Surface topography and energy of grain boundary triple junctions in copper tricrystals

Copper tricrystals with a common crystallographic axis 〈1 0 0〉, 〈1 1 0〉, 〈1 1 1〉 were fabricated by the Bridgman technique. Utilizing atomic force microscopy, the topography of the grain boundary triple junctions for the three crystallographic systems were investigated. The grain boundary free surface energy and the grain boundary triple line energy were determined for specific crystallographies of tricrystals.     

Electron-beam-induced current study of small-angle grain boundaries in multicrystalline silicon

Recombination activity of small-angle grain boundaries (SA GBs) in multicrystalline silicon (mc-Si) was studied by means of electron-beam-induced current (EBIC) technique. In the as-grown mc-Si, the EBIC contrasts of special Σ and random GBs were weak at both 300 and 100 K, whereas those of SA GBs were weak (<3%) at 300 K and strong (30–40%) at 100 K. In the contaminated mc-Si, SA GBs showed stronger EBIC contrast than Σ and R GBs at 300 K. It is indicated that SA GBs possess high density of shallow levels and are easily contaminated with Fe compared to other GBs.     

Stress-driven migration of simple low-angle mixed grain boundaries

We investigated the stress-induced migration of a class of simple low-angle mixed grain boundaries (LAMGBs) using a combination of discrete dislocation dynamics simulations and analytical arguments. The migration of LAMGBs under an externally applied stress can occur by dislocation glide, and was observed to be coupled to the motion parallel to the boundary plane, i.e. tangential motion. Both the migration and tangential velocities of the boundary are directly proportional to applied stress but independent of boundary misorientation. Depending on the dislocation structure of the boundary, either the migration or tangential velocity of the boundary can switch direction at sufficiently high dislocation climb mobility due to the dynamics of dislocation segments that can climb out of their respective slip planes. Finally, we show that the mobility of the LAMGBs studied in this work depends on the constituent dislocation structure and dislocation climb mobility, and is inversely proportional to misorientation.     

Mobility of Σ5 tilt grain boundaries: Inclination dependence

Molecular dynamics simulations were performed to examine the influence of grain boundary inclination on the stress-driven motion of flat <0 1 0> Σ5 tilt asymmetric of different inclinations in Ni. The inclination dependence of the mobility, boundary diffusivity and energy are correlated. Special properties are associated with low index boundary planes.     

The disconnection mechanism of coupled migration and shear at grain boundaries

The mechanism of coupled migration and shear is studied in a range of [0 0 0 1] tilt boundaries in hexagonal close-packed metal using atomic-scale computer simulation. Symmetrical tilt boundaries spanning the low- and high-angle regimes and comprising regular arrays of grain boundary dislocations are simulated. For each misorientation, θ, the perfect boundary (pristine) is investigated as well as one containing a disconnection. Both types of structures are subjected to incremental applied strains to determine the stress that produces coupled migration and shear. The stress for motion in the pristine case, entailing nucleation, is higher than the Peierls stress for motion when disconnections are present. We conclude that the applied stresses in our simulations exert a Peach-Koehler force on pre-existing disconnections, thereby providing a feasible mechanism with a well-defined driving force that produces coupled migration and shear. This mechanism is feasible for the lower-angle boundaries studied, and facile for the high-angle cases.     

Nanohardness of copper in the vicinity of grain boundaries

The nanohardness in the vicinity of grain boundaries in high purity Cu was investigated. It was found that the nanohardness increases while approaching the grain boundary, the characteristic distance at which the grain boundary influences the nanohardness being in the range of few micrometers.     

A scanning force microscopy study of grain boundary energy in copper subjected to equal channel angular pressing

We employed a scanning force microscopy technique to determine the ratio of grain boundary and surface energies in copper using the thermal grooving method. Samples of ultrafine grain copper obtained by four passes of equal channel angular pressing were heat treated in a reducing atmosphere at 400 °C for 15 min and at 800 °C for 2 h. The average dihedral angles of the grain boundary grooves after the former and the latter heat treatments were 152.4 ± 6.3° and 164.2 ± 4.3°, respectively, which can be translated into the difference by a factor of 1.8 in average grain boundary energies. This difference implies that the grain boundaries in ultrafine grain copper produced by equal channel angular pressing are in a state of high non-equilibrium that cannot be fully relaxed after a short annealing at 400 °C, but that undergoes significant relaxation after annealing at 800 °C.     

Motion of a grain boundary facet in aluminum

The shape and migration of a capillary-driven 20.8°〈1 0 0〉 tilt grain boundary in aluminum bicrystals were investigated in situ in a scanning electron microscope at elevated temperatures. The moving boundary assumed a semi-faceted configuration composed of a singular facet connected to a curved boundary section joined at a distinct edge. At constant temperature the boundary with a facet moved steadily and its shape remained self-similar. Both the facet length and the connecting angle between the facet and the curved boundary section did not change over the investigated temperature range. The capillary-driven migration of the semi-faceted grain boundary in “quarter-loop” geometry was analyzed. The obtained results revealed that the major factor responsible for the formation of the mobile facet was the energy difference between the facet and the curved boundary. The temperature dependence of the facet mobility was determined. The migration activation enthalpy of the investigated capillary-driven planar boundary/facet was a factor of two higher than the activation enthalpy for migration of a geometrically similar boundary driven by an applied stress.     

AISI304连铸薄带组织与取向的EBSD分析

利用EBSD取向成像技术,分析了双辊连铸法生产的AISI304奥氏体连铸薄带的两相组织结构及取向特点.结果表明,ND-TD方向上,不同的位置铁素体和奥氏体两相组织中晶界取向差在0°～15°的小角度晶界较多,表明薄带内部存在大量亚结构;薄带内部不同位置两相中的晶粒生长方向不同.     

Coarsening of second phase in a two-phase Zr–2.5Nb: On the role of phase boundaries

Deformed two-phase Zr–2.5Nb was subjected to 700 °C annealing and associated coarsening. After primary recrystallization, the second-phase body-centered cubic β was well dispersed on the grain boundaries of hexagonal α matrix. Prolonged annealing led to the presence of the β, mainly at the α tri-junctions. The two-phase coarsening was associated with clear trends of widening in the second-phase size distribution and changes in phase boundary nature. The former is contrary to what is expected. Changes in phase boundary nature were through increased concentration of 45°[0 0 1] phase boundaries—good-fit interfaces, present in larger β particles, which were estimated to have higher three-dimensional lattice coincidence and hence lower energy. Simple analytical modeling, assuming dissolution-controlled particle coarsening and a lower interfacial energy of the phase interface with the 45°[0 0 1], was shown to explain these experimental observations.     

The role of grain boundary plane orientation on intergranular corrosion of symmetric and asymmetric [1 1 0] tilt grain boundaries in directionally solidified pure copper

Susceptibility to intergranular corrosion of asymmetric grain boundaries in pure copper was well correlated with a geometric criteria, i.e. the effective interplanar spacing of grain boundaries rather than coincidence site lattice (CSL) scheme if we rationally assume that the grain boundary structure relaxes to be formed by low-index planes, therefore resulting in high interplanar spacing.     

Segregation engineering enables nanoscale martensite to austenite phase transformation at grain boundaries: A pathway to ductile martensite

In an Fe–9 at.% Mn maraging alloy annealed at 450 °C reversed allotriomorphic austenite nanolayers appear on former Mn decorated lath martensite boundaries. The austenite films are 5–15 nm thick and form soft layers among the hard martensite crystals. We document the nanoscale segregation and associated martensite to austenite transformation mechanism using transmission electron microscopy and atom probe tomography. The phenomena are discussed in terms of the adsorption isotherm (interface segregation) in conjunction with classical heterogeneous nucleation theory (phase transformation) and a phase field model that predicts the kinetics of phase transformation at segregation decorated grain boundaries. The analysis shows that strong interface segregation of austenite stabilizing elements (here Mn) and the release of elastic stresses from the host martensite can generally promote phase transformation at martensite grain boundaries. The phenomenon enables the design of ductile and tough martensite.     

单道次轧制变形对Hastelloy C-276合金晶界特征分布的影响

采用电子背散射衍射(EBSD)技术研究了固溶处理的Hastelloy C-276合金经2.5%~90%变形后在1150℃退火15min的晶界特征分布(GBCD)。结果表明,在5%变形条件下,总的特殊晶界比例最高为67.1%,其中特殊晶界主要是Σ3晶界,Σ9和Σ27晶界比例较低。当变形量小于15%时,晶粒组织处于回复或部分再结晶,晶粒组织粗大,特殊晶界比例随变形量变化较大。在此阶段,回复或部分再结晶有利于晶界的迁移,从而产生更多特殊晶界。当变形量大于15%后,随变形的增加,晶粒组织细小,特殊晶界比例在50%左右。这是由于晶粒发生完全再结晶后,大角度晶界增多,不利于特殊晶界的形成。因而,Hastelloy C-276合金晶界特征分布优化变形范围应小于15%,更有利于特殊晶界的产生和分布。