Showing papers on "Grain size published in 1979"

Dynamic recrystallization during creep of single-crystalline halite: An experimental study

Abstract: Single crystals of pure and impure halite have been dynamically recrystallized during compression creep at temperatures between 250° and 790°C and stresses between 1.5 and 120 bars. Recrystallization was found to occur by two different mechanisms: at lower temperatures and stresses the new grains result from the rotation of subgrains without grain boundary migration (rotation recrystallization), and at higher temperatures and stresses the final texture results from the migration of the high-angle grain boundaries of the rotated subgrains. Migration recrystallization was shown to occur for critical stress and temperature conditions, allowing rapid grain boundary migration. A curve separates the two domains in the σ, T plane and moves to higher temperatures and stresses for crystals of higher impurity content; for natural crystals, only rotation recrystallization can occur. In each recrystallization regime the recrystallized grain size is uniquely related to the applied stress, thus yielding two different geopiezometers, which should not be applied indiscriminately to natural tectonites to determine lithospheric or mantle deviatoric stresses. The experimental results are interpreted by the Lucke et Stuwe theory for impurity-controlled grain boundary migration.

Dynamic recrystallization kinetics

Abstract: Dynamic recrystallization in two austenitic stainless steels has been monitored by (i) observations of the high-temperature stress/strain behaviour, and (ii) metallographic measurements on rapidly quenched, partially reacted specimens. Particular effort has been put into assessing the influence of pre-existing grain size on the kinetics of dynamic recrystallization at a specified temperature and strain rate. A theory due to Cahn, which is intended to describe the kinetics of transformations where nucleation is strictly confined to pre-existing grain boundaries, has been applied to the dynamic recrystallization problem. Some measure of agreement with observation is obtained but only on the proviso that proper attention is paid to repeated nucleation at the recrystallization front, and that a distinct variation of active grain boundary area during the reaction is recognized and considered.

The process of crack initiation and effective grain size for cleavage fracture in pearlitic eutectoid steel

Abstract: The process of cleavage crack initiation and the character of the effective grain size which controls the fracture toughness of pearlitic eutectoid steel has been investigated using smooth tensile and precracked Charpy impact specimens. The results demonstrated that initial cracking in both specimens was largely the result of shear cracking of pearlite;i.e., localized slip bands in ferrite promoted cracking of the cementite plates, which was then followed by tearing of the adjacent ferrite laths. Such behavior initially results in a fibrous crack. In the tensile specimen, the initiation site was identified as a fibrous region which grew under the applied stress, eventually initiating an unstable cleavage crack. In precracked impact specimens, this critical crack size was much smaller due to the high state of stress near the precrack tip. Fracture mechanics analysis showed that the first one or two dimples formed by the shear cracking process can initiate a cleavage crack. Using thin foil transmission electron microscopy, a cleavage facet was found to be an orientation unit where the ferrites (and the cementites) of contiguous colonies share a common orientation. The size of this orientation unit, which is equal to the cleavage facet size, is controlled by the prior austenite grain size. The influence of austenite grain size on toughness is thus explained by the fact that the austenite grain structure can control the resultant orientation of ferrite and cementite in pearlitic structures.

Grain and sub-grain size variations across a mylonite zone

Abstract: The results are reported of a combined optical and electron microscopy study of microstructural variations across a quartz mylonite zone with increasing shear strain. The mylonite developed by recrystallization of the deformed quartz grains with increasing shear strain. It was found in a given specimen that the size of recrystallized grains and of sub-grains were always smaller in electron micrographs. The possible reasons for this are discussed. The size of both features decreased with increasing shear strain irrespective of the microscope used. However the density of unbound dislocations remained constant. A marked grain size reduction occurred in phyllosilicate rich layers. Variations in sub-grain size were observed within the relict old grains which remained at low shear strains. These are thought to reflect stress intensification adjacent to grain boundaries during deformation. The relict grains recrystallized at higher strains. Stresses were estimated from grain and sub-grain sizes and from the dislocation density. The results indicate that estimates based on grain size are unreliable if phyllosilicates inhibit the growth of grains during recrystallization, and that the dislocation densities are altered during uplift and are unlikely to give meaningful estimates. It is also concluded that the microstructures reflect stress gradients present during the formation of the mylonite, that is the initiation and propagation of the shear zone and that these were subsequently replaced by strain rate gradients.

Deformation of Polycrystals

Abstract: SUMMARY Deformation of polycrystals is treated as the average behavior of an agglomerate of individual crystallites and their interaction across the grain boundaries. After considering the effect of grain boundaries on yield, in a rather general way the similarities between stress strain curves of single crystals and polycrystals are discussed, including the implication for commonly used stress strain relationships. Since - as known from polyslip in single crystals - the polycrystal curve consists of two stages (II and III) which are differently affected by deformation temperature, these two stages are treated separately. The range of athermal hardening (stage II) is described quantitatively by incorporating the processes of dislocation storage near the grain boundary into the well established stage II theories of single crystals. It is concluded from this discussion, that the influence of grain size on yielding and on work hardening at low strains is phenomenologically well explored and rather well understood theoretically. The range of temperature dependent work hardening (stage III) is interpreted as being the stage where the dislocation structure evolves towards a well defined cell structure, which approaches a steady state pattern as strain proceeds. The influence of temperature and strain rate on steady state appears to be the same for single crystals (deformed in polyslip) and polycrystals in the normal range of grain sizes. Although the differences between different materials can be related to the values of certain material parameters, understanding of stage III suffers from our ignorance of the basic dislocation mechanisms which control cell formation. In the last part effects of very large strains are shortly considered. Texture development is taken as an example how information about details of the deformation of the grains can be deduced from a comparison of experimentally obtained and of theoretically derived pole figures. Eventually it is discussed that at very large strains new effects appear, which are beyond the scope of the common theoretical concepts of work hardening and plastic flow.

Static recrystallization and restoration after hot deformation of Type 304 stainless steel

Abstract: Specimens of Type 304 stainless steel have been deformed in torsion at temperatures of 950–1150°C and annealed for different times before quenching or continuing the deformation. The kinetics of static recrystallization and the recrystallized grain size have been determined by quantitative metallography. Increasing strain and decreasing original grain size both reduce recrystallization time and recrystallized grain size. Increasing strain rate and decreasing temperature of deformation have a similar, but smaller effect. Annealing temperature also strongly influences recrystallization time (activation energy 425 kJ mol−1), but not recrystallized grain size. The effects of all the experimental variables are described in terms of simple parametric relationships. The kinetics of static restoration have been determined both from hardness tests on quenched specimens and from interrupted torsion tests. The extent of restoration is related in a non-linear manner to fraction recrystallized. This arises par...

Microstructural influences on superplasticity in Ti-6AI-4V

Abstract: The strong dependence of the superplastic behavior of metals and alloys on grain size has been demonstrated, and it is now well known that a fine grain size is normally a requirement for superplasticity. However, the microstructure of certain alloy systems such as Ti-6A1-4V cannot always be adequately characterized by a single parameter such as grain size. In two-phase α β alloys such as Ti-6A1-4V, other microstructural parameters such as volume fractions of the two phases, grain aspect ratio, grain size distribution and crystallographic texture may also influence superplasticity. For example, if “grain switching” is an important deformation mechanism in superplastic flow as suggested by Ashby and Verall, then factors such as grain aspect ratio and range of grain sizes would be expected to have an effect on superplastic behavior. In this study, these microstructural features were determined for several different heats of Ti-6Al-4V, and the corresponding superplastic properties were evaluated in terms of their fully characterized microstructure. The flow stress as a function of strain rate, strain rate sensitivity exponent (m) as a function of strain rate and total elongation on properties were found to be strongly influenced by microstructural parameters such as grain aspect ratios, grain size and grain size distribution.

Differential stress determined from deformation‐induced microstructures of the Moine Thrust Zone

Abstract: The dislocation structure, recrystallization, and elongation of quartz grains in tectonites from three localities along the Moine thrust fault have been analyzed by transmission electron and optical microscopy. The dislocation density, 5×108 cm−2, and the grain size after recrystallization, 15 μ, in the basal quartzite unit at the Stack of Glencoul are independent of distance from the fault, to the maximum sampling distance of ∼100 m. The differential stress level determined from the deformation-induced microstructures is on the order of 100 MPa. The magnitude of the differential stress at the fault decreases by a factor of 2 from Knockan Crag to Loch Eriboll, a distance of 50 km. At the Stack of Glencoul, grains in the basal quartzite are progressively elongated and recrystallized approaching the fault, corresponding to a progressive increase in strain. At 100 m from the fault, the aspect ratio of relict quartz grains is ∼1:1; at 0.01 m it is 85:1. A quantitative estimate of the strain, based on the aspect ratios of the relict quartz grains, as a function of distance from the fault has been used to calculate strain rate and temperature profiles. For a creep activation energy of 0.19 MJ mol−1 the temperature decreases away from the fault with a gradient of 7.5×10−3 °C cm−1. For a differential stress of 100 MPa, a steady state thrust sheet velocity of 11 cm yr−1 would be required to produce this gradient. At 100 m from the fault, 5% of the quartzite is recrystallized from 1-mm to 15-μm grains; at 0.01 m it is 100% recrystallized to 15-μm grains. This small grain size probably favored grain boundary deformation mechanisms over grain matrix mechanisms and thus helped to concentrate the strain at the fault. Systematic changes in the elongation and recrystallization of quartz grains in Lewisian Gneiss and Moine Schist are obscured by the constraints imposed by other mineral phases. The small size of the grains after recrystallization, highly serrated grain boundaries, and large densities of free dislocations suggest that very little static recovery followed the plastic deformation event.

Mechanical behavior of superplastic ultrahigh carbon steels at elevated temperature

Abstract: Ultrahigh carbon (UHC) steels have been investigated for their strength and ductility characteristics from 600 to 850°C. It has been shown that such UHC steels, in the carbon range 1.3 to 1.9 pct C, are superplastic when the microstructure consisted of fine equiaxed ferrite or austenite grains (∼1 μm) stabilized by fine spheroidized cementite particles. The flow stress-strain-rate relations obtained at various temperatures can be quantitatively described by the additive contributions of grain boundary (superplastic) creep and slip (lattice diffusion controlled) creep. It is predicted that superplastic characteristics should be observed at normal forming rates for the UHC steels if the grain size can be stabilized at 0.4 μm. The UHC steels were found to be readily rolled or forged at high strain-rates in the warm and hot range of temperatures even in the as-cast, coarse grained, condition.

Tempered martensite embrittlement in phosphorus doped steels

Abstract: In this paper the effect of phosphorus on tempered martensite embrittlement of Ni−Cr steels is reported. It is shown that the measured degree of embrittlement depends on the phosphorus concentration, test temperature, grain size, and austenitizing temperature. Although reducing the prior austenite grain size tends to reduce the observed embrittlement, this can be offset by the fact that the low austenitizing temperatures used to produce the fine grain size cause an increased amount of impurity segregation. It is further shown that bulk phosphorus concentrations below 100 wppm may be required to avoid embrittlement of this type in ultra-high strength steels.

Grain size limits for pseudosingle domain behavior in magnetite: Implications for paleomagnetism

Abstract: A theoretical model of grain size dependence of domain transitions in magnetite is presented. This domain model is used to predict the grain size range for pseudosingle domain (PSD) behavior in magnetite. For cubic magnetite particles, the single domain (SD) to two-domain (TD) transition occurs at ∼800 ± 200 A, the TD-three-domain transition occurs at ∼ 1500 A, and the PSD-"true" multidomain (MD) transition occurs at \sim 8\mu m. These results are in fair agreement with experimental results within a factor of 2. Our results indicate that the grain size range for the canonical case of a PSD grain (a TD grain with a single 180° wall) or "PSARK" (subdomain magnetic moment) ranges from ∼700 A for cubic particles of magnetite to \sim3.2 \mu m for a length: width ratio of 5:1. Comparison with grain size distributions of magnetite grains in igneous rocks suggest that the PSARK can be the major contributor to the remanence of these grains.

Silt-clay aggregates on Mars

Abstract: Viking observations suggest abundant silt and clay particles on Mars. It is proposed that some of these particles agglomerate to form sand size aggregates that are redeposited as sandlike features such as drifts and dunes. Although the binding for the aggregates could include salt cementation or other mechanisms, electrostatic bonding is considered to be a primary force holding the aggregates together. Various laboratory experiments conducted since the 19th century, and as reported here for simulated Martian conditions, show that both the magnitude and sign of electrical charges on windblown particles are functions of particle velocity, shape and composition, atmospheric pressure, atmospheric composition and other factors. Electrical charges have been measured for saltating particles in the wind tunnel and in the field, on the surfaces of sand dunes, and within dust clouds on earth. Similar, and perhaps even greater, charges are proposed to occur on Mars, which could form aggregates of silt and clay size particles

Initial stages of recrystallization in aluminum of commercial purity

Abstract: In commercial aluminum with a purity of 99.4 pct, the formation and growth of recrystallization nuclei were studied by techniques such asin-situ annealing in a high voltage electron microscope, transmission electron microscopy and light microscopy. Sample parameters were the initial grain size (370 and 19 microns) and the degree of deformation (50 and 90 pct reduction in thickness by cold-rolling). It was found that the initial grain boundaries and high angle boundaries within the original grains are preferential sites for recrystallization nuclei, and that the effect of such sites is enhanced by the FeAl3 particles present in the commercial aluminum as impurities. The nucleation temperatures determined by high voltage electron microscopy and transmission electron microscopy decrease markedly when the initial grain size is decreased both after 50 and 90 pct cold rolling; a less pronounced temperature decrease is obtained by increasing the degree of deformation. The size of the recrystallization nuclei, the recrystallization temperature and the recrystallized grain size are reported for the four sample states, and finally the structural and kinetic observations are discussed.

The oxidation resistance of fine-grained sputter-deposited 304 stainless steel

Abstract: The air oxidation of free standing 0.1 cm thick sputter-deposited and wrought 304 stainless steel specimens was studied and the long term oxidation weight gains of the sputterdeposited material were much less than weight gains for the wrought material at 800, 900, and 1000°C. The amount of scaling was also much less on the sputtered material and a thin, adherent oxide formed. The oxide on the sputtered material was more uniform in composition and was higher in chromium and manganese compared to oxide on wrought stainless and eventually formed MnCr2O4 after long periods of exposure. No stratified oxide layers, as typically observed in wrought stainless steel, formed on the fine-grained sputtered material. The improved scaling resistance of the sputter-deposited steel was attributed to a combination of grain boundary enhancement of chromium diffusion, reduced stresses in the oxide and mechanical keying of the oxide to closely spaced grain boundaries. The stability of grain size for the sputtered material (grain size ≤ 6 μm) also contributed to the better oxide adherence.

Subgrain coalescence and the nucleation of recrystallization at grain boundaries in aluminium

Abstract: An experimental study has been made of recrystallization processes which have been initiated at grain boundaries in aluminium samples. The samples had an initial grain size of between 30 and 130 $\mu $m and were deformed 50% by rolling before annealing at 573 K. The microstructures developed in the region of the grain boundaries, after annealing times chosen just to initiate recrystallization, have been investigated with transmission electron microscopy. A major part of the investigation has been the identification of a number of different types of potential recrystallization nuclei at these sites. It has been established that the formation of most of these recrystallization nuclei involves modifications to the subgrain structures adjacent to grain boundaries. A model is developed for the formation of recrystallization nuclei at grain boundaries by processes involving subgrain coalescence. The microstructures predicted by the model are in accord with those observed experimentally.

Oxidation of high-chromium Ni-Cr alloys

Abstract: The oxidation of binary Ni-Cr alloys containing 44 and 50 wt. % Cr has been studied over a range of oxygen partial pressures at temperatures between 800 and 1100°C. The effects of cold work, surface preparation, and distribution of the Cr-rich second phase have been studied. The oxidation behavior is complex and cannot be described by a single model. The oxide grows by short-circuit diffusion as well as bulk transport through Cr 2 O 3 scales. The scale-growth mechanism includes extensive metal-oxide separation requiring Cr vapor transport to the scale, compressive stresses within the oxide which result in scale bulging and cracking, and the formation of a second oxide layer which results in voids being incorporated into the scale. Any factor which reduces the oxide grain size, such as cold work, finer distribution of the Cr-rich α phase or reduced oxygen pressure, results in an increased oxidation rate of binary alloys because of an increased number of grain-boundary short-circuit diffusion paths.

Strength‐Grain Size Relations in Polycrystalline Ceramics

Abstract: A model is presented to explain the strength-grain size behavior of ceramics exhibiting duplex microstructures and of ceramics with uniform coarse-grained microstructures. Using an energy balance analysis, the model shows that in most ceramics an hitiipl flaw contained within a single large grain will initially propagate and then arrest after entering the region of increased fracture-surface energy. The arrested flaw will subsequently propagate to failure at the value OF the critical stress intensity factor. Using the data in the literature, a strength-(grain size)−1/2 plot for MgAl2O4 was generated.

Hardening mechanisms of hard gold

Abstract: Electrodeposited hard gold with 0.6 at.% cobalt has a hardness about four times that of annealed bulk gold and this high hardness cannot be reproduced by standard metallurgical methods. By measuring the hardness for gold and gold alloys subjected to various quenching, annealing, and deformation processes, all common hardening mechanisms such as solution hardening, precipitation hardening, strain hardening, and ’’voids’’ hardening were eliminated as possible major hardening contributors with the exception of the grain size effect. Pure gold with grain sizes ranging from 200 A to 3 μm were prepared using sputtering deposition by varying the substrate temperature during deposition from 55 to 310 °C. Larger grain sizes from 5 to 200 μm were prepared by annealing cold‐drawn gold wire at 300–750 °C. The hardness versus grain diameter followed the Hall‐Petch relation up to a grain size of 0.1 μm. Beyond that, the hardness increased less rapidly. At the grain size of electrodeposited hard gold of 250–300 A, the sputtered pure gold gave the same hardness value also. Therefore, the grain size effect accounts for the observed high hardness of electrodeposited hard gold, with other mechanisms accounting for only small alterations.

Influence of carbide thickness on impact transition temperature of ferritic steels

Abstract: A large number of ferrite–pearlite steels have been examined and measurements made of grain size, pearlite volume fraction, degree of precipitation hardening, and thickness of grain-boundary carbid...