Reversed anisotropy of grain boundary properties and its effect on grain boundary engineering

Data on anisotropy of grain boundary properties are frequently published. In some cases, when they show the reverse course of structural dependence than is expected, they can seem confusing. Examples of this “reversed anisotropy” found for grain boundary segregation, diffusion and migration are presented. We demonstrate that the reversed anisotropy of grain boundary properties is caused by the compensation effect. This can have serious consequences for grain boundary engineering. It is also suggested that grain boundaries can be classified specifically and generally solely on the basis of well-defined thermodynamic parameters.     

Transformations of grain boundary dislocation pile-ups in nano- and polycrystalline materials

A theoretical model is suggested which describes several types of transformations of grain boundary dislocation pile-ups at triple junctions of grain boundaries in (super) plastically deformed nanocrystalline and polycrystalline materials. Ranges of parameters of defect configurations are revealed at which the transformations considered are energetically favourable. The role of transformations of grain boundary dislocation pile-ups at triple junctions of grain boundaries in plastic deformation processes in nanocrystalline and polycrystalline materials is discussed with special attention being paid to the influence of such transformations on competition between different deformation mechanisms in nanocrystalline materials.     

晶界特征分布对哈氏合金力学性能的影响

采用扫描电镜、背散射电子衍射、透射电镜等手段对Hastelloy X(HX)合金的晶界特征分布、拉伸断口形貌及位错分布等进行研究。结果表明,HX合金室温拉伸断口由局部裂纹、微孔和大量的韧窝构成,微孔的形成与材料内部第二相颗粒分布有关。组织为晶内共格孪晶型的材料开裂是裂纹源扩展引起的,而组织为非共格孪晶型的样品是微孔聚集导致的开裂。分析表明,不同晶界特征分布的样品室温力学性能的差异,主要是晶粒尺寸和一次碳化物分布作用的结果。     

Optimization of grain boundary character distribution for intergranular corrosion resistant 304 stainless steel by twin-induced grain boundary engineering

The effects of process parameters, pre-strain, annealing temperature, time, etc. on grain boundary character distribution (GBCD) and intergranular corrosion in thermomechanical treatment were examined during grain boundary engineering of type 304 austenitic stainless steel. Slight pre-strain annealing at a relatively low temperature resulted in excellent intergranular corrosion resistance due to optimized GBCD, i.e. the uniform distribution of a high frequency of coincidence site lattice boundaries and consequent discontinuity of random boundary network in the material. The optimum distribution can be formed by introduction of low energy segments on migrating random boundaries during twin emission and boundary–boundary reactions in the grain growth without generation of new random boundaries.     

Correlations between the crystallographic texture and grain boundary character in polycrystalline materials

A method is presented to determine the misorientation probability distribution function in polycrystalline materials based on a known, analytical or numerical, representation of the associated orientation probability distribution function, i.e., texture. The proposed formulation incorporates the local grain-to-grain orientation correlations by combining local or macroscopic statistical information, and finds a natural interpretation through the well-known stereographic projection (pole-figure) representation. The proposed formulation distinguishes between antiparallel crystallographic orientations, as well as cone-angle and polar angle misorientations. For fiber-textured samples, it is quantitatively shown that highly oriented samples are equivalent to polycrystals with a high density of low-angle misorientations, while completely random (untextured) materials are equivalent to microstructures with a high probability of large-angle misorientations.     

A study of low-strain and medium-strain grain boundary engineering

Grain boundary engineering (GBE) processing schedules, involving low-strain (5% deformation) iterative treatments, have been carried out on copper. Misorientation and grain boundary plane statistics have been derived, plus tensile and hardness measurements. The Σ3 length fraction and Σ9/Σ3 number ratio decreased during the first two processing iterations, whereas maximum GBE misorientation statistics were achieved after three processing iterations. Analysis of mechanical properties data revealed an accumulation of strain energy throughout the first three processing iterations, sufficient to provide enough driving force for extensive Σ3ⁿ interactions. The density of Σ3 boundaries had a larger effect on the rate of hardening than did the density of grain boundaries. This finding indicates the effectiveness of Σ3 interfaces as barriers to plastic flow, which plays an important role in the early stages of GBE processing. Data from samples that had undergone the low-strain iterations were also compared to medium-strain (25% deformation) processing iterations.     

Grain boundary engineering based on fractal analysis for control of segregation-induced intergranular brittle fracture in polycrystalline nickel

The effect of grain boundary microstructure on the fracture resistance of sulfur-doped polycrystalline nickel was investigated using specimens with different grain boundary microstructures to reveal the usefulness of grain boundary engineering for control of segregation-induced intergranular brittle fracture of polycrystalline materials. The sulfur-doped polycrystalline nickel specimen with more homogeneous fine-grained structure and a higher fraction of low-Σ coincidence site lattice (CSL) boundaries shows higher fracture resistance than the specimen with coarse-grained structure and a lower fraction of low-Σ CSL boundaries. It was found that high-energy random boundaries play a key role as the preferential crack path in fracture processes. The resistance to sulfur segregation-induced intergranular brittle fracture was evaluated by analyzing the fractal dimension of random boundary connectivity in the polycrystalline nickel specimens studied. The fractal dimension of random boundary connectivity decreases with increasing fraction of low-Σ CSL boundaries, resulting in the generation of a higher fracture resistance by restricting more frequent branching and deflection of propagating crack path along random boundaries from the main crack.     

Arrest of weld-decay in 304 austenitic stainless steel by twin-induced grain boundary engineering

A twin-induced grain boundary engineered 304 austenitic stainless steel with a high frequency of coincidence site lattice boundaries produced by a one-step thermomechanical process was arc-welded. The heat-affected zone was examined by orientation imaging microscopy and corrosion tests. The results indicated that the intergranular corrosion due to sensitization in the heat-affected zone were perfectly suppressed, because the high frequency of coincidence site lattice boundaries and the optimum grain boundary character distribution were stable in the grain boundary engineered stainless steel during arc-welding. The grain boundary engineering can arrest the weld-decay of austenitic stainless steel effectively.     

Mechanical properties of amorphous and polycrystalline multilayer systems

Amorphous and polycrystalline multilayer structures containing materials with metallic (Cr, Cr₃C₂), ionic (Al₂O₃) and covalent (SiC) bonding have been prepared by magnetron sputtering and ion plating in a dual-source apparatus. Up to 1000 layers have been deposited with a constant total thickness of 2.3 μm.

Below a single-layer thickness of 10–30 nm the mechanical properties stress and hardness show strong variations. On one hand it is possible that below a certain thickness the mechanical properties of a single layer change (A. Pan and J.E. Greene, Thin Solid Films, 78 (1981) 25–34). On the other hand electrical resistance and electron spin density measurements indicate that electronic effects may be involved. An attempt is made to explain the observed correlations by transport mechanisms of the electrons, by saturation of dangling bonds with delocalized electrons and by changes in the electronic band structure.     

Mechanism of twinning-induced grain boundary engineering in low stacking-fault energy materials

The current status of “grain boundary engineering” is overviewed, i.e. the deliberate manipulation of grain boundary crystallography in polycrystals in order to produce a material containing grain boundaries which have superior properties compared to average boundaries. A particular focus of attention is annealing twinning in low stacking-fault energy (SFE) materials as a means to achieve this aim, since the role of such twinning in improving the grain boundary network has not hitherto been satisfactorily explained. The twinning is discussed from the viewpoint of strain retention, imposition of crystallographic constraints at grain junctions, “relative specialness” rather than “absolute specialness” and proposal of a new “Σ3 regeneration model” to explain how twins can enhance the Σ3 boundary fraction in the network.     

The influence of bondcoat and topcoat mechanical properties on stress development in thermal barrier coating systems

A finite-element study has been undertaken to investigate the stress development within a TBC system consisting of an EB-PVD YSZ topcoat and a Pt-aluminized diffusion bondcoat. Particular attention has been paid to the role of variables such as the elastic anisotropy within the topcoat, interface roughness, variation in creep strength of the bondcoat and the volumetric strains associated with the formation of the thermally grown oxide (TGO). Bond coat oxidation and thermal loading during cooling give rise to significant tensile stresses within the topcoat and tensile tractions at the TGO interfaces. Bondcoat creep, as distinct from yield and plastic behaviour, was the dominant stress relaxation process, and strong bondcoats (in creep) tended to show higher tensile stress levels. Another important factor determining thermal barrier coating stress levels was the level of elastic anisotropy of the topcoat: an elastic isotropic yttria-stabilized zirconia gave rise to considerably higher stresses than a transversely isotropic topcoat.     

TbHx晶界扩散细晶钕铁硼磁体的组织结构与性能

通过调整粉末粒度控制烧结钕铁硼磁体的晶粒尺寸,研究烧结温度对磁性能的影响.在磁体表面涂覆TbHx然后进行晶界扩散,研究晶粒细化对TbHx晶界扩散磁体性能的影响.结果 表明:Tb原子扩散进入主相晶粒边缘区域,使主相晶粒外延层产生磁硬化;晶粒细小磁体中的Tb元素均匀分布于晶界,形成连续的重稀土薄层,起到良好的去磁耦合作用,从而提高磁体内禀矫顽力.因此,细晶粒磁体晶界扩散后矫顽力提升幅度大,且剩磁下降较小,具有好的综合磁性能.     

The influence of the grain boundary phase on the mechanical properties of Si3N4–MoSi2 composites

The microstructure and mechanical properties of Si₃N₄–MoSi₂ composites doped with two different sintering additive systems, Y₂O₃–Al₂O₃ and Lu₂O₃, were investigated. It was found that the composite doped with Y₂O₃–Al₂O₃ had an amorphous grain boundary phase, while the grain boundary phase of the Lu₂O₃-doped composites was completely crystallized. The Si₃N₄–MoSi₂ composite containing Lu₂O₃ had higher elastic modulus and better creep resistance at elevated temperatures (>1000 °C) than the composite doped with Y₂O₃–Al₂O₃. This is attributed to the crystallization and higher softening temperature of the Lu₂O₃-doped grain boundary phase compared with that doped with Y₂O₃–Al₂O₃. However, the toughness and strength were not influenced significantly by the grain boundary phase. The inclusion of MoSi₂ particles in Si₃N₄ can improve their fracture toughness through residual stresses induced by the coefficient of thermal expansion mismatch of Si₃N₄ and MoSi₂. The strength decreased significantly at temperatures over 1000 °C due to the brittle–ductile transition of the MoSi₂ phase.     

超细晶铁素体钢的力学性能及成形性能研究

采用形变强化铁素体相变超细晶轧制工艺,获得了750MPa级超细晶铁素体钢板,其超细晶铁素体体积比高达93%,晶粒尺寸仅为1μm,并具有优异的成形性能。对超细晶铁素体钢组织的精细分析发现,因铁素体晶界随晶粒超细化而减薄,使得其晶界对材料的力学性能具有双重影响,一方面,屈服强度随着晶界总长度的增加而提高;另一方面,又因晶界减薄而降低。研究表明,超细晶铁素体钢的组织性能关系虽然与霍尔-佩奇关系相吻合,但存在较大偏差,主要表现为斜率显著下降。文章对斜率下降的原因进行了机理分析。