钛合金动态塑性变形过程中绝热剪切带的形成机理

材料在应变速率高于102 s-1时的塑性变形被称为动态塑性变形.区别于准静态塑性变形,动态塑性变形涉及的复杂的高度局域化变形机制对材料的性能与寿命具有显著影响.绝热剪切带作为材料动态塑性变形过程中产生的特殊的变形结构,它对材料性能的影响引起了人们的高度关注.钛合金具有优良的力学性能,被用作结构材料,广泛应用于诸多行业,由于应用面广,钛合金会经常面临动态载荷产生绝热剪切带而失效,从而缩短使用寿命.因此,研究钛合金中绝热剪切带的形成机制对延长钛合金的使用寿命,改善钛合金的力学性能具有重要意义.然而,由于动态塑性变形的瞬时性、内部应力的复杂性等,还原绝热剪切带的形成过程具有相当大的难度.同时,钛合金结构的复杂性、变形过程中相的不稳定性等因素均提高了观察其内部绝热剪切带的难度.通过大量的实验观察与模拟计算,目前较为普遍的观点为钛合金中绝热剪切带的形成机制为动态再结晶.而动态再结晶的过程目前有四种主流的观点,分别是传统动态再结晶、连续动态再结晶、孪生动态再结晶与相变诱发动态再结晶.针对不同的动态再结晶方式,研究者们建立了基本的变形模型与理论依据,并找到了一定的实验证据.本文通过总结近年来学者们对钛及钛合金动态塑性变形行为研究的典型成果,重点介绍了绝热剪切带的形貌与性能及其形成的不同机制.同时对钛合金中绝热剪切带的几种不同的形成机制及其研究中存在的问题进行了分析讨论,旨在为未来的研究探索提供有用的参考.     

钛/钢爆炸复合界面的微观组织结构和力学行为

首次在TEM下直接观察和研究了钛(TA2)/钢(A3)界面结合层内的微观组织结构,从而在更微观的尺度上确立了界面结合层内的组织结构模型。揭示了爆炸复合界面通过局部熔化和扩散的物理冶金过程实现其“冶金结合”的机制。所建立的温度场模型可用以预测和分析界面结合层内的微观组织结构。 第一次利用TEM直接观察并研究了界面结合层内TA2例所产生的绝热剪切带(ASB)内的微观组织结构,结合ASB内的形变热力学条件首次利用动态再结晶理论和起塑变形理论阐明了ASB内细小(<0.1μm)等轴晶粒组织的产生机制及ASB内大剪切应变机制。首次基于材料热粘塑性本构失稳理论对材料本身的物理-力学-热学性能及其晶体结构相耦合进行综合分析,阐明了界面结合层内仅在TA2侧产生、ASB而在A3侧从未产生ASB的机制。 通过对界面微观断裂过程的动态观察和分析,揭示了其不同波形状态界面的微观断裂机制。 深入系统地研究了界面扩散反应区内的微观组织结构和反应相的形成、生长规律。所得结论可指导TA2/A3复合材的工业生产和应用。 本研究中有关金属在冲击载荷下的塑性变形机制和力学行为(孪生、绝热剪切等)的研究结果对研究金属在高应变率冲击载荷下的力学冶金行为具有指导意义。     

高应变速率下TC11钛合金动态剪切行为与性能

采用分离式霍普金森压杆（Hopkinson Bar）装置系统,对TC11钛合金进行室温高应变速率（700-2100s^-1）动态剪切试验,通过光学显微镜、显微硬度分析仪、扫描电镜研究了TC11钛合金动态剪切行为、绝热剪切带微观组织与性能。结果表明：TC11钛合金随应变速率的提高绝热剪切敏感性增加;绝热剪切带由过渡区域的变形拉长组织和中间部位的细小晶粒组织组成,具有清晰的剪切变形流线,宽度约为10μm;绝热剪切带内的显微硬度值高于基体组织,是,由应变速率强化和应变强化与热软化相互作用的结果。     

动态载荷下7005铝合金力学行为及数值模拟

本工作研究了7005铝合金在应变速率为(1～5)×103s-1条件下的力学行为。结果表明,7005铝合金有明显的应变速率敏感性。利用最小二乘法求得了材料的Johnson-Cook本构参数;应用ABAQUS软件,研究了高应变速率下的帽型试样的绝热剪切变形历程。数值模拟得到的应力-应变曲线与实验结果吻合。温度场的计算为绝热剪切带内微观组织是否发生动态再结晶提供了依据,当应变速率为15 000 s-1时,在120～240μs内,相比初始温度,试样的平均温升达到405℃。通过对冲击后试样的微观组织观测发现,绝热剪切带中有大量的等轴晶,具有典型的再结晶组织特征。7005铝合金在高应变速率下的变形温升和动态再结晶软化行为,将为其在汽车碰撞构件中的应用提供指导作用。     

高速切削淬硬钢切屑变质层微结构及形成机理

为了揭示高速切削过程中切屑形成以及刀具-切屑界面摩擦机理,对剪切区内剪切带及白层进行了微观观察和分析.利用光学显微镜,SEM,电子探针,TEM和X射线衍射等方法对高速切削淬硬钢锯齿状切屑剪切变形区内变质层的微细组织进行了观察,对变质层的微结构本质及其形成机理进行了分析.研究表明:在锯齿状切屑锯齿之间的第一变形区存在绝热剪切带,由切屑基体到剪切带中心依次出现马氏体板条、沿剪切方向拉长并被位错分割的板条和等轴晶粒等不同的微结构特征;在切屑底部的第二变形区存在白层,其微结构显示了非晶组织特征.剪切带形成过程中没有相变发生,其形成是基于一种旋转式动态再结晶机制,白层的形成过程中发生了非晶转变和马氏体相变,形成机理属于相变机制.     

工业纯钛TA2剪切带中微观组织的演变

剪切变形局域化是结构材料经受冲击时的一种重要失效机制,为研究密排六方晶体结构金属材料的绝热剪切带形成条件与扩展规律,采用HOPKINSON压杆装置对精加工后的工业纯钛帽形样品进行高速冲击,利用扫描电镜和高分辨透射电镜研究了剪切带形貌和剪切带微观组织的演化过程.结果表明,工业纯钛TA2经高速冲击后,在帽形样品的韧带部位形成了明显的剪切带,剪切带组织由细小的再结晶晶粒组成,剪切带内没有相变发生,剪切带内的动态再结晶过程通过渐进式亚晶位相差再结晶机制完成.     

变形条件对2519A铝合金动态力学性能与组织演化的影响

为研究温度与应变率对2519A铝合金动态力学行为及组织演化的影响,采用霍普金森压杆对2519A铝合金进行了不同温度(-90~350℃)、不同应变率下的动态冲击压缩实验,分析了该合金的动态力学性能,并结合金相显微镜与透射电镜对合金在冲击变形后的微观组织进行分析。结果表明:在250~350℃的高温环境冲击下,合金的流变应力迅速下降,组织以形变带为主,同时组织内伴随有明显的动态回复和动态再结晶。在20~150℃的环境中进行动态冲击,合金变形时组织出现了典型的绝热剪切带特征。在室温、应变率达到8200s-1时,应变率强化效果发生转变。随着温度降至-90℃,在绝热剪切带内的组织出现了长度较短、连续性差的微裂纹,同时组织内的长条状第二相粒子发生不同程度的脆性断裂。     

高熵合金动态载荷下变形机制的研究进展

因具有一系列特殊的结构和性能,高熵合金在短短十几年间就从一种新型的合金设计理念成为了高性能结构材料的明日之星.近年来,研究者们相继开展了高熵合金的动态力学行为和变形机制的研究,旨在推进高熵合金的实用化进程、夯实高熵合金动态力学行为的理论基础并进一步丰富高熵合金的内涵.本文综述了高熵合金动态变形机制的研究进展,对高熵合金在动态载荷下的位错运动、孪生变形、应变诱发相变以及绝热剪切效应进行了总结和分析.在此基础上,本文认为高熵合金在动态载荷下的变形机制与其在准静态载荷下的变形机制之间,既有相互关联的相似性,又有值得关注的差异性.具体而言,动态变形由位错运动主导的高熵合金,因受到热激活机制、拖曳机制和位错间强相互作用的影响,而具有显著的应变率效应、应变敏感性和强应变硬化能力.其中,动态载荷下的位错运动会受到晶格畸变、短程有序、第二相等一系列微结构的影响.此外,层错能较低的面心立方型高熵合金的动态变形过程、亚稳高熵合金的动态变形过程以及难熔高熵合金的动态力学行为,还会分别受到孪生变形、应变诱发相变效应以及热效应和变形局域化引发的绝热剪切效应的显著影响.     

镁合金塑性变形力学行为与微观组织研究进展

介绍了镁合金在单轴压缩、单轴拉伸、轧制和挤压条件下塑性变形的力学行为及微观组织结构演变规律。简述了镁合金中二次拉伸孪生现象以及各种变形条件下孪生与孪生变体类型的选择规律。基于对镁合金位错滑移、机械孪生及动态回复与再结晶行为的认识,对镁合金力学行为的各向异性、轧制与挤压成型能力的影响规律进行了探讨,强调了初始织构对变形机制、动态再结晶及成型能力的重要影响。最后讨论了析出强化镁合金塑性变形与强韧化机理。     

激光立体成形TC4钛合金的力学特性与破坏机理

对激光立体成形TC4钛合金在较宽温度(173～1173K)和应变率(0.001～10~5s~(-1))范围内,分别进行压缩、拉伸和剪切试验。结果表明:该合金无明显的力学各向异性;静态和动态加载时均存在明显的拉伸-压缩力学不对称性;其强度低于锻造和挤压成形TC4钛合金。微观分析表明:动态压缩和剪切加载时该合金易出现绝热剪切变形,但试验温度的升高会抑制绝热剪切带的产生;静态拉伸时断口为含韧窝和准解理台阶的混合性形貌,而动态拉伸时则有韧窝而没有解理台阶。动态压缩断裂机理是剪切带内形成空位和裂纹,裂纹沿α/β界面扩展形成断面。     

装甲用镁合金抗弹性能表征体系探讨

介绍了高应变率载荷条件下镁合金的吸能特性及变形特征；论述了对镁合金抗弹性能有重要影响的动态强度、高应变率能量吸收率、高应变率变形断裂特征和动态强度等科学问题；就镁合金在装甲领域的应用研究做了初步探讨。     

镁合金动态力学性能的研究现状及发展方向

概述了镁合金高应变加载条件下的塑性变形微观机制和损伤形式.综述了国内外镁合金动态力学性能的研究现状.针对镁合金动态力学性能的不足之处,指出未来镁合金动态力学性能研究的发展方向为:系统研究现有牌号镁合金的动态力学性能;探讨合金元素、加工工艺和热处理对镁合金动态力学性能的影响;构建镁合金动态本构方程.     

Failure of AA 6061 and 2099 aluminum alloys under dynamic shock loading

Microstructural aspects of the deformation and failure of AA 6061 and AA 2099 aluminum alloys under dynamic impact loading are investigated and compared with their responses to quasi-static mechanical loading in compression. Cylindrical specimens of the alloys, heat-treated to T4, T6 and T8 tempers, were subjected to dynamic compressive loading at strain rates of between 2800 and 9200 s⁻¹ and quasi-static compressive loading at a strain rate of 0.0032 s⁻¹. Plastic deformation under the dynamic impact loading is dominated by thermal softening leading to formation of adiabatic shear bands. Both deformed and transformed shear bands were observed in the two alloys. The shear bands offer preferential crack initiation site and crack propagation path in the alloys during impact loading leading to ductile shear fracture. While cracks propagate along the central region of transformed bands in AA 6061 alloy, the AA 2099 alloy failed by cracks that propagate preferentially along the boundary region between the transformed shear bands and the bulk material. Whereas the AA 2099 alloy shows the greatest propensity for adiabatic shear banding and failure in the T8 temper condition, AA 6061 alloy is most susceptible to formation of adiabatic shear bands and failure in the T4 temper. Deformation under quasi-static loading is dominated by strain hardening in the two alloys. Rate of strain hardening is higher for naturally aged AA 6061 than the artificially aged alloy, while the strain hardening rate for the AA 2099 alloy is independent of the temper condition. The AA 2099 alloy shows a superior mechanical behaviour under quasi-static compressive loading whereas the AA 6061 shows a higher resistance to impact damage.     

镁合金在大变形和高应变率下塑性变形研究进展

介绍了强应变塑性大变形下镁合金研究现状.重点综述了在较高应变率及冲击载荷作用下关于镁合金变形的研究情况,同时也比较详细地综述了在不同温度、不同载荷作用下镁合金塑性变形特征及其物理机制.最后简要介绍了几个描述材料在较高应变率和冲击载荷作用下变形行为的数学表示式,并就镁合金作为结构材料的研究说明了作者的一些看法.     

Effects of nano-sized α<Superscript>2</Superscript> (Ti<Superscript>3</Superscript>Al) particles on quasi-static and dynamic deformation behavior of Ti-6Al-4V alloy with bimodal microstructure

A bimodal microstructure containing very fine α²(Ti³Al) particles was produced by over-aging a Ti-6Al-4V alloy. The effects of α² precipitation on quasi-static and dynamic deformation behavior were investigated in comparison with an unaged bimodal microstructure. Quasi-static and dynamic torsional tests were conducted on them using a torsional Kolsky bar. The quasi-static torsional test data indicated that the over-aged bimodal microstructure showed higher fracture shear strain than the unaged bimodal microstructure, while their maximum shear stresses were similar. Under dynamic torsional loading, both maximum shear stress and maximum shear strain of the over-aged microstructure were higher than those of the unaged microstructure. The possibility of the adiabatic shear band formation under dynamic loading was quantitatively analyzed by introducing concepts of critical shear strain, absorbed deformation energy, and void initiation. In the over-aged microstructure, the energy required for forming adiabatic shear bands was higher than that in the unaged microstructure, thereby lowering the possibility of the adiabatic shear band formation. The α² precipitation in the over-aged microstructure was effective in both the improvement of quasi-static and dynamic torsional properties and the reduction in the adiabatic shear banding, which suggested a new approach to improve ballistic performance of Ti alloy armor plates.     

Effects of particulate reinforcement and strain‐rates on deformation and fracture behavior of Aluminum 6061‐T6 under high velocity impact

Aluminum matrix composites (AMC) exhibit an attractive combination of mechanical and physical properties such as high stiffness and low density, which favors their utilization in many structural applications. Thus, increasing the structural applications of AMC is the driving force for the need to adequately understand their deformation and failure mechanisms under various types of loading conditions. In this study, plastic deformation of alumina particle reinforced Aluminum 6061‐T6 matrix composite is investigated and compared to that of an un‐reinforced Aluminum 6061‐T6 alloy at high strain‐rates under compressive loading. Dynamic stress‐strain curves are obtained using direct impact Split Hopkinson Pressure Bar (SHPB). Particulate reinforcement increases the deformation resistance of the aluminum alloy at high strain‐rates. Strain localization along narrow adiabatic shear bands is observed in both the reinforced and un‐reinforced alloy. Whereas the microstructure of shear bands in un‐reinforced alloy showed finer grain size compared to that of the bulk material, the shear bands observed in the AMCs are darker than the bulk material and the reinforcing particles are observed to be more closely spaced along the shear bands.     

Microstructure and mechanical behavior of ECAP processed AZ31B over a wide range of loading rates under compression and tension

In this work, a commercial magnesium alloy, AZ31B in hot-rolled condition, has been subjected to severe plastic deformation via four passes of equal channel angular pressing (ECAP) to modify its microstructure. Electron backscatter diffraction (EBSD) was used to characterize the microstructure of the as-received, ECAPed and mechanically loaded specimens. Mechanical properties of the specimens were evaluated under both compression and tension along the rolling/extrusion direction over a wide range of strain rates. The yield strength, ultimate strength and failure strain/elongation under compression and tension were compared in detail to sort out the effects of factors in terms of microstructure and loading conditions. The results show that both the as-received alloy and ECAPed alloy are nearly insensitive to strain rate under compression, and the stress–strain curves exhibit clear sigmoidal shape, pointing to dominance of mechanical twinning responsible for the plastic deformation under compression. All compressive samples fail prematurely via adiabatic shear banding followed by cracking. Significant grain size refinement is identified in the vicinity of the shear crack. Under tension, the yield strength is much higher, with strong rate dependence and much improved tensile ductility in the ECAPed specimens. Tensile ductility is even much larger than the malleability under compression. This supports the operation of 〈c + a〉 dislocations. However, ECAP lowers the yield and flow strengths of the alloy under tension. We attempted to employ a mechanistic model to provide an explanation for the experimental results of plastic deformation and failure, which is in accordance with the physical processes under tension and compression.     

镁合金的塑性变形机制和孪生变形研究

概述了镁合金的塑性变形机制,介绍了镁合金的主要孪生系及其表征技术,详细分析了变形温度、变形速率、受力方向和晶粒尺寸等对镁合金孪生变形的影响,讨论了孪生变形对镁合金塑性变形、动态再结晶、力学性能与断裂的影响。孪生通常发生在粗大晶粒中,晶粒细化可以激活镁合金中的非基面滑移,抑制孪生变形和降低镁合金的各向异性,指出细晶镁合金的研制和工业化生产是变形镁合金发展的重要方向。     

Effects of strain rates on dynamic deformation behavior of Cu-20Ag alloy

Copper alloy is widely used in high-speed railway,aerospace and other fields due to its excellent electri-cal conductivity and mechanical properties.High speed deformation and dynamic loading under impact load is a complex service condition,which widely exists in the field of national defense,military and industrial application.Therefore,the dynamic deformation behavior of the Cu-20Ag alloy was inves-tigated by Split Hopkinson Pressure Bar (SHPB) with the strain rates of 1000-25000 s-1,high-speed hydraulic servo material testing machine with the strain rates of 1-500 s-1.The effect of strain rate on flow stress and adiabatic shear sensitivity was analyzed.The results show that the increase of strain rate will increase the flow stress and critical strain,that is to say,the increase of strain rate will reduce the adiabatic shear sensitivity of the Cu-20Ag alloy.The Cu-Ag interface has obvious orientation relationship with (111)Cu//(111)Ag;((1)11)Cu//((1)11)Ag;((2)00) Cu//((2)00)Ag and[0(1)1]Cu//[0(1)1]Ag with the increase of strain rate.The increase of strain rate promotes the precipitation of Ag and increases the number of interfaces in the microstructure,which hinders the movement of dislocations and improves the stress and yield strength of the Cu-20Ag alloy.The concentration and distribution density of dislocations and the precipitation of Ag were the main reasons improve the flow stress and yield strength of the Cu-20Ag alloy.     

GCr15SiMo钢的动态及高温力学行为EI北大核心CSCD

以GCr15SiMo钢为对象,研究热处理工艺对其微观组织的影响规律,并利用霍普金森杆和GNT100-2型高温拉伸试验机,分析不同组织结构GCr15SiMo钢的动态及高温力学行为。结果表明:淬火温度由800℃升高至920℃,GCr15SiMo钢中M_(3)C型碳化物颗粒的质量分数由2.319%减少至0%;动态压缩过程中,GCr15SiMo钢的失效应变均随应变速率的增加而增大,在真应变分别为0.2和0.8时,随着淬火温度的升高,GCr15SiMo钢流变应力分别下降13.45%,21.44%,27.49%和31.79%,流变应力迅速下降主要与组织结构和动态压缩变形时的绝热剪切机制有关;在高应变速率条件下,GCr15SiMo钢的宏观变形由镦粗转变为沿45°方向的剪切破坏,绝热剪切机制是导致变形行为变化的主要原因之一,且组织结构是影响材料绝热剪切敏感性的关键因素之一;GCr15SiMo钢动态压缩变形过程中形变升温在117~333℃之间,M_(3)C碳化物颗粒回溶是其高温性能呈现抗拉强度增加、屈服强度降低的关键因素之一;淬火温度为920℃时,GCr15SiMo钢的组织为均匀一致的孪晶马氏体,孪晶马氏体中的亚晶界可有效阻碍位错运动,在拉伸应力作用下表现出明显的应变硬化现象,应力-应变曲线较淬火温度800℃时呈现更显著的上升趋势。