Cyclic grain boundary migration during high temperature fatigue—I. Microstructural observations

Experiments were conducted on high purity lead at room temperature using reverse bending and torsion fatigue at low cyclic frequencies (⩽ 1.50 Hz). Metallographie observations after testing show that there is a one-to-one correspondence between the markings from grain boundary migration and the number and pattern of cyclic loading, and this correspondence is maintained up to s>100 cycles. Grain boundary sliding occurs in each cycle in addition to the migration, and this leads to the development of broad triple point folds. If the strain amplitude is maintained constant, it is shown that the average distance migrated in each cycle increases as the imposed frequency is decreased. The distance migrated is often exceptionally large in the first cycle of testing, and there is often a similar large initial displacement if the test is interrupted for periods of time from 1 to 24 h and then continued. For large grain sizes ( ≳ 2000 μm), the migration markings may lead to a zig-zag pattern where the individual segments lie fairly close to 45° to the stress axis. A model is described which accounts for the one-to-one correspondence and which is consistent with a fine structure observed within the migration markings.     

Precipitation at interphase boundaries

Three problems in precipitation at interphase boundaries are examined. 1) The classu argument as to the particular phase in which such a precipitate nucleates is shown to be irrelevant; except in a special situation, the critical nucleus must normally penetrate both phases forming the interphase boundary. 2) The relative penetrations into the two phases achieved during growth can be very different than those expected during nucleation; hence, deductions about the nucleation process based upon observations on growth morphologies can be quite misleading. 3) The observations of Honeycombe and others that the nucleation of carbides at austenite: ferrite boundaries occurs predominantly at the low energy, immobile broad faces of ledges rather than at the higher energy, mobile risers of ledges are accounted for theoretically on the basis of the high velocities of the risers preventing nucleation. Example calculations on a Ti-Ni alloy indicate that precipitation at the risers of ledges may become possible in substitutional systems, but only at lower homologous temperatures, and if the migration of these boundaries is still controlled by volume diffusion while nucleation is controlled by interfacial diffusion.     

An in situ field-ion microscope study of the recovery behavior of ion-irradiated tungsten and tungsten alloys

Five grades of tungsten specimens with different purity levels (resistivity ratios R of 5 × 10⁴, 1.5 × 10⁴, 50, 15 and 5) were irradiated in situ with 30 keV W⁺ ions to a dose of typically 5 × 10¹² ion cm⁻² at 18 K. Examination with a low-temperature field-ion microscope (FIM) showed the isochronal-annealing spectra of the specimens to result from a large self-interstitial atom (SIA) flux at ~ 38 K, followed by significant SIA flux from ~ 50 to 80 K and a small amount of additional recovery up to 120 K. The spectra for these five different R value specimens were essentially identical between 18 and 120 K. High-purity W specimens (R = 5 × 10⁴) doped with 5 × 10⁻⁵ to 1 × 10⁻⁴ atom fraction carbon showed only a small reduction in the amount of recovery observed for the long-range migration peak at 38 K. The isochronal recovery spectra for WRe alloy specimens (5 × 10⁻³ and 3 × 10⁻² atom fraction Re) were radically different from the isochronal recovery spectra of pure W specimens. For both alloys the recovery of the Stage I long-range migration peak at 38 K was strongly suppressed; for the 3 × 10⁻² atom fraction alloy, all recovery from 18 to 120 K was virtually eliminated. This result indicated that during the long-range migration substage at 38 K tightly-bound, immobile SIA-Re complexes were formed that suppressed the SIA-SIA reaction. However, this effect was only observed at these high Re atom concentrations. The lack of any significant differences for the annealing spectra of the five purity-levels of undoped tungsten and the appearance of impurity effects only in the extremely concentrated W alloys (i.e. 5 × 10⁻³ to 3 × 10⁻² atom fraction Re) indicated that the early Stage II recovery (45–120 K) observed in the FIM isochronal-annealing spectra of self-ion irradiated high-purity W was intrinsic in nature. Because of the highly inhomogeneous SIA distribution of the W⁺ ion damage, the SIA-SIA interaction during Stage I long-range migration at 38 K appeared to be the dominant trapping mechanism. The early Stage II SIA recovery was therefore attributed to the migration or dissolution of these SIA clusters.     

Cyclic deformation and fracture in Pb-Sn solid solution alloy

The cyclic deformation behavior of Pb-2 pct Sn solid solution alloy has been investigated. Results indicate that grain boundary migration and sliding occur in a systematic manner, giving rise to a series of migration markings on the surface of deformed specimens. Extensive cracking and cavitation takes place at migrating boundaries leaving an array of microcavities at high strain amplitudes. This feature was not observed at the low strain amplitudes investigated. Dynamic recrystallization and the evolution of a diamond grain configuration from an initial random distribution of grains were observed. These microstructural observations are discussed in relation to their influence on high temperature fatigue fracture. Formerly with IBM Thomas J. Watson Research Center, Yorktown Heights, NY 10598.     

Creep at very low rates

The creep rate in a land-based power station must be less than 10⁻¹¹ s⁻¹. At these low rates of deformation the transport of matter occurs by the migration of vacancies rather than by the glide of dislocations. A quantitative understanding of these diffusional processes is, therefore, important. First type of diffusional creep (Nabarro-Herring (N-H)): the sources and sinks of vacancies are grain boundaries. The vacancies may diffuse through the bulk of the grain or along the grain boundaries (Coble (C)). Second type (Harper-Dorn (H-D)): the vacancies diffuse from edge dislocations with their Burgers vectors parallel to the major tensile axis to those with Burgers vectors perpendicular to this axis. The coherence of the polycrystalline aggregate is maintained by sliding along the grain boundaries. The three mechanisms of vacancy migration, grain boundary sliding, and dislocation glide may all interact. The theories of N-H and C creep in pure metals are established and confirmed, but H-D creep and grain boundary sliding are less well understood. Practical engineering materials are usually strengthened by precipitates that accumulate on grain boundaries and slow down creep in complicated ways. This article is based on a presentation made in the workshop entitled “Mechanisms of Elevated Temperature Plasticity and Fracture,” which was held June 27–29, 2001, in Dan Diego, CA, concurrent with the 2001 Joint Applied Mechanics and Materials Summer Conference. The workshop was sponsored by Basic Energy Sciences of the United States Department of Energy.     

Thermotransport in grain boundaries

Thermotransport effects in grain boundaries in lead were studied by means of a temperature gradient (≤60 K cm⁻¹) lying parallel to the planes of the boundaries. The temperature gradient was observed to result in grain boundary sliding, migration and enhanced boundary diffusion. These effects are interpreted in terms of climb and/or glide motion of grain boundary dislocations induced by the gradient. The behaviour of boundaries in temperature gradients seems to differ from the behaviour reported in concentration gradients (diffusion induced boundary migration) suggesting different mechanisms to operate in both cases.     

The Effect of Solute Atoms on Aluminum Grain Boundary Sliding at Elevated Temperature

Grain boundary sliding (GBS) is an important deformation mechanism for elevated temperature forming processes. Molecular dynamics simulations are used to investigate the effect of solute atoms in near grain boundaries (GBs) on the sliding of Al bicrystals at 750 K (477 °C). The threshold stress for GBS is computed for a variety of GBs with different structures and energies. Without solute atoms, low-energy GBs tend to exhibit significantly less sliding than high-energy GBs. Simulation results show that elements which tend to phase segregate from Al, such as Si, can enhance GBS in high-energy GBs by weakening Al bonds and by increasing atomic mobility. In comparison, intermetallic forming elements, such as Mg, will form immobile Mg-Al clusters, decrease diffusivity, and inhibit GBS.     

Cavitation at grain and phase boundaries during superplastic flow of an aluminum bronze

Cavities have been observed to form at grain and phase boundaries under certain strain rate conditions during superplastic tensile deformation of a Cu-9.5 pct Al-4 pct Fe aluminum-bronze. The cavities form preferentially at α-β interfaces or triple junctions involving both phases. The process of cavitation is associated with grain boundary sliding and cavity nucleation probably occurs at points of stress concentration in the sliding interfaces. The ductility is not markedly impaired by the cavities because the high strain-rate sensitivity of the material inhibits the interlinkage of cavities at high strains. A range of strains and strain rates for superplastic forming processes has been determined at which the volume fraction of cavities present was tolerable.     

Nanometer grain boundary sliding in Cu: [011] symmetric tilt boundaries,misorientation dependence and anisotropy

Sliding on [011] symmetric tilt boundaries in Cu has been studied between 323 and 584 K, using an electron microscope technique and bicrystals of an aged Cu-1.05%Fe-0.45%Co alloy. The b.c.c. FeCo particles formed by aging on boundaries suppress the sliding so that it amounts to less than 1 nm. The sliding has been detected by observing a change in the direction of Moiré fringes formed by the matrix {111} and the particle {110}. Finely dispersed f.c.c. FeCo particles in grains abutting boundaries completely prevent crystal dislocations playing any role in boundary sliding. The sliding increases with increasing temperature and saturates to a value determined by the size and distribution of the boundary particles and the applied shear stress. A boundary with a higher energy can slide at a lower temperature, as low as 350 K. A curve showing the ease of sliding against the misorientation angle is similar, with cusps, to the energy vs misorientation angle curve. The sliding parallel to the tilting axis, [011], occurs at lower temperatures than that in the direction normal to the tilting axis.     

A theoretical study of grain growth in porous solids during sintering

The processes of grain boundary migration, pore drag and pore/boundary separation are described on the basis of the phenomenological equations for boundary migration and surface diffusion. Cylindrical pores on triple grain junctions are assumed to represent the open porosity during intermediate-stage sintering. It is found that cylindrical pores can hardly detach from migrating boundaries. Three-dimensional closed pores, however, which predominate during final stage sintering, can separate from migrating grain junctions. The separation process is modelled numerically and the conditions for separation are formulated. Analytical approximations for the pore mobility are shown to describe the numerical results well. They serve to establish effective mobilities of grain boundaries bearing pores in various configurations. Classical theories of grain coarsening are modified by using these effective mobilities. Mechanical constitutive models of sintering contain the grain size as an internal variable. The present analysis leads to an evolution equation for the average grain size, which depends on the volume fraction of the pores and on their configuration.     

Intergranular fracture in 4340-type steels: Effects of impurities and hydrogen

A study has been made of the conditions which lead to intergranular brittle fracture in 4340-type steels at an ultra high yield strength level (200 ksi, 380 MPa) in both an ambi-ent environment and gaseous hydrogen. By means of Charpy impact tests on commercial and high purity steels, and by Auger electron spectroscopy of fracture surfaces, it is con-cluded that one-step temper embrittlement (OSTE or “500°F embrittlement”), and low K intergranular cracking in gaseous hydrogen are primarily the result of segregation of P to prior austenite grain boundaries. Segregation of N may also contribute to OSTE. Most, if not all, segregation apparently occurs during austenitization, rather than during tem-pering. Elimination of impurity effects by use of a high purity NiCrMoC steel results in an increase inK _th for hydrogen-induced cracking by about a factor of five (to the range 130 to 140 MNm^-3/2). These observations are discussed in terms of our understanding of the mechanisms of OSTE and hydrogen-assisted cracking. H. C. FENG, now deceased, was formerly with Research Staff, LRSM, University of Pennsylvania. S. K. Banerji was formerly Post-Doctoral Fellow.     

Migration of liquid film and grain boundary in Mo-Ni induced by temperature change

Migration of liquid films and grain boundaries in liquid phase sintered 95Mo-5Ni (wt%) alloy occurs if the sintered specimens are heat-treated at temperatures above or below those of the initial sintering treatment. Behind the migrating boundaries, solid solutions in equilibrium at the heat-treatment temperature are deposited on the parent grains and the process is analogous to discontinuous precipitation and diffusion induced grain boundary migration (DIGM). The migration rate is varied by changing the sintering temperature while keeping the heat-treatment temperature constant; it increases parabolically with the expected composition difference between the initial and the final solid solutions. This result agrees with Hillert's proposal that the coherency strain energy in a diffusion layer in the retreating grain is the driving force. The observed migration rate and retardation effect due to the boundary curvature also agree in an order of magnitude with the coherency strain energy as the driving force. The results show that chemically induced migration of liquid films between the grains can be readily controlled experimentally and analyzed theoretically in terms of well known thermodynamic and kinetic laws.     

Atomistic simulations of grain boundary migration in copper

While the motion of twist boundaries can be readily studied by atomistic simulations with molecular dynamics (MD) under the action of an elastic driving force, the approach fails for tilt boundaries. This is due to the interaction of the elastic stress with the grain boundary (GB) structure, which causes plastic strain by GB sliding. A novel concept, the orientation correlated driving force, is introduced to circumvent this problem. It is shown that this concept can be successfully applied to the study of the migration of tilt boundaries. The migration behavior of several twist and tilt GBs was investigated. The transition from low-to high-angle boundaries can be captured, and a structural transition of tilt boundaries was found at high temperatures, which also affected the migration behavior. The results compare well with experimental results of the motion high-angle boundaries, but for low-angle boundaries, the agreement is poor. This article is based on a presentation made in the “Hillert Symposium on Thermodynamics & Kinetics of Migrating Interfaces in Steels and Other Complex Alloys,” December 2–3, 2004, organized by The Royal Institute of Technology in Stockholm, Sweden.     

Grain boundary migration with precipitation and dissolution of a liquid phase in MoNi alloy

When 85Mo15Ni (by weight) alloys prepared by liquid phase sintering at 1380°C are heat-treated at 1520°C, the grain boundaries and liquid films between the grains migrate, leaving behind them a new MoNi solid solution with Ni content higher than that in the initial solid formed during the liquid phase sintering treatment. The grain boundaries migrating during this discontinuous dissolution of the liquid phase have a flat shape. When the temperature change is reversed by first sintering at 1520°C and subsequently heat-treating at 1380°C, the grain boundaries and liquid films again migrate, with the composition change of the solid reversed. Liquid precipitates form at grain boundaries and migrate with them, their size and total volume increasing during the migration. The grain boundaries have a curved shape and their initial migration rate is considerably higher than that during the discontinuous dissolution. This contrasting migration behaviour between the discontinuous dissolution and precipitation is attributed to the coherency strain energy operating locally on both grain boundary segments and liquid droplets, and to the high mobility of fine liquid droplets.     

Yield point behavior in extruded aluminum rod

The microstructure of aluminum, extruded under industrial conditions at 250°C has been investigated in relation to the purity of the billet. Electron microscopy was used to observe the substructure and Kikuchi diffraction techniques were used to measure boundary angles and thus distinguish between recrystallization and repolygonization for samples of two different purities (99.7 pct Al and 99.99 pct Al) extruded under identical conditions. High tensile flow stresses of about 8000 to 9000 psi (55 to 62 MN/m²) were observed in specimens taken from the first sections of the high purity extrusion. These high strength levels were attributed to the presence of fine microstructure. When small recrystallized grains (0.5 to 2.0 μm diam) were present a yield drop was observed. This phenomenon is associated with the condition where nearly all the dislocations are likely to be immobile. The absence of a yield point in the 99.7 pct purity aluminum extruded under the same conditions as the 99.99 pct purity aluminum is due to the existence of fine subgrains instead of the fine recrystallized structure. A small yield point in 99.7 pct aluminum was induced by subsequent heat treatment resulting in the formation of small recrystallized grains of similar character to those in the higher purity extrusion.     

Diffusion induced grain boundary migration (DIGM) and liquid film migration (LFM) in an Al-2.07 wt% Cu alloy

Diffusion induced grain boundary migration (DIGM) and liquid film migration (LFM) in an Al-2.07 wt% Cu alloy during isothermal annealing after down and up quenching from an equilibrated state at 620°C in the α + liquid phase field has been studied by light and scanning electron microscopy. Down quenching to 520, 540 and 560°C resulted in grain boundaries migrating by DIGM against their curvature from one grain into another leaving behind alloyed zones. Down quenching to 590 and 605°C resulted in liquid films migrating against their curvature from one grain into another leaving behind alloyed zones. Boundaries were also observed to migrate at these temperatures. Up quenching to 630 and 640°C resulted in liquid films migrating from one grain into another leaving behind dealloyed zones. The migration distance, s, for both boundaries and liquid films was observed to decrease monotonically with annealing time (t) and to obey a power law relationship, s = Ktⁿ, with annealing time. The exponent n falls between 0.20 and 0.25. For a given annealing time the migration rates of liquid films by LFM were about twice as fast as those of grain boundaries by DIGM. With respect to driving force for boundary and liquid film migration, the coherency strain energy developed in the grain being consumed does not seem to be great enough to drive the reactions against the curvature observed.     

The effect of triple junction type on grain-boundary sliding and accomodation in aluminium tricrystals

The topological method of analysis of grain boundary triple junctions during deformation of polycrystals is proposed in our work. The method is based on the mutual orientation of all the boundaries forming junctions and of the loading axis. Comparative investigations of grain boundary sliding and accomodation behaviour of two types of aluminium tricrystals under high temperature creep conditions have been carried out from the point of view of the proposed classification of triple junctions. It is shown that the type of junction not only affects the kinetics of sliding but also determines the variety of the primary accomodation which proves to be constant when loading and temperature change.     

Cooperative phenomena at grain boundaries during superplastic flow

In situ observations in a scanning electron microscope (SEM) performed on different microstructural scales in Pb—62%Sn specimens, superplastically deformed in single shear and in simple tension, showed sliding of grain groups [cooperative grain boundary sliding (CGBS)], rotation of grain groups (cooperative grain rotation) and cooperative grain boundary migration (correlated migration of sliding grain boundaries). The observed macroscopic pattern of the CGBS surfaces is consistent with predictions of slip-lines field theory. The progress of the sliding of grain blocks at the scale of grain groups can be modeled in terms of cellular dislocations. The micromechanism for such sliding and the migration of sliding grain boundaries at the scale of the individual interface might be interpreted from the viewpoint of grain boundary dislocations.     

A metallographic study of diffusion-induced grain boundary migration in the Fe-Zn system

Diffusion-induced grain boundary migration has been studied by zincification of pure iron. The temperature dependence of the rate was measured at 460–650°C A change in morphology was observed between 460 and 650°C and may explain the fact that the phenomenon changed character at higher temperatures. The temperature dependence of v/D^hδ may be decisive, v being the rate of migration. D^h the grain boundary diffusivity and δ the thickness of grain boundaries. The concentration behind the migrating boundaries was studied as function of the distance from the surface by microprobe measurements and allowed the grain boundary diffusivity to be evaluated as function of temperature.Fine zinc-rich grains of ferrite sometimes nucleate on the surface a form and complete fine-grained surface layer. After a while it disappears by abnormal grain growth but the increased zinc content remains.     

Yield point behavior in extruded aluminum rod

The microstructure of aluminum, extruded under industrial conditions at 250°C has been investigated in relation to the purity of the billet. Electron microscopy was used to observe the substructure and Kikuchi diffraction techniques were used to measure boundary angles and thus distinguish between recrystallization and repolygonization for samples of two different purities (99.7 pct Al and 99.99 pct Al) extruded under identical conditions. High tensile flow stresses of about 8000 to 9000 psi (55 to 62 MN/m²) were observed in specimens taken from the first sections of the high purity extrusion. These high strength levels were attributed to the presence of fine microstructure. When small recrystallized grains (0.5 to 2.0 μm diam) were present a yield drop was observed. This phenomenon is associated with the condition where nearly all the dislocations are likely to be immobile. The absence of a yield point in the 99.7 pct purity aluminum extruded under the same conditions as the 99.99 pct purity aluminum is due to the existence of fine subgrains instead of the fine recrystallized structure. A small yield point in 99.7 pct aluminum was induced by subsequent heat treatment resulting in the formation of small recrystallized grains of similar character to those in the higher purity extrusion. Formerly Research Assistant, Department of Mechanical Engineering, University of Waterloo, Waterloo, Ontario.