态再结晶过渡;合金拉伸断口由混合断裂转变为明显韧性断裂,压缩断口由解理断裂转变为准解理断裂?大挤压比可以获得良好的综合性能和细致均匀的组织,抗拉强度为310MPa,屈服强度为200MPa,延伸率为14%,并且拉伸压缩不对称性得到缓解,有利于镁合金的二次塑性加工?     

不同路径等通道转角挤压镁合金的结构与力学性能

为了研究等通道转角挤压时不同工艺路径对镁合金微观结构及性能的影响 ,采用模角φ =12 0°的模具 ,以A ,BA,BC,C四种工艺路径对AZ31镁合金进行了等通道转角挤压 ,分析测试了室温下挤压试样的微观结构及性能 .结果表明 ,相比于A ,BA,C路径挤压 ,BC 路径挤压容易实现较多的挤压道次和变形量 ;多道次挤压后 ,镁合金的晶粒得到显著细化 ,力学性能也显著改善 ,但不同路径的影响不同 .当挤压 12道次时 ,BC,BA 路径挤压试样的屈服强度显著下降 ,延伸率大幅度提高 ;A ,C路径挤压试样的屈服强度变化较小 ,延伸率的提高幅度也小 .     

优异力学性能的AZ31镁合金挤压板材:多类型织构的作用

采用横向梯度挤压（TGE）和传统挤压（CE）工艺制备Mg–3Al–1Zn（AZ31）镁合金板材,系统地研究了镁合金在挤压工艺中流变和动态再结晶行为,并对挤压AZ31镁合金板材的微观组织、织构和力学性能进行了分析。结果表明,由于在横向梯度挤压工艺中引入了沿板材横向额外流速和沿挤压方向流速差,板材具有细小晶粒的微观组织和多种类型的织构。板材横向从边缘到中心基极逐渐偏离法线方向,在板材中心区域达到最大倾角65°。此外,除了横向梯度挤压板材中心区域外,板材基极沿挤压方向向横向偏转40°–63°。与传统挤压板材相比,横向梯度挤压板材具有高的延展性和应变硬化指数（n值）,低的屈服强度和Lankford值（r值）。由于横向梯度挤压板材在变形过程中基面滑移和拉伸孪晶容易被激活,板材延伸率最高可达41%,屈服强度低至86.5 MPa。     

AZ31镁合金轧制板材各向异性力学性能研究

在几种厚度的AZ31镁合金轧制板材上沿不同方向取样进行常温单向拉伸和压缩实验,研究了AZ31镁合金轧制板材的各向异性力学性能。基于晶体塑性理论,探讨了织构对金属板材宏观各向异性的影响。分析表明,轧制镁合金板材具有明显的各向异性力学性能及拉压不对称性。在轧制(RD)方向的抗压及抗拉屈服强度明显小于横向(TD),各个方向的抗拉屈服强度明显大于抗压屈服强度。不同轧制工艺对板材的力学性能影响较显著,主要表现在屈服应力不同和延伸率不同。基于实验结果与晶体塑性理论,本文从多角度分析了轧制工艺对AZ31镁合金各向异性力学性能及拉压不对称性行为的影响。     

高应变速率下钛-钢复合板界面组织特征及变形机制

在高应变速率下,钛-钢复合板不同材料以不同的变形机制协调变形,结合界面起到至关重要的作用.本文分析研究了高应变速率下钛-钢复合板的界面组织特征和变形机制.结果表明:在钢侧,随着应变速率的提高,小角度(3°~10°)晶界含量增多,织构组分{1-12}〈2-41〉逐渐演变为织构{6-65}〈38-6〉和{111}〈1-10〉.在钛侧,随着应变速率的提高,出现了明显的形变孪晶组织,三种形变孪晶如{11-21}〈1-100〉拉伸孪晶、{11-22}〈11-23〉压缩孪晶和{10-12}〈10-11〉拉伸孪晶产生的难易程度不一样,变形机制由常规的"孪生变形为主"转变为"位错滑移与孪生变形共存"的复合变形模式.在结合界面处,随着应变速率的提高,需要适应由两侧产生的不同变形抗力,才能够实现连续变形而不致使材料发生破坏,其主要的协调机制依靠结合界面及附近晶粒的滑移实现变形.     

异速比对异步轧制AZ31镁合金板材组织和织构的影响

采用不同异速比对AZ31镁合金板材进行异步轧制,并将轧后样品进行显微组织和X射线衍射分析,研究异速比对镁合金板材组织和织构转变的影响. 结果表明:异速比的变化对晶粒形貌影响较大但晶粒细化效果不明显;当异速比为2.800时,板材内出现了长条晶粒;快速辊侧{0002}基面织构强度高于慢速辊侧,且板材两侧表面{0002}晶面的偏转方向相反;异速比对基面织构的强度影响显著,随着异速比的增大,基面织构的强度先增加后下降. 这种特殊的织构变化与异步轧制过程中沿厚度方向引入的剪切变形有关.     

不同取向条件下Al-Cu-Mg-Sc-Zr合金薄板的组织与性能

采用室温拉伸性能测试、金相组织观察、透射电子显微分析以及取向分布函数(ODF)测定研究T3态Al-Cu-Mg-Sc-Zr合金(2524SZ-T3)薄板在不同取向条件下的显微组织和力学性能.研究结果表明:合金薄板在与轧制方向呈30°,60°和90°方向上的强度较0°和45°方向上的强度稍低,伸长率则在60°方向上最高,薄板的抗拉强度、屈服强度和伸长率平面各向异性指数分别为0 9％,3 3％和6.6％; 2524SZ-T3态铝合金薄板具有明显的再结晶织构｛110}〈111〉和CubeND旋转立方织构{001}〈310〉,同时还保留较弱的轧制织构即Goss织构{011} <000〉;不同取向条件下2524SZ-T3合金薄板的平面各向异性与合金的晶粒结构以及晶体学织构密切相关.     

Effect of initial orientation on microstructure and mechanical properties of AZ31 Mg alloy sheets via accumulated extrusion bonding

In this study, effects of initial orientation on microstructure evolution and mechanical properties of AZ31 Mg alloy sheets via accumulated extrusion bonding(AEB) was systematically studied. The samples with RD and TD parallel to extrusion direction(ED) were labeled as RED and TED, respectively. RD and TD pieces alternately stacked was named as RTED. The results revealed that under three-dimensional compressive stress, {10-12} tensile twinning dominated the first stage deformation in container. ...     

AZ91铸造镁合金屈服行为实验研究

摘要：在室温下,本文通过压剪试件（SCS）法对AZ91铸造镁合金进行不同加载速率下（4.8/48/480mm/min）的复合压剪实验,对AZ91铸造镁合金复杂应力状态下的力学行为进行研究。再结合单轴拉伸实验、单轴压缩实验和“帽型”试件剪切实验的结果,得到AZ91铸造镁合金的实验初始屈服面。通过实验,以及实验屈服面、Ellipse准则理论屈服面和极限应变能强度理论（LESET）屈服面的比较,可以发现：在本文研究的应变速率范围内（0.01～1s-1）,AZ91铸造镁合金弹性阶段具有明显的应变率负敏感性,初始屈服强度随着应变速率的增大而减小;Ellipse准则对AZ91镁合金屈服的判定优于LESET,可以比较准确的描述AZ91铸造镁合金在复合压剪应力状态下的屈服行为;由实验屈服面和Ellipse准则理论屈服面可以初步判断,正应力对AZ91铸造镁合金的屈服具有促进作用,而静水压力对其有抑制作用。     

AZ31B镁合金I型材室温弯曲成型研究

由于镁合金AZ31B材料在拉伸与压缩时的力学性能差异,使AZ31B型材在室温下成型规律很难掌握.文章以AZ31B镁合金I型材为研究对象,通过对I型材弯曲过程的数值分析,研究了I型材成型过程与加强筋的高度与厚度的关系,同时对成型过程中的受力、回弹现象及翘曲做了深入的分析,得出了对工程有实用性的结论,可以指导工程实际.     

Mechanical behavior and microstructure evolution for extruded AZ31 sheet under side direction strain

AZ31 magnesium alloy sheets were prepared by a conventional extrusion (CE) and a novel integrated extrusion with side direction strain (SE). The microstructure characterizations, crystallographic texture and mechanical property tests were carried out and compared between the extruded Mg alloy sheets processed by CE and SE. The results indicated that the SE sheets exhibited an excellent combination of strength and ductility. To reveal the side strain effect, the finite element model was employed to investigate the effective stress and strain behavior of the AZ31 magnesium alloy sheets during CE and SE processes. It was found that the SE process was effective in weakening the stress and strain concentration. This implied that it developed an additional side direction strain through the sheet thickness during the hot extrusion. Meanwhile, the side strain shear paths could promote the local accumulation of dynamically recrystallized grains and increase the random high-angle boundaries to achieve weak (0002) basal texture. Important factors including the side strain path and extrusion parameters need to be taken into account to understand the deformation mechanism and microstructure evolution.     

Microstructure and tensile behavior of hot extruded AZ91 alloys at room temperature

AZ91 alloys were prepared by hot extrusion and its microstructure and tensile behavior at room temperature were investigated. Compared to as-cast ingot, the grain size of hot-extruded material is morerefined, the intermetallic phase Mg₁₇Al₁₂ is broken and dispersed discontinuously. Both strength and elongation of AZ91 are improved by hotextrusion. Tensile behavior and fracture surface of the experimental material were studied. Due to the change in microstructure, the fracture mechanism of extruded material is different from that of as-cast ingot, the latter is mainly a brittle fracture. Ductile fracture plays arole in hot-extruded AZ91 failure at room temperature.     

Characteristics of AZ31 Mg alloy joint using automatic TIG welding

The automatic tungsten-inert gas welding (ATIGW) of AZ31 Mg alloys was performed using a six-axis robot. The evolution of the microstructure and texture of the AZ31 auto-welded joints was studied by optical microscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and electron backscatter diffraction. The ATIGW process resulted in coarse recrystallized grains in the heat affected zone (HAZ) and epitaxial growth of columnar grains in the fusion zone (FZ). Substantial changes of texture between the base material (BM) and the FZ were detected. The {0002} basal plane in the BM was largely parallel to the sheet rolling plane, whereas the c-axis of the crystal lattice in the FZ inclined approximately 25° with respect to the welding direction. The maximum pole density increased from 9.45 in the BM to 12.9 in the FZ. The microhardness distribution, tensile properties, and fracture features of the AZ31 auto-welded joints were also investigated.     

Influence of slip system combination models on crystal plasticity finite element simulation of NiTi shape memory alloy undergoing uniaxial compression

The influence of various slip system combination models on crystal plasticity finite element simulation of Ni Ti shape memory alloy subjected to uniaxial compression deformation is investigated according to three combinations of slip systems, including combination of {010}100and {110}111slip modes, combination of {110}100and {110}111slip modes and combination of {110}100, {010}100and {110}111slip modes, which consist of 18, 18 and 24 slip systems, respectively. By means of simulating mechanical response,strain distribution, stress distribution and Schmid factor, it can be found that in terms of simulation accuracy,combination of {110}100and {110}111slip modes is in good agreement with combination of {110}100,{010}100and {110}111slip modes. The contribution of {110}100slip mode to plastic strain is primary in plastic deformation of Ni Ti shape memory alloy, whereas {010}100slip mode, which makes small contribution to plastic deformation, can be regarded as the unfavorable slip mode. In the case of large plastic strain, the {010}100slip mode contributes to the formation of(001) [010] texture component, while {110}100and {110}111slip modes facilitate the formation of γ-fibre(111) texture.     

AZ31镁合金无缝管挤压动态再结晶数值模拟研究

通过热压缩实验得出温度在300~450℃,应变率为0.01~1 s-1时的应力-应变曲线,建立了AZ31镁合金的动态再结晶模型。该模型用于AZ31镁合金无缝管挤压过程中动态再结晶过程的数值模拟,并通过金相观察得以实验验证。结果表明,在挤压速度确定的情况下,挤压温度对动态再结晶分数的影响最为明显。随着挤压温度的升高,动态再结晶体积分数明显增大。预测的动态再结晶体积分数与实验结果吻合。     

Twinning,grain orientation,and texture variations in Mg alloy processed by pre-rolling

A pre-rolling(PR) process with a small pre-strain was applied along the transverse direction(TD) and the extrusion direction(ED) of non-basal and basal textured AZ61 Mg alloys at room temperature to investigate the induced twinning behavior. The microstructural evolution of the alloys was used to evaluate the grain-boundary effect in terms of the texture variation. The results demonstrated that■ extension twinning was introduced by the PR process and that the volume fraction of twins increased with increasing thickness reduction. The subdivision of grains via twinning induced grain reorientation that generated a prominent basal texture with the c-axis parallel to the normal direction(ND) after PR. The textured Mg alloy with this c-axis//TD feature can promote twinning activity. The twinning performance was critical to initiate plastic deformation, therefore to determine deformation behavior at room temperature. The deformation mechanism was also addressed related to the extension twin lamellae.