Fine Subgrain Structure Research Articles

(α+α′) 2相ステンレス鋼の結晶粒微細化

The microduplex process of a Fe-16%Cr-5%Mo-8%Co-2%Ni alloy has been investigated. The alloy used has a fully ferritic structure after quenching from 1573 K, and then austenite (followed by martensitic transformation during air cooling) decomposes from the ferrite during reheating up to the temperature range of (α+γ) tow phase.If the ferrite, however, was cold rolled about 90% prior to the reheating, strain induced precipitation of Laves phase occurs in the temperature range 850-950 K. On 923 K aging, the precipitates play an important role in the suppression of subgrain growth, resulting in the fine subgrain structure. It was also shown that austenite nucleates at the subgrain boundaries. On the other hand, in the temperature range 1000-1300 K, austenite immediately nucleates along slip bands and deformation twins, resulting in the microduplex structure of ferrite and martensite. At temperatures above 1300 K where the recrystallization of ferrite matrix can preferentially occur, however, a structure abruptly coarsened. Consequently, to obtain the microduplex structure, it is likely important that the nucleation of austenite should take place prior to the recrystallization of ferrite matrix.

Quartz deformation and the recognition of recrystallization regimes in the Flinton Group conglomerates, Ontario

The clastic metasedimentary rocks that form the Flinton Group occur as arcuate belts within the Grenville Structural Province, southeastern Ontario. Metamorphic isotherms transect these rocks and allow comparison of deformation over an approximate temperature range of 550–650 °C. Two geometrically discrete but temporally continuous deformation phases contribute significantly to pebble deformation within the conglomeratic units. There is a general increase in both finite strain, over which there is a strong compositional control, and the frequency of D2 minor structures with increasing metamorphic temperature. D1 is characterized by the creation of a subgrain structure and progressive misorientation of the subgrains as strain accumulates. Ultimately this leads to formation of high-angle boundaries defining recrystallized grains of a size similar to that of the precursor subgrains. D2 produces a new, finer subgrain structure, elongated grains, grain boundary migration (bulging), and small recrystallized grains. Empirical relationships between stress and subgrain diameter suggest that D1 is a low stress (5–6 MPa) deformation and D2 is a higher stress (12–80 MPa) deformation. This emphasizes that rotation recrystallization defines a low-temperature, low-stress regime, whereas migration recrystallization is typical of higher temperatures and stresses. Although dislocation creep predominates in most pebble types, evaluation of the deformation in terms of non-uniform flow laws can explain the compositional control of strain in the quartzite pebbles and suggests that significant grain boundary sliding occurred.

Delamination effects in grinding

Delamination of the surface layers of brass workpieces was observed during single-grit grinding. In most instances, complete delamination did not occur and the delaminated material was still attached to the workpiece. Transmission electron microscope examination of the ground surface layers showed that they had a fine subgrain structure; no evidence of a dislocation-free surface layer was obtained. Delamination was found to occur in the fine-grained surface layers, and it is suggested that this occurs by a process of shear separation within the fine subgrain structure. The manner in which the delaminated sheets were observed to lift away from the workpiece surface seems to indicate the presence of both residual tensile and compressive stresses within the surface layers.

Transmission electron microscope studies in special ceramics

The use of high temperature special ceramics which are usually complex materials based on oxides, nitrides, carbides and borides of silicon and aluminum, is critically dependent on their thermomechanical and other physical properties. The investigations of the phase diagrams, crystal structures and microstructural features are essential for better understanding of the macro-properties. Phase diagrams and crystal structures have been studied mainly by X-ray diffraction (XRD). Transmission electron microscopy (TEM) has contributed to this field to a very limited extent; it has been used more extensively in the study of microstructure, phase transformations and lattice defects. Often only TEM can give solutions to numerous problems in the above fields, since the various phases exist in extremely fine grains and subgrain structures; single crystals of appreciable size are often not available. Examples with some of our experimental results from two multicomponent systems are presented here. The standard ion thinning technique was used for the preparation of thin foil samples, which were then investigated with JEOL 200A and Siemens ELMISKOP 102 (for the lattice resolution work) electron microscopes.

Ordering transformations and mechanical properties of Ti3Ai and Ti3Al-Nb alloys

Phase transformations and the kinetics of domain growth were studied in near stoichiometric Ti3Al and in a similar alloy containing about 5 at. pct Nb (Cb). The alloys were quenched from the β and from the α+ β fields and were subsequently annealed in the α2 field to study the ordering transformation. The critical temperature (T c) for ordering was found to be between 1125 and 1150° for both alloys. When quenched from aboveT c the microstructure of the stoichiometric compound contained massive martensite with small antiphase domains of average size 8 × 10— μm. On annealing the quenched structures in the range 700 to 1000°, domain coalescence occurred, the domains growing approximately as the square root of the annealing time. The activation energy for the domain growth process was found to be 64.6 ± 6 Kcal/mole (2.68 ± 0.25 × 105 J/mole). On quenching the alloy containing Nb the β transforms to a fine acicular martensite. On annealing, antiphase domain coalescence within the martensite plates and the simultaneous recrystallization of the martensite resulted in a fine subgrain structure even after annealing at 900° for up to 3 h. The mechanical properties and the fracture modes of the two alloys tested at 700° were correlated with the observed microstructural changes. The effects of Nb in this alloy are to slow the domain growth kinetics, to reduce the planarity of slip, and to increase nonbasal slip activity.

The shear mode of ductile fracture in materials with few inclusions

The mechanism of the shear mode of ductile fracture has been investigated in sheet tensile specimens of an α brass containing few inclusions. The techniques of optical metallography, scanning electron microscopy and transmission electron microscopy of both thin foils and replicas were all used in order to obtain as complete a picture of the fracture mechanism as possible. It was found that a zone of shear deformation, made up of many intense shear bands, developed across the specimen neck and fracture occurred within this zone along the shear bands. These shear bands consisted of material with a fine subgrain structure. No voids were found in the necked region of either deformed or fractured specimens yet the fracture surfaces were covered with elongated dimples. It is therefore suggested that the dimples on the fracture surface are not a precursor of the fracture but form only during the final process of separation.

The nature of the white-etching surface layers produced during reaming ultra-high strength steel

The white-etching surface layers on reamed holes, in ultra-high strength steel, produced under both cutting and rubbing conditions have been studied. Transmission electron microscope examination of the layers has revealed that both are produced by intense plastic deformation. The layers had a fine subgrain structure which was body-centred-cubic and there was a general loss of carbides within the layers. For the purpose of structure identification these white-etching layers were compared with (i) white-etching layers produced by abrasion, (ii) martensite and (iii) white-etching surface layers produced by abusive grinding.

A New Model for Initiation and Growth of Fatigue Cracks

Abstract The initiation and growth of fatigue cracks are often associated with localized fatigue-induced or pre-existing soft layers (∼0·1 μm thick). The deformation of specimens, consisting of soft layers hundreds of μm thick sandwiched between harder regions, simulated many features of fatigue on a larger scale and demonstrated that slip-band extrusions were produced when soft layers were ‘squeezed out’ at the surface. It is concluded that fatigue cracks initiate and grow (in stage I and stage II) by a mechanism of intrusion which occurs when soft layers are ‘sucked in’ as a result of preferential tensile deformation in the soft layers. This deformation is discussed in terms of macroplasticity theory and dislocation theory. It is concluded that the production of fine sub-grain structures in soft layers during fatigue greatly reduces the constraint normally experienced by thin soft layers and permits extensive deformation in these layers at relatively low stresses. The proposed model accounts for most of...

Influence of Ingot Processing on the Structure of As-Cast Al-Zn-Mg-Cu-Cr Alloy

A study has been carried out of the effect of thermal and mechanical treatments on the structure of as-cast Al–Zn–Mg–Cu–Cr high purity alloy. It was found that Cr distribution depends on the thermal treatments the alloy is given; moreover it has been shown that a dense precipitation of Cr rich incoherent particles is very effective in determining the very fine subgrain structure of the final wrought product.

Fine subgrain structure adjacent to fatigue cracks

Deformation microstructures adjacent to fatigue cracks in 2024-T4 and 7075-T6 aluminum and Ti-6A1-4V alloys have been examined by transmission electron microscopy. The data show that material within 1–2 μm of a tensile-mode crack consist of very fine (0.1 μm) subgrains in all three materials. The average misorientation between the subgrains is 2–4°. The presence of this substructure, which seems to be independent of initial metallurgical condition, is used to explain the weak dependence of stage II crack growth rates on the initial condition in high stacking fault energy materials. No evidence of precipitate reversion was observed adjacent to the fatigue cracks in 7075-T6 aluminum.

Al-Ti合金における降伏点現象

A yield point phenomenon was observed in Al-0.16 wt%Ti alloy when specimens were stretched at room temperature after cold working and annealing under the limited conditions, for example, 1 hr at 280 to 300°C. The phenomenon was most clearly observed in a specimen annealed at 280°C rather than at higher temperatures. Electron microscope observation revealed that the annealed structure of the specimen consisted of very fine subgrains of a few microns, inside of which the dislocation density was low. Therefore, it is concluded that, in addition to locking of dislocations by added titanium, the fine subgrain structure contributes greatly to the phenomenon.