反复轧制过程中TA1纯钛板的组织演化及强化机制

采用反复轧制工艺制备了超细晶TA1纯钛板。通过金相、透射电镜、X射线衍射、扫描电镜等手段,分析了纯钛板在反复轧制过程中,不同的应变量所对应的组织形貌特点,并测试了强度、塑性,观察了宏观断口与微观形貌。结果表明:纯钛在常规轧机上经过反复轧制可显著细化晶粒,晶粒尺寸由轧制前的80μm降至120 nm;强度则随着轧制应变量的增加而提高,当Von Mises等效应变为2.4时,平均屈服强度提高到678 MPa,是轧制前粗晶的3倍多;位错及其交互作用是细化晶粒的主要机制,在高密度位错区域由于位错的交互作用而形成了位错胞和亚晶粒,最终演变成超细晶粒;细晶强化和加工硬化是导致纯钛轧制后强度显著提高的主要原因。     

异步轧制工业纯钛的组织与力学性能

采用异步轧制方式在室温下轧制TA1工业纯钛,获得平均晶粒尺寸小于300nm的超细晶组织,显著提升了材料的强度,并探讨了超细晶的形成和强化机制。经轧下量为87％的异步轧制后,材料内部形成均匀分布的狭长剪切带,组织中主要为等轴超细晶,亦存在少量拉长晶粒和小角度晶界;材料的显微硬度比原始热轧态提高了60％,抗拉强度提高至740MPa以上。     

室温ECAP变形纯钛孪生行为研究

室温下对纯钛进行多道次等径弯曲通道变形（ECAP）,分别采用光学显微镜（OM）、透射电镜（TEM）、电子背散射衍射仪（EBSD）、室温拉伸和显微硬度观察,测试纯钛变形过程组织演变和力学性能变化规律,探讨纯钛室温变形机制和孪生行为。结果表明,纯钛ECAP变形过程中出现■拉伸孪晶和■压缩孪晶,随着挤压道次的增大,孪晶数量先增大后减小。孪晶的出现有效改变晶格取向,激发进一步位错滑移,辅助塑性变形过程,使纯钛显微组织有效细化,经过4道次ECAP变形,平均晶粒尺寸由约63.79μm细化至约2.81μm。1道次变形后晶粒细化效果最显著,平均晶粒尺寸比变形前减小约94%;随着变形道次的增加,晶粒细化效果减弱,4道次变形后平均晶粒尺寸累积减小约95.6%。同时,大量位错、孪晶和亚晶的形成,使得位错、孪晶以及亚晶之间的相互作用加强,显著提高了纯钛的屈服强度和显微硬度,4道次变形后,屈服强度从215 MPa增加到600 MPa,增幅为179%;显微硬度从HV 129增加到HV 200。由于1道次变形后晶粒细化效果最显著,并且出现大量孪晶和位错,屈服强度与硬度的增幅也最大。     

微合金钢超细组织的控制轧制

探讨了采用应变诱导轧制及应变诱导轧制与常规控轧技术相结合获得超细晶铁素体组织的可行性,研究了冷却速度对应变诱导铁素体晶粒长大的影响,实验结果表明,采用应变诱导轧制可显细化铁素体组织,将应变诱导轧制技术与常规控轧工艺相结合所获得的超细晶组织更为理想。通过实验室模拟轧制,已获得体积分数为９７％、晶粒尺寸达０．９２μｍ的超细铁素体组织,ＴＥＭ分析发现,超细晶铁素体内位错密度较低并有少量小角度晶界存在。     

轧制工艺对工业纯钛组织性能的影响

利用现有轧钢设备轧制纯钛板坯,借助光学显微镜研究轧坯的组织形貌,发现轧制钛坯材料的头部与尾部微观组织形貌变化情况相同,即心部组织为锯齿状α片群,α片间还保留少量β相,角部微观组织基本是等轴晶粒,侧边中间位置均为变形α晶粒,且晶粒大于角部、小于心部。透射电镜分析发现轧制钛坯微观组织中出现大量位错,α晶粒内有较多的析出相和孪晶组织,但未见板条状结构。X射线衍射分析表明轧制变形过程中出现了较强的变形织构。沿平行和垂直于轧制方向的取样拉伸,结果发现纯钛坯横向平均屈服强度和抗拉强度高于纵向。     

超细晶纯锆热压缩变形行为及组织演变研究

采用Gleeble-3500热模拟机对等通道转角挤压(ECAP)+旋锻复合细化工艺制备的超细晶纯锆,在温度为300～450℃、应变速率为1×10~(-3)～1×10~(-1) s~(-1)的条件下进行压缩实验,分析了超细晶纯锆在热变形过程中流变应力特点及显微组织演变规律。并使用人工神经网络建立了超细晶纯锆压缩本构模型。结果表明:变形初期流变应力迅速升高,达到峰值应力后逐渐进入稳态流变阶段。随着温度的升高和应变速率的降低,稳态流变应力降低。超细晶纯锆屈服强度显著高于粗晶纯锆;超细晶纯锆的应变速率敏感指数m值为0.028～0.132,高于粗晶纯锆,而且低应变速率和高温有利于提高超细晶纯锆的塑性;透射电镜(TEM)结果表明:随着变形温度的升高和应变速率的降低,超细晶纯锆呈现明显的动态回复与再结晶,晶内位错密度减小,晶界逐渐清晰,晶粒尺寸逐渐增大。基于人工神经网络的压缩本构模型预测结果表明:预测值与实验值的平均相对误差(AARE)为0.5385%,相关系数(R)为0.9991,模型预测精度高。     

板条马氏体大变形轧制工艺的晶粒细化机制

分析了采用板条马氏体大变形轧制工艺制备超细晶钢板时的显微组织演变过程及其晶粒细化机制。结果表明,该工艺包含3个具有不同晶粒细化机制的工艺过程：①轧前预淬火 回火使原始奥氏体晶粒分裂为均匀细小的板条马氏体,板条晶内部含有大量吸附着碳原子的位错;②大变形轧制细化、破碎板条马氏体,并进一步增加了组织中的位错密度;③在轧后再结晶退火时,基体中的高密度位错提供了超常的驱动力,使再结晶晶核尺寸小于1μm,400℃和500℃退火后钢板的晶粒尺寸分别为52nm和316nm。     

超细晶高强度纯钛的开发

<正>日本丰桥技术科学大学的研究人员研究了通过控制母相的组织而不添加其他元素,即利用动态及静态再结晶机理,在以往的加工热处理方法的基础上,施加大应变,且为多向锻造(MDF)的晶粒细化方法。该方法适用于制备超细晶高强度纯钛(以下称为MDF纯钛)。采用MDF方法及传统的热处理工艺制备了两种纯钛,平均晶粒尺寸约100 nm。钛为密排六方结构,     

热处理对ARB制备超细晶1060工业纯铝组织和性能的影响

采用累积复合轧制(ARB)技术制备超细晶1060工业纯铝,研究了热处理对超细晶工业纯铝组织和性能的影响.结果显示,室温下1060工业纯铝经过ARB8道次轧制后,材料的晶粒尺寸由轧制前的38 μm细化至0.42μm,抗拉强度增加2.5倍,延伸率下降.ARB轧制后不同道次试样经150℃x 1 h热处理后,改善了材料层间界面的结合强度,使抗拉强度略有增加..ARB5道次轧制后试样在200℃以下热处理1 h,显微组织处于回复阶段,晶粒尺寸在0.478O.58μm范围内,机械性能稳定;在200℃以上热处理时,晶粒发生再结晶,晶粒长大,材料的性能恢复.     

超细晶纯钛疲劳裂纹扩展及裂纹尖端组织演变

采用疲劳裂纹扩展实验研究剧烈塑性变形制备的4种超细晶纯钛的疲劳裂纹扩展行为,通过观察不同组织超细晶纯钛的疲劳裂纹扩展路径,并对裂纹尖端塑性区进行显微硬度分布实验和显微组织观察,分析不同组织超细晶纯钛疲劳裂纹扩展的微观机制。结果表明,等径弯曲通道变形（equal channel angular pressing,ECAP）纯钛和旋锻后400℃退火纯钛的疲劳裂纹扩展路径曲折,而ECAP+旋锻变形纯钛和旋锻后300℃退火纯钛的疲劳裂纹扩展路径平直。循环载荷作用下,超细晶纯钛裂纹尖端区域的显微硬度和微观组织变化显著。其中ECAP+旋锻变形纯钛的组织位错密度和显微硬度降低,而ECAP变形和旋锻后退火纯钛的组织位错密度和显微硬度均升高。超细晶纯钛疲劳裂纹扩展过程中组织结构对位错运动的阻碍导致裂纹扩展方向改变,且裂纹尖端区域位错密度为显微硬度变化的主要影响因素。     

Deformation Structures of Pure Titanium during Shear Deformation

The deformation microstructure of commercial pure (CP) titanium formed in the theoretical shear zone of an equal channel angular pressing (ECAP) die during 3 or 4 passes is investigated by electron backscatter diffraction (EBSD) and transmission electron microscopy (TEM). The typical feature of the microstructure is that ultrafine grains coexist with coarse elongated grains with a high density of low-angle grain boundaries (LAGBs). Dislocation tangle zones (DTZs), dislocation cells (DCs), and subgrains are generated during shear deformation. The primary twin type has been found to be { 10[`1]2 }. \left\{ {10\bar{1}2} \right\}. Grain refinement appears to progress by continuous dynamic recrystallization (CDRX), in which dislocation movement to LAGBs leads to their evolution into high-angle grain boundaries (HAGBs).     

500 MPa级高延性方管用钢的开发及加工硬化行为

 为了开发满足二次加工性能要求的500 MPa级高延性方管用钢,采用OM、SEM和TEM等对500 MPa级高延性方管用钢制管前后的组织与性能进行分析,研究了其强化机制与加工硬化机理。结果表明,两种试验钢的组织均由铁素体和少量珠光体组成,低C-低Mn-Nb、Ti微合金化试验钢铁素体晶粒与珠光体球团尺寸更加细小,第二相析出物尺寸稍大,位错密度相似。两种试验钢制管前力学性能相似,低C-低Mn-Nb、Ti微合金化试验钢屈强比较高;制管后低C-低Mn-Nb、Ti微合金化试验钢加工硬化程度显著,屈服强度、抗拉强度分别增加了45与26 MPa,伸长率降低6.0%,高C-高Mn-Nb微合金化试验钢屈服强度、抗拉强度分别增加了22与10 MPa,伸长率降低4.0%。固溶强化与细晶强化是两种试验钢最主要的强化机制,由晶粒细化引起的强度增量占总强度的52.9%~61.8%,由固溶强化引起的强度增量占总强度的17.2%~25.3%;析出强化与位错强化对强度的贡献较小。制管后低C-低Mn-Nb、Ti微合金化试验钢位错强化增加显著,达到了82 MPa,明显高于高C-高Mn-Nb微合金化试验钢位错强化的贡献(65 MPa);对于制管用途而言,高C-高Mn-Nb微合金化试验钢制管后综合力学性能更加优异。     

Intercritical Rolling Induced Ultrafine Lamellar Structure and Enhanced Mechanical Properties ofMedium-Mn Steel

The medium-Mn steel with ferrite and austenite structure was rolled in the intercritical region down to dif- ferent rolling reduction. The microstructure and mechanical properties of the rolled steels were investigated by scan- ning electron microscopy, transmission electron microscopy, X-ray diffraction and tensile tests. It was found that the ferrite and austenite structure gradually evolved into an ultrafine structure from the random directional lath structure to lamellar structure with lath longitudinal direction parallel to the rolling direction with increasing rolling strain. It was found that the thickness of the laths was gradually refined with increasing rolling strain. The lath thickness is about 0. 15 9m stored with high density dislocations and the austenite volume fraction of the steel is about 24% after 80% rolling reduction. Furthermore, it was interesting to find that yield strength, tensile strength and total elongation of the 80% rolled medium-Mn steel are about 1000 MPa, 1250 MPa and 24%, respectively, demonstrating an excellent combination of the strength and ductility. Based on the microstructure examination, it was proposed that the grain refinement of the medium-Mn steels could be attributed to the duplex structure and the low rolling temperature. Analysis of the relationship between the microstructure and the mechanical properties indicated that the high yield strength mainly resulted from the ultrafine grain size and the high density dislocation, but the improved ductili- ty may be attributed to the large fractions of austenite retained after intercritical rolling.     

HB450低合金超高强耐磨钢回火过程中的组织性能演变

  利用透射电镜（TEM）、扫描电镜（SEM）等试验方法,研究了新型低成本HB450低合金超高强耐磨钢在不同回火条件下组织和性能演变规律,并分析了其强韧变化机制。结果表明：回火对试验钢的硬度和强度均有较大影响。在200~250℃回火,试验钢表现出良好的强韧性配合,满足国标工程机械用高强度耐磨板GB/T24186—2009中NM450级别要求;在250℃回火时,试验钢出现硬化迹象。透射电镜分析表明,在该温度下回火,析出物中除了有大量弥散分布的ε 碳化物外,还出现了少量30~50nm的（Nb,Ti）（C,N）粒子,该2类粒子对试验钢的强化起着较大的作用。     

高应变率下纯钛动态压缩力学性能各向异性

利用Instron电子拉伸机和分离式霍普金生（SHPB）压杠实验装置,研究了准静态和动态压缩条件下织构多晶纯钛板TA2的力学性能,对织构多晶纯钛板法向ND、轧向RD、横向TD等三个方向进行了压缩实验,得到了不同应变率下的应力-应变曲线。结果表明对于轧制和退火纯钛板,准静态和动态压缩力学性能均表现出明显的各向异性,且规律一致：均为ND方向屈服强度最大,TD次之,RD方向最小。     

Nanoscale Cementite Precipitates and Comprehensive Strengthening Mechanism of Steel

This article summarizes the state of the art of the comprehensive strengthening mechanism of steel. By using chemical phase analysis, X-ray small-angle scattering (XSAS), room temperature organic (RTO) solution electrolysis and metal embedded sections micron-nano-meter characterization method, and high-resolution transmission electron microscopy (TEM) observation, the properties of nanoscale cementite precipitates in Ti microalloyed high-strength weathering steels produced by the thin slab continuous casting and rolling process were analyzed. Except nanoscale TiC, cementite precipitates with size less than 36 nm and high volume fraction were also found in Ti microalloyed high-strength weathering steels. The volume fraction of cementite with size less than 36 nm is 4.4 times as much as that of TiC of the same size. Cementite with high volume fraction has a stronger precipitation strengthening effect than that of nanoscale TiC, which cannot be ignored. The precipitation strengthening contributions of nanoscale precipitates of different types and sizes should be calculated, respectively, according to the mechanisms of shearing and dislocation bypass, and then be added with the contributions of solid solution strengthening and grain refinement strengthening. A formula for calculating the yield strength of low-carbon steel was proposed; the calculated yield strength considering the precipitation strengthening contributions of nanoscale precipitates and the comprehensive strengthening mechanism of steels matches the experimental results well. The calculated σ _s  = 630 to 676 MPa, while the examined σ _s  = 630 to 680 MPa. The reason that “ultrafine grain strengthening can not be directly added with dislocation strengthening or precipitation strengthening” and the influence of the phase transformation on steel strength were discussed. The applications for comprehensive strengthening theory were summarized, and several scientific questions for further study were pointed out.     

Effects of lanthanum addition on microstructure and mechanical properties of as-cast pure copper

As-cast Cu-La alloys with La contents in the range of 0–0.32 wt.% were fabricated by vacuum melting method. The effects of La on microstructure and mechanical properties of as-cast pure copper were investigated using optical microscopy(OM), scanning electronic microscopy(SEM), transmission electron microscopy(TEM), X-ray diffraction(XRD) and tensile test. The results showed that La had obvious effects on the solidification microstructure and the grain refinement of as-cast pure copper. With the increase of La content, the ultimate tensile strength, the yield strength and the microhardness increased gradually, but the elongation increased first and then decreased while La content exceeded 0.089 wt.%. The improvement of mechanical properties was attributed to the effect of grain refinement strengthening, solid solution strengthening, second phase strengthening and purifying. However, excessive adding La would deteriorate the elongation owing to the excessive Cu6 La phases.     

Regularities of Formation and Degradation of the Microstructure and Properties of New Ultrafine-Grained Low-Modulus Ti–Nb–Mo–Zr Alloys

Abstract—Regularities of the formation of ultrafine-grained (UFG) and submicrocrystalline (SMC) structures in new nickel-free low-modulus Ti–Nb–Mo–Zr titanium β alloys under the action of plastic deformation have been studied. Temperature–time ranges of the development of dynamic recrystallization processes under the simultaneous action of temperature and plastic deformation are determined. A type-II recrystallization diagram of the Ti–28Nb–8Mo–12Zr alloy is constructed and analyzed. It is shown using scanning electron microscopy and the electron backscatter diffraction method that the UFG structure with an average grain size of no more than 7 μm and high fraction of high-angle grain boundaries is formed in the investigated alloys as a result of longitudinal rolling, followed by annealing for quenching. It is found that the formation of the UFG structure leads to a significant increase in the strength and plastic characteristics of these alloys. The regularities of the formation of UFG and SMC structures in titanium β alloys Ti–28Nb–8Mo–12Zr and industrial VT30 under the action of plastic deformation by the helical rolling method are studied. It is shown that the helical rolling of the VT30 alloy leads to the formation of the homogeneous UFG state as opposed to the Ti–28Nb–8Mo–12Zr alloy, where this method causes structure softening with micropores and microcracks formed in the central region. It is possible to form a nanostructured state with an average grain size of about 100 nm in Ti–Nb–Mo–Zr titanium β alloys using the high-pressure torsion method.