用等径角挤压变形法制备纳米晶金属结构材料的组织演变

用实验方法研究了工业纯铁在等径角挤压变形(路径C)过程中晶粒细化过程。实验结果表明，经4道次剪切变形后开始出现纳米级晶粒。晶粒细化过程为：原始粗晶粒→晶粒被剪切变形带分割→位错线分割滑移带→位错线发展为位错墙，把变形带分割成细小的亚晶→亚晶界的位错密度增加→形成大角度晶界的纳米晶粒。测试了不同变形道次下材料的显微硬度值。     

工业纯钛ECAP变形组织与织构演变的EBSD分析

利用电子背散射衍射(EBSD)对采用90°模具以C方式制备的等径弯曲通道变形(ECAP)各道次工业纯钛(CP-Ti)试样的组织及织构演变进行表征。结果表明:1道次变形后,ECAP组织并不均匀,既有拉长的粗晶,又有细小的等轴晶。随着道次的增加,组织变得细小均匀,从而达到细化晶粒的效果;工业纯钛ECAP变形初始阶段,粗大的晶粒破碎,产生位错缠结和位错胞,使小角度晶界增加。随着道次的增加,位错不断地向亚晶界运动,亚晶间产生相对滑动和转动,最终形成具有大角度晶界的超细晶组织,使小角度晶界减少,大角度晶界增加。工业纯钛原始试样具有双峰基面织构,晶体的c轴由法向方向(ND)向挤出方向(ED)偏转约15°,4道次变形后变为剪切织构,晶体的基面与剪切面平行,最终形成织构组分为(1120)[1101]。     

剪切变形方式下低碳钢表面纳米结构的形成

对低碳钢板材进行表面球磨处理，利用电子显微术研究剪切变形方式下深度方向的组织演变。结果表明，低碳钢的纳米化过程包括：位错通过增殖、运动、湮灭和重组形成位错墙；位错墙逐渐转变成小角度和大角度晶界，将原始粗晶分割成亚微晶；亚微晶内重复上述过程使晶粒继续细化，最终形成等轴状、取向接近随机分布的纳米晶组织。外力作用方向并未改变...     

45钢等径弯曲通道变形及组织细化研究

研究了等径弯曲通道(ECAP)变形后45钢中先共析铁素体及珠光体组织的演变特征.结果表明,ECAP变形4道次后,片层状的珠光体组织演变成了超细的渗碳体颗粒均匀分布于亚微晶铁素体基体的组织.先共析铁素体由原始的平均晶粒尺寸约为30 μm演变为大角度晶界分离的、平均晶粒尺寸约为0.4μm的超细晶组织.ECAP变形后,先共析铁素体首先在其内部会形成具有薄片层界面(LBs)的板条位错胞甚至亚晶组织.进一步变形时位错胞或亚晶可继续细化.再进一步变形时通过晶界滑移和晶粒旋转的方式可以获得具有大角度晶界分离的、等轴的超细晶组织.     

铸态304L奥氏体不锈钢等径角挤压变形研究

 研究了铸态304L奥氏体不锈钢在等径角挤压(ECAP)变形过程中显微组织的演变过程。结果表明,经4道次剪切变形后树枝晶破碎、原始粗大晶粒碎化。显微组织的变化过程可归纳为：原始粗晶粒→晶粒被滑移带分割→位错发展形成高密度位错墙,与滑移带共同作用形成胞块结构→应变增加形成层片状界面→形成大角度晶界的细小晶粒。表明铸态304L奥氏体不锈钢经ECAP变形后塑性变形机制主要由滑移完成。     

等径角挤压变形奥氏体不锈钢的组织演变

 用实验方法研究了奥氏体不锈钢在等径角挤压冷变形(路径RC)过程中组织变化。实验结果表明：当剪切方向与孪晶带方向成一定角度时，在剪切力的作用下，孪晶逐渐由大块孪晶→由剪切带分割的孪晶(楼梯状)→小块状→奥氏体亚晶或马氏体晶粒；部分孪晶在剪切力作用下，剪切带可直接碎化成具有大角度位向差的细小晶粒(奥氏体亚晶+马氏体晶粒)，可发生马氏体相变；当剪切方向与孪晶带方向相同时，孪晶带区域也可发生马氏体转变；3道次变形后，具有明显特征的孪晶已很少，此后继续进行剪切变形，孪晶碎化组织(含马氏体)和奥氏体剪切滑移带(含碎化晶粒)的变形以剪切滑移方式进行，当奥氏体的滑移遇到阻力时，可局部形成局部形变孪晶来协调变形；随变形道次的增加，马氏体转变也越多，在多次剪切以及道次中的交叉滑移作用下，马氏体板条逐渐被高密度位错墙分割而碎化成细小的晶粒；8道次变形后，可获得60~230 nm的等轴晶粒。     

TWIP钢室温静态拉伸下的组织形态和变形机制

在室温下对退火Fe-24Mn-1Si-1.5Al-0.045CTWIP钢进行了不同程度的拉伸变形,采用JEM-2100透射电子显微镜对变形后的组织形貌进行表征和分析。研究结果表明:在变形初期,晶粒内存在着大量位错,它们相互缠结,呈胞状结构。在此阶段,位错滑移为主要变形机制。随着变形量的增加,形变孪晶在晶界等处形成,孪生机制被激活,孪生和滑移机制相互竞争。双孪生系统在大多数晶粒内先后被激活,孪生和滑移机制相互交割,起到动态细化晶粒的作用,使强度显著提高。在变形后期,试验钢的变形机制主要是TRIP效应,以及孪生与滑移的相互作用而诱发了去孪生机制,层状组织出现,孪晶特征减弱,从而导致样品的局部变形和失效。     

Cu-P-Cr-Ni-Mo耐候钢双相区多道次轧制模拟的组织演变

 为了实现Cu-P-Cr-Ni-Mo耐候钢的铁素体晶粒细化从而充分提高其强塑性,通过热模拟压缩试验,利用金相、SEM、EBSD等微观组织分析方法研究了其在双相区的多道次压缩变形过程中的组织演变。结果表明,试验钢在变形过程中,第二相（马氏体、贝氏体）呈条带状分布于铁素体基体上,随着道次增加,铁素体晶粒逐步细化,第5道次变形后得到1.8 μm左右的超细晶铁素体。前期铁素体晶粒细化的主要机制是形变强化铁素体相变,即多道次的累积大变形使组织内畸变能增大,铁素体形核点增多,促进铁素体快速析出,形成细小铁素体晶粒;后面几道次变形中,随着应变量继续增大,在铁素体晶粒内形成大量亚晶界,且亚晶界逐步累积扭转成大角度晶界,分割原来的粗大晶粒,发生铁素体连续动态再结晶细化。     

等通道转角挤压组织演变规律的研究进展

综述了金属结构材料和功能材料基体相晶体结构、层错能、Hollomon参数lnZ对等通道转角挤压ECAP变形组织演变规律影响的研究进展,试样基体相的晶体结构对变形组织的演变起重要的影响作用。随着应变量的增大,密排六方结构金属先形成形变孪晶、再启动优先存在的但被阻塞的滑移系统;面心立方结构金属的位错滑移主导着组织演变与晶粒细化过程,先形成亚晶界,再增大组织取向差,最终形成大角度晶界。在高层错能材料中,随着Hollomon参数lnZ增大,位错运动受到抑制,驱使变形机制从位错滑移逐渐转变成形变孪晶;当Z参数减小时,在ECAP高层错能材料中会形成微尺度的剪切带。在低层错能材料中形成丰富的孪晶,极低层错能的材料形成宏观剪切带。而中等层错能材料的变形机制则取决于Z值的高低。分析了ECAP过程动态再结晶的影响因素,认为γm·ln2Z30不宜作为ECAP过程是否发生动态再结晶的判据,ECAP过程动态再结晶的影响因素还有待进一步研究,如弄清ECAP过程温升规律、分析淬火保存ECAP变形组织将有助于研究ECAP动态再结晶。     

铝合金等通道挤压晶粒细化机制

本文讨论铝合金在等通道挤压过程中的晶粒细化机制。发生的晶粒细化主要通过三种机制完成：1)取向分裂诱发形变带;2)应变集中产生的宏观或微观剪切带;3)高角度晶界随应变增加。形变条件和路径、模具几何及材料参数决定形变组织的演化。亚结构和显微剪切带的取向与模具剪切面一致但在原则上与材料的晶体位错滑移系统无关。形变带的晶体取向倾向接近在路径A下稳定织构的取向。在高应变,由于显微组织的压缩和拉长造成的晶界面积增加成为主要晶粒细化机制。变形至一定应变后,形变进入稳态,晶粒细化不再发生。     

Adiabatic shear bands on the titanium side in the titanium/mild steel explosive cladding interface: Experiments, numerical simulation, and microstructure evolution

The microstructure and microtexture in adiabatic shear bands (ASBs) on the titanium side in the titanium/mild steel explosive cladding interface are investigated by means of optical microscopy, scanning electron microscopy/electron backscattered diffraction (SEM/EBSD), and transmission electron microscopy (TEM). Highly elongated subgrains and fine equiaxed grains with low dislocation density are observed in the ASBs. Microtextures (25 deg, 75 deg, 0 deg), (70 deg, 45 deg, 0 deg), and (0 deg, 15 deg, 30 deg) formed within the ASBs suggest the occurrence of the recrystallization. The grain boundaries within ASBs are geometrically necessary boundaries (GNBs) with high angles. Finite element computations are performed to obtain the effective strain and temperature distributions within the ASBs under the measured boundary conditions. The rotation dynamic recrystallization (RDR) mechanism is employed to describe the kinetics of the nanograins’ formation and the recrystallized process within ASBs. During the deformation time (about 5 to 10 μs), the following processes take place: dislocations accumulate to form elongated cell structures, cell structures break up to form subgrains, and subgrains rotate and finally form recrystallized grains. The small grains within ASBs are formed during the deformation and do not undergo significant growth by grain boundary migration after deformation.     

Influences of cell walls and grain boundaries on transient responses of an if steel to changes in strain path

The effects of changes in strain path on plastic behaviour in sheets of an interstitial-free steel with two widely different grain sizes were investigated. The sheets were prestrained in rolling and, apart from supplementary tests, they were tested in uniaxial tension at 90° to the rolling direction. The results support the following conclusions. The magnitude of the increase in reloading yield stress and amplitude of the subsequent reduction in work hardening depend on the strength of dislocation walls generated in the prestrain rather than the grain size. The walls are more effective barriers to dislocation glide in freshly activated slip systems than to glide in the original slip systems operating in the prestrain. The primary cause of the subsequent reduction in hardening rate is disruption and partial dissolution of the original dislocation substructure. The final recovery in hardening rate is caused by generation of a new substructure compatible with the new deformation mode.     

Cyclic deformation of ferritic steel—I. Stress-strain response and structure evolution

The complete CSS-curve has been established for a low alloyed structural steel tested under plastic strain control. The curve which can be divided into three separate regimes has a plateau region between cyclic strain levels of 10⁻⁴ and 8·10⁻⁴. PSBs are formed on the specimen surface when the plastic strain range corresponds to the plateau regime. The PSBs are sites for crack nucleation. The substructure evolution as seen going along the CSS-curve and with accumulating numbers of cycles is documented in detail and includes: dislocation loops, veins, walls including the ladder-like walls usually associated with PSBs, labyrinths, cells, subgrains, banded cells and subgrains. At low and intermediate plastic strain ranges the surface grains contain a more fine-scaled substructure and develop features which appear in the interior at higher plastic strain ranges. At larger cyclic strain levels microbands and noncrystallographic deformation bands become dominating features. Heavily displaced and serrated grain boundaries are observed at intermediate and high plastic strains both in interior- and surface grains containing microbands, banded cells or banded subgrains.     

Geometrically necessary,incidental and subgrain boundaries

During the deformation of polycrystals, the grains break up into domains within which the selection of operative slip systems differs. The domains then subdivide into “cell blocks”. Locally, each group of cell blocks comes near to fulfilling the Taylor criterion when taken collectively, but the number of active glide systems in any one cell block is fewer than predicted. The boundaries between cell blocks and/or domains accommodate the lattice misorientations which result from glide on the different slip system combinations. They are therefore named “geometrically necessary bpundaries”. They, like all boundaries capable of accommodating variable lattice misorientations, are composed of dislocations. Microscopically, such boundaries appear as “dense dislocation walls” and “microbands”. Geometrically necessary boundaries are distinguished from ordinary dislocation cell boundaries by the absence of a change of glide systems across the latter. In materials deforming with a cell structure, ordinary dislocation cell boundaries as well as traditional “deformation bands” arise from the mutual trapping of dislocations into low-energy configurations. Such cell boundaries or walls are therefore named “incidental dislocation boundaries”. The misorientation across incidental boundaries is typically much smaller than for geometrically necessary boundaries. A further distinction is their respective on the flow stress. The average spacing of dislocation cell walls is inverse proportional to the flow stress whereas geometrically necessary boundaries obey the Hall-Petch relationship. Since they tend to occur more frequently the incidental boundaries typically control the flow stress. At increasing strain the angles between dislocation cells increase and different slip system combinations can operate in neighbouring cells. Cell walls are then no longer incidental boundaries but geometrically necessary boundaries. Such boundaries are termed “subgrain boundaries”.     

Low temperature strain behavior of Pb thin films on a substrate

The microstructural changes occurring during cooling from 300 to 100 K in a 0.2 μm thick polycrystalline Pb film deposited on a Si₃N₄ substrate were studied by a combination of transmission electron microscopy and X-ray diffraction technique. The tensile strain induced in the film upon cooling due to the thermal expansion coefficient mismatch between the film and the substrate was observed to be relaxed by dislocation glide. Most of the dislocations were observed to glide across the grains on 111 planes that are inclined at an angle of ∼ 70 deg to the film surface, and on 111 planes that are nearly parallel to the film surface. All the dislocation motions are confined in each grain by the surface oxide, the substrate, and grain boundaries. Some observations suggested that these dislocations emanate from grain boundaries. At 100 K, the density of dislocations introduced in the grains with diameters larger than ∼0.6 μm was found to be roughly constant (about 10¹⁰/cm²), while no dislocations were observed in grains smaller than ∼0.6 μm. The observed dislocation density can account for the amount of strain relaxed which was measured by the X-ray diffraction technique. It was also found that almost all the dislocation glide events involved in the thermal cycling process are reversible. This explains a previous X-ray diffraction result that no work hardening effect was observed in Pb films during the thermal cycling at low temperature. The yield stresses of Pb films as determined by the strain measurements are about three times higher than those expected by a simple dislocation pinning model. Based on the dislocation motion observed in this work, the yield stresses of the films were re-evaluated as a function of film thickness and grain size using an energy criterion model. This model took into account the effects of the surface oxide and substrate on dislocation     

Microstructure Evolution of Surface Gradient Nanocrystalline by Shot Peening of TA17 Titanium Alloy

A surface gradient nanocrystalline structure (SGNS) was obtained by shot peening (SP) on the TA17 near α titanium alloy to improve its surface properties. The effect of shot peening time was investigated by characterizing the grain size of the surface nanocrystalline layer, the thickness of the severe plastic deformation (SPD) layer, the microstructure evolution of the SGNS and the hardness change. The experimental results show that the grains of TA17 titanium alloy can be refined to a nano-scale of about 22 to 26 nm when shot peened at 0.6 MPa pressure for 5 to 10 minutes. The thickness of the SPD layer increases from 55 to 88 μm with the SP duration from 5 to 10 minutes and tends to be saturated afterward. The SGNS is composed of a surface nanocrystalline layer and a transition layer. During the SP treatment, the coarse grains are first divided into small blocks by intersection of twins, then dislocation walls, dislocation tangles and dislocation bands lead to the formation of low-angle grain boundaries, which subdivide the subgrains into a finer scale until nanograins with a stable size are obtained. The topmost surface hardness can be improved significantly to twice the hardness of the matrix due to both grain refinement and work-hardening.

     

Dynamic restoration mechanisms in Al-5.8 At. Pct Mg deformed to large strains in the solute drag regime

An Al-5.8 at. pct Mg (5.2 wt pct Mg) alloy was deformed in torsion within the solute drag regime to various strains, up to the failure strain of 10.8. Optical microscopy (OM) and transmission electron microscopy (TEM) were used to analyze the evolution of the microstructure and to determine the dynamic restoration mechanism. Transmission electron microscopy revealed that subgrain formation is sluggish but that subgrains eventually (ε ≈ 1) fill the grains. The “steady-state” subgrain size (λ ≈ 6 μm) and misorientation angle (θ ≈ 1.6 deg) are reached by ε ≈ 2. These observations confirm that subgrains eventually form during deformation in the solute drag regime, though they do not appear to significantly influence the strength. At low strains, nearly all of the boundaries form by dislocation reaction and are low angle (θ < 10 deg). At a strain of 10.8, however, the boundary misorientation histogram is bimodal, with nearly 25 pct of the boundaries having high angles due to their ancestry in the original grain boundaries. This is consistent with OM observations of the elongation and thinning of the original grains as they spiral around the torsion axis. No evidence was found fordiscontinuous dynamic recrystallization, a repeating process in which strain-free grains nucleate, grow, deform, and give rise to new nuclei. It is concluded that dynamic recovery in the solute drag regime gives rise togeometric dynamic recrystallization in a manner very similar to that already established for pure aluminum, suggesting that geometric dynamic recrystallization may occur generally in materials with a high stacking-fault energy (SFE) deformed to large strains.     

Dislocation structures in the strain localized region in fatigued 70/30 brass and the interaction with grain boundary

Long-term fatigue tests of polycrystalline 70/30 brass were carried out under low strain amplitudes in vacuum, and dislocation structures of the strain localized regions developed were examined by means of transmission electron microscopy In the planar dislocation structures, the strain localized regions (SLRs) bounded by a pair of parallel active glide layers were frequently observed in favorably oriented grains. Where the SLRs impinge on grain boundaries (GBs), extrusion-type deformations were sometimes formed notwithstanding the restraints of the neighboring grains. On the basis of observations, the mechanism of crack initiation at the GB is discussed.