单相fcc金属晶内裂纹萌生与扩展的TEM原位分析

用ＴＥＭ拉伸方法,对具有ｆｃｃ结构的３０４Ｌ钢,进行了晶内裂纹萌生与裂纹志愿的原位观察,结果表明：晶内开裂时首先形成无位错区,裂纹在无位错区 形成,裂尖护 裂尖前晶面变形行为由原位观察和原位选区衍射的结果进行分析,裂纹沿（１００）晶面扩展时,裂尖前（１００）同发生倾转,（１１１）晶面局部弹性弯曲,裂尖钝化;（１００）倾声讨在以致开裂,钝化裂尖扩展,（１１１）局部弯曲有所恢复。     

Effect of microstructure on crack propagation in high-temperature fatigue of directionally solidified Ni-based superalloy

In order to clarify the crack propagation properties of an anisotropic material (Ni‐based directionally solidified superalloy), longitudinally loaded specimens (L‐specimens) and transversely loaded specimens (T‐specimens) with a crack are subjected to high temperature fatigue. The crack propagation rate is reasonably well correlated with the effective stress intensity factor range regardless of the propagation direction (specimens L and T), the stress range and the stress ratio. However, the crack propagation rate shows a notable fluctuation particularly in the T‐specimens. It is at most about five times faster than the average. The fracture surface features can be classified into four types with three transgranular and one intergranular types. In the former, though the crack is along the {100} or {110} planes on a macroscopic scale, it threads through the {111} or {100} planes on a microscopic scale. Crack propagation is notably accelerated in the intergranular region, while deceleration is caused by crack branching.     

Computer simulation of atomic properties and dynamic behavior of interstitial clusters in Ni*

The atomic structure and dynamic behavior of interstitial clusters, i.e., a bundle of <110> crowdions, have been investigated in a model Ni lattice. An extended dislocation loop was obtained after full relaxation of a loop of hexagonal shape, consisting of four dislocation segments lying on {111} slip planes and two dislocation segments on {100} slip planes. The dislocation segments on {111} slip planes are extended, but the segments on {100} slip planes are not extended. By observing the motion of a dislocation loop under axially symmetrical shear stress, the Peierls stress for the dislocation loop was obtained. Also, a diamond-shaped dislocation loop was constructed in the model lattice, consisting of four dislocation segments on {111} slip planes and no segments on {100} slip planes. The Peierls stress for this diamond-shaped dislocation loop was found to be less than that for the hexagonal-shaped dislocation loop. This revised version was published online in June 2006 with corrections to the Cover Date.     

Grain boundary structure dependent fracture in nanocrystalline Au films

Nanocrystalline Au films were in situ strained in a high resolution transmission electron microscope, which demonstrated that the diffusion-assisted intergranular fracture was the dominant failure mode. Grain orientation with respect to grain boundaries (GBs) imposes important effect on the crack propagation and blunting. The low surface energy and high diffusion mobility of {111} planes lead to a notch-like crack. The stress concentration at the tip may help breaking {111} planes layer by layer and thus advance the crack. Cracks can be diverted from the preset path by GBs and grow into the grain interior, which has never been revealed by other experiments and molecular dynamics simulations.     

甲烷浓度对CVD金刚石薄膜晶体学生长过程的影响

采用X射线衍射技术、电子背散射衍射技术和扫描电镜分别观察了不同甲烷浓度条件下沉积的CVD自支撑金刚石薄膜的宏观织构、微区晶界分布和表面形貌.研究了金刚石晶体{100}面和{111}面生长的晶体学过程.研究表明,{100}面通过吸附活性基团CH^2- 2,而{111}面通过交替吸附活性基团CH^-3和CH^-3后脱氢堆积碳原子.低甲烷浓度时,{111}面表面能低于{100}面,使{111}面生长略快于{100}面.甲烷浓度升高,动力学作用增强使{100}面生长明显快于{111}面,使金刚石薄膜产生{100}纤维织构;同时显露的{100}面平行于薄膜表面,竞争生长使位于晶体侧面的{111}面由于相互覆盖而减小,形成了不同于单晶体自由生长的薄膜表面形貌组织.     

Orientation dependence of fracture toughness measured by indentation methods and its relation to surface energy in single crystal silicon

Fracture toughness of silicon crystals has been investigated using indentation methods, and their surface energies have been calculated by molecular dynamics (MD). In order to determine the most preferential fracture plane at room temperature among the crystallographic planes containing the 〈001〉, 〈110〉 and 〈111〉 directions, a conical indenter was forced into (001), (110) and (111) silicon wafers at room temperature. Dominant {110}, {111} and {110} cracks were introduced from the indents on (001), (011) and (111) wafers, respectively. Fracture occurs most easily along {110}, {111} and {110} planes among the crystallographic planes containing the 〈001〉, 〈011〉 and 〈111〉 directions, respectively. A series of surface energies of those planes were calculated by MD to confirm the orientation dependence of fracture toughness. The surface energy of the {110} plane is the minimum of 1.50 Jm⁻² among planes containing the 〈001〉 and 〈111〉 directions, respectively, and that of the {111} plane is the minimum of 1.19 Jm⁻² among the planes containing the 〈011〉 direction. Fracture toughness of those planes was also derived from the calculated surface energies. It was shown that the K _IC value of the {110} crack plane was the minimum among those for the planes containing the 〈001〉 and 〈111〉 directions, respectively, and that K _IC value of the {111} crack plane was the minimum among those for the planes containing the 〈011〉 direction. These results are in good agreement with that obtained conical indentation.     

Influence of microstructure and texture on mechanical properties of aluminium alloy 2124 + 5%SiC particulate composites

Abstract

A study of texture, microstructure, mechanical properties, and crack propagation mechanisms was carried out on aluminium alloy 2124 reinforced with 5 vol.-%SiC particles (3 μm). Three fabrication techniques have been used to produce the composites. composite I wasfabricated by blending followed by hot isostatic pressing. composite II was fabricated by mechanical alloying followed by hot isostatic pressing. composite III wasfabricated by agglomeration of aluminium powder by mechanical alloying followed by blending with SiC and hot isostatic pressing. All three composites were hot rolled to nominally 12.5 mm thick plate. Similar textures were observed for all composites. A model of the observed texture is {001} (211), {111} (211), and {211} (111) for rolling, side, and transverse planes respectively. Composite I showed a homogeneous distribution of SiC particles. Transmission electron micrographs of composite I showed good interface bonding, stacking faults present in SiC particles, and segregation of aluminium, oxygen, copper, and magnesium to the interface. composites II and III showed an inhomogeneous distribution of SiC particles. The elastic modulus was slightly higher in the (211) direction than in the (111) direction. The fracture toughness of composite I was higher in the (211) crack direction whereas that for composite II and composite III was higher in the (111) crack direction. Secondary crack propagation modes follow the crystallographic orientations of {100} and {111} planes.     

The role of vicinal Σ3 boundaries and Σ9 boundaries in grain boundary engineering

Σ 3 grain boundary planes and triple junctions containing Σ 3 boundaries have been investigated in grain boundary engineered brass using an adaptation of electron backscatter diffraction (EBSD) data combined with a single surface trace analysis methodology. The data have been analysed from the standpoint of whether or not the interface planes in the Σ 3 boundaries are close to {111} (i.e. coherent annealing twins), vicinal to {111} (i.e. a small deviation from the {111} reference structure) or not on {111}. At triple junctions composed of Σ 3/Σ 3/Σ 9 it was shown that a combination of one {111} and one vicinal-to-{111} Σ 3 was more likely to occur than two {111} Σ 3s or two not-{111} Σ 3s. The explanation for the preferred {111}/vicinal-to-{111} combination is that mobile Σ 9 boundaries with high deviations encounter {111} Σ 3s, and in consequence regenerate a Σ 3 at the triple junction which must be vicinal-to-{111} according to the crystallographic constraints at the junction. Analysis of the not-{111} Σ 3s indicated that more than half the boundaries in this category could not have {211}{211}, {774}{855}, {111}{511}, {001}{221} or {110}{411} planes. The possible distribution of these planes types, based on information from the single-surface trace analysis, had a high rank correlation coefficient, 0.925, with previous data from nickel which was based on a two-surface, full boundary planes analysis.     

DD6单晶高温合金振动疲劳性能及断裂机理

研究[001]取向的DD6单晶高温合金的室温振动疲劳S-N曲线,并获得了其室温振动疲劳极限。利用体视显微镜、扫描电子显微镜、背散射衍射等手段对DD6单晶高温合金振动疲劳断裂机制进行分析。结果表明:采用S-N法估算得到的[001]取向的DD6单晶高温合金室温振动疲劳极限约为337.5MPa。振动疲劳裂纹断口呈现单个或多个沿{111}晶体学扩展平面组成的形貌特征,断口上分为疲劳源区和疲劳扩展区两个阶段,裂纹在应力最大截面处的表面或内部缺陷处萌生,呈单源特征,疲劳扩展区呈现类解理断裂特征,未出现典型的疲劳条带特征。说明沿{111}晶面滑移是DD6单晶高温合金室温振动疲劳断裂的主要变形机制,断口上的类解理扩展平面以及微观上类解理花样是DD6单晶高温合金室温振动疲劳断裂的主要特征。     

Deformation and fracture of Al-8Fe rapidly solidified alloys

The effects of the rapid solidification on the deformation and fracture of Al-8Fe alloys, from TEM fracture specimens, have been studied. The most general conclusion which can be drawn in this study is clearly in agreement with a plastic deformation mechanism. Crack propagation occurs by localized plastic rupture mechanisms which result from enhanced slip along {111} planes. Crack propagation occurs within the deformed zone either by the nucleation, growth and coalescence of holes ahead of the crack-tip, or through the emission of dislocation from the crack-tip. The resulting fracture is along the active {111} slip planes. The principal effect of secondary phases (Al₁₃Fe₄) on the fracture propagation in Al-8Fe alloys was that the secondary phases increased the stress level at which plastic deformation occurs at the crack-tip and increased the stress level at which the crack propagates. This work clearly shows that in order to obtain coarse intermetallic precipitates in the specimens after ageing heat treatments the crack propagation and deformation processes occur at lower stresses compared to as-received rapidly solidified samples.     

铸钢中(Mn,Fe)S单晶体的发现与研究

用SEM505扫描电子显微镜观察并发现了ZG25钢铸件皮下显微气孔壁上（Mn，Fe）S单晶体的自然形态。平衡态面心立方（Mn，Fe）S单晶体是由八个{111}晶面和六个{100}晶面组成的十四面体。（Mn，Fe）S单晶体自然形态是Oh—m3m类立方晶系对称图形新发现的典型实例。填补了晶体学单晶十四面体晶形的空白。     

Morphology of C60 Crystals Grown from the Vapor Phase

Abstract

The morphology of C₆₀ crystals grown from the vapor phase have been studied. In all observations, only hexagonal and rectangular shaped crystal faces were found. Very different morphology, highly faceted {111} faces and flat {100} faces were observed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). the highly regular shape and similar distance between all neighboring macrosteps observed for the {111} faces can be explained by taking into account that edges of two adjacent {111} and {100} planes can act as step sources.     

Ductile-brittle behavior at the (1 1 0)[0 0 1] crack in bcc iron crystals loaded in mode I

Fracture experiments performed at room temperature on four test samples made of Fe-3wt.%Si single crystals with an edge crack (1 1 0)[0 0 1] (crack plane, crack front) showed approximately 45° deflections of the crack from the initial crack plane (1 1 0). This behavior appeared to be independent on loading rate. Fractographic analysis confirmed that the cracks were deviated along {1 0 0} planes and the fracture was accompanied by dislocation slip and by twinning. 3D simulations at 300 K by molecular dynamic technique in bcc iron with edge cracks of equivalent orientation indicated that the crack itself could contribute to understanding of this behavior by three processes: twinning on oblique {1 1 2} planes, which hindered growth of the original crack, and by emission of dislocations on oblique {0 1 1} and {1 2 3} planes, which led to separation of the {1 0 0} planes and might cause decohesion and subsequent cleavage fracture along the mentioned planes.     

Effects of temperature and ferromagnetism on the γ-Ni/γ′-Ni3Al interfacial free energy from first principles calculations

The temperature dependencies of the γ(f.c.c.)-Ni/γ′-Ni₃Al(L1₂) interfacial free energy for the {100}, {110}, and {111} interfaces are calculated using first-principles calculations, including both coherency strain energy and phonon vibrational entropy. Calculations performed including ferromagnetic effects predict that the {100}-type interface has the smallest free energy at different elevated temperatures, while alternatively the {111}-type interface has the smallest free energy when ferromagnetism is absent; the latter result is inconsistent with experimental observations of γ′-Ni₃Al-precipitates in Ni–Al alloys faceted strongly on {100}-type planes. The γ(f.c.c.)-Ni/γ′-Ni₃Al interfacial free energies for the {100}, {110}, and {111} interfaces decrease with increasing temperature due to vibrational entropy. The predicted morphology of γ′-Ni₃Al(L1₂) precipitates, based on a Wulff construction, is a Great Rhombicuboctahedron (or Truncated Cuboctahedron), which is one of the 13 Archimedean solids, with 6-{100}, 12-{110}, and 8-{111} facets. The first-principles calculated morphology of a γ′-Ni₃Al(L1₂) precipitate is in agreement with experimental three-dimensional atom-probe tomographic observations of cuboidal L1₂ precipitates with large {100}-type facets in a Ni-13.0 at.% Al alloy aged at 823 K for 4096 h. At 823 K this alloy has a lattice parameter mismatch of 0.004 ± 0.001 between the γ(f.c.c.)-Ni-matrix and the γ′-Ni₃Al-precipitates.     

A model for the 〈100〉 crystallographic tunnel etching of aluminium

When high purity aluminium foil is subjected to DC electrochemical etching in hot aqueous chloride solution at temperatures above 60 °C, the 100 crystallographic tunnel growth of an etch pit is achieved. This crystallographic tunnel growth behaviour arises due to the following reason. The tunnel tip region where the active corrosion reaction takes place tends to be made of {111} planes which minimize the surface energy, whilst the subsequent tunnel wall is made of {100} planes, because the passive oxide film formed on the {100} planes, along which the biaxial elastic modulus is the smallest, would be least attacked by corrosion.     

Temperature dependence of impact deformation behavior of single crystal Ni based superalloy

The impact test was carried out to investigate the intermediate temperature brittleness of single crystal Ni based superalloy. The samples were impacted at the velocity of ～5?m?s^??1. The results showed that the impact toughness also exhibited intermediate temperature brittleness, which is similar to the situation in tensile. The samples showed the highest impact toughness at 600°C but exhibited the lowest impact toughness at 760°C. Results showed that the variety of impact toughness was due to the deformation mechanism. At 600°C, a/2? dislocation slips on the {111} slip system were attributed to the high impact toughness; however, a/2? dislocation slips from octahedral {111} planes to cubic {100} planes resulted in significant work hardening, leading to the decrease of impact toughness.     

The unusual near-threshold FCG behavior of a single crystal superalloy and the resolved shear stress as the crack driving force

An investigation of the fatigue crack growth (FCG) behavior of PWA 1480 single crystal nickel base superalloy was conducted. Typical Paris region behavior was observed above a δK of 8 MPa√m. However, below that stress intensity range, the alloy exhibited highly unusual behavior. This behavior consisted of a region where the crack growth rate became essentially independent of the applied stress intensity. The transition in the FCG behavior was related to a change in the observed crack growth mechanisms. In the Paris region, fatigue failure occurred along {111} facets, however at the lower stress intensities, (001) fatigue failure was observed. A mechanism was proposed, based on barriers to dislocation motion, to explain the changes in the observed FCG behavior. The FCG data were also evaluated in terms of a recently proposed stress intensity parameter, K_rss. This parameter, based on the resolved shear stresses on the slip planes, quantified the crack driving force as well as the mode I ΔK, and at the same time was also able to predict the microscopic crack path under different stress states.     

MODE I AND MIXED MODE I/II FATIGUE CRACKING IN Ni3Al(CrB) SINGLE CRYSTALS

Abstract— Nominal mode I and mixed mode I/II fatigue tests were carried out using the intermetallic compound Ni₃Al(CrB) in the form of single crystal specimens. The effects of crystal orientation and load mode on fatigue crack initiation and growth were studied. The fracture surfaces of the single crystals were characterized by a cleavage-like appearance and cracking occurred either on a single {111} plane or on multiple {111} planes irrespective of whether mode I or mixed mode I/II loadings were applied. It was found that the crack initiation and growth behaviour are dependent on both crystal orientation and applied loading mode. The cracking behaviour predicted by three mixed mode fracture criteria (MTS, SED and G criteria) in polycrystalline materials under mixed mode loading can be understood from the present results on single crystals.     

Transgranular fracture in low temperature brittle fracture of high nitrogen austenitic steel

Morphological features of transgranular fracture facets in low temperature brittle fracture of 18Cr–18Mn–0.7N high nitrogen austenitic steel have been studied by means of scanning electron microscopy and their formation mechanisms have been discussed. The transgranular fracture facets are fairly coarse compared with intergranular fracture facets and annealing twin boundary fracture facets. There are parallel steps and river patterns on the facets. Dual-surface observation indicated that these patterns are parallel to {111} planes and of strict crystallographic features. Microstructure observation shows that there are a lot of planar deformation structures formed prior to low temperature transgranular fracture. Transgranular fracture originates from microcracks formed at the intersections of the deformation structures on different {111} planes. These microcracks propagate toward adjacent microcracks on different {111} planes, forming transgranular fracture facets with steps and river patterns.     

The role of grain orientation in microstructure evolution of pure aluminum processed by equal channel angular pressing

A new approach describing the role of crystallographic orientation in the microstructural refinement of commercially pure aluminum during the successive passes of equal channel angular pressing (ECAP) is introduced. The study is based on analysis of X-ray diffraction texture data that is used to calculate the geometrical position of crystallographic slip planes with respect to the shearing plane of the ECAP die. The angular deviations of {111} slip planes from the macroscopic deformation plane for different processing routes were calculated and compared. The microstructure evolution was investigated using electron back-scattered diffraction (EBSD). The grain size and grain boundary character distribution obtained for each processing route are related to the angles between {111} planes and the shearing plane. It was shown that the more effective routes in grain refinement have higher angles between {111} slip planes and the shearing plane.