Showing papers on "Grain boundary strengthening published in 1971"

On grain boundary sliding and diffusional creep

Abstract: The problem of sliding at a nonplanar grain boundary is considered in detail. The stress field, and sliding displacement and velocity can be calculated at a boundary with a shape which is periodic in the sliding direction (a wavy or stepped grain boundary): a) when deformation within the crystals which meet at the boundary is purely elastic, b) when diffusional flow of matter from point to point on the boundary is permitted. The results give solutions to the following problems. 1) How much sliding occurs in a polycrystal when neither diffusive flow nor dislocation motion is possible? 2) What is the sliding rate at a wavy or stepped grain boundary when diffusional flow of matter occurs? 3) What is the rate of diffusional creep in a polycrystal in which grain boundaries slide? 4) How is this creep rate affected by grain shape, and grain boundary migration? 5) How does an array of discrete particles influence the sliding rate at a grain boundary and the diffusional creep rate of a polycrystal? The results are compared with published solutions to some of these problems.

Temper embrittlement in a Ni-Cr steel containing phosphorus as impurity

Abstract: Phosphorus segregates to prior austenite grain boundaries in ferrite during embrittling treatments (below 1000°F) and contributes to temper embrittlement of a Ni-Cr-C-P steel. The phosphorus is confined to a width of 5 to 10A, at a very high concentration of approximately 5 wt pct (0.1 monolayer) in the vicinity of the grain boundaries; segregation of nickel is also observed to a lesser degree and spread over a distance somewhat larger than 50a. Nickel segregation to the grain boundaries appears to be of minor consequence either in terms of its own effect or in terms of its interaction with phosphorus. Fracture appearance transition temperatures of the various samples are directly correlatable to the phosporous content at grain boundaries through a linear plot. Deembrittlement of the steel is found to consist of reversing the boundary segregation and driving the phosphorus and nickel away from the prior austenite boundaries. The amount of phosphorus segregated at grain boundaries of the embrittled Ni-Cr-C-P steel is found to be compatible with an equilibrium segregation model.

Effects of Grain Growth on the Distribution of Nb in BaTiO3 Ceramics

Abstract: Measurements of Nb diffusion into large- and small-grained BaTiO3 disks show a high ratio of grain boundary to bulk diffusivity. Well defined X-ray diffraction lines are found in Nb-doped BaTiO3 only when significant grain growth occurs during sintering. When grain growth of a 0.65 μm grain size powder is limited at ∼1 μm, excess line broadening results. This is attributed to the simultaneous presence of Nb-free and Nb-rich regions. Because of its low bulk diffusivity little Nb penetrates into the original BaTiO3 grain cores, and a solid solution forms only in the regions of recrystallization. When grain growth is limited, the “sintering reaction” results in a non-homogeneous system; when appreciable grain growth occurs, most of the original grain cores are eliminated and an essentially uniform system is obtained.

Grain boundary dislocations: Their effect on vacancies and sliding

Abstract: When a grain interior dislocation meets a grain boundary it can move along the latter by a mixture of climb and glide. Vacancies are then cmitted or absorbed and sliding is caused in amounts depending on the orientation of the dislocation's Burgers vector, which in turn depends on the grain orientation. The consequence is that emission and absorption of vacancies by grain boundaries, and also sliding, depend on the orientation with respect to the applied stress of the grains as well as of the grain boundaries.

Grain Boundary Segregation of Carbon in Iron

Abstract: The distribution of C14 in a series of a-phase Fe-C alloys was studied autoradiographically. Excess carbon was present near grain boundaries in specimens equilibrated at 500‡ and 600‡C; the amount of carbon near the grain boundaries was found to decrease as the equilibration temperature increased. Electron microscopy of the specimens subsequent to the autoradiography showed that no grain boundary precipitates were present. The results are compared with the theories of nonequilibrium vacancy-aided segregation and Gibbsian adsorption, and also with McLean's model of grain boundary segregation. It is shown that the technique measures directly the quantity of thermodynamic interest, the total excess solute per unit area of grain boundary.

Discontinuous precipitation of M23C6 in austenitic steels

Abstract: Grain boundary precipitation of M23C6 has been studied in a 20% Cr-35% Ni stainless steel with two grain sizes during creep deformation at 700°C as well as during an ordinary ageing treatment at 700°C. A special etching technique was applied which showed how the grain boundary precipitation gave rise to depletion of alloying elements in a zone of uniform thickness, independent of the carbide distribution, and with a gradual decrease of the depletion towards the grain interior. At some places the carbide precipitation and grain boundary migration co-operated and in these cases there was a sharp change in alloying content across the grain boundary. This process was more frequent in creep tested samples and the degree of co-operation was larger in the coarse-grained material where even a few cases of lamellar, eutectic-like precipitation was observed. Such a grain size dependence is expected theoretically and is caused by the large difference in diffusivity between carbon and the other alloying elements. It is proposed that the various degrees of co-operation between carbide precipitation and grain boundary migration are all examples of discontinuous precipitation. The various proposed mechanisms for grain boundary migration during discontinuous precipitation are discussed on the basis of the present results.

Altering the time cycle of heat treatment by controlling grain boundary and subboundary structure

Abstract: The spectrum of grain and subgrain boundary structures in the matrix phase can frequently be changed by processes such as texturing and polygonization. These changes can markedly alter the morphology, kinetics of nucleation and/or growth and even the identity of the precipitate formed during subsequent heat treatment. The numerous effects of grain and subgrain boundary structure upon precipitation processes are summarized and discussed. When the boundary structure is of the dislocation type many of the noncrystallographic effects are qualitatively explicable in terms of Cahn’s theory of nucleation at isolated dislocations or by simple extensions of this theory; but a more detailed study of the strain energy interactions between a plate-like nucleus and a dislocation boundary is needed for a quantitative understanding of these effects. Some anomalies which are presently unexplained are noted. At general high-angle grain boundaries, on the other hand, the effects of boundary structure are simpler and better understood. Grain boundary allotriomorphs are normally the only morphology present. A question remains about the detailed mechanism of nucleation, but the relative roles of volume diffusion directly to allotriomorphs and of volume diffusion aided by interfacial diffusion in the growth kinetics of allotriomorphs are becoming increasingly well established.

On the generation and motion of grain boundary dislocations in steel

Abstract: Electron microscopy of a 0.2% C Nb-steel slightly deformed at various temperatures has given additional evidence for generation and conservative movement of dislocations in grain boundaries. These phenomena, which seem to occur preferentially in the range 250°C-300°C, strongly suggest that grain boundary sliding in metals may receive an important contribution from the conservative movement of grain boundary dislocations, as has been put forward by Gleiter, Hornbogen and Baro (1968) and Gleiter (1970), even though these processes are probably confined to limited grain boundary areas.

Altering the time cycle of heat treatment by preannealing prior to grain growth

Abstract: Preannealing is found to alter the time cycle of grain growth in lead and lead-tin alloys by modifying the kinetics of grain boundary migration. Aging at a low temperature prior to annealing at a high temperature shows the influence of residual strains in the early stage of grain growth. A model is proposed to explain the deviation of the growth kinetics from the theoretical t1/2 law when the samples are annealed immediately after primary recrystallization in terms of a superposition of a strain-induced and a surface tension-induced grain boundary migration. Preannealing by using multiple anneals at the same temperature enables simulation of many different alloys by changing the amount of impurities segregated at grain boundaries. In zone refined material, a good agreement between the Cahn theory and the experimental results is observed; in lead-tin alloys, grain growth kinetics are understood in terms of a superposition of the influences of the solute and of other lattice defects.

Grain boundary dislocation spirals in high-angle (001) twist boundaries in gold

Abstract: Bardeen–Herring type grain boundary dislocation source (or sink) spirals were found in high-angle (001) twist boundaries in gold by transmission electron microscopy. The possible roles that these configurations could play in supporting grain boundary point-defect source (or sink) action and boundary migration were pointed out.

Effect of grain size on serrated yielding in an Al-Mg-Si alloy

Abstract: The effect of grain size on the characteristics of serrated yielding in an Al-Mg-Si alloy has been studied. The critical strain for the onset of serrated yielding was found to be independent of grain size in quenched specimens. With increasing grain size, a transition from the periodic locking form of serrated yielding to irregular serrated yielding was found to occur. The amplitude and strain period of the periodic locking serrations decreased with increasing grain size.

The initiation of intergranular failure in inconel-600

Abstract: The nature of slip interaction with the grain boundaries was examined on the polished surface of Inconel-600 pulled in tension at 370°C and at the initial strain rate of 1.65 × 10−6 sec−1. It was shown that the grain boundaries could not always accommodate slip taking place at the boundary regions. This led to the separation of grains at the boundary. Depending on the detailed morphology of the carbides, these intergranular cracks could form at axial strains as low as 10 pet. Since the cracking takes place in the inert atmosphere, it is concluded that the grain boundaries are inherently embrittled by the formation of carbides and/or by solute segregation. There was no evidence of grain boundary sliding, but when carbides were absent highly localized slip in the boundary region gave the appearance of grain boundary offsets. The implications of these observations to stress corrosion cracking are discussed.

Effect of Redundant Microstructure on Electromigration‐Induced Failure

Abstract: Aluminum films with grain boundaries staggered throughout the film thickness, and hence with nonoverlapping sites for electromigration‐induced failure, were prepared by vacuum deposition in layered or extremely fine‐grained form. Mean lifetimes before the occurrence of electrical opens were an order of magnitude larger than those for control films having non‐redundant grain structure. Heat‐treatment effects which reduced lifetimes to values comparable to control lifetimes were interpreted in terms of either a lifetime minimum for grain size ∼ 0.6 μ or disruption of the layered structure.

Influence of Microstructure on the Mechanical Properties and Stress Corrosion Susceptibility of 7075 Aluminum Alloy.

Abstract: : Matrix and grain boundary features were systematically varied, using appropriate heat treatment, to evaluate their significance in changing the mechanical and stress corrosion properties of 7075 aluminum alloy. Maximum strength is associated with a Guinier-Preston zone matrix. The precipitate-free-zone adjacent to high angle grain boundaries was found to have only a slight effect on yield and tensile strength and a greater influence on hardness. Stress corrosion susceptibility was studied in a chloride ion environment over a 0.7 - 3.5 pH range. Differences in pH-dependent susceptibility associated exclusively with grain boundary structure for material of highest strength were observed. Grain boundary precipitate spacing (inversely related to precipitate density) was found to be of primary importance to susceptibility. The relative importance of crack initiation to the over-all stress corrosion process was also found to be dependent on grain boundary microstructure. These results indicate that improved properties for Al-Mg-Zn type alloys can be attained by a desirable combination of matrix and grain boundary structure. (Author)

Kinetics and mechanisms of pore growth in MgO

Abstract: Isothermal pore growth occurs in MgO when polycrystalline compacts are annealed in air for long periods of time at temperatures between 1450 and 1650°C. Fractographic examination of the microstructures demonstrates that the pores are trapped along the grain boundaries and at the intersections. The growth occurs as a result of pore dragging by grain boundary in combination with transport of atoms by surface diffusion. The contribution from lattice diffusion and vapour transport becomes increasingly important as the temperature is raised. Finally, as the pores grow in size, they inhibit the normal grain growth in MgO.

The influence of applied stress and stress sense on grain boundary precipitate morphology in a nickel-base superalloy during creep

Abstract: Creep induced instability of strengthening precipitates at grain boundaries is of general concern in the applications of many high temperature alloys. Having shown that the general validity of the existing mechanism for such an instability in nickel-base superalloys may be considered suspect, this paper reports and discusses the effects of both tensile and compressive creep on γ′ grain boundary precipitate morphology in an alloy consisting of γ′ (Ni3Al) precipitates in a γ (nickel solid solution) matrix. We find that the uniform distribution of γ′ precipitates is altered by the application of uniaxial creep stress, with the stress-induced precipitate morphology depending strongly on stress sense. Tensile creep results in the dissolution of γ′ precipitates at grain boundaries aligned more or less transverse to the stress axis, with an accompanying increase in volume fraction of γ′ precipitates at grain boundaries oriented parallel to, or almost parallel to the stress axis. In contrast, the reverse change in morphology occurs during compressive creep. The observed morphology changes and their dependence on stress sense are shown to be consistent with the flow of chromium atoms from grain boundaries that are under normal compression towards grain boundaries that are under normal tension. The results conclusively demonstrate that Herring-Nabarro type diffusion in multiphase, polycrystalline alloys can cause chemical changes in grain boundary regions which, in the extreme, result in phase changes at grain boundaries. The results and proposed mechanism are discussed in terms of the findings of other investigations.