Showing papers on "Grain boundary strengthening published in 1974"

The annealing of dislocations in high-angle grain boundaries

Abstract: Hot-stage electron microscopy is used to study the annealing behaviour of lattice dislocations in high-angle grain boundaries in Ni, Ni 5% Al, austenitic stainless steel and Al 0·9 % Mg. The observations indicate that individual dislocations are incorporated in a boundary by a spreading of their cores or a dissociation into partial dislocations with small Burgers vectors. This process results in a rearrangement of the grain boundary structure by grain boundary diffusion. It is shown that this process enables a random grain boundary to incorporate more than 1012 dislocations cm−2 at the recrystallization temperature.

Mechanism of steady-state grain growth in aluminum

Abstract: Grain growth, defined as the increase in volume of the average grain, is found, in its steadystate, to be directly proportional to the time of isothermal annealing. During steady-state grain growth the grain corners are found all to be quadruple points, the grain edges all triple lines and the ratio of corners to faces to edges to be 6:7:12. The rate constant for steady-state grain growth is shown to be calculable from first principles and from properties that can be measured independently of the growth observation. It is the product of four individual constants, namely: 1) a dimensionless topological constant ⊝ that is characteristic of steady-state grain growth in any material, 2) the mobility ώ of the average grain boundary in the specific material, 3) the surface tension y of the average grain boundary in the specific material and 4) a dimensionless structural constant σ which expresses the curvature of surface of the grain boundary in the array of grain forms obtaining in the specific specimen of the material and which can be determined metallographically. The topological changes that constitute steady-state growth are shown to exist as a logical sequence of simple events.

Effects of austenitizing temperature and austenite grain size on the formation of athermal martensite in an iron-nickel and an iron-nickel-carbon alloy

Abstract: The effects of austenitizing conditions on the kinetics at the start of martensite formation in Fe-31Ni and Fe-31 Ni-0.28C alloys have been studied using electrical-resistance measurements during cooling of the specimens to follow the course of the transformation. The primary object of the study was to decide whether or not a change in austenitizing temperature, in the absence of a change in austenite grain size, has any effect on the Ms temperature or the burst characteristics of athermal martensite. It is concluded that it does not, suggesting that the potential nuclei (embryos) of martensite are mechanically stable crystal defects. Another interesting observation is that when the austenite grain size is small, the Mb temperature increases with increasing grain size and the burst is always small. When the austenite grains are coarse, the Mb temperature is independent of the grain size and the burst is large. It is suggested that this phenomenon is a result of the elastic shear stress concentration being related to the size of the first martensite plate and, in turn, to the size of the austenite grain.

Cohesion margin of copper

Abstract: Measurements on the mechanical properties of copper-bismuth alloys show that the only effect of bismuth within the solid solution range is to reduce the true tensile strength of the alloys—there is no detectable effect on plastic behaviour or on the elastic limit. Intergranular fracture occurs by the formation of grain boundary crack nuclei, these being seen in alloys containing as little as 0·002 wt.-% Bi. In addition, by measuring the effect of bismuth on surface and grain boundary energies of copper, it is shown that bismuth is present at the grain boundaries at monolayer levels, as true Gibbs segregation. It appears that the brittleness is due simply to reduced grain boundary cohesion. The measured relative cohesion of the copper-bismuth alloy is approximately half that of pure copper: such a reduction in grain boundary cohesion therefore converts ductile copper into a brittle metal, and thus defines a margin of cohesion. Reduced grain boundary cohesion favours the nucleation of cracks where ...

Grain boundary diffusion and growth of intermetallic layers: Nb3Sn

Abstract: A study of the formation of Nb3Sn layers at the junction of pure Nb and a Cu–Sn alloy is presented, and the effect of grain boundary diffusion on the kinetics of the formation of the layers is assessed. Data at 700–800 °C imply that the layer thickness increases with a t0.35 time dependence. This deviation from the usual t1/2 law for a process whose rate is limited by volume diffusion through the growing layer had previously been explained using analytical expressions for grain boundary diffusion into a semi‐infinite crystal. A numerical analysis of grain boundary diffusion with geometry appropriate to an intermetallic layer is presented, and the results imply that a fixed array of grain boundaries in the layer cannot cause appreciable deviation from a t1/2 law for most systems. The flux to the growth interface required to form the layer normally causes the diffusion processes in the layer to approach steady‐state conditions, which, in the case of fixed boundaries, precludes a large deviation from the t1/...

Calculated grain size-dependent vacancy supersaturation and its effect on void formation

Abstract: In order to study the effect of grain size on void formation during high-energy electron irradiations, the steady-state point defect concentration and vacancy supersaturation profiles have been calculated for three-dimensional spherical grains up to three microns in size. In the calculations of vacancy supersaturation as a function of grain size, the effects of internal sink density and the dislocation preference for interstitial attraction have been included. The computations show that the level of vacancy supersaturation achieved in a grain decreases with decreasing grain size. The grain size dependence of the maximum vacancy supersaturation in the centre of the grains is found to be very similar to the grain size dependence of the maximum void number density and void volume swelling measured in the central regions of austenitic stainless steel grains. This agreement reinforces the interpretation that the grain size effect is due primarily to the depletion of point defects from the grain interi...

Physics of Electromigration

Abstract: This survey paper on electromigration describes factors which govern the rate of electromigration and therefore relate to the lifetime of conductors stressed at high current density. These include the type of metal conductor, the conductor cross sectional area, lattice, grain boundary and surface diffusion effects, the addition of alloying elements, temperature and current density as well as the thermal conductivity of the substrate. The effect of gradients in temperature, current density, conductor composition and grain size on conductor lifetime are also discussed.

Theory of high temperature intercrystalline fracture under static or fatigue loads, with or without irradiation damage

Abstract: Hull and Rimmer’s theory of high temperature creep fracture by grain boundary void growth and Skelton’s theory of high temperature fatigue fracture have been extended The present theory includes the effect on void growth of volume diffusion and of vacancy generation by irradiation damage or plastic deformation within the grain interiors The growth rate of cylindrical as well as spherical voids is calculated We find that the presence of volume diffusion or of vacancy generation within the grain does not increase very much the growth rate of spherical voids but under certain conditions the growth rate of cylindrical voids is increased markedly Unlike Skelton, we find that vacancy production within the grains does not decrease the critical radius for the growth of a grain boundary void in push-pull fatigue (Likewise intra-granular vacancy production does not decrease the critical radius of grain boundary voids in creep) Since in push-pull fatigue void growth is observed in the case of radii smaller than the critical value it is concluded that the stress concentrations that are produced by grain boundary sliding (according to the theory of Raj, Ashby, and Gifkins) are responsible for this growth Whether voids aided by stress concentrations can continue to grow beyond a certain size depends critically upon the “roughness” of the grain boundary It is strongly recommended that grain boundary internal friction peak studies be carried out in connection with void growth and high temperature fracture experiments In particular, grain boundary voids are more likely to grow in push-pull fatigue (or in creep) if the value of the grain boundary sliding activation energy is close to that of volume self diffusion rather than if the sliding activation energy is close to the grain boundary self diffusion activation energy

Stress corrosion crack velocity and grain boundary precipitates in an Al-Zn-Mg alloy

Abstract: The fracture kinetics of Al-5.5 Zn-2.5 Mg alloys submersed in 3 pct NaCl-H2O solutions were varied by heat treatment. The steady state velocity, on a plot of velocity vs stress intensity, was compared with microstructure and it was found to be inversely proportional to the volume of MgZn2 in the grain boundary. This behavior suggests that grain boundary precipitates can act as sacrificial anodes to retard intergranular stress corrosion cracking.

Faceting of high-angle grain boundaries in the coincidence lattice

Abstract: Faceting of high angle grain boundaries in SIGMA = 3 and SIGMA = 5 coincidence lattices of the fcc structure was studied using thinfilm bicrystal specimens of controlled geometry. A number of relatively high index boundaries in the coincidence lattices (containing relatively low planar densities of coincidence sites) was found to break up into low energy facets corresponding to low index planes of the coincidence lattices (containing high densities of coincidence lattice sites). These results are consistent with a general expectation that the grain boundary energy decreases as the planar density of coincidence sites increases, i.e., the two-dimensional periodicity of the boundary becomes shorter, and long-ranged distortions are reduced. (auth)

Analysis of the Grain Size Dependence of the Yield Stress in Copper-Aluminum and Copper-Nickel Alloys

Abstract: Grain size dependence of the yield stress in polycrystalline copper-aluminum and copper-nickel alloys is measured at 77,194 and 293° K. It is found that the Hall-Petch relation is valid for these materials. The Petch slope ky is nearly independent of the temperature, but increases with increasing solute concentration. In copper-aluminum alloys the increase in ky is nearly proportional to the square root of the solute concentration, while in copper-nickel alloys it is proportional to the concentration itself. The concentration dependence of ky in Cu-Al alloys is considerably larger than that in Cu-Ni alloys. None of previously proposed theories on the Petch slope can explain these results. The experimental results will be compared with the calculation in the following paper.

The effect of austenite grain size on microcracking in martensite of an Fe-1.22C alloy

Abstract: Austenitic grain sizes of ASTM No. 9 and coarser were produced in an Fe-1.22 pct C alloy austenitized by immersion in molten lead at 1640†F (893°C), a temperature just above theA cm for this alloy, for periods between 20 s and 1 h. Microcracking sensitivity,Sv, measured as crack area/unit volume martensite, was determined as a function of grain size in brine quenched specimens. Two locations of microcracks were observed in this investigation: 1) intragranular, resulting from the impingement of one martensite plate with another, and 2) grain boundary or intergranular resulting from the impingement of martensite plates at prior austenite grain boundaries. Intragranular microcracking sensitivity, the subject of previous investigations, increased and became the dominant type of cracking with increasing grain size, and reached a constant level for grain sizes of ASTM No. 4.5 and coarser. Total microcracking sensitivity, consisting of both intragranular and grain boundary microcracks, also increased with increasing grain size, then decreased to approach the intragranular value for grain sizes coarser than ASTM No. 3.5. On the other end of the scale, grain boundary microcracking made up a much larger proportion of the total microcracking in the fine grained specimens.

The effect of an air environment on the creep and rupture behavior of a nickel-base high temperature alloy

Abstract: The behavior of wrought and cast polycrystalline Udimet 700 tested in static tension at 1200 K (1700†F) has been determined. An air environment decreases rupture life and ductility, except in very coarse-grained cast specimens, because of premature failure by stress-assisted grain boundary oxidation and cracking. In very coarse-grained cast specimens greater life and ductility are found in air than in vacuum, presumably due to the paucity of transverse grain boundaries and to some type of surface hardening effect. Wrought specimens exhibit greater grain boundary sliding, hence more cracking, shorter life, and lower ductility, than cast specimens in any environment. This is attributed to differences in grain boundary topography.

The relationship between intergranular cavitation and superplastic flow in an industrial copper base alloy

Abstract: Intergranular cavitation has been observed during the superplastic deformation of a fine grain sized (1 μm) Cu-2.8% Al-1.8% Si-0.4% Co alloy when tested at temperatures ≥500° C. High voltage electron microscopy revealed that the cavities could be nucleated at twin boundary/grain boundary intersections. The maximum elongation occurs at a higher temperature than that of the maximum strain-rate sensitivity and this is explained in terms of grain-boundary migration, at the higher temperature, which restricts the cavitation process. This explanation was put forward on the basis of texture analysis which was used to study the deformation characteristics at the temperatures of maximum elongation and strain-rate sensitivity. The final fracture mode is shown to change with test temperature: (i) at 400° C no cavitation occurs and fracture is by ductile rupture, (ii) at 500 to 550° C cavitation occurs and fracture is by the interlinkage of voids by an intergranular void sheet (IVS) mechanism and (iii) at 800° C grain growth occurs and fracture occurs by the propagation and interlinkage of grain-boundary cracks along the grain boundaries.

Grain boundary dislocations and the mechanism of sliding in symmetrical [011] tilt Al bicrystals

Abstract: SUMMARY Observations of grain boundary dislocations in [011] tilt bicrystals of aluminium after high temperature shear tests have led to a concept for grain boundary sliding that involves extrinsic dislocations moving through an array of structural dislocations. Each extrinsic dislocation consists of a group of structural dislocations with closer than normal network spacing caused by the presence of an extra dislocation with the same Burgers vector. The extrinsic dislocation exhibits a long range strain field and can move individually through the array of structural dislocations. The movement involves displacements in the structural network analogous to atom displacements during crystal slip. The width of the extrinsic dislocation on the grain boundary plane is dependent on the degree of accommodation of the extra dislocation by the structural array and this appears to increase with increasing density of structural dislocations.

High-temperature deformation of polycrystalline magnesium oxide

Abstract: The mechanical behaviour of two types of polycrystalline MgO was studied in compression over the temperature range 1200–1400°C. Differences in behaviour between hot-pressed specimens and specimens which were hot-pressed, then annealed, were attributed to differences in the structure of the grain boundary regions. Results indicate that creep in the hot-pressed specimens was controlled by a grain boundary sliding mechanism characterized by the movement of dislocations along grain boundaries, with a stress exponent n = l·8 and an apparent activation energy for the process of ∼70 kcal/mol. Creep in the specimens which were also annealed was found to be controlled by a dislocation climb mechanism within the grains, with a stress exponent n = 7 and an apparent activation energy of ∼110 kcal/mol.

Effect of prior austenitic grain boundary composition on temper brittleness in a Ni-Cr-Sb steel

Abstract: Grain boundary segregation during temper embrittlement of an Sb-containing, Ni-Cr steel has been examined both by Auger electron analysis and by chemical analysis by neutron activation of residues of surface layers dissolved by etching intercrystalline fracture surfaces. No grain boundary segregation of either alloying additions or impurities was detected during austenitization or tempering. Redistribution of Cr, Ni, and Sb between carbide and ferrite was observed during tempering, but no grain boundary segregation was noted. Both Ni and Sb were observed to segregate to the boundaries during embrittling. The segregated Sb was shown to be uniformly distributed along the prior austenitic grain boundaries and to control the ductile brittle transition temperature of the alloy studied. Ni segregating to the prior austenitic boundaries during embrittling was shown to be localized in a phase other than the ferritic portions of the boundaries. A possible location was shown to be the ferritecarbide interfaces in the grain boundaries. Weakening of these normally tenacious carbide and ferrite interfaces could account for the change in mode of brittle failure from transcrystalline cleavage to intercrystalline along the prior austenitic grain boundaries that is observed in temper brittle steels.

Onset of serrated yielding in Mg-10Ag alloy

Abstract: The onset of serrated yielding in Mg-10Ag alloy is found to be in agreement with the existing strain-ageing theories; however, the static strain-ageing model seems to be a better approach than the dynamic strain-ageing model. There is little dependence of yield and flow stress on grain size over most of the temperature range. The critical strain to serrations is also effectively independent of grain size.