晶粒尺寸对Cu-30%Zn合金形变孪晶厚度的影响

对于面心立方结构的纳米金属,晶粒尺寸对孪生厚度(孪生核)的影响虽已有研究,但仍有待深入。本论文以Cu-30%Zn合金为模型材料,通过高压扭转变形技术、等径角挤压连同轧制技术变形得到晶粒尺寸在5~500nm的样品。透射电子显微镜观察发现:变形孪晶的片层厚度随晶粒尺寸的减小而减小,当晶粒尺寸小于20nm以后,孪晶厚度为(111)晶面间距(层错);另外,层错存在于各个不同尺寸范围的晶粒内,表明层错不受晶粒尺寸影响。研究结果表明在低层错能超细晶材料中,孪生变形是通过从晶界连续发射不全位错(层错)形成的。     

高应变速率下等径角挤压高纯粗晶铝中的形变孪晶与退火孪晶

在室温下对铸态高纯粗晶铝进行一道次高应变率动态等径角挤压(D-ECAP)变形,利用电子背散射衍射技术(EBSD)研究挤压过程中所形成的孪晶。结果表明:利用D-ECAP能够在粗晶铝中同时制备出形变孪晶和退火孪晶,但两者在形态、Kernel平均取向差(KAM)以及与相邻晶粒的取向差三个方面存在较大差异。D-ECAP高应变率和大剪切变形使高层错能铝中形成了百微米级的形变孪晶,形变孪晶的形态为透镜状,后续变形使得孪晶界偏离∑3 60°〈111〉取向关系且KAM值主要集中于0.6°~1.8°。高应变率剪切变形下形成的大量层错和复杂的位错组态以及高形变储存能在变形温升的作用下促进了退火孪晶的形成。退火孪晶的形态较不规则,但孪晶界的取向关系更接近于∑3 60°〈111〉且KAM值主要集中于0.2°~0.5°。     

单晶Al在剪切变形时微观变形机制的分子动力学模拟

运用分子动力学模拟方法,研究Al在剪切变形下的微观变形机制,模拟初期铝变形主要以层错为主,孪晶数量较少。模拟进行到一定程度,由于晶体内部分切应力达到孪晶形核所需分切应力,孪晶开始大量出现,且以单层孪晶为主,多重孪晶为辅共同作用。并且出现近几年实验观察到的新缺陷结构五重孪晶,得出单晶Al在剪切变形下也会实现五重孪晶形核,生长。随剪切应变的进一步加大,形成的五重孪晶在晶体内部持续存在一段时间,但不会一直保留,先是转化为四重孪晶,并最终消亡。在模拟剪切一个完整周期后,材料内部出现取向异性的新晶粒,在此条件下实现晶粒细化。     

硅纳米晶微观结构缺陷的高分辨电子显微学研究

利用离子注入结合后续高温退火的方法成功地制备出包埋在二氧化硅(SiO_2)基质中的硅纳米晶.利用透射电子显微学对所制备的硅纳米晶(离子注入浓度为3×10~(17)cm~(-2))的微观结构缺陷进行了详细的研究.通过高分辨像分析发现:较大的纳米晶(直径＞6nm)中存在很多面缺陷,主要为孪晶与层错.孪晶包括一次孪晶、二重孪晶、三重孪晶及五重孪晶.层错分为内禀和外禀两种类型,并讨论了内禀层错占多数的原因.除了面缺陷以外,还有一部分纳米晶中存在位错.     

TiAl基合金中γ/γ界面关系的定量分析

γ/γ界面对于 (γ α2) 双相TiAl基合金的力学性能具有重要影响.本工作利用汇聚束电子衍射技术对一种含有Mn,Nb合金元素的 (γ α2) TiAl基合金中具有六种不同晶体学取向的γ相及γ/γ界面关系进行了实验唯一测定.结果表明,仅利用一套〈334〉高阶劳厄带花样就可以完全决定该合金中γ层片的晶体学取向.该合金中γ/γ界面有四种,即绕〈111〉旋转120°、旋转180°呈镜面对称真孪晶、旋转60°呈镜面对称的伪孪晶和反相畴界面.统计分析表明,真孪晶γ/γ界面的比例最高,伪孪晶γ/γ界面和旋转120°γ/γ界面较少.TiAl合金中偶尔也会出现γ/γ反相畴界.     

层错能对Fe-Mn-C系TRIP/TWIP钢变形机制影响

对三种不同层错能(SFE)Fe-Mn-C系TWIP钢的变形机制进行了研究.结果表明:在淬火态下,TWIP钢组织为全奥氏体,奥氏体晶粒内存在少量退火孪晶.TWIP钢的层错能随着C、Mn含量的增加而增加.层错能为7 mJ/m2时,变形后出现大量ε马氏体,且随着应变量的增大,ε马氏体峰增强,表现为单一的TRIP效应;层错能为12 mJ/m2时,应变诱导γ→ε→α或γ→α的转变及形成少量形变孪晶,表现为TRIP/TWIP效应;层错能为18 mJ/m2时,变形后形成大量形变孪晶,表现为单一的TWIP效应,抗拉强度和延伸率分别达到851 MPa及49%.随着层错能增加,TWIP钢的断裂机制由沿晶断裂转变为以韧窝为主的塑性断裂.     

镍基高温合金微孪晶形成机制的研究进展

镍基高温合金具有优良的成分兼容性、良好的组织稳定性、抗氧化和抗腐蚀性能,被广泛用于航空发动机和地面燃气轮机的涡轮叶片等关键的热端部件。沉淀相γ'对位错运动的阻碍是镍基高温合金的主要强化作用之一。一般而言,这种阻碍作用不仅与γ'相的形貌、体积分数及尺寸有关,也取决于γ'相与位错的交互作用。通常这种交互作用机制可分为三种:切割机制、Orowan绕过机制和热激活攀移机制。当不同类型的位错切割γ'相时,在γ'相中会形成不同的高能缺陷,能够阻碍位错运动,延缓材料软化。这类结构或成分缺陷包括:反相畴界(APB)、复杂层错(CSF)、超点阵内禀层错(SISF)、超点阵外禀层错(SESF)和微孪晶。微孪晶化(Micro-twinning)是镍基高温合金中一种重要的变形机制,主要发生在中温高应力条件下。此外,中温拉伸变形过程中也有微孪晶产生。早期研究表明,微孪晶的产生与SESF有关,可以认为SESF是"胚体孪晶",且SESF是由a/3〈112〉超点阵不全位错切入γ'相产生的。基于溶质原子短程扩散的原子重排(Reordering)机制被用来解释微孪晶的形成,即a/6〈112〉不全位错切入γ'相中先产生CSF,而后CSF通过原子重排转变为SESF,最终形成微孪晶。最近的研究表明,在微孪晶产生过程中,Co和Cr原子会在成分偏析和柯氏气团的作用下发生长程扩散,因此有学者指出微孪晶的形成是原子重排短程扩散机制和偏析主导的长程扩散共同作用的结果。同时,对于高温合金微孪晶机制的研究,研究人员不再局限于其形成机制,而对微孪晶的长大机制有了进一步的理解。共格的纳米孪晶界作为金属材料中的一种特殊缺陷,可以有效阻碍位错运动,从而强化材料,这种强化方式已经在纳米铜、TWIP钢以及Ti Al合金中得到应用。研究人员发现,孪晶能够强化固溶强化的镍合金;同时,有学者发现镍基高温合金中退火孪晶界对位错运动有明显的阻碍作用。因此,微孪晶化有望成为一种强化镍基高温合金的方法。本文归纳了镍基高温合金中微孪晶形成机制的发展和演变,分析了不全位错、内禀层错、外禀层错、复杂层错、元素偏析以及柯氏气团(Cottrell atmospheres)在微孪晶化中所起的作用,同时也阐述了孪晶界面处元素偏析在孪晶长大中的作用。此外,本文还综述了微孪晶在镍基高温合金强化中的作用,指出了通过微孪晶强化高温合金过程中存在的问题,展望了微孪晶在高温合金强化中的应用,为研究高温合金的中温变形机制和孪晶强化机制提供参考。     

高锰Hadfield钢单向拉伸变形及断裂行为研究

本研究测试了高锰Hadfield钢室温下在大应变速率(分别为6×10~(-3)s~(-1)、6×10~(-4)s~(-1)、3×10~(-5)s~(-1)和6×10~(-6)s~(-1))范围内的单向拉伸变形的力学响应行为,分析了合金的变形行为及裂纹萌生与扩展规律。结果表明:在不同应变速率下均存在动态应变时效现象,且延伸率具有正的应变速率敏感性。拉伸变形后,奥氏体晶粒内产生了大量位错和层错,以及细小且相互平行的形变孪晶。应变硬化率随真应变的增加依次表现为"减小—增大—减小"三个演变阶段。其中,第二阶段的增大现象是形变孪晶的急剧增加而形成孪生硬化所致。垂直于拉伸变形方向分布的高密度滑移带是裂纹萌生的主要区域。裂纹扩展以沿垂直拉伸方向的穿晶形式为主,结合沿孪晶方向进行。高锰Hadfield钢的主要变形机制是滑移与孪生的相互竞争。     

CdZnTe晶体缺陷的透射电子显微分析

采用透射电子显微镜对CdZnTe晶体材料的缺陷特性进行了分析。在（111）面的透射电镜明场像中，观察到了棱柱位错环、位错墙、沉淀相、层错及倾斜的孪晶界面。应力是位错形成的主要原因，棱柱位错环的产生是由于沉淀相粒子在基体上产生错配应力；而位错网络与位错墙是两种热应力联合作用于晶体边缘的结果。晶体生长过程中，液固界面生长形态从平界面向胞状界面发展产生的沉淀相衬度不同于由于Te原子溶解度的回退产生的沉淀相衬度。CdZnTe晶体中的堆跺层错和孪晶与固液界面的稳定性相关。     

CVD金刚石的晶体形态及界面位向关系

综述了CVD金刚石的晶体形态及界面位向关系.用SEM观察CVD金刚石晶粒形态主要有立方体、长方体、八面体、立方八面体、孪晶八面体、十面体和二十面体颗粒以及球形金刚石,讨论了晶粒的形成条件.用TEM观察则主要有单晶体、孪晶八面体和五重孪晶体形态.界面位向关系主要有:Si(001)//Diamond(001),Si<110>//Diamond<110(50078018);Si(111)//Diamond(111),Si<110>//Diamond<110>和Si(110)//Diamond(110),Si[110]//Diamond[111].     

Cryogenic deformation microstructures of 32Mn–7Cr–1Mo–0.3N austenitic steels

The cryogenic deformation microstructures of impact and tensile specimens of 32Mn–7Cr–1Mo–0.3N austenitic steel were investigated using light microscopy and transmission electron microscopy. The results show that the deformation microstructures of the impact specimens are mainly composed of stacking faults, network dislocation, slip bands, and a few mechanical twins and -martensite. These microstructures cross with each other in a crystal angle. The deformation microstructures of the tensile specimens consist only of massive slip bands, in which a few mechanical twins and -martenite are located. Because of the larger plastic deformation the slip band traces become bent. All the deformation microstructures are formed on the {111} planes and along the <110> orientation.     

Deformation bands in Inconel 718

Abstract

The deformation bands formed under monotonic loading in the homogenised and the aged Inconel 718 have been investigated using transmission electron microscopy. The crystallographic behaviour of the deformation bands is discussed. The orientations of the deformation bands were observed in tranmission electron micrographs as follows: two orientations of <112> bands and one orientation of <110> bands on the {O 11} plane; two orientations of <110> bands on the {DOl} plane, and three orientations of <110> bands on the {111} plane. Deformation bands were classified into <112> and <110> bands, theformer corresponding to the projections of edge traces of {111} planes, and the latter corresponding to the projections of inclined deformation {111} planes. Both twins and dislocations were observed in the deformation bands.

MST/2002     

Superlattice stacking fault formation and twinning during creep in γ/γ′ single crystal superalloy CMSX-4

This paper describes further insight which has been gained into the formation of deformation twins during high temperature creep in a high volume fraction single crystal superalloy (CMSX-4) and correlation with the nature of superlattice stacking faults also observed. In general it is found that the formation of high temperature twinning can always be associated with the loading orientation in which SESF formation is expected from a determination of the sign of the shear stress. One can rationalise the reason for this twinning being associated with extrinsic stacking faults, by consideration of the critical radius of stacking fault loop nucleation. In particular calculations at 1223 K demonstrate a minimum in radius being associated with four overlapping faulted planes for the extrinsic case. Although the process by which twin formation occurs has not been observed, it is shown that twin formation can take place by the passage of extrinsic stacking faults within the precipitate which operate on every alternate plane of the structure. If one considers formation to occur via a pole mechanism similar to that in a face centred cubic structure a mechanism for this occurrence is postulated due to the fact that climb of the helix structure as it rotates around a pair of suitable matrix dislocations, will amount to double that which occurs in the single pole case.     

Precipitation of Epsilon Copper in Ferrite Antibacterial Stainless Steel

The precipitation of epsilon copper at 1023 K ageing in ferrite antibacterial stainless steel was investigated by a combination of electron microscopy and micro-Vickers hardness measurement. The results show that epsilon copper precipitation occurs within 90 s, Complex multilayer structure confirmed as twins and stacking faults on {111}ε-Cu planes was observed in the precipitates. The precipitates grow by the lengthwise enlargement of a set of parallel layers, having [111]ε-Cu and [112]ε-Cu preferred growth orientations. The volume fraction of precipitates f formed within 120 min can be predicted by a modified Avrami equation （In1/1-f= kt ＋ b）. Simultaneously, substituent atom clusters with a size of 5-10 nm was found to occur in the solution and cause matrix strain. The precipitate morphology and distribution on the surface of ferrite antibacterial stainless steel are associated with surface crystallographic orientation of the matrix. The precipitates are predominantly located within the ferrite grains of 〈110〉 orientation. The precipitates located on {111}α-Fe surface planes have sphere or ellipse shape.     

铜单晶体在循环形变中形成的形变带

系统研究了循环形变铜单昌体中宏观形变带的产生规律和特征及相应的位错结构,并对其形成机制进行了综合探讨。结果表明,在不同取向铜单昌体的循环形变中,形变带ＤＢＩ近似沿主滑移面发展。而ＤＢⅡ的惯习面接近传统的扭折面｛１０１｝,两者成严格的正交分布。     

Precipitation and deformation behaviour of cast alloy 718 during creep and thermal exposure

Abstract

The precipitation, deformation, and fracture behaviour of cast alloy 718 during creep rupture tests was investigated, in comparison with thermal exposure tests. Inhomogeneous deformation bands appeared during monotonic or cyclic deformation of alloy 718. The bands were identified as mechanical twins, which are known to be responsible for crystallographic failure during creep rupture at and below 649°C. However, crystallographic failure was observed at temperatures up to 760°C in the present study. No crystallographic failure was observed at and above 816°C. Precipitation of δ phase was observed on deformation bands following creep rupture tests at and above 704°C. The difference in failure mode below 760°C and above 816°C is assumed to be caused by the precipitation of δ phase on the bands. A few discrete δ particles on the bands during 704°C creep rupture tests were not sufficient to prevent decohesion along the twin/matrix interface, and therefore crystallographic failure still occurred. In contrast with little or no precipitation below 704°C, needlelike or platelike δ phase precipitated at and above 816°C. It is postulated that the precipitation of δ phase restricted successive deformation. Since δ phase precipitates on {111} planes where major deformation occurs, this phase usually grows according to the following orientation relationship: (010)_δ∥(111)_γ, [100]_δ∥[11¯0]_γ. Restriction of deformation by the precipitation of δ phase caused the change in failure behaviour at and above 816°C.     

The effect of stacking fault energy on the microstructural development during room temperature wire drawing in Cu,Al and their dilute alloys

The effect of stacking fault energy (SFE) on the evolution of microstructures during wire drawing at room temperature has been studied in pure aluminium, pure copper and Cu-2.2% Al andCu-4.5% Al alloys which covers a range of SFE values from 4 to 166 mJ m^–2. The compositions are expressed in atomic parts per million by weight. The microstructures have been characterized from samples obtained by deforming rods of these materials to true wire drawing strain values of up to 1.47. A decrease in the SFE value changes the deformation mechanisms from the formation of cell structure and their size refinement in a high SFE material to the formation of deformation bands and deformation twins in a low SFE materials. The Cu-2.2% Al alloy deforms by deformation bands at low true strain values while deformation twins within the bands control the deformation mechanisms at higher true strain values. The alloy, Cu-4.5% Al, with the lowest SFE value deforms only by deformation twins even at low true strain values and the presence of overlapping and intersecting deformation twins are the dominating features as the rods are drawn to higher true wire drawing strains.     

SiCp/AZ91D镁基纳米复合材料的室温拉伸行为及塑性变形机理

为得到高强度和高塑性的镁基复合材料,通过高能超声分散法和金属型重力铸造工艺制备了SiC纳米颗粒分散均匀的SiCp/AZ91D镁基纳米复合材料,并进行T4固溶热处理和室温拉伸。利用扫描电子显微镜(SEM)、透射电子显微镜(TEM)对试样拉伸后的显微组织和塑性变形机理进行观察与研究。结果表明:T4态SiCp/AZ91D镁基纳米复合材料室温下抗拉强度达到296 MPa,伸长率达到17.3%。经室温拉伸变形后复合材料基体微观组织中出现了大量的孪晶和滑移,孪生和滑移是复合材料塑形变形的主要机制。在室温拉伸过程中,α-Mg基体中SiC纳米颗粒周围形成高应变场,高应变场内形成大量位错和堆垛层错,这些位错和堆垛层错在拉伸应变的作用下演变成大量的滑移带和孪晶,这是SiCp/AZ91D镁基纳米复合材料在室温下具有高塑性的微观塑性变形机理。     

Microstructure of SiC Whiskers with Different Cross-sections Perpendicular to the Whiskers Axis

Microstructure of β-SiC whiskers with differ-ent cross-sections perpendicular to their growingdirection was studied in detail by transmission elec-tron microscopy (TEM).It was indicated that therewere three types of cross-sections:round,hexagonal and trigonal.The whiskers with roundand hexagonal cross-sections had a high density ofplanar faults lying on the (111) close packed planesperpendicular to the whisker axis.There existed afew stacking faults on the other {111} planes insome hexagonal whiskers.The whiskers withbicrystals were also found in hexagonal whiskers.The microstructure of trigonal SiC whiskers wasbasically perfect but there were a few intrinsic stack-ing faults on the (11) planes (mostly) and (111)planes.The characters of electron diffraction pat-terns of β-SiC whiskers with different cross-sec-tions were reasonably analyzed using a reciprocalspace model with continuous diffraction streaksalong the [111] reciprocal direction.     

Annealing twin boundary cracking in the low-temperature brittle fracture of a high-nitrogen bearing austenitic steel

Crack propagation paths in the low temperature brittle fracture of 18Cr-18Mn-0.7N austenitic steel were investigated by means of scanning electron microscopy. Corresponding relationships of the fracture facets with microstructures were established by the simultaneous observation of the fracture surface and the microstructure of the adjacent side surface. It was shown that the annealing twin boundary cracking occurred during fracture. A great deal of twins formed during solution treatment of the steel, with steps several microns high on the twin boundaries, and a considerable amount of planar deformation structures developed on {111} planes in the fracture process. The fracture facets of the annealing twin boundary are fairly flat and smooth, with bent steps of micron-scale height, and a pattern of three sets of parallel straight-lines intersecting at 60°. But there is no river pattern on the facets. The bent steps result from partial propagation of crack along steps that are developed on annealing twin boundaries during solution treatment, while the line-pattern is the intersection traces of the planar deformation structures with the fracture facet. It is believed that the annealing twin boundary cracking is attributed to the stress concentration arising at the intersection of planar deformation structures and the annealing twin boundaries.