Papers by Keyword: Low Angle Boundaries

Abstract: Effects of low angle boundaries (LABs) on the stress rupture properties of bicrystals of a nickel-based third generation single crystal superalloy at 1093 °C/158 MPa were investigated. The results show that the effect of LABs on the stress rupture elongation of the alloy is higher than that of the stress rupture life at 1093 °C/158 MPa. As the misorientation angle of the LABs reaches 9.0°, the stress rupture life of the alloy with LABs can still retain nearly 50% of that with LABs of 0° at 1093 °C/158 MPa; while the stress rupture elongation of the alloy with LABs drops obviously when the misorientation angle of the LABs is larger than 6.5°. The fracture surfaces of stress ruptured alloy with LABs of 0°~2.9° are characterized by dimple features, while those with LABs of 6.5°~12.3° all exhibit intergranular fracture features. Apparent dimple features can be observed at the intergranular fracture surface of the alloy with LABs of 6.5° and the elongation of it is high. However, obvious dendrite features can be observed at the intergranular fracture surfaces of the alloy with LABs of 7.6°~12.3° and the elongations of them are relatively low.

Abstract: The specimens of low angle boundaries were machined from the second generation single crystal superalloy DD6 blades. The microstructures of low angle boundaries (LAB) were investigated from three scales of dendrite, γ′ phase and atom with optical microscopy (OM), scanning electron microscope (SEM), transition electron microscope (TEM) and high resolution transmission electrion microscopy (HREM). The results showed that on the dendrite scale LAB is interdendrite district formed by three dimensional curved face between the adjacent dendrites. On the γ′ phase scale LAB is composed by a thin layer γ phase and its bilateral imperfect cube γ′ phase. On the atom scale LAB is made up of dislocations within several atom thickness.

Abstract: Structure and configuration of boundary dislocations on the low angle tilt grain boundaries in alumina were considered based on the ideas that the boundary is composed of regularly arrayed edge dislocations and that the dislocations could dissociate into partial dislocations as well as glide dislocations in bulk. Moreover, the structure of the dissociated boundary dislocations were evaluated by the calculations based on an elastic theory. The calculations indicated that the largeness of the stacking fault region between partial dislocations formed by the dissociation will decrease with increasing tilt angles. It can be said that the idea and calculations used here will be powerful in considering the dislocation structure of low angle tilt grain boundaries that are not or are difficult to be identified.

Abstract: Two methods of automatically determining boundary alignments from EBSD maps are discussed and shown to produce comparable results. Measurements of Al-0.1Mg and IF steel, deformed at room temperature, confirm that the alignment of low angle boundaries is primarily a function of the deformation mode, rather than the crystallography. During the high temperature deformation of aluminium, the lagbs maintain a large angle of inclination to the rolling direction (>35o) even at large strains, which is consistent with the boundaries being of transient character during deformation.

Abstract: The prior literature on abnormal grain coarsening (AGC) at low volume fractions (f) of stable second phase particles in high purity Al alloys is reviewed and reanalyzed in the light of developments in modeling particle inhibition of grain boundary migration. With the usual assumptions (i) of incoherent particles that retain their shape on contact with the grain boundaries and (ii) that all the grain boundaries are equally mobile, it appears impossible to account for process of AGC. Normal grain coarsening (NGC) is shown to be less inhibited by the particles than is AGC. This idea is explored using a new but simple model of particle inhibition by curvature removal. The curvature of the smallest grains is always larger than that of the larger grains. Two possible hypotheses to overcome this difficulty are proposed: First the possible change of shape of particles on slowly moving grain boundaries, of grains with near 14 neighbors should, after a small increment of NGC, promote AGC at low values of the volume fraction f. The second hypothesis involves the observed high density of immobile, low angle grain boundaries (LAGBs) found in recent experiments on high purity Al-Fe-Si alloys cast with very coarse grain sizes. These alloys undergo rapid AGC even at higher values of f (> 0.01). These LAGBs are expected to inhibit the shrinkage of many of the small grains, whose loss is the fundamental mechanism of NGC.