Number Of Low-angle Boundaries Research Articles

The Number of Subgrain Boundaries in the Airfoils of Heat-Treated Single-Crystalline Turbine Blades.

In the present study, the dendrites deflection mechanism from the mold walls were subjected to verification regarding its heat-treated turbine rotor blades. The number of macroscopic low-angle boundaries created on the cross-section of the blades’ airfoil near the tip was experimentally determined and compared to the number of low-angle boundaries calculated from a model based on the dendrites deflection mechanism. Based on the Laue patterns and geometrical parameters of airfoils, the number of low-angle boundaries occurring at the upper part of the blades airfoil after heat treatment was calculated. This number for the analyzed group of blades ranged from 5 to 9.

Correlation between microstructure and soft magnetic parameters of Fe-Co-Ni-Al medium-entropy alloys with FCC phase and BCC phase

The magnetic property and microstructure of as-cast FeCoNiAlx (x = 0.2, 0.4, 0.6, 0.8, 1.0) MEAs prepared by copper mold suction were investigated. For x ≤ 0.6 alloys, the soft magnetic properties monotonously decrease with increasing Al content, while a turning point of improving magnetic property occurs at x = 0.8. XRD result shows that Al addition changes the phase constitution. The alloys (x ≤ 0.4) consist of single FCC phase. FeCoNiAl0.6 (x = 0.6) and FeCoNiAl0.8 (x = 0.8) alloys are composed of FCC phase and BCC phase. FeCoNiAl1.0 (x = 1.0) alloy consists of single BCC phase. Furthermore, more serious lattice distortion is present in FCC phase than that in BCC phase. It results in the deterioration of soft magnetic property by increasing the coercivity (Hc) and the hysteresis loss (Pu), and decreasing the initial permeability (μi) and the maximum permeability (μm). The grain-boundary character distribution of FeCoNiAl0.6 (x = 0.6) alloy and FeCoNiAl0.8 (x = 0.8) alloy was studied by electron backscatter diffraction (EBSD). Less strain concentration is observed in x = 0.8 alloy than that in x = 0.6 alloy because of containing much smaller number of low angle boundaries (LAB) with a misorientation angle between 2° and 5° (5.71%) than 36.6% in x = 0.6 alloy. It leads to an improvement of soft magnetic property by decreasing the Hc, the Pu, and increasing the μi and the μm of x = 0.8 alloy. Energy dispersive spectroscopy (EDS) result shows that, in both FeCoNiAl0.6 (x = 0.6) and FeCoNiAl0.8 (x = 0.8) alloys, the enrichment of Fe and Co elements and the depletion of Ni and Al elements are observed in FCC phase, while Fe and Co elements are depleted and Ni and Al elements are enriched in BCC phase. Moreover, a close value of ferromagnetic elements, (Fe + Co + Ni)%, is obtained in FCC phase (≈85%) and BCC phase (≈78%) in both FeCoNiAl0.6 (x = 0.6) and FeCoNiAl0.8 (x = 0.8) alloys. Combined with results of EDS and magnetic properties of x = 0.6 and x = 0.8 alloys, it can conclude that the saturation magnetization (Ms) of BCC phase is larger than that of FCC phase.

Microstructure development in a high-nickel austenitic stainless steel using EBSD during in situ tensile deformation

Plastic deformation of surface grains has been observed by electron backscatter diffraction technique during in situ tensile testing of a high-nickel austenitic stainless steel. The evolution of low- and high-angle boundaries as well as the orientation changes within individual grains has been studied. The number of low-angle boundaries and their respective misorientation increases with increasing strain and some of them also evolve into high-angle boundaries leading to grain fragmentation. The annealing twin boundaries successively lose their integrity with increasing strain. The changes in individual grains are characterized by an increasing spread of orientations and by grains moving towards more stable orientations with 〈111〉 or 〈001〉 parallel to the tensile direction. No deformation twins were observed and deformation was assumed to be caused by dislocation slip only.

Electron backscatter diffraction investigation of duplex-phase microstructure in a forged Zr-2.5Nb alloy

Microstructural features of a duplex-phase Zr-2.5Nb alloy were investigated in detail using electron channeling contrast (ECC) imaging and electron backscatter diffraction (EBSD) technique in an emission gun scanning electron microscope (FEGSEM). The excellent resolution provided by the FEGSEM promises the combined utilization of both techniques to be quite adequate for characterizing the duplex-phase microstructures. Results show that the microstructure of the Zr-2.5Nb alloy is composed of bulk a grains (majority) in equiaxed or plate shape and thin β films (minority) surrounding the bulk grains, with their average grain size and thickness measured to be 1.4 µm and 72 nm, respectively. Analyses on α-grain boundaries reveal a number of low angle boundaries, most of which belong to deformation-induced dislocation boundaries. Measurements on relative proportions of various Burgers boundaries suggest very weak (if any) variant selection during β → α cooling, which should be related to deformation-induced higher nucleation rate of α phases. Compared to earlier attempts, more satisfactory indexing of fine β phases (down to nanoscale) is attained by the FEGSEM-based EBSD. Examples are presented to clearly reveal well-obeyed Burgers orientation relationships between adjacent a and β phases. Finally, it is deduced that continuing application of the FEGSEM-based EBSD to duplex-phase Zr alloys could help clarify controversies like the deformation priority of the two phases.

A study aimed at determining and understanding the fracture behaviour of an Al–Li–Cu–Mg–Zr alloy 8090

The S-L fracture toughness of aluminium–lithium based alloys is generally poor and this has limited their applicability, particularly in aerospace where good damage tolerance is required. The low S-L toughness has been attributed variously to grain boundary precipitation, planar slip and lithium segregation. This work examined the S-L fracture behaviour of an 8090 Al–Li-based alloy in 34–45mm plate. The results confirmed that the S-L fracture toughness decreases from the centre to the surface of plate material and increases with double ageing. Fracture is principally intergranular, with both ductile and brittle components occurring, but some transgranular fracture also occurs and this produces steps in the fracture plane. Changes in the relative proportions of brittle and ductile intergranular fracture, as well as in the amount of transgranular fracture, accompany the changes in toughness. However, the decrease in fracture toughness across the plate is accompanied principally by an increase in the relative proportion of brittle intergranular fracture, while the toughening produced by double ageing is accompanied principally by an increase in the amount of transgranular fracture. Evidence of coarse slip, indicative of slip planarity, was seen from slip steps and dislocation structures. However, planar slip was seen only towards the centre of the plate and not towards the surface. The level of planar slip was not reduced markedly by double ageing. Texture varied with position across the plate. There were also a larger number of low angle boundaries towards the centre of the plate than towards the surface. The hardness did not change across the plate. The results could not be explained fully in terms of either the grain boundary precipitate theory or the planar slip model, but were generally consistent with the lithium segregation model. However, the basic tenet of this model is that the level of embrittlement is influenced by the grain boundary structure, and the results did not indicate a substantial difference between the boundaries that failed in a ductile manner and those which failed by brittle fracture. This suggests that the factors which affect lithium segregation may be more complex than originally envisaged.

An Analysis of Strain Path Effects on Static Recrystallisation in Hot Worked Aluminium Alloy AA5052 Using Forward and Reverse Torsion

The effects of strain path reversal, using forward and reverse torsion, on the microstructure evolution in the aluminium alloy AA5052 have been studied using high resolution electron backscatter diffraction. Deformation was carried using two equal steps of forward/forward or forward/reverse torsion at a temperature of 300°C and strain rate of 1s-1 to a total equivalent tensile strain of 0.5. Sections of the as-deformed gauge lengths of both test specimens were then annealed at 400°C for 1 hour in a salt bath in order to investigate their subsequent recrystallisation response. In both strain path histories the deformation substructure in the grains analysed consisted of microband arrays within an equiaxed dislocation cell structure. The material subjected to forward/forward deformation did, however, have a slightly greater number of low angle boundaries, i.e. boundaries < 15° misorientation, whilst the forward/reverse material had some grains containing little evidence of substructure. On annealing both materials had significantly reduced levels of low angle boundaries but only the forward/forward material had an increased number of high angle boundaries and a reduced grain size, indicating recrystallisation had only occurred in this material. This would suggest that the deformation microstructure within the forward/forward condition was sufficient to initiate and maintain recrystallisation whilst the microstructure produced by the forward/reverse test contained insufficient nuclei or internal energy to produce a recrystallised material within 1 hour. Further work is now required at different annealing times in order to determine if the major effect of strain path is on retarding nucleation, growth or both.

Grain Boundaries with High Misorientation and Low Mobility

Island grains have been studied in iron samples that had been treated by critical-strainannealing and in commercial silicon iron alloy sheets after incomplete secondary recrystallisation. Such islands remain behind because their boundaries have such a low mobility that the grains cannot shrink away in the time available during annealing. Misorientations of these islands in relation to the grains surrounding them were measured using EBSD. Similar results were observed in both materials. A small number of low angle boundaries were found and also many twin boundaries. The most remarkable observation, however, was the presence of many general high angle boundaries that did not correspond to any evident coincidence relation.

The structural characteristics of Bi-based 2212 phase whiskers

We have succeeded in studying the structural characteristics of a single thread of Bi-based 2212-phase whisker by X-ray diffraction. It is found that the structural characteristics of a single thread of whisker coincide with those of a 2212-phase single crystal. The single thread of whisker is a 2212 plate-like single crystal and is stacked in a layered structure. There are a number of low-angle boundaries in the BC-plane, but few in the AB-plane of the whisker.