Survey of computed grain boundary properties in face-centered cubic metals—II: Grain boundary mobility

The absolute grain boundary mobility of 388 nickel grain boundaries was calculated using a synthetic driving force molecular dynamics method; complete results appear in the Supplementary materials. Over 25% of the boundaries, including most of the non-Σ3 highest mobility boundaries, moved by a coupled shear mechanism. The range of non-shearing boundary mobilities is from 40 to 400 m/s GPa, except for Σ3 incoherent twins which have mobilities of 200–2000 m/s GPa. Some boundaries, including all the 〈1 1 1〉 twist boundaries, are immobile within the resolution of the simulation. Boundary mobility is not correlated with scalar parameters such as disorientation angle, Σ value, excess volume or boundary energy. Boundaries less than 15° from each other in five-dimensional crystallographic space tend to have similar mobilities. Some boundaries move via a non-activated motion mechanism, which greatly increases low-temperature mobility. Thermal roughening of grain boundaries is widely observed, with estimated roughening temperatures substantially among boundaries.     

Interphase boundary fracture and grain boundary precipitation of Ni3Al(γ′) phase in β-NiAl bicrystals

The effect of the crystallography of film-like Ni₃Al(γ′) precipitates along grain boundaries of NiAl(β) on the fracture stress at room temperature was examined using β bicrystals with controlled orientations. The selected variant of γ′-film satisfied the Kurdjumov–Sachs (K–S) relation with a neighbouring β crystal, and deviated from the relation with an adjacent β crystal. In the course of tensile deformation at ambient temperature, fracture occurred preferentially at the (β/γ′) interphase boundary deviating from the K–S relation, which showed no plastic flow, and the fracture stress decreased with increasing deviation angle. In contrast, slip transfer from γ′-film to β crystal across coherent (β/γ′) interface was observed, when the variant of γ′-film satisfied the K–S relation with both neighbouring β crystals. To clarify the relation between the interphase boundary fracture and the deviation angle from the K–S relation, the boundary structure of incoherent (β/γ′) interfaces was discussed using O-lattice theory and transmission electron microscopic observations.     

Effect of piled-up dislocations on strain induced boundary migration (SIBM) in deformed aluminum bicrystals with originally ∑3 twin boundary

Using aluminum bicrystal specimens with ∑3 twin boundary at the initial orientation, deformation and recrystallization experiments were performed in order to clarify the contribution of piled-up dislocations at the grain boundary (GB) to the occurrence of the strain induced grain boundary migration (SIBM). In the tensile deformation of the specimen along the <011> axis to a strain of 20%, an inhomogeneously deformed structure was developed along the GB. After annealing, the SIBM was observed along the original GB. These screw dislocations passed easily through the GB because of the special orientational and geometric conditions, whose motion acted as the stress relief mechanism against the imposed strain. It was found that the SIBM was caused by the edge component of dislocations piled up against the GB.     

Effect of coherency on interphase boundary sliding in NiAl(β) bicrystals with film-like Ni3Al(γ′) precipitate along boundary

Effect of the crystallography of Ni₃Al(γ^′) precipitates along grain boundaries of NiAl(β) on the interphase boundary sliding of (β/γ^′) interface was examined using β bicrystals. Interphase boundary sliding occurred preferentially at incoherent (β/γ^′) interface and the sliding displacement increased with increasing deviation angle from the Kurdjumov–Sachs orientation relationship.     

Twin boundary: Stronger or weaker interface to resist fatigue cracking?

The fatigue cracking mechanisms at twin boundaries (TBs) are investigated by considering the stacking fault energy, dislocation slip mode and the difference in the Schmid factors (DSF) between matrix and twin. In Cu and its alloys, the occurrence of TB cracking becomes much more frequent with either increasing the alloying component or the DSF. Considering the interactions between dislocations and TBs, a semiquantitative relationship based on the statistical experimental results is established to describe the TB cracking mechanism.     

Mechanism for phase boundary sliding and its relevance to diffusion-solution zone in SPD

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Effects of grain boundary η phase on the mechanical properties of Fe-Ni-Cr alloy

通常认为Fe-Ni—Cr合金在过时效后晶界上析出的η相为裂纹源，恶化合金性能．通过对一种Fe-Ni—Cr合金进行过时效后晶界上析出η相的研究发现，由于晶界η相与合金基体保持共格关系，η相对合金的室温性能影响不大；并且在高温变形条件下η相形成凸起，阻碍晶界滑动，减小在晶角处的应力集中，从而提高合金的高温塑性．断口扫描电镜观察表明，晶界η相的析出使合金高温断裂方式由原来的沿晶脆性断裂转变为穿晶韧窝状断裂．通过在合金中添加Nb，可以在晶界上析出更为弥散的η相，从而使得合金在高温下获得相当高的塑性．     

Morphological instability ofδ-ferrite/γ-austenite interphase boundary in low carbon steels

The dynamic behavior of the δ-ferrite/γ-austenite interphase boundary during δ/γ transformations was observed in situ with a confocal scanning laser microscope. The morphological instability and the growth of the finger pattern of the δ/γ interphase boundary were mainly focused on and studied in this work. It was found that the incoherent δ/γ interphase boundaries were always unstable with finger-like morphology during δ→γ transformation, which developed along δ-GBs at low supercoolings (<7 K) and even into the δ-matrix at higher supercoolings for the transformation. At the same time, the finger spacing was measured and analyzed by using the classical constitutional supercooling and instability theories, which show good agreement between the observations and theories.     

Weld pool boundary and weld bead shape reconstruction based on passive vision in P-GMAW

A passive visual sensing system is established in this research, and clear weld pool images in pulsed gas metal arc welding （ P-GMA W） can be captured with this system. The three-dimensional weld pool geometry, especially the weld height, is not only a crucial factor in determining workpiece mechanical properties, but also an important parameter for reflecting the penetration. A new three-dimensional （3D） model is established to describe the weld pool geometry in P-GMAW. Then, a series of algorithms are developed to extract the model geometrical parameters from the weld pool images. Furthermore, the method to reconstruct the 3 D shape of weld pool boundary and weld bead from the two-dimensional images is investigated.     

Grain boundary pinning by Al6Mn precipitates in an Al–1wt%Mn alloy

The grain boundary area per unit of volume, S_V, and the interface area per unit of volume of Al₆Mn precipitates, S_VP, were measured in a high purity Al–1wt%Mn alloy for an annealing time of 3600 s at temperatures ranging from 500 to 620 °C. The experimental results show very good agreement with the expression proposed by Rios [Acta Metall. 35 (1987) 2805]: S_VL=3S_VP where S_VL is the limiting grain boundary area per unit of volume. This expression is easier to verify experimentally than its equivalent form: R_L=r/6f, where r is the particle radius, f is the volume fraction of the particles and R_L is the limiting grain radius.     

Effects of temperature boundary conditions on equiaxed dendritic growth in phase-field simulations of binary alloy

By the phase-field approach, the dendritic growth in binary alloy melt was simulated respectively using two types of temperature boundary conditions, i.e., the constant temperature boundary by which the boundary temperature was fixed at the initial temperature, and Zero-Neumann temperature boundary. The influences of the temperature boundary conditions on numerical results are investigated. How to choose appropriate temperature boundary conditions is proposed. The results show that: 1) when the computation region is limited to a changeless size, the Zero-Neumann and constant temperature boundary conditions lead to the different dendritic growth behaviors, and the Zero-Neumann condition is preferable to the constant temperature condition; 2) when the computation region is enlarged continually with the computational time according to the increasing thermal diffusion scale, the two types of temperature boundary conditions achieve the consistent tip velocities and tip radii, and they both are appropriate choices.     

Simulation of interphase boundary of Ni75AlxV25-x alloys using microscopic phase-field method

The evolution of ordered interphase boundary （IPB） of Ni75AlxV25-x alloys was simulated using the microscopic phase-field method. Based on the atomic occupation probability figure on 2D and order parameters, it was found that the IPB formed by different directions of θ phase has great effect on the precipitation of γ′ phase. The γ′ phase precipitated at the IPB that is formed by [100]θ direction where the （001）θ plane is opposite, and then grows up and the shape is strap at fmal. The IPB structure between γ′phase and θ phase is the same. There is no γ′ phase precipitate at the IPB where the （002）θ and （001）θ planes are opposite, the ordered IPB is dissolved into disordered area. There is γ′ phase precipitation at the IPB formed by the [001]θ and [100]θ directions, and the IPB structure is different between γ′ phase and the different directions of θ phase. The IPB where （001）γ′, and （100）θ plane opposite does not migrate during the γ′ phase growth, and γ′ phase grows along [100]θ direction.     

Grain boundary embrittlement by Mn and eutectoid reaction in binary Fe–12Mn steel

The grain boundary embrittlement in a binary Fe–12Mn is due to the grain boundary segregation of Mn. During tempering at 400 °C (higher than the equilibrium eutectoid reaction temperature 247 °C), reverted austenite particles were formed at lath and grain boundaries through the equilibrium reaction of lath martensite to ferrite + austenite. Surprisingly, hydrostatic pressure, which is induced by the transformation of epsilon martensite to austenite during heating at the tempering temperature, resulted in the nonequilibrium eutectoid reaction producing α-Mn precipitates at the interface between lath martensite and the transformed austenite during the tempering. The segregation concentration kinetics of Mn formed a convex profile due to the active grain boundary precipitation of the reverted austenite particles and the α-Mn particles, which act as a sink for the segregated Mn. Finally, the convex segregation profile of Mn corresponded to the concave profile of intergranular fracture strength.     

Tin-induced dynamic embrittlement of Cu–7% Sn bicrystals with a Σ5 boundary

The kinetic model for dynamic embrittlement predicts that the cracking rate is proportional to the diffusivity of the embrittling species along the grain boundary. Diffusion-bonded bicrystals of Cu–7% Sn with a Σ5 (031)/[100] symmetrical tilt boundary were used to test the model and to examine the mechanism of the cracking process. The bicrystals, in which surface-segregated tin was the embrittling species, were tested at 265°C in vacuum parallel and perpendicular to the tilt axis. Cracking occurred parallel to the tilt axis, the fast-diffusion direction, by the propagation of a sharp crack at a maximum rate of ∼2 μm/s and at a stress intensity of less than 3.5 MPa √m. Cracking appeared to be continuous, suggesting that the tin diffusion occurs in the core of the crack tip. No cracking occurred perpendicular to the tilt axis, i.e. the slow-diffusion direction; here, plastic creep occurred with the formation of cavities at the grain boundaries. The hypothesis of a grain-boundary-diffusion process leading to cracking is supported, and the susceptibility to this type of cracking appears to be extremely sensitive to grain boundary structure.     

Phase composition and structure of grain boundary of oversintered Y3Al5O12 ceramics

Phase composition and microstructures of grain boundary of oversintered yttrium aluminum garnet （Y3Al5O12, YAG） ceramics by vacuum sintering at 1 850 ℃ were investigated. For synthesizing YAG, grain boundary is a key factor for YAG ceramics. The morphology of grain boundary was observed by SEM, TEM and its composition was analyzed by EDS. It is identified that the grain boundary is composed of a-AI2O3 and yttrium aluminum perovskite （YAP, YAlO3） eutectics. At the edge of YAG crystal grain, YAG phase is decomposed into perovskite YAP and α-Al2O3 during high temperature sintering. Due to refractive indexes of YAP and α-Al2O3 phases in wide grain boundary are different from those of YAG, the transmittance of oversintered YAG ceramics is lower than that of YAG ceramics sintered at 1 750 ℃.