Showing papers on "Grain boundary strengthening published in 1988"

The kinetics of ferrite nucleation at austenite grain boundaries in Fe-C alloys

Abstract: The nucleation kinetics of grain boundary allotriomorphs of proeutectoid ferrite at austenite grain faces have been measured in three high purity Fe-C alloys as a function of isothermal reaction time and temperature. Several correction techniques, including discrimination between different nucleation sites and the effect of carbon diffusion fields on further nucleation of ferrite, were incorporated into a stereological procedure utilizing the SchwartzJSaltykov size distribution analysis. This analysis enabled the number of ferrite particles per unit unreacted grain boundary area to be obtained as a function of isothermal reaction time, and thus the time-dependent nucleation kinetics to be obtained as a function of temperature and carbon concentration. These rates were then compared with those predicted by classical heterogeneous nucleation theory using various models for the critical nucleus. It was concluded that viable critical nuclei must have predominately low energy interphase boundaries. Only a very small fraction of the austenite grain face area appears to be capable of supporting nucleation.

Computer simulation of microstructural evolution in thin films

Abstract: The nature of the microstructure of a thin film strongly affects its functionality in electronic applications. For example, the rate of electromigration-induced failure is a function not only of the grain size in an interconnect line, but also of the width and shape of the grain size distribution. We are developing techniques which allow prediction of the relationships between the conditions for thin film processing and the topology and geometry of resulting grain structures. We have simulated two-dimensional microstructural evolution by determining the location of grain boundaries after nucleation and growth of crystalline domains. We have allowed for nucleation under a variety of conditions including constant nucleation rates, decreasing nucleation rates and instantaneous saturation of nucleation sites. We have also allowed for increasing and decreasing growth rates which depend in various ways on the domain size. We have simulated grain growth in two-dimensional structures by allowing boundary and triple point motion in order to reduce the total grain boundary area. The rate and nature of the initial stages of grain growth are strongly affected by the conditions for nucleation and growth. Eventually, however, grain growth appears to proceed as expected from analytical treatments.

Study on the substructure evolution and flow behaviour in type 316L stainless steel over the temperature range 21-900°C

Abstract: Tensile specimens of type 316L stainless steel with a grain size of 5.0 μm have been deformed at a constant strain rate of 10−3 s−1 over the temperature range 21–900°C and by differential strain-rate test technique over strain rates from about 10−5 to 10−3 s−1 at temperatures in the range 750–900°C. The normalized yield and flow stresses against temperature plots exhibit three regions. While in regions I and III the stresses decrease with increasing temperature, they increase with increasing temperature in region II. Transmission electron microscopy studies on deformed specimens show that at small strains the dislocations generated at grain boundaries have characteristic distributions: in region I the dislocations are confined to the vicinity of the grain boundary, in region II the dislocations are spread into the grain interior, and in region III the dislocations rearrange to form walls. The evolution of substructure and the work-hardening behaviour are explained by considering both intragranula...

Grain growth and alignment in hot deformed Nd‐Fe‐B magnets

Abstract: Formation of grain oriented Nd‐Fe‐B magnets from melt‐spun ribbons by hot deformation has been studied using electron microscopy. It is shown that deformation and alignment of Nd2Fe14B magnets result from a combination of plastic deformation, grain boundary migration, and grain boundary sliding. Due to a limited number of available slip systems in this material, samples with large grains deform less easily. Small grain size materials, as encountered in the melt‐spun ribbons, are well suited for die‐upsetting to produce oriented magnets.

The effect of boron on the chemistry of grain boundaries in stoichiometric Ni3Al

Abstract: X-ray spectra have been obtained, using a 1 nm diameter electron beam, from thin foils of stoichiometric Ni3Al with and without boron (0·35 at.%). The boron addition produces nickel enrichment at grain boundaries, which implies a disordering of this region. Disordering of the grain boundary and the consequent expected increase in grain boundary dislocation mobility are suggested to be the reason for the ductility improvement of Ni3Al due to boron.

Evaluation of the effects of segregation on austenite grain boundary energy in Fe-C-X alloys

Abstract: A scanning Auger microprobe study has been made of the segregation of substitutional alloying elements to austenite grain boundaries in Fe-C-X alloys (where X = Mn, Ni, Si, Co, and Mo). The grain boundary enrichments in X are considerably smaller than those previously estimated from the thermal grooving method but appear consistent with other SAM results in the literature. Mo exhibits the highest enrichment factor, those for Si and Mn are intermediate, and no appreciable grain boundary enrichment of either Co or Ni is observed. In view of the special relevance of this information to nucleation kinetics of austenite decomposition products at austenite grain boundaries, the reductions in grain boundary energy attending the measured enrichments are evaluated using the model of interactive segregation of interstitial and substitutional solutes formulated by Guttmann and McLean. These calculations were performed under two different (limiting) conditions: (i) equilibrium segregation of both solutes is fully achieved at the isothermal reaction temperatures, and (ii) the boundary concentration of X is fully inherited from the austenitizing temperature and only paraequilibrium segregation of carbon is achieved. Various characteristics of interactive segregation are also discussed in terms of the interaction and binding energies of each solute.

Preparation and properties of fine grain β- CuAlNi strain- memory alloys

Abstract: A method has been developed to produce grain sizes as small as 5 μm in alloys of β-CuAlNi. The alloys were of eutectoid composition and a procedure was developed for determining the composition of a eutectoid alloy having any required value for transition temperature (M s ). The thermo-mechanical treatment involved two sequential stages of warm rolling followed by recrystallization. The alloys produced were single phase β-type with no second phase being present. Characteristic two-stage stress-strain curves were obtained for most of the specimens. It was generally found that the tensile strength and strain to failure increased with decreasing grain size according to a Hall-Petch type relationship down to a grain size of 5 μm. A fracture strength of 1200 MPa and a fracture strain of 10 pct were obtained in the best alloy. It was found that the major recovery mode, whether pseudoelastic or strain-memory, did not have any significant effect on the total recovery obtained. Recovery properties were not affected significantly by decreasing grain size, and 86 pct recovery could still be obtained at a grain size of around 10 μm. Grain refinement improved the fatigue life considerably, possibly due to the high ultimate fracture stress and ductile fracture mode. A fatigue life of 275,000 cycles could be obtained for an applied stress of 330 MPa and a steady state strain of 0.7 pct. At fine-grain sizes most of the fractures were due to transgranular-type brittle fracture and micro void-type ductile fracture, depending on the alloy composition. It was suggested that the difference between the alloys was due to differences in oxygen segregation at the grain boundaries.

A theoretical investigation of two-dimensional grain growth in the ‘gas’ approximation

Abstract: A master equation for grain growth is suggested for the one-particle distribution of grain areas and topological classes in two-dimensional polycrystals with uniform properties of grain boundaries. The ‘collision’ term for a self-similar mode (normal grain growth) is formulated within the ‘gas’ approximation, assuming equal probabilities of neighbour switchings for all the grain boundaries and ignoring mutual arrangement of grains.

Special grain boundaries in YBa2Cu3O7

Abstract: The orientation distribution produced in strings of YBa2Cu3O7−x often referred to as 123, crystals produced from a melt is not random. The individual crystals are kinematically free to rotate during the coalescence process and it is found that coincidence site lattice misorientations are selected. This observation suggests that grain boundaries in 123 have structures related to those in other crystalline materials.

Effect of austenite grain boundary mobility on hardenability of steels containing vanadium

Abstract: A correlation has been established between the rate of grain boundary migration during austenitisation and the hardenability of steels containing 0·2–0·3%C, 1·5–1·7%Mn, up to 0·35% V, and small additions of Al or Ti. Interaction between the austenite grain boundaries and pinning particles was investigated using transmission electron microscopy and segregation to the austenite grain boundaries was examined using Auger electron spectroscopy. It has been concluded that the velocity of grain boundary migration during austenitisation influences the extent of equilibrium segregation to the austenite grain boundaries which, in turn, affects the hardenability. Pinning of the austenite grain boundaries enabled the potential hardenability effect of the alloying elements to be increased. Mechanisms have been discussed for the ways in which segregation, particularly of V, occurs to pinned or immobilised austenite grain boundaries, and the conditions by which most effective grain boundary pinning can be achiev...

The influence of grain size and fracture surface geometry on the near-threshold fatigue crack growth in ferritic steels

Abstract: Fatigue crack growth in the threshold region is very much affected by the microstructure A large portion of this microstructural influence is due to changes in the crack closure stress intensity Kcl The present paper is concerned with the influence of the grain size on the near-threshold fatigue crack growth in ferritic steels, with special emphasis on the influence of the microstructurally induced fracture surface roughness on the crack closure stress intensity The experiments were performed on ferritic steels with grain sizes varying between 8 and 82 μm An increase in the nominal and effective fatigue crack growth threshold values with increasing grain size was found Quantitative fractographic measurements showed that the fracture surface roughness rises with the grain size independently of the stress intensity level The results also indicate that Kcl is proportional to H 1 3 , where H is the fracture surface roughness as expressed by the mean standard deviation of height By rationalizing the experimental data, ΔKth has been quantified in terms of grain size effects and the fracture surface roughness

The effects of strain on grain misorientation texture during the grain growth incubation period

Abstract: The effects of post-recrystallization strains of 2% and 7% on the microtexture and grain misorientation texture of an austenitic stainless steel are investigated. The time to onset of grain growth is decreased by two to three orders of magnitude after the additional strain. An electron back-scattering technique is used to collect grain orientations as a function of strain and post-strain annealing time and the results show that the proportions of coincident site lattice (CSL) boundaries increase during the post-strain anneals. Most of the CSLs are potentially ‘special’ types and tend to occur in clusters, with an increased tendency for clustering as the end of the grain growth incubation period is approached. It is therefore proposed that special boundary clustering is instrumental in the initiation of anomalous grain growth.

Creep rupture strength and grain-boundary sliding in austenitic 21 Cr-4Ni-9Mn steels with serrated grain boundaries

Abstract: The effect of grain-boundary strengthening on the creep-rupture strength by modification of the grain-boundary configuration is studied using austenitic 21 Cr-4Ni-9Mn steel in the temperature range from 600 to 1000° C in air. Grain-boundary sliding is also examined on a steel with serrated grain boundaries during creep at 700° C. The improvement of creep-rupture strength by the strengthening of grain boundaries is observed at high temperatures above 600° C. The 1000 h rupture strength of steels with serrated grain boundaries is considerably higher than that of steels with straight grain boundaries, especially at 700 and 800° C. The strengthening by serrated grain boundaries is effective in retarding both the crack initiation and the crack propagation at 700° C, while it does not improve the life to crack initiation at 900° C. Grain-boundary sliding is considerably inhibited by the strengthening of grain boundaries at 700° C. The amount of it in steels with serrated grain boundaries is less than about one-third of that of steels with straight grain boundaries at the same creep strain. The stress dependence of grain-boundary sliding rate in the steady-state regime is also examined from the steels with these two types of grain-boundary configuration.

Grain-size dependence of fracture stress in anisotropic brittle solids

Abstract: The stress concentrations that occur at grain boundaries due to thermal expansion anisotropy and elastic stress concentration are discussed, and the stress intensity factor that results from these stresses is estimated. The procedure for the stress intensity factor calculation is based on the model in which a spherical crystal (grain) is forced into a cavity of equal size possessing annular or radial cracks emanating from the boundary. The stress intensity factor equation thus obtained is extended to include the effect of elastic stress concentration due to the presence of a cavity, and is subsequently used to predict the grain-size dependence of strength in anisotropic brittle ceramics. In assessing the degradation of strength with increasing grain size in non-cubic ceramics, it is shown that, in addition to grain size, the effect of pre-existing crack size must also be considered. Cubic ceramics, on the other hand, are known to exhibit no thermal expansion anisotropy and, based on the present model, their strength is predicted to be governed by the pre-existing flaw size, rather than the grain size.

Correlation of grain boundary defect structure with boundary orientation in Ba2YCu3O7−x

Abstract: Correlations between observed defect structures and the orientation of planes involved in grain boundary formation in single phase, nearly ideal density Ba2YCu3O7−x are presented. Three types of grain boundary structures are observed: (1) coherent boundaries with only a low‐energy network of dislocations at the interface, which are associated with boundaries not involving a basal plane face, (2) semicoherent boundaries surrounded by a 50–1500 A wide band of dislocation loops on the c‐axis side of the basal‐plane‐faced boundary, and (3) incoherent boundaries where severe local strain has produced substantial plastic deformation and generation of small voids, again associated with basal‐plane‐faced boundaries. These results are interpreted using a model for local stress based on the known highly anisotropic thermal contraction of the material during cooling from sintering temperatures.

Local texture changes associated with grain growth

Abstract: Recently it has been shown that grain growth is accompanied by macroscopic texture changes. Such changes will necessarily produce modifications in the proportions of ‘special’ grain boundaries and consequently may affect significantly the kinetics of grain growth and particularly anomalous growth. After new contributions from computer modelling processes are taken into account, it becomes clear that the onset of grain growth for a fixed driving force is a complex function of particle pinning, effects from solutes, surfaces and residual strains, grain-size distribution and texture-boundary mobility. The aim of this paper is to provide insight into the relation between grain texture-grain misorientation texture and anomalous grain growth in the alloy Nimonic PE16 where, for the particular heat-treatment conditions employed, particle pinning may be ignored. Macroscopic orientation measurements reveal only the overall texture; the present work considers microstructural regions of anomalous growth and texture measurements arecollated on a grain-specific basis (microtexture). From this grain-specific data the grain-misorientation textures (GMT) (i. e. the grain-boundary counterparts of an inverse pole figure) are also computed. The experimental results demonstrate that two types of microtexture may exist for different regions of anomalous growth within the same specimen. These differences are rationalized by noting that further grain growth-inducing heat treatments cause a change from the first texture to the second. Clear evidence is thus provided that there are micro textural changes associated with anomalous grain growth. The grain-boundary textural data show some deviation from a random distribution, particularly for boundaries between small grains (SS boundaries) as compared to boundaries between large and small grains (LS boundaries). These differences between the SS and ls groups extend to the proportions of geometrically special boundaries in each group, with the SS group containing a higher proportion of special (therefore higher than average mobility and lower energy) boundaries than the ls group. This trend was particularly apparent for boundaries that fell into the second texture group. The implication from these results is that in this case anomalous growth will tend to stagnate because the boundaries of small grains surrounding the large grain (SS boundaries) are on average of lower energy than the boundaries that border the large grain (LS boundaries). Finally, it is suggested, by analogy with primary recrystallization, that the definition of grain growth in terms of an energy-reduction criterion should be extended to encompass the sum of all available sources of energy minimization. Hence both grain-boundary textural rearrangements and grain-boundary migration are included.