Interdiffusion in two-layer Pd/Ag films II. “Cold” homogenization mechanisms

Interdiffusion processes in thin epitaxial polycrystalline Pd/Ag films in the temperature range 20–500°C are studied by transmission electron microscopy, electron diffraction and electrical resistance methods. Homogenization is investigated both during condensation and under conditions of postcondensation annealing.The basic processes of homogenization associated with GB diffusion along migrating boundaries. It is found that in real polycrystal films with wide spectrum of grain sizes few mechanisms can occur simultaneously or subsequently: recrystallization induced diffusion, diffusion induced grain boundary migration, activation of bulk diffusion in fine grain clusters, bulk diffusion through interphase boundary. The conditions for prevailing one of them can be provided by changing condensation and postcondensation annealing temperatures or by choosing certain grain size.     

Interdiffusion in two-layer Pd/Ag films I. The effects of interphase boundaries

Interdiffusion processes in epitaxial single-crystal Pd/Ag thin films within the temperature range 20°–500°C are studied by transmission electron microscopy, electron diffraction and electrical resistance methods. Homogenization is investigated both during deposition and during annealing.The mass transfer kinetics is found to depend significantly on the original structure of the interphase boundary (dislocations present or not) and on its reconstruction due to interdiffusion. Regular networks of misfit dislocations at the interface can retard interdiffusion, while network failures lead to acceleration of homogenization process. The vacancy flows play a decisive role in these processes. The effects of interphase dislocations on the DIGM, DIR and RID processes are discussed.The numerical study of electrical resistance variations during annealing is carried out. The concentration profiles are plotted for a nontrivial dependence of interdiffusion coefficient on concentration. The numerical results are compared with the experimental data.     

The stability of triple junctions during the grain boundary diffusion process

The grain boundary diffusion in a system with triple junctions is considered in such a geometry, in which the flows of diffusing atoms meet at the triple line. The solutions of the diffusion equation is given in the frameworks of Fisher's model and under the assumption of quasi-stationary distribution of the diffusing atoms along the grain boundaries. The change of the mechanical equilibrium at the triple junction due to the increase of the concentration of solute atoms is considered. It is shown that under some circumstances the triple junction looses its stability with respect to migration in the direction to the diffusion source. The stability diagrams in the segregation-diffusivity parameter space are plotted.     

In situ TEM observations of dislocation/anti-phase boundary interactions

L2₁-ordered Fe₅₉Mn₁₇Al₂₄ (in at%) single crystals were in situ strained at either 300?K or 700?K in a transmission electron microscope. At 300?K, the strain was accommodated by the glide of four-fold dissociated super-dislocations, whereas, at 700?K, the strain was accommodated by the glide of a/2?111? partials. Dislocation pile-ups occurred at a/4?111? thermal anti-phase boundaries (APBs). Screw super-dislocations frequently cross-slipped when they encountered the thermal APBs, while mixed dislocations tended to be pinned at them. This impediment is attributed to the creation of new APB segments when dislocations pass through the curvy thermal APBs.     

Grain boundary interdiffusion in the case of concentration-dependent grain boundary diffusion coefficient

The grain boundary diffusion in a binary system which exhibits a grain boundary phase transition is considered in the framework of Fisher's model. The kinetic law of the growth of the grain boundary phase and the distribution of the diffusant near the grain boundary are calculated. The method of determining of the concentration dependence of the grain boundary diffusion coefficient from the experimentally measured penetration profiles of the diffusant along the grain boundaries is suggested. The experimental results on Zn diffusion in Fe(Si) bicrystals, Ni diffusion in Cu bicrystals and grain boundary grooving in Al in the presence of liquid In are discussed in light of the suggested model.     

Theory and simulation of coupled grain boundary migration and grain rotation in low angle grain boundaries

Abstract

Microstructure evolution due to coupled grain boundary migration and grain rotation in low angle grain boundaries is studied through a combination of molecular dynamics and phase field modeling. We have performed two dimensional molecular dynamics simulations on a bicrystal with a circular grain embedded in a larger grain. Both size and orientation of the embedded grain are observed to evolve with time. The shrinking embedded grain is observed to have two regimes: constant dislocation density on the grain boundary followed by constant rate of increase in dislocation density. Based on these observations from the molecular dynamics simulations, a theoretical formulation of the kinetics of coupled grain rotation is developed. The grain rotation rate is derived for the two regimes of constant dislocation density and constant rate of change of dislocation density on the grain boundary during evolution. The theoretical calculation of the grain rotation rate shows strong dependence on the grain size and compares very well with the molecular dynamics simulations. A multi-order parameter based phase field model with coupled grain rotation is developed using the theoretical formulation to model polycrystalline microstructure evolution.     

An investigation of misfit dislocations in Al on (001) GaAs grown by chemical beam epitaxy

We report a transmission electron microscope study of the morphology and interfacial structure of Aluminium grown on (001) GaAs by chemical beam epitaxy (CBE). The Al grows in islands for all thicknesses deposited, and exhibits four distinct orientation relationships with respect to the substrate. One of these orientation relationships becomes dominant as growth progresses, with (011)_Al parallel to (001)_GaAs. Misfit dislocations can be seen in the interface between this orientation and the substrate with Burgers vector 1/4(110)_GaAs, and a crystallographic analysis shows that these dislocations are associated with interfacial steps of height 1/2[001]_GaAs. In (001)Al on (001)GaAs, the existence of these dislocations has in the past been regarded as evidence for the existence of a rigid-body shift of the Al in the interfacial plane. Using cross-sectional high-resolution TEM, it is shown that this shift is not present in the (011) orientation. The similarity in the microstructure and crystallography of the (001) and (011) orientations leads us to suggest that there is also no shift in (001) Al on (001)GaAs. This is in conflict with previous investigations of this system using a wide variety of techniques.     

Grain boundary structures in silicon carbide: Verification of the extended boundary concept

A grain boundary layer of ca. 0.5 nm in thickness is present in B+C added SiC and SiC without any sintering aids. Since these materials do not show a significant strength-decrease at high temperatures, Ikuhara et al. presumed that the layer is not a second phase of sintering aids or impurities but a reconstructed structure formed to reduce the high energy of the grain boundary, and they called such a boundary an extended boundary. The concept of the extended boundary, however, has not yet been generally accepted for lack of convincing evidence. In the present work, the elements analysis of the boundary layer was made and some additional collateral verifications were conducted in order to inspect the extended boundary concept.     

Analysis of kinetics regimes in grain boundary self-diffusion†

The evolution of self-diffusion concentration depth profiles into a bulk polycrystalline material from a thin-film tracer source was studied using the lattice Monte Carlo method. The onset of the Type-B kinetics regime from the Type-A kinetics regime was investigated for a cubic grain model, which can be expected to capture the three-dimensional nature of the problem better than the usual grain boundary slab model. The solutions of Whipple–Le Claire (derived for the constant source condition), Suzuoka (derived for the instantaneous source condition), Bokstein and colleagues (derived for the constant source condition) and Levine and MacCallum (derived for the constant source condition) are tested. All these solutions describe the tail region of the profiles well for this model (provided that a simple empirical factor in the case of the first three solutions is introduced). The intermediate Type-AB kinetics regime proposed by Divinski and Larikov between the Type-A and Type-B kinetics regimes was also investigated but could not be located from analyses of best-fit exponents. Forcing the fitting nonetheless to the tail regions shows that their analysis needs some further refinement before application to this model.     

Misorientation Dependence of the Grain Boundary Energy in Magnesia

Geometric and crystallographic data obtained from a well annealed magnesia polycrystal have been used to specify the five macroscopic degrees of freedom for 4665 grain boundaries. The results indicate, that for this sample, the five parameter grain boundary character space is fully occupied. A finite series of symmetrized spherical harmonics has been used to approximate the misorientation dependence of the relative grain boundary energy. Best fit coefficients for this series were determined by assuming that the interfacial tensions at each triple junction are balanced. The grain boundary energy function shows Read-Shockley behavior at small misorientations and a broad minimum near the 3 misorientation. Furthermore, misorientations about the ‹100› axis create boundaries with relative energies that are less than those created by misorientations about the ‹110› or ‹111› axes.     

High-resolution transmission electron microscopy investigation of the nanostructure of undoped and Pt-doped nanocrystalline pulsed laser deposited SnO2 thin films

Pulsed laser deposition (PLD) was used to grow nanocrystalline SnO₂ thin films onto alumina substrates. The reactive PLD process was carried out at different substrate deposition temperatures (T_d) between 20 and 600 °C under an oxygen background pressure of 150 mtorr. The same PLD technique was used to produce SnO₂ films in situ-doped with Pt (at the level of ∼2 at. %) through the concomitant ablation of both SnO₂ target and Pt strips. Conventional and high-resolution transmission electron microscopy (HRTEM) observations have revealed that the microstructure of the PLD SnO₂ films is highly sensitive to their deposition temperature. Indeed, its changes from a porous granular structure with extremely fine equiaxed grains (∼4 nm diameter), at T_d=20 °C to a very compact and textured columnar structure characterized by SnO₂ columns (∼25 nm diameter) composed of grains of ∼12 nm of diameter, at T_d=600 °C. In addition, the PLD SnO₂ films were found to exhibit the highest nanoporosity at T_d=300 °C which also coincides with the granular-to-columnar microstructural transition. On the other hand, the microstructure of the Pt-doped SnO₂ films, deposited at 300 °C, was found to contain a high density of defects, such as twin boundaries and edge dislocations. By combining HRTEM and EDS microanalysis, we were able to show that the Pt-dopant self-organizes into spherical nanoparticles (1-2 nm diameter) randomly distributed at the SnO₂ grain boundaries. Finally, doping the films with such platinum nanoclusters is found to affect the SnO₂ nanostructure by particularly reducing the SnO₂ mean grain size (from ∼10 nm when undoped to ∼6 nm for the doped films).     

Effects of the triple junction types on the grain boundary carbide precipitation in a nickel-based superalloy,a statistical analysis

The morphology evolution of carbide precipitated on grain boundary nearby different triple junctions in grain boundary engineering (GBE) treated nickel-based Inconel Alloy 690 aged at 715°C for different time was investigated by scanning electron microscopy and electron backscatter diffraction. The results show that, the diversity of triple junction types was increased by GBE significantly. The size and morphology of grain boundary carbide were not only affected by the grain boundary character, but also the nearby grain boundary character at the triple junction. The higher Σ values of the nearby grain boundaries, the larger carbide precipitated on the other grain boundary. Based on the experimental results, the effects of grain boundary characters and triple junction types on the carbide precipitation behaviours are discussed.     

Atomic scale investigation of grain boundary structure role on intergranular deformation in aluminium

The role that grain boundary (GB) structure plays on the directional asymmetry of an intergranular crack (i.e. cleavage behaviour is favoured along one direction, while ductile behaviour along the other direction of the interface) was investigated using atomistic simulations for aluminium 〈1 1 0〉 symmetric tilt GBs. Middle-tension (M(T)) and Mode-I crack propagation specimens were used to evaluate the predictive capability of the Rice criterion. The stress–strain response of the GBs for the M(T) specimens highlighted the importance of the GB structure. The observed crack tip behaviour for certain GBs (Σ9 (2 2 1), Σ11 (3 3 2) and Σ33 (4 4 1)) with the M(T) specimen displayed an absence of directional asymmetry which is in disagreement with the Rice criterion. Moreover, in these GBs with the M(T) specimen, the dislocation emission from a GB source at a finite distance ahead of the crack tip was observed rather than from the crack tip, as suggested by the Rice criterion. In an attempt to understand discrepancy between the theoretical predictions and atomistic observations, the effect of boundary conditions (M(T), Mode-I and the edge crack) on the crack tip events was examined and it was concluded that the incipient plastic events observed were strongly influenced by the boundary conditions (i.e. activation of dislocation sources along the GB, in contrast to dislocation nucleation directly from the crack tip). In summary, these findings provide new insights into crack growth behaviour along GB interfaces and provide a physical basis for examining the role of the GB character on incipient event ahead of a crack tip and interface properties, as an input to higher scale models.     

利用Pr_(70)Cu_(30)晶界扩散改善烧结钕铁硼废料矫顽力的研究

钕铁硼磁体制备过程中出现的部分块体废料由于矫顽力较低,性能难以满足使用要求.本文主要通过晶界扩散技术来提高废料磁体的矫顽力.采用Pr_(70)Cu_(30)合金作为扩散介质,对烧结钕铁硼废料磁体进行了晶界扩散处理,研究了扩散温度、扩散时间和回火时间对扩散后的磁体性能的影响.结果显示,800℃下扩散3 h,磁体的矫顽力从原来的7.88 kOe(1 Oe=79.5775 A/m)提升至11.55 kOe,提升幅度为46.6%,同时剩磁没有明显降低.扩散后回火对矫顽力的提升有一定的作用.800℃下扩散4h后的磁体在500℃回火3h后,最高矫顽力可达11.97 kOe,比原磁体废料提高了51.9%,接近成品磁体的水平.显微组织分析证实了晶界扩散的作用.扩散处理后的磁体中,主相晶粒间形成了连续晶间相,起到有效的磁隔离作用,有利于矫顽力的提高.研究还发现,Pr_(70)Cu_(30)晶界扩散虽然可以使磁体腐蚀电位上升,但也会增加腐蚀电流密度,不利于磁体抗腐蚀性的改善.本文工作对于提高材料的成品率具有重要意义.     

晶界对纳米多晶铝中冲击波阵面结构影响的分子动力学研究

用分子动力学方法研究了纳米多晶铝在冲击加载下的冲击波阵面结构及塑性变形机理.模拟研究结果表明:在弹性先驱波之后,是晶界间滑移和变形主导了前期的塑性变形机理;然后是不全位错在界面上成核和向晶粒内传播,然后在晶粒内形成堆垛层错、孪晶和全位错的过程主导了后期的塑性变形机理.冲击波阵面扫过之后留下的结构特征是堆垛层错和孪晶留在晶粒内,大部分全位错则湮灭于对面晶界.这个由两阶段塑性变形过程导致的时序性塑性波阵面结构是过去未见报道过的. 关键词： 晶界 塑性变形 冲击波阵面 分子动力学     

The effect of surface contact conditions on grain boundary interdiffusion in a semi-infinite bicrystal

The Kirkendall effect is conditioned by active diffusion as well as by active sources and sinks for vacancies. In the case of grain boundaries under the condition of negligible bulk diffusion, the Kirkendall effect is highly localized and responsible for the formation of an extra material wedge in the grain boundary, which may lead to high stress concentrations. The Kirkendall effect in grain boundaries of a binary system is described by a set of partial differential equations for the mole fraction of one of the diffusing components and for the stress component normal to the grain boundary completed with the respective initial and boundary conditions. The contact conditions of the grain boundary with the surface layer acting as source of one of the diffusing components can be considered as equilibrium ones ensuring the continuity of generalized chemical potentials of both diffusing components. Thus, the boundary conditions are determined by the difference in chemistry (i.e. how the thermodynamic parameters depend on chemical composition) of the grain boundaries and of the surface layer. The simulations based on the present model indicate a drastic influence of the chemistry on the grain boundary interdiffusion and Kirkendall effect.     

In situ TEM study of reversible and irreversible electroforming in Pt/Ti:NiO/Pt heterostructures

Experimental verification of the microscopic origin of resistance switching in metal/oxide/metal heterostructures is needed for applications in non‐volatile memory and neuromorphic computing. Numerous reports suggest that resistance switching in NiO is caused by local reduction of the oxide layer into nanoscale conducting filaments, but few reports have shown experimental evidence correlating electroforming with site‐specific changes in composition. We have investigated the mechanisms of reversible and irreversible electroforming in 250–500 nm wide pillars patterned from a single Ta/Ti/Pt/Ti‐doped NiO/Pt/Ta heterostructure and have shown that these can coexist within a single sample. We performed in situ transmission electron microscopy (TEM) electroform‐ ing and switching on each pillar to correlate the local electron transport behavior with microstructure and composition in each pillar. DFT calculations fitted to electron energy loss spectroscopy data showed that the Ti‐doped NiO layer is partially reduced after reversible electroforming, with the formation of oxygen vacancies ordered into lines in the 〈110〉 direction. However, under the same probing conditions, adjacent pillars show irreversible electroforming caused by electromigration of metallic Ta to form a single bridge across the oxide layer. We propose that the different electroforming behaviors are related to microstructural variations across the sample and may lead to switching variability. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)