Internal ductile failure mechanisms in steel cold heading process

The occurrence of internal ductile failure in cold-headed products presents a major obstacle in the fast expanding cold heading (CH) industry. This internal failure may lead to catastrophic brittle fracture under tensile loads despite the ductile nature of the material. Comprehensive testing and investigation methodologies were used to this work to reveal the complicated interplay of process and material parameters contributing in the initiation and propagation of internal ductile failure in six CH quality AISI steel grades.The metallurgical and microscopic investigations showed that internal ductile failure occurs progressively by void nucleation and growth mechanisms with increasing plastic strain inside the highly localized adiabatic shear bands (ASBs). The void nucleation occurs by decohesion at second-phase particles, inclusion–matrix interfaces, grain boundaries and by particle or inclusion cracking. Therefore, the number and morphology of any inclusions and second-phase particles are key factors in material formability.The metallurgical investigations showed that under compressive loading conditions, the nature of the metal flow pattern promotes different rates of material flow around the inclusions and stringers which supports decohesion and void nucleation since the early stages of deformation. At advanced stages of deformation, the metal flow pattern contributes to the ASB localization in supporting void growth and coalescence along the band leading to narrow void sheets.All tested materials in this work experienced ductile failure by void nucleation and coalescence, forming cracks along the ASBs. The ductile failure of each material was the result of the contribution of all the mechanisms of void nucleation at the inclusion–matrix interface, second phase–matrix interface and at the grain boundaries. However, the level of contribution of each mechanism in the final ductile failure varied depending on material properties and their microstructure.     

冶金因素影响绝热剪切带形成的金相观察

对在相同爆炸复合条件下几种不同状态的金属所产生的绝热剪切带(ASB)的显微组织进行了观察及显微硬度测定，探讨了不同的相组成、预变形程度和晶体结构对绝热剪切带形成的影响。结果表明：ASB的形态、大小和数量受材料的预冷变形程度、成分、晶体结构、相组成等因素的影响；ASB的硬度值均高于基体。     

不同取向疲劳态铜单晶高速冲击下的绝热剪切带

采用分离式Hopkinson压杆装置（SHPB）和扫描电镜电子通道衬度（SEM—ECC）技术研究了不同取向疲劳态铜单晶高应变率压缩下形成的绝热剪切带（ASB）．实验表明，ASB形成的临界应变与晶体取向有关，接近压缩临界双滑移取向晶体需要的临界应变最小，单滑移和压缩共轭双滑移取向的次之，共面双滑移取向的最大．本实验条件下形成的ASB内部典型的位错组态为位错胞结构，未观察到再结晶现象．根据空间位向，ASB可以分为3类：第1类非常接近铜晶体疲劳时形成的第2类形变带（DBII）平面，其临界应变最小；第2类ASB位向或者比较接近DBII平面或者比较接近第1类形变带（DBI）平面，其临界应变居中；第3类ASB位向与DBI和DBII平面均不接近，其临界应变最大．     

Phenomenon of transformed adiabatic shear band surrounded by deformed adiabatic shear band of ductile metal

The coexistent phenomenon of deformed and transformed adiabatic shear bands（ASBs） of ductile metal was analyzed using the JOHNSON-COOK model and gradient-dependent plasticity（GDP）. The effects of melting point, density, heat capacity and work to heat conversion factor were investigated. Higher work to heat conversion factor, lower density, lower heat capacity and higher melting point lead to wider transformed ASB and higher local plastic shear deformation between deformed and transformed ASBs. Higher work to heat conversion factor, lower density, lower heat capacity and lower melting point cause higher local plastic shear deformation in the deformed ASB. Three reasons for the scatter in experimental data on the ASB width were pointed out and the advantages of the work were discussed. If the transformed ASB width is used to back-calculate the internal length parameter in the GDP, undoubtedly, the parameter will be extremely underestimated.     

高速冲击下钢板的微观组织及绝热剪切带

用烧结的钨合金弹丸垂直冲击45钢与30CrMnMc钢两种钢板，45钢钢板穿孔周围的微观组织沿径向可分为熔化快凝层、再结晶细晶层、变形细晶层，形变层及正常基体组织，该材料中没有发现绝热剪切带．30CrMnMo钢板穿孔周围的微观组织表面是熔化快凝层，其下是扩散层、形变层和基体组织，钢中形成了几类绝热剪切带，由于冲击波和反射波的作用、复杂的塑性变形及侵彻过程本身的原因，它们具有不同的方向和宽度冲击45钢钢板和30CrMnMo钢钢板的弹丸的变形形式相差很大，冲击45钢钢板的弹丸被墩粗且损耗较慢，冲击30CrMnMo钢钢板的弹丸为剪切变形，损耗速度快．分析表明，绝热剪切带的发生与材料的强度、冲击下材料的应变量有关．     

30MnCrNiMoB低合金钢绝热剪切带的形成机制与微结构

研究了３０ＭｎＣｒＮｉＭｏＢ低合金钢在穿甲弹冲击下绝热剪切带（ＡＳＢ）的形成机制。指出在高速变形（冲击）条件下ＡＳＢ的形成是应变硬化、热软化和位错动态发时导致的应变软化等因素相互耦合产生的塑性失稳现象。进一步确认ＡＳＢ内具有周期性微观结构，并说明ＡＳＢ中周期性开裂与微结构的关系。     

热处理对铀铌合金变形局域化的影响

对多温度点人工时效(200℃/2 h、400℃/2 h、600℃/2 h)铀铌合金在冲击载荷作用下的绝热剪切带形成机理进行了研究,采用Hopkinson压杆装置对铀铌合金帽形样品进行了冲击加载,利用光学显微镜和扫描电镜对冲击后的回收样品进行了显微组织观察。结果表明,显微组织对剪切带的形成具有重要影响,只在200℃/2 h与600℃/2 h帽形样品台阶处观察到不同类型的绝热剪切带,400℃/2 h样品在冲击过程表现出韧性断裂与脆性断裂的特征,某些区域已经接近或达到熔点。     

Effects of constitutive parameters on adiabatic shear localization for ductile metal based on JOHNSON-COOK and gradient plasticity models

1Introduction Adiabatic shear band(ASB)is a very narrow zone with a high concentration of shear strain.It is believed that ASB is formed by a process of thermo-mechanical instability.ASB can be observed in the process of dynamic deformation of various fer…     

高速弹丸冲击诱发TC32钛合金中绝热剪切带的组织演化行为

采用光学显微镜(OM),扫描电子显微镜(SEM)和透射电子显微镜(TEM)研究了Ti-5Al-3Mo-3Cr-1Zr钛合金在高速弹丸冲击后的组织特征和演变行为。在弹坑周围,绝热剪切带(ASB)和剪切应力一直呈半圆状分布。观察到绝热剪切带中尺寸较大等轴晶粒和细长板条亚晶粒的旋转细化过程。采用聚焦离子束(FIB)技术准确地从ASB中的裂纹尖端取样制备TEM样品,在裂纹尖端区域周围发现非晶区、非晶-纳米晶过渡区域、细小纳米晶区共存。计算结果表明,绝热剪切带内温度升高可导致微观组织熔化,快速淬火后形成非晶区和细小的纳米晶。由于绝热剪切带中的显微组织是细小的等轴晶和非晶,具有较高的强度,使形变带与基体之间成为相对较弱的区域,绝热剪切带中的裂纹也主要在该区域萌生,裂纹通过微孔洞旋转联结的方式扩展。     

321-15CrMoR爆炸焊接复合板结合界面区的显微组织分析

采用金相显微镜、扫描电镜、电子探针和显微硬度计,对爆炸焊接321-15CrMoR复合板结合界面进行了研究,探讨爆炸焊接过程的金属物理学机理.结果表明,界面呈波形;界面附近基体组织产生了剧烈的塑性变形;321一侧存在绝热剪切带(ASB),ASB在局部发展成为微裂纹,波峰存在一薄层"白亮带",ASB和"白亮带"均由细小等轴晶粒组成;界面附近原子存在短程扩散和熔化长程扩散现象;界面区显微硬度显著提高.这些特征保证了复板和基板之间的快速优质冶金结合.     

Quantitative calculation of local shear deformation in adiabatic shear band for Ti-6Al-4V

JOHNSON-COOK（J-C） model was used to calculate flow shear stress-shear strain curve for Ti-6Al-4V in dynamic torsion test. The predicted curve was compared with experimental result. Gradient-dependent plasticity（GDP） was introduced into J-C model and GDP was involved in the measured flow shear stress-shear strain curve, respectively, to calculate the distribution of local total shear deformation（LTSD） in adiabatic shear band（ASB）. The predicted LTSDs at different flow shear stresses were compared with experimental measurements. J-C model can well predict the flow shear stress-shear strain curve in strain-hardening stage and in strain-softening stage where flow shear stress slowly decreases. Beyond the occurrence of ASB, with a decrease of flow shear stress, the increase of local plastic shear deformation in ASB is faster than the decrease of elastic shear deformation, leading to more and more apparent shear localization. According to the measured flow shear stress-shear strain curve and GDP, the calculated LTSDs in ASB are lower than experimental results. At earlier stage of ASB, though J-C model overestimates the flow shear stress at the same shear strain, the model can reasonably assess the LTSDs in ASB. According to the measured flow shear stress-shear strain curve and GDP, the calculated local plastic shear strains in ASB agree with experimental results except for the vicinity of shear fracture surface. In the strain-softening stage where flow shear stress sharply decreases, J-C model cannot be used. When flow shear stress decreases to a certain value, shear fracture takes place so that GDP cannot be used.     

钛合金电磁铆接数值模拟

为了更深入地认识钛合金铆钉成型时镦头内温度场的变化及其对绝热剪切带形成的影响,在ABAQUS平台上建立了3D有限元模型,综合考虑了材料的应变硬化、应变率硬化、热软化效应及摩擦和结构的非线性.计算表明:剪切带内应变高度集中,温升高达500℃,高温区域与剪切带基本重合,最高温度超过了其再结晶温度.     

Dynamic modelling of shear band formation and tool-tip vibration in ultra-precision diamond turning

This paper proposes a dynamic model to correlate the two basic physical phenomena in ultra-precision diamond turning, i.e. the formation of adiabatic shear band (ASB) and high frequency tool-tip vibration (HFTTV). The conventional approach explains the former using a static model without consideration of the latter. In this paper, a dynamic model is developed to reflect how the ASB and the HFTTV interactively affect each other. To illustrate the validity of this model, a novel experimental method is proposed and the effect of HFTTV on cutting force, surface roughness and chip morphology of ASBs is discussed in terms of the variation of strain rates.     

Microstructural characteristics of post-shear localization in cold-rolled 316L stainless steel

The microstructural evolution in a cold-rolled 316L stainless steel during shear localization was comprehensively studied using transmission electron microscopy (TEM). The TEM results indicate that the main substructure inside a shear band consists of elongated lath, fine rectangular and equiaxed subgrains. The substructure at an early stage of shear banding was found to strongly depend upon existing defects, especially deformation twin patterns. These twin structures determine the initial dimensions of the elongated subgrains inside the shear bands. The coexistence of both rectangular and equiaxed subgrains suggests that no melting occurred though the predicted temperature was much higher than the bulk melting temperature. Dynamic/static recovery and continuous dynamic recrystallization are thought to be the main mechanisms by which these substructures form inside the shear bands. A new mechanism for nanostructure deformation of subgrains within shear bands is proposed to explain the temperature divergence between the experimental and calculated results.     

基于混合位移-压力有限元的厚板精冲数值模拟和损伤断裂预测

在MSC／MARC有限元分析软件中建立了轴对称精冲有限元模型，利用基于混合位移一压力的更新Lagrange有限元方法对精冲局部剧烈塑性变形进行模拟，克服了剪切自锁和体积自锁，结合网格自适应技术，对剪切区进行网格细化，精确捕捉了应变集中现象并通过MARC用户子程序实现了Schiffmann损伤功密度的半耦合计算，预测了精冲过程中材料损伤的发展，并预测了不同工艺参数下零件的塌角高度、光亮带高度和精冲力．最后，利用模拟得出的最佳工艺参数获得了合格的零件．     

高碳硅锰钢的动态力学性能

通过扫描电镜、霍普金森压杆和拉伸试验机等仪器研究了不同温度淬火高碳硅锰钢的组织、变形程度和动态力学性能。研究表明,随淬火温度提高,试验钢中残留碳化物数量减少,在880 ℃时,试验钢的残留碳化物全部溶解。动态力学试验后,880 ℃淬火试样因残留碳化物固溶,动态强度增幅较小,对应变速率不敏感,由均匀变形转为破碎。绝热剪切带内和远离绝热剪切带的区域均存在孔洞,远离剪切带区域的孔洞无序分布,且伴随有均匀分布的碳化物。靠近绝热剪切带区域的碳化物存在分布不均匀的情况,且绝热剪切带内部的孔洞沿着热量传播方向扩展,形成沿剪切带分布的裂纹。     

Simulation of slip band evolution in duplex Ti–6Al–4V

Formation of slip bands plays an important role in deformation and fatigue processes of duplex Ti–6Al–4V. In this study, shear-enhanced crystal plasticity constitutive relations are proposed to account for the slip softening due to breakdown of the short-range order between titanium and aluminum atoms. A hybrid strategy is developed which allows the softening to occur in slip bands only within the primary α phase, with the degree of localization depending on the specific polycrystalline initial-boundary-value problem and the requirements for compatibility of each grain or phase with its neighbors. The proposed model is calibrated by performing finite-element (FE) simulations on an experimentally studied Ti–6Al–4V alloy. The slip behavior of a Ti–6Al–4V sample subjected to an in situ (scanning electron microscopy (SEM)) tensile test is investigated. A two-dimensional (2-D) FE with 3-D crystal plasticity relations is constructed to represent the microstructure of the Ti–6Al–4V sample. Due to the lack of access to fully 3-D microstructure, a generalized plane-strain condition is used in the FE model which assumes columnar grains that are free of net traction in the direction normal to the surface. The assumption of columnar grains significantly reduces the computational cost. The contours of effective plastic strain are compared with the surface SEM micrographs from experiments at various strain levels. It is shown that the proposed approach for modeling slip bands qualitatively captures experimentally observed slip band behavior.     

聚能射流穿甲后超高强度钢靶板的损伤特征及其机理

超高强度钢靶板在聚能射流穿甲后损伤特征的研究表明,在穿甲过程中,弹孔周围白层的温度超过了相变点,形成了晶粒尺度在20nm左右的马氏体 残余奥氏体混合组织.绝热剪切带（ASB）中的剪切仅发生在相当窄的层面上,其宽度约为2μm,平均切应变量为110,应变速率≥2.24×106s-1;光学显微镜下显示的剪切带宽度是集中剪切变形区（LSDZ）及其热影响区（HAZ）的总宽度.聚能射流穿甲过程中,靶板材料破坏包含了如下几个相互重叠的过程:与射流接触的靶板周围局部区域材料沿射流方向上的整体协调变形;射流与靶板之间的剪切断裂和弹孔表面部分靶材的熔化;集中在极薄层面卜高度局域化的剪切变形;应力波在靶表面反射造成的正向开裂.     

动态载荷下Al—Li合金剪切变形局部化

采用Ｈｏｐｋｉｎｓｏｎ压杆装置,对４种时效处理的Ａｌ－Ｌｉ合金在冲击压缩试验时变形局部化产生的力学条件和微结构进行了研究,结果指出,４种处理的合金均产生两种类型的剪切带即形变剪切带和白剪切带,它们是在局部化过程中不同形变阶段下形的。两者无本质上的差别。白色剪切带内未发生相变。大应变和高应变率是产生力剪切带的必要条件,即材料在一定的高应变率下形变到一定程度时才能形成剪切带,电镜研究表明,剪切带内无畸     

Heterogeneous deformation in twinning-induced plasticity steel

The influence of deformation localization during tensile deformation on the development of crystallographic texture in twinning-induced plasticity (TWIP) steel is reported. Whereas Al-added TWIP steel deforms homogeneously before the critical strain of onset of serrations, plastic deformation in Al-free TWIP steel occurs by propagation of isolated Portevin–Le Chatelier bands from the early stage of plastic deformation. The crystallographic texture evolution of both steels is, however, very similar. Deformation leads to the development of a Cu-type texture characterized by the strengthening of the rotated Brass component rather than the pure Brass component.