Effects of anisotropy on the equilibrium shape of nanoscale pores at grain boundaries

Molecular dynamics simulations have been performed to study the interaction between a faceted pore and an anisotropic grain boundary (GB). Nickel was chosen as a convenient model system. In order to establish the equilibrium crystal shape (ECS) of the pore, studies were also conducted on isolated pores. Isolated pores were found to be subject to the nucleation inhibition of equilibration that has been predicted by Rohrer et al. (J Am Ceram Soc 2000;83:214, 2001;84: 2099). This work shows that configurations close to the ECS can be obtained if supersaturation within a pore is artificially increased by adding mobile adatoms to the internal surfaces of the pores. In the case of pores located at GBs, the nucleation energy barriers to facet displacement are not present for facets in contact with the GB at the triple line, but may still persist for facets that have no contact with the GB. This problem can be overcome by approaching the equilibrium shape from different initial configurations. The configuration of the GB in the vicinity of the pore has been found to be essentially planar, indicating that GB puckering in the vicinity of anisotropic pores is not generally necessary. The present calculations show that incompatibilities between misoriented pore facets that meet at the triple line with the GB are easily accommodated by local atomic rearrangements at the disordered region of intersection with the GB.     

Grain boundary relaxation of hydrogen precharged mild and Cr steels

Internal friction was applied to trace the formation and propagation of the hydrogen-induced degradation of commercial mild and 5% Cr steels. Hydrogen charged and then degassed specimens were subjected to internal friction measurements at the grain boundary relaxation temperature range and the irreversible effects of hydrogen precharging on the grain and phase boundary relaxation processes were studied. From the comparison of the internal friction and the hydrogen permeation data, the critical limits of hydrogen pretreatment corresponding to the different state of the grain boundary degradation were evaluated. At lower critical limit (associated with the minimum of hydrogen apparent diffusivity), the formation of near grain boundary dislocations and the grain boundary decohesion occurred. At critical hydrogen precharging causing the formation of microvoids, annihilation of dislocations and vacancies within the grain boundary voids led to the recovery of internal friction and hydrogen diffusivity. The obtained results agreed with the earlier observed effects of hydrogen precharging on pure iron and low alloy steel, despite the apparent difference in the grain boundary structure and chemistry of those materials.     

Utilizing the SIMS technique in the study of grain boundary diffusion along twist grain boundaries in the Cu(Ni) system

The secondary ion mass spectrometry (SIMS) technique was used to study grain boundary diffusion along (100) twist grain boundaries in the Cu(Ni) system. Concentration profiles of Ni down Cu twist grain boundaries with nominal disorientation angles of 10°, Σ5 (36.87°), and 45°, were measured using the SIMS technique. The average activation energy for grain boundary diffusion, Q_b, was found to be 245±22, 140±10, and 102±15 kJ/mol, for the 10°, Σ5, and 45° twist grain boundaries, respectively. The average grain boundary diffusion pre-exponential term, sδD_bo, was found to be 9.6±1.24×10⁻⁹, 1.1±0.17×10⁻¹⁴, and 1.3±0.36×10⁻¹⁶ m³/s, for the 10°, Σ5, and 45° twist grain boundaries, respectively.     

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.     

Investigation of the effect of the types and densities of grain boundary carbides on grain boundary cavitation resistance of AISI 321 stainless steel under creep–fatigue interaction

The effect of MX (where X=C, N) and M₂₃C₆ and their densities at the grain boundaries on creep–fatigue behavior of AISI 321 stainless steel are investigated. The creep–fatigue lives of fine MX and coarse MX aged alloys were longer than those of M₂₃C₆ aged alloy under the same test conditions. In order to better understand the difference in the creep–fatigue lives between the tested alloys, microstructural observations are conducted by scanning electron microscope (SEM) and transmission electron microscope (TEM). The differences in the creep–fatigue lives of the alloys are due to the stronger cavitation resistance of MX carbides compared with that of M₂₃C₆ carbides. From the microstructural observations, it is verified that formation and growth of grain boundary cavities in MX carbides are more retarded than in M₂₃C₆ carbides. Therefore, it is suggested that the types of carbides are a more prominent factor than the density of carbides for grain boundary cavitation in austenitic stainless steels.     

A crystallographic mechanism for fatigue crack propagation through grain boundaries

A crystallographic model is proposed which takes into account both crack-plane twist and tilt effects on crack retardation at grain boundaries. The twist and tilt angles of the crack-plane deflection at a grain boundary are the key factors that control the path and growth rate of a short crack. Because of crack-plane twist, the area between the traces on the grain-boundary plane of the crack planes across the boundary has to be fractured in order for the crack to propagate through the boundary. This presents significant resistance to crack growth. As the area to be fractured increases with the extent of crack growth beneath the surface of observation, the grain boundary could still resist crack growth after the crack tip has passed the grain boundary on the surface, until the crack propagates through the whole boundary below the surface. A grain boundary with a large twist component could cause a short crack to arrest or branch. Studies of short fatigue crack growth in an Al–Li 8090 alloy plate provide evidence that supports the model.     

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.     

FE approach and experiments for an axisymmetric micro-yoke part forming using grain element and grain boundary element

Until present, an axisymmetric yoke part of electro-magnetic micro-speaker/receiver had manufactured by machining through the material’s removal. However, this machining process have some problems such as long production time, high precision of tool and costs. In this study, a program based on finite element method developed to establish milli-structure forming technology for micro-yoke component, one of the key parts in the multi-functioned micro-speaker/receiver. This developed program is applied for the milli-part forming in order to replace the machining process of micro-yoke. A new numerical approach is proposed to simulate intergranular milli-structure in forming the miniature yoke, in which grain elements and grain boundary elements are introduced. The results of the finite element analysis for micro-yoke part are confirmed by a series of experiments.     

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.     

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.