Grain boundary complexions and pseudopartial wetting

The important class of grain boundary (GB) complexions includes the few nanometer thick layers having composition which strongly differs from that of the abutting grains. Such GB complexions are frequently called intergranular films (IGFs) and can be observed close to the lines of wetting, prewetting and premelting complexion transitions in the bulk phase diagrams. In the majority of systems, the direct transition between complete and partial GB wetting takes place (by changing temperature, pressure, etc.). However, in certain conditions the so-called pseudopartial (or pseudoincomplete, or frustrated complete) GB wetting appears in a phase diagram between complete and partial wetting. In case of pseudopartial GB wetting, the thin GB layer of a complexion (IGF or 2-D interfacial phase) can coexist with large droplets (or particles) of the wetting phase with a non-zero dihedral (contact) angle. Thus, such IGFs can be observed in the two-phase (or multiphase) fields of bulk phase diagrams, in the broad intervals of concentrations, temperature and/or pressure. The IGFs driven by the pseudopartial GB wetting can drastically modify the properties of polycrystals. In this review, we discuss this phenomenon for the technologically important Fe–Nd–B-based hard magnetic alloys, WC–Co cemented carbides and Al-based light alloys.     

Grain boundary diffusion: recent progress and future research

Grain boundary (GB) diffusion often controls the evolution of structure and properties of engineering materials at elevated temperatures. A knowledge of diffusion characteristics of GBs and deep fundamental understanding of this phenomenon are critical to many materials applications. In this paper we give an overview of boundary diffusion theory with emphasis on the interpretation of concentration profiles measured in diffusion experiments. We consider the most important situations encountered in boundary diffusion experiments, such as diffusion in the B and C regimes and diffusion in the presence of segregation. We also discuss the recent progress in the atomistic interpretation of GB diffusion. We conclude with an outlook for future research in this area.     

Grain boundary effect on the electrical properties of boron-doped polysilicon films

The effect of grain boundary width has been accounted for and a modified simple model of average carrier concentration is presented considering the transport mechanism of charge carriers by thermionic emission only. It is found that the electrical properties of polysilicon are very sensitive to doping concentration when the grain size is small and the effect of grain boundary width on electrical properties increases as the grain size decreases. The inclusion of grain boundary width in resistivity and mobility formulae also gives better results near the critical doping concentration. The proposed model gives better agreement between experimental data and theoretical results.     

Towards the diffusion source cost reduction for NdFeB grain boundary diffusion process

Aiming at improving the performance/cost ratio in grain boundary diffusion process(GBDP),the critical RE containing Pr-Al-Cu alloy,less expensive RE containing La-Al-Cu alloy and non-RE Al-Cu alloy were employed as the diffusion sources.The preliminary results show that the coercivity was successfully enhanced from 1000 kA/m to 1695,1156 and 1125 kA/m by Pr70Al20Cu10,La70Al20Cu10 and Al75Cu25(at.%) alloys diffusion,respectively,due to the formation of(Nd,Pr)-Fe-B,La2 O3 and c-Nd2 O3 phases respectively,after diffusion.It is also found that the corrosion resistance can be improved by Al-Cu diffusion due to the positive effects of Al and Cu elements in grain boundary.The present results demonstrated the various coercivity enhancement mechanisms for the GBDP based on different diffusion sources,and provided feasible solutions for cost reduction of GBDP and NdFeB production by saving RE resource.     

Stress relaxation of thin film due to coupled surface and grain boundary diffusion

A numerical scheme, based on an energy statement, is developed to simulate thin film morphology evolution and stress relaxation due to concurrent surface diffusion and grain boundary diffusion. Different from previously published works, this paper also explores the effects of mobility ratio of the two processes and the dihedral angle at the surface-grain boundary triple junction. The range of mobility ratio, in which the stress relaxation process is limited by either surface diffusion or grain boundary diffusion, is determined. It is found that, when the stress relaxation is limited by the rate of surface diffusion, the dihedral angle at the surface-grain boundary junction plays a significant role. A scheme of using both experimental and numerical results to determine grain boundary diffusivity is also presented. As an example, we obtain the activation energy and diffusivity for grain boundary diffusion in Cu thin film.     

The relation between the distribution of dihedral angles and the wetting angle during liquid phase sintering

This study presents a Monte Carlo method to simulate the effect of the wetting angle on a dihedral angle distribution and on a degree of the grain boundary penetration of a liquid phase during liquid phase sintering. For the high probability for wetting, the number of the grain boundary penetration of a liquid phase increased while the wetting angle decreased and a few bonded grains maintained a stable grain boundary with high dihedral angles. This finding contradicts the classical prediction that smaller values of mean dihedral angles produce superior wettability.     

Correlation between the intergranular brittleness and precipitation reactions during isothermal aging of an Fe–Ni–Mn maraging steel

Evolution of the intergranular brittleness of an Fe–10Ni–7Mn (weight pct) maraging steel was correlated with precipitation reactions during isothermal aging at 753 K. Intergranular brittleness of the Fe–Ni–Mn steel raises after aging treatment which occurs catastrophically at zero tensile elongation in the underaged and peakaged steels. The intergranular failure is attributed to grain boundary weakening due to the formation of coarse grain boundary precipitates associated with solute-depleted precipitate-free zones during isothermal aging. Further, evidences of planar slip bands were found within the grains of a peakaged specimen loaded by tensile deformation. Those inhomogeneously deformed bands were identified to apply large strain localization in the soft precipitate-free zones at grain boundaries which is assumed to fascinate microcracks initiation at negligible macroscopic strains in the underaged and peakaged steels. During further aging, concurrent reactions including (i) overaging of matrix precipitates, (ii) spheroidization of grain boundary precipitates, (iii) growth of precipitate-free zone in width and (iv) diffusional transformation to austenite take place which increase tensile ductility after prolonged aging.     

Grain boundary sliding and migration in copper: the effect of vacancies

The atomic structure and mechanism of the interface sliding of the Σ = 5(2 1 0)[0 0 1] symmetric tilt grain boundary (GB) in copper and its interaction with vacancies at an elevated temperature has been studied using a computationally efficient potential based on the Embedding Atom Method in connection with the finite temperature Monte Carlo technique. Grain boundary sliding is performed for pure copper as well as copper containing a vacancy at a selected position. The discontinuous changes of the GB energy at certain sliding distances are associated with GB migrations. Elevated temperature reduces the grain boundary sliding/migration energy by a factor of about 2 but does not increase the rate of migration. Migration of the GB is mediated by the flow of atoms along the interface in coordination with the atoms in bulk. The sliding and migration properties partially depend on the position of the vacancy in the GB core. We found that the grain boundary sliding energy profile in the presence of a vacancy placed at the interface increased the GB energy, but reduces the sliding energy. The sliding process invokes the interface migration in such a way that the vacancy effectively migrates to a more convenient position and reduces the GB energy.     

An explanation of the wetting and the mutual diffusion mechanisms of liquid metals using ab-initio atomic orbital calculation

In this report, in order to clarify fundamentally the wetting phenomenon between pure material surfaces, we propose a new method for the experimental evaluation and consider quantitatively the wetting mechanism from the microscopic standpoint of the chemical reaction between interfacial atoms. Ab-initio molecular orbital calculation was performed to explain the wetting and the mutual diffusion mechanisms. It was found that the calculated binding energies were in good agreement with the interaction energies estimated from measured contact angles, and it was also cleared that the hybridization of the interfacial d-orbitals was one of the most important elements of the mutual diffusion.     

Diffusion induced grain boundary migration in the Ag–Zn system

Diffusion induced grain boundary migration (DIGM) has been studied in the Ag–Zn system by exposing polycrystalline Ag to Zn vapor with a Ag-25 wt.% Zn alloy as the source of Zn. The time and temperature dependence of the migration distance has been studied in the temperature range 660 to 810 K. The composition profile was obtained on the sheet cross-section along a line perpendicular to the edge to determine D_bδ at each temperature. Similarly, the Zn concentration profile was obtained from the region swept by the migrating grain boundary. The coherency strain energy, the total chemical free energy change and the effective free energy change were calculated. The regular solution model was used for calculating the free energy change. It has been observed that a fraction of the total free energy has been used for volume diffusion in front of the migrating grain boundary. The instantaneous rate of migration has been observed to be directly proportional to the chemical free energy change and the coherency strain energy. The instantaneous rate of migration versus the composition graph has indicated that the driving force for DIGM in the Ag–Zn system is the coherency strain energy.The fine-grained layer formed at the surface follows a parabolic growth behavior. The diffusion coefficients calculated from the composition profile as well as from the rate of growth of the fine-grained layer are of the same order of magnitude. The diffusivity values are four to six orders of magnitude higher than the volume diffusion coefficients. From the activation energy and the diffusivities it is clear that DIGM in the Ag–Zn system occurs by the diffusion of Zn along the grain boundaries of polycrystalline Ag.     

Grain boundary optimization induced substantial squareness enhancement and high performance in iron-rich Sm-Co-Fe-Cu-Zr magnets

Increasing iron content has been witnessed an essential method to improve the remanence of 2:17-type Sm-Co-Fe-Cu-Zr magnets,however,the inferior squareness factor accompanied with the increased iron content turns into a neck sticking problem.In this work,the grain boundary optimization induced sub-stantial squareness enhancement from 63.4％to 91.4％,and consequently an excellent maximum energy product of 32.63 MGOe have been achieved in iron-rich Sm-Co-Fe-Cu-Zr magnets via tuning solution process.It is clearly revealed that the grain boundary(GB)phases as well as the micro-twins'density in grain interiors can be controlled and interprets the enhancement mechanism of squareness.     

Grain boundary segregation engineering in metallic alloys: A pathway to the design of interfaces

Grain boundaries influence mechanical, functional, and kinetic properties of metallic alloys. They can be manipulated via solute decoration enabling changes in energy, mobility, structure, and cohesion or even promoting local phase transformation. In the approach which we refer here to as ‘segregation engineering’ solute decoration is not regarded as an undesired phenomenon but is instead utilized to manipulate specific grain boundary structures, compositions and properties that enable useful material behavior. The underlying thermodynamics follow the adsorption isotherm. Hence, matrix-solute combinations suited for designing interfaces in metallic alloys can be identified by considering four main aspects, namely, the segregation coefficient of the decorating element; its effects on interface cohesion, energy, structure and mobility; its diffusion coefficient; and the free energies of competing bulk phases, precipitate phases or complexions. From a practical perspective, segregation engineering in alloys can be usually realized by a modest diffusion heat treatment, hence, making it available in large scale manufacturing.     

Breaking the purity-stability dilemma in pure Cu with grain boundary relaxation

High purity metals are increasingly demanded in modern manufacturing industries, but their processing and applications are limited by a dilemma that purer metals are thermally and mechanically less stable. The reduced stability of pure metals originates from the weakened drag effect of impurity atoms on the mobility of grain boundaries (GBs) that are hard to stabilize without alloying. Following recent studies on stabilizing nanograined metals by tailoring structures of GBs, here we report that structural relaxation of GBs breaks the purity-stability dilemma in pure Cu. Contrary to the conventional impurity effect, thermal stability and hardness of nanograined Cu samples with relaxed GBs increase (rather than decrease) with higher purities. The discovered anomalous impurity effect, owing to suppression of GB relaxation process with impurity atoms, offers an alternative vector to stabilizing purer metals for advanced processing and applications.     

Microstructural impact on intergranular corrosion and the mechanical properties of industrial drawn 6056 aluminum wires

The influence of individual manufacturing steps during industrial wire drawing processes on the mechanical and corrosion properties of the 6056 aluminum alloy was investigated. These steps demonstrated an essential influence on the microstructure, and thus, the susceptibility to intergranular corrosion (IGC). No clear correlation between IGC susceptibility and hardness was observed. Although the highest resistance against intergranular attack was determined for those alloys in the solution annealed condition, pitting corrosion was identified to occur. Subsequent artificial aging of the solution annealed and quenched wires reintroduced IGC susceptibility; this phenomenon was attributed to the occurrence of galvanic coupling between the noble Cu-phases, located on the grain boundary, and the anodic grain boundary area.     

等离子体增强原子层沉积技术制备过渡金属薄膜的研究进展

原子级的处理对应用于计算和数据存储的最先进的电子设备,以及与物联网、人工智能和量子计算相关的新兴技术正变得越来越重要。等离子体增强原子层沉积(PEALD)是一种原子级表面沉积技术,由于其较高的反应活性以及较低的沉积温度日益受到研究者的关注。本文介绍了PEALD技术的基本原理以及相对于其他薄膜沉积技术的优势,之后从前驱体和基底材料的影响等方面介绍了利用PEALD制备Ti、Co、Ni、Cu、Ru、Pd、Ag、Ta、Ir和Pt等过渡金属薄膜以及它们在微电子领域的应用现状,最后进行了总结和展望。     

Iterative solution of systems of equations in the dual reciprocity boundary element method for the diffusion equation

In this paper the diffusion equation is solved in two-dimensional geometry by the dual reciprocity boundary element method (DRBEM). It is structured by fully implicit discretization over time and by weighting with the fundamental solution of the Laplace equation. The resulting domain integral of the diffusive term is transformed into two boundary integrals by using Green's second identity, and the domain integral of the transience term is converted into a finite series of boundary integrals by using dual reciprocity interpolation based on scaled augmented thin plate spline global approximation functions. Straight line geometry and constant field shape functions for boundary discretization are employed. The described procedure results in systems of equations with fully populated unsymmetric matrices. In the case of solving large problems, the solution of these systems by direct methods may be very time consuming. The present study investigates the possibility of using iterative methods for solving these systems of equations. It was demonstrated that Krylov-type methods like CGS and GMRES with simple Jacobi preconditioning appeared to be efficient and robust with respect to the problem size and time step magnitude. This paper can be considered as a logical starting point for research of iterative solutions to DRBEM systems of equations. © 1998 John Wiley & Sons, Ltd.