Temperature-dependent surface diffusion near a grain boundary

Metal surface evolution is described by a nonlinear fourth-order partial differential equation for curvature-driven flow. The standard boundary conditions for grain-boundary grooving, at a grain–grain–fluid triple intersection, involve a prescribed slope at the groove axis. The well-known similarity reduction is no longer valid when the dihedral angle and surface diffusivity depend on time due to variation of the surface temperature. We adapt a nonlinear fourth-order model that can be discerned from symmetry analysis to be integrable, equivalent to the fourth-order linear diffusion equation. The connection between classical symmetries and separation of variables allows us to develop the correction to the self-similar approximation as a power series in a time-like variable.     

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.     

Grain boundary plane distributions and single-step versus multiple-step grain boundary engineering

The ‘five-parameter’ (i.e. both misorientation and grain boundary plane) distribution in type 304 austenitic stainless steel has been measured and evaluated for an ‘as-received’ (AR) specimen and specimens undergoing both single-step grain boundary engineering processing (SSGBE) and multiple-step grain boundary engineering processing (MSGBE) comprising three iterations. The results showed that the fundamental requirement for twinning-related GBE is to maximise concomitantly the proportion of both Σ3 and Σ9 boundaries, which in turn supports the development of special planes in the grain boundary network. 1 1 0 and 1 1 1 tilt and twist boundaries play a key role in the formation of ‘special’ grain boundary planes. MSGBE added increased proportions of Σ3 boundaries and resulted in development of different characteristics in the planes distribution compared to SSGBE. These modifications are likely to result in improved grain boundary properties after MSGBE compared to SSGBE.     

Atomic structure of a CeO2 grain boundary: the role of oxygen vacancies

Determining both cation and oxygen sublattices of grain boundaries is essential to understand the properties of oxides. Here, with scanning transmission electron microscopy, electron energy-loss spectroscopy, and first-principles calculations, both the Ce and oxygen sublattices of a (210)Σ5 CeO(2) grain boundary were determined. Oxygen vacancies are shown to play a crucial role in the stable grain boundary structure. This finding paves the way for a comprehensive understanding of grain boundaries through the atomic scale determination of atom and defect locations.     

Atomic mechanisms of local structural rearrangements in strained crystalline titanium grain

The nucleation and development of plastic deformation in a crystalline grain of titanium (Ti) during uniaxial tension has been studied by molecular dynamics (MD) simulations with the interatomic interaction described using the embedded atom method. Specific features of the generation of local structural rearrangements in the grain at various straining rates are revealed. It is established that there is a threshold deformation level at which local structural rearrangements begin to nucleate in the crystal, which is accompanied by a jumplike decrease in the potential energy. Because of the inertial character of the accommodation processes, this threshold value increases with the loading velocity.     

Theory of boundary diffusion controlled grain rotation in a 'bamboo' structure

Abstract

The theory of grain boundary diffusion controlled rotation of an orthogonal bicrystal about its common boundary has been extended to the case of cylindrical geometry. The analysis for this simple 'bamboo' geometry enables predictions to be made with a higher level of certainty than is usual for other diffusion controlled processes. Since bamboo structures are easy to fabricate, this suggests an indirect method of estimating boundary diffusion coefficients based on experimental measurement of rotation rates. A numerical analysis is presented and the dependence of the rotation rate on bending moment and wire radius is determined. The variation of the local stress and diffusion fluxes over the boundary is calculated. The conditions where experimental measurements are likely to provide a viable method of estimating grain boundary diffusion coefficients are predicted.     

Origin of grain boundary motion during diffusion bonding by hot pressing

The microstructural changes during diffusion bonding of Al₂O₃ fibres with boron-doped Ni₃Al plates by hot pressing were investigated. Particular attention was paid to the conversion of the bonding surfaces to an interior grain boundary and its migration during the hot-pressing treatment. The microstructure changes were found to depend on the grain size of the matrix material prior to hot pressing. Fine-grained matrix material led to a fast migration of the bonded interface in order to establish force equilibrium at grain boundary junctions at the prior surfaces. In coarse-grained material, the bonded interface moved after much larger hot pressing time by strain-induced boundary migration due to the accumulated plastic-strain during prolonged hot-pressing treatment. The fibres were found to strongly impede migration of the bonded interface, which can interfere with the perfection of the bonding process.     

Lattice and grain boundary diffusion of copper in thin aluminum films

The lattice D₁ and grain boundary δD_b diffusivities of Cu in Al thin films at 130–185°C are calculated from measurements employing Auger electron spectroscopy and Ar ion beam etching. The calculated values are D₁ = 0.065 cm² s^-1 × exp(-122 kJ/RT) and δD_b = 4.5 × 10^-9 cm³s^-1 exp(-97.4 kJ/RT). The D₁ value is 3–5 times larger at 130–185°C than that predicted by an extrapolation of radioactive tracer measurements of large grain bulk specimens at 433–652°C. The higher value measured here is attributed to the higher density of subgrain defect structures in the thin film.     

Applications of grain boundary diffusion studies to soldering and thermocompression bonding

This paper describes the application of grain boundary diffusion and oxidation to the technologically important topics of solderability and thermocompression bondability. Copper leads plated with 0.73 μm of gold were studied. Arrhenius plots were constructed and extrapolated to expected storage conditions. A storage life of greater than 2 a at 50°C is predicted for both properties. The time to loss of solderability is given by t_s = 1.6 x 10^-7 exp(0.8 eV/kT) h and the time to loss of bondability by t_B = 4 x 10^-10 exp(1.0 eV/kT) h. The effective activation energies 0.8 and 1.0 eV compare with 0.91 eV for the grain boundary diffusion of copper through gold. Loss of solderability and loss of bondability were modeled as a two-step series event. First the copper diffuses to the gold surface via its grain boundaries. The copper then oxidizes. Auger analysis indicates that the surface oxide is Cu₂O.     

Oxygen diffusion blocking of single grain boundary in yttria-doped zirconia bicrystals

Yttrium-doped zirconia bicrystals with [001] symmetric tilt Σ 5 grain boundaries were fabricated by a diffusion bonding technique, and oxygen diffusion behavior across the grain boundary was measured by secondary ion mass spectrometry (SIMS), tracing ¹⁸O isotope. It was found that the ¹⁸O fraction across the boundary exhibited explicit decrease around the boundary plane, indicating that the oxygen-diffusion is blocked by the presence of the Σ 5 grain boundary. This is the first experimental detection of oxygen diffusion blocking at a single grain boundary in zirconia ceramics. From high-resolution transmission electron microscopy observations and energy dispersive X-ray spectroscopy analysis, neither amorphous layers nor Si impurity segregation were found at the grain boundary. The grain boundary blocking effect of the Σ 5 boundary must be an intrinsic feature arising from its core structure and yttrium solute segregation of the grain boundary.     

Deformation response of grain boundary networks at high temperature

The deformation response of random grain boundary networks as a function of temperature and strain rate is explored using molecular dynamics atomistic simulations and an embedded atom method interatomic potential. We find that deformation at higher temperatures promotes both dislocation emission and grain boundary accommodation processes. The results allow estimating the activation energies and volumes for the deformation process. We find activation energy values for the deformation process similar to those for grain boundary diffusion and activation volumes consistent with an atomic shuffling mechanism. Our results suggest a picture of the deformation process as governed by the combination of the applied stress and thermally activated processes.     

Production of very fine grained AISI 304 steel with high special grain boundary density by grain boundary engineering

Abstract

AISI 304 stainless steel was subjected to grain boundary engineering by applying cycles of calibre rolling and subsequent heat treatment. After three cycles the grain size started to decrease, and after the fourth cycle a very fine grained material having high fraction of special grain boundaries was produced. Due to the short heat treatment at 850°C, only partial recrystallization occurred after the first three cycles, which was proven by the large amount of low angle boundaries. The stored elastic strain energy helped the grain boundary movement and the formation of annealing twins in the fourth cycle, which caused the formation of very fine grained structure with a large amount of special grain boundaries.     

The role of partial grain boundary dislocations in grain boundary sliding and coupled grain boundary motion

We study the process of grain boundary sliding through the motion of grain boundary dislocations, utilizing molecular dynamics and embedded atom method (EAM) interatomic potentials. For a Σ = 5 [001]{310} symmetrical tilt boundary in bcc Fe, the sliding process was found to occur through the nucleation and glide of partial grain boundary dislocations, with a secondary grain boundary structure playing an important role in the sliding process. While the homogeneous nucleation of these grain boundary dislocations requires shear strain levels higher than 7%, preexisting grain boundary dislocations are shown to glide at applied shear levels of 1.5%. The glide of the dislocations results in coupled motion of the boundary in the directions parallel and perpendicular to itself. Finally, interstitial impurities and vacancies were introduced in the grain boundary to study the effects on the sliding resistance of the boundary. While vacancies and H interstitials act as preferred nucleation sites, C interstitials do not. Both hydrogen and C interstitials stop dislocation glide whereas vacancies do not. A detailed study of the dynamic properties of these dislocations is also presented.     

First-principles calculation of grain boundary energy and grain boundary excess free volume in aluminum: role of grain boundary elastic energy

We examined the grain boundary energy (GBE) and grain boundary excess free volume (BFV) by applying the first-principles calculation for six [110] symmetric tilt grain boundaries in aluminum to clarify the origin of GBE. The GBE increased linearly as BFV increased. The elastic energy associated with BFV, namely the grain boundary elastic energy, was estimated as a function of BFV and the shear modulus. The grain boundary elastic energies were close in value to the GBEs. The charge density distributions indicated that the bonding in the grain boundary region is significantly different from the bonding in the bulk. The grain boundary elastic energies were 15–32% higher than the GBEs. This overestimation of the grain boundary elastic energy is caused by the characteristics of the electronic bonding at the grain boundary, which is different from bonding in the bulk. We have concluded that GBE results mainly from the grain boundary elastic energy.