Effect of grain size on cavitation in superplastic Zn-Al eutectoid

The effect of initial grain size on cavitation during superplastic deformation in two commercially available Zn-Al eutectoid alloys has been studied using metallography and precision density measurements. Cavitation was found to be minimal for initial grain sizes below about 5 m. Superplastic deformation caused grain growth in both alloys under all testing conditions, and when the grain size exceeded about 8 m a significant level of cavitation was produced. The grain size and extent of cavitation increased with increasing strain along the specimen gauge length, with cavities concentrated in regions adjacent to the fracture tip. Although never very large, the cross-sectional area at fracture increased with increasing levels of cavitation. It was concluded that cavitation in Zn-Al eutectoid results from incomplete accommodation of grain-boundary sliding when excessive grain growth leads to restricted grain-boundary diffusion and/or to restricted grain-boundary migration.     

Continuous creep cavity nucleation by stochastic grain-boundary sliding

Creep cavitation in metals and ceramics is generally considered to occur by the nucleation, growth, and coalescence of grain-boundary cavities. By considering grain-boundary slidings as the process driving force, a stochastic model is proposed for continuous cavity nucleation in metals and ceramics subjected to creep loading. The nucleation rate is shown to be directly proportional to the number of grain-boundary sliding events. The dependence of the number of cavities on grain boundary sliding displacement, creep strain, and time are established and compared with available experimental data of alumina, copper, and copper alloys. This comparison supports the contention that creep cavity nucleation in metals and ceramics does originate from stochastic grain-boundary sliding.     

Transient cavity growth in ceramics under compression

The transient cavity growth behaviour of liquid phase-sintered ceramics subject to compressive loads is examined. Three possible sources of transient behaviour are suggested, and their ranges of applicability evaluated. By considering the values of the characteristic time for individual transient modes, it has been determined that transient cavity growth in ceramics probably originates from transient grain-boundary sliding. Assuming that the creep-induced cavities nucleate and grow on grain boundaries that are parallel to the loading axis, a transient cavity growth model is developed on the basis that the local stress which drives cavity growth is induced by transient sliding of adjacent grain boundaries. Results of the proposed model are compared with small-angle neutron scattering measurements of a hot-pressed silicon carbide and a liquid phase-sintered alumina, both of which contain a continuous, amorphous grain-boundary phase. The different cavity growth behaviours observed in these ceramics are discussed in conjunction with transient grain-boundary sliding.     

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

Intergranular cavitation has been observed during the superplastic deformation of a fine grain sized (1 m) 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.     

On the formation of the diamond grain configuration during high temperature creep and fatigue

A study has been made of the influence of test variables on the formation of the diamond grain configuration during high temperature creep and fatigue deformation of a wide variety of metals. The proposed mechanisms for the formation of this interesting grain morphology are reviewed. It is concluded that the diamond grain configuration arises from a balance between grain-boundary sliding, grain-boundary mobility, intragranular deformation and defect imbalance across the grain boundaries and that it tends to be stabilized by intergranular cavitation. While the phenomenon occurs during high temperature fatigue in a variety of metals irrespective of their crystal structure, during creep it has been observed only in to h c p metals. It is surmised that the occurrence of the diamond array of grain boundaries during creep deformation in h c p metals is aided by the limited number of slip systems which leads to high defect imbalances in adjacent grains and consequently high driving forces for grain-boundary migration. On the basis of quantitative metallography involving measurements of the number of edges per grain section, the number of grains meeting at vertices, angular distribution histograms and grain-boundary lengths in different angular orientations with respect to the stress axis in "annealed" and "diamond" microstructures, it is concluded that the shape of the "diamond" grain is essentially the same as that of the "annealed" grain but in a distorted form.     

Electrical properties of the grain boundaries of oxygen ion conductors: Acceptor-doped zirconia and ceria

This article reviews the current understanding of the electrical properties of the grain boundaries of acceptor-doped zirconia and ceria, however, with an emphasis on the grain-boundary defect structure. From an electrical point of view, a grain boundary consists of a grain-boundary core and two adjacent space-charge layers. The grain-boundary cores of acceptor-doped zirconia and ceria are positively charged, probably owing to the oxygen vacancy enrichment there. Oxygen vacancies are therefore depleted in the space-charge layer. The grain-boundary conductivities of acceptor-doped zirconia and ceria are at least two orders of magnitude lower than the corresponding bulk values, depending on temperature and dopant level. Such a phenomenon is due to the facts: (1) that oxygen vacancies are severely depleted in the space-charge layer, and (2) that the grain-boundary impurity phase blocks the ionic transport across the grain boundaries by decreasing the conduction path width and constricting current lines. In materials of high purity, the effect of the space-charge depletion layer is dominant; however, in materials of normal purity, the effect of the grain-boundary impurity phase is dominant. A Schottky barrier model satisfactorily explains all the phenomenological observations of the grain-boundary electrical properties of materials of high purity, and experimental evidence soundly supports the model. Various factors (alumina addition and grain size) influencing the grain-boundary electrical properties are discussed, and some special aspects of nanocrystalline materials are highlighted.     

The Measurement of Compressive Creep Deformation and Damage Mechanisms in a Single-Phase Alumina Part II Correlation of creep cavitation and grain boundary sliding

It has been theorized that stochastic grain boundary sliding (GBS) is the primary driving force for the nucleation, growth, and coalescence of cavities located on the grain boundaries of polycrystalline ceramics undergoing creep. This paper reports on the results of co-ordinated measurements of both GBS and creep cavitation during the creep of a single-phase alumina. Constant compressive stress creep experiments were performed at a temperature of 1600 °C, and stress levels of 70, 100, and 140 MPa. Small angle neutron scattering measurements (SANS) show that cavities nucleate continuously due to creep at all three stress levels, and that since negligible cavity growth was measured, creep cavitation appears to be ruled by a nucleation rather than a growth process. Also, at a constant creep temperature, the number and volume of cavities measured was observed to decrease with a decrease in the applied stress. GBS displacements reported in Part 1 of this paper [1] are related to the number of cavities nucleated per unit volume and shown to relate directly, thereby providing experimental evidence that GBS may act as the driving force for creep cavitation.     

Cavitation in creep

Metals often fail in service under creep conditions because of the formation of cavities on the grain boundaries which are approximately normal to the applied stress. This phenomenon of creep cavitation is becoming of increasing technological importance. As a result a complete understanding of it is desirable so that alloys with improved cavitation resistance can be designed. This paper reviews the development of our present understanding of the phenomenon which is one of nucleation, growth and linkage leading to failure. Several mechanisms of nucleation, such as at grain-boundary ledges or precipitates, have been postulated and experimental evidence in support of each has been cited. Similarly, deformation- or vacancy-controlled growth mechanisms have been discussed. It is apparent from the literature thatno single mechanism is applicable, indeed, the work discussed here suggests that several mechanisms may operate and each may become dominant at different stages of the creep life. Finally, the status of research into nickel-base superalloys is reviewed with reference being made to such effects as regenerative heat-treatment.     

The Nature of the Nonsingularity of Inner Interfaces in Hydroxyapatite Ceramics

The morphology of inner interfaces in hydroxyapatite (HA) based calcium phosphate ceramics has been studied by transmission electron microscopy. Grain boundaries in the ceramics have been shown to have a vicinal character, which is related to the mechanism of secondary recrystallization in the material: layer growth of grains via sequential motion of elementary steps on planes corresponding to the \(\{ 1\bar 100\} \) HA prism faces, which grow through transitions of atoms from adjacent grains that are in contact through their planes with large Miller indices. The recrystallization process may be accompanied by a “collision” of vicinal growth surfaces of grains with relatively large misalignment angles and the formation of grain boundaries nanofaceted by prism planes of adjacent grains. The recrystallization process in such a case should be expected to continue in the grain with a smaller nonsingularity of the growth front. Grain boundaries may allow for a match between planes differing in Miller indices, nd_h1k1l1 ≈ md_h2k2l2, and the formation of grain-boundary Pumphrey dislocations, which compensate for the size mismatch between interplanar spacings and/or misalignment of the planes. The observed characteristic grain match configurations are typical of both ceramics produced by sintering HA powders and HA films produced by ion sputtering.     

Characterization of creep cavity shape in hot-pressed silicon carbide using small-angle neutron scattering

Fine-grained silicon carbide with a continuous second-phase grain-boundary film was crept under compressive loading at 1600° C. The shape of the resultant grain-boundary cavities was characterized using small-angle neutron scattering. During the early stages of creep the cavities grew more rapidly in the plane of the grain boundary, as evidenced by an elongation of the isointensity contours and an increase in the radius of gyration,R _D, along the direction of the applied compressive stress. During the latter stages of creep the cavities grew more rapidly perpendicular to the grain-boundary plane, as evidenced by a gradual reduction in the scattering anisotropy and by an increase inR _D perpendicular to the compressive stress axis relative toR _D parallel to the compressive stress axis. Cavity aspect ratios calculated from the ratios of theR _D values parallel to and perpendicular to the compressive stress axis are shown to support a recent model of cavity growth in a viscous grain-boundary film.     

Improvement of creep-rupture properties of wrought cobalt-based HS-21 alloys at high temperatures

The improvement of creep-rupture properties by serrated grain boundaries is investigated using wrought cobalt-based HS-21 alloys in the temperature range 816 to 1038° C (1500 to 1900°F). Serrated grain-boundaries are produced in the early stage of the grain-boundary reaction (GBR) by a heat treatment. Specimens with serrated grain boundaries have superior creep-rupture properties compared with those with normal straight grain boundaries. The rupture lives of specimens with serrated grain boundaries are more than twice as long as those of specimens with straight grain boundaries. The rupture elongation is considerably improved by serrated grain boundaries especially at lower temperatures. A ductile grain-boundary fracture is observed in specimens with serrated grain boundaries, while brittle grain boundary facets prevail in specimens with straight grain boundaries.     

Effect of grain-boundary diffusion acceleration during recrystallization in submicrocrystalline metals and alloys prepared by severe plastic deformation

The results of studying the effect of grain-boundary diffusion acceleration during the annealing of submicrocrystalline (SMC) materials prepared by severe plastic deformation techniques are described. It is shown that the grain-boundary diffusion coefficient during recrystallization of SMC materials depends on the density of lattice dislocations and the pattern and rate of migration of the grain boundaries. It is found that, in SMC metals that undergo anomalous grain growth during annealing, the grain-boundary diffusion coefficient increases and its activation energy decreases. In SMC materials in which the recrystallization process exhibits conventional behavior, the diffusion properties of grain boundaries hardly differ from the equilibrium properties. Expressions describing the dependence of the grain-boundary diffusion coefficient on the migration rate of grain boundaries, as well as the thermodynamic and crystallographic parameters of the material, are derived. The calculation results are compared with the experimental data.     

A creep rupture model accounting for cavitation at sliding grain boundaries

An axisymmetric cell model analysis is used to study creep failure by grain boundary cavitation at facets normal to the maximum principal tensile stress, taking into account the influence of cavitation and sliding at adjacent inclined grain boundaries. It is found that the interaction between the failure processes on these two types of adjacent facets reduces the failure time significantly when cavitation is creep constrained. In all cases the time to cavity coalescence on transverse facets appears to be a useful lower bound measure of the material life-time. Sliding at the boundaries of the central grain of the cell model is accurately represented; but in some computations a stress enhancement factor is used to incorporate also the effect of sliding between surrounding grains. The influence of grain boundary viscosity is included in the model and it is found that even in the absence of sliding, cavitation on inclined boundaries may significantly reduce the failure time.     

Improving the resistance to intergranular cracking and corrosion at elevated temperatures by grain-boundary-engineering-type processing

Failure of structural components operating under high mechanical loading and/or in aggressive environments can often be attributed to intergranular degradation, e.g. by creep, corrosion, fatigue or brittle cracking. The present article is focussed on oxygen-diffusion-controlled grain-boundary attack, for example, of a nickel-based superalloy leading to intercrystalline oxidation or rapid cracking by dynamic embrittlement. Since grain-boundary diffusion depends on the crystallographic orientation relationship between adjacent grains, the grain-boundary-engineering approach was applied to reduce the susceptibility to grain-boundary attack. The relevant mechanisms are discussed in terms of modifying the network of general high-angle and so-called special grain boundaries taking the results of cracking experiments on bicrystals into account.     

Fracture of a tin-lead eutectic alloy in Region III of superplastic flow

With the aid of scanning electron microscopy, cavitation and fracture behaviour in the Sn-Pb eutectic alloy, whose reduction in area of cross-section before failure is close to 100%, has been investigated in Region III of superplastic flow (where both the elongationto-fracture and the strain-rate sensitivity index decrease with increasing strain rate). It has been demonstrated that, although it decreases, grain-boundary sliding persists in this range as the strain rate is increased. At all strain rates the final failure was due to tearing by plastic flow of the inter-cavity ligaments, but the interlinkage of cavities along the graininterphase boundaries decreased with increasing strain rate. The features of cavitation and fracture did not differ much from an earlier study on a pseudo-single phase copper alloy, although copper alloys usually fail non-ideally, i.e., a large area of cross-section is present at fracture.     

Grain-boundary sliding measurements in Al2O3 by machine vision photogrammetry

The nucleation, growth and coalescence of grain-boundary cavities is the primary damage mechanism observed during creep of structural ceramics. Furthermore, grain-boundary sliding (GBS) has been identified as the driving force process. Although the creep characteristics of structural ceramics have been extensively studied, very little is known about the details of GBS during creep and how GBS relates to cavitation kinetics. This paper presents the results of a study using a machine vision system to measure Mode II GBS displacements in a Lucalox Al₂O₃. Specifically, sliding displacements as large as 0.4 m were measured. The measured displacements indicate that some grain boundaries experienced shear strains and strain rates of 4200% and 2.3×10^–2 s^–1, respectively. The techniques utilized for these measurements are described in detail, and data gathered during a 2 1/2 h compressive creep test under a stress of 138 MPa at 1600 °C are presented and discussed.     

An evaluation of the grain-boundary sliding contribution to creep deformation in polycrystalline alumina

The occurrence of grain-boundary sliding during creep in fine grained alumina was examined by inscribing marker lines on the tensile surfaces of specimens, prior to testing in four-point bending mode. There was considerable microstructural evidence for the occurrence of grainboundary sliding and grain rotation during creep deformation. Experimental measurements of the offsets in the marker lines at grain boundaries reveal that the grain-boundary sliding contribution to the total strain during creep deformation is 70 ± 6.2%. The extensive grain boundary sliding observed, together with the other mechanical properties, suggests that polycrystalline alumina exhibits superplastic characteristics. Several possible rate controlling mechanisms are examined critically in light of the present results and it is concluded that creep occurs either by an independent grain-boundary sliding mechanism or by an interface controlled diffusion mechanism.     

Creep-rupture properties and grain-boundary sliding in wrought cobalt-base HS-21 alloys at high temperatures

The effects of serrated grain boundaries on the creep-rupture properties of wrought cobaltbase HS-21 alloys were investigated at 1311 and 1422 K. The amount of grain-boundary sliding and the initiation and growth of grain-boundary cracks were also examined during creep at 1311 K. Specimens with serrated grain boundaries exhibited longer rupture life and larger rupture ductility than those with straight grain boundaries, but these specimens had almost the same rupture life and rupture ductility under lower stresses at 1422 K, because serrated grain boundaries were also formed in specimens with originally straight grain boundaries. The average amount of grain-boundary sliding during creep at 1311 K increased with time (or with creep strain), but was almost the same in both specimens with serrated grain boundaries and those with straight grain boundaries at the same creep strain. Grain-boundary cracks or voids initiated in the early stage of creep in those specimens at 1311 K. Therefore, the strengthening by serrated grain boundaries at high temperatures above about 1311 K was attributed to the retardation of growth and linkage of grain-boundary cracks and voids.     

Grain-boundary de-segregation and intergranular cohesion in Si-Al-O-N ceramics

The influence of heat-treatment on high-temperature creep and sub-critical crack growth in hot-pressed Si-Al-O-N ceramics has been analyzed from microstructural evidence and determination of stress exponents and activation energies. The most significant change is the suppression of cavitation during creep and of the cavity-interlinkage mechanism for slow crack propagation. A creep mechanism of grain-boundary diffusion is characterized by stress exponent n=1 and unusually high activation energy >820 kJ mol^–1. The microstructural origin of the transformation in grain-boundary dominated properties is mainly the removal of triple-junction glassy residues within which cavities are nucleated. This is caused by grain-boundary diffusion of metallic impurities (Mg, Mn, Ca) into a surface silica oxidation layer, and consequent crystallization of the remaining glass components as . There is a continued improvement in grain-boundary cohesion and increased difficulty of grain-boundary diffusion following the stage at which triple-junction glass is removed. The resultant ceramics, in addition to superior mechanical behaviour, have an increased temperature for application due to a marked reduction in susceptibility to dissociation above 1400° C.     

Superplasticity and cavitation in AZ31 Mg alloy at elevated temperatures

Superplasticity of AZ31 alloy at elevated temperatures was examined by uniaxial tension test. Optical microscopy (OM) and scanning electronic microscopy (SEM) were employed to observe the cavities and topography during the superplastic deformation of AZ31 alloy, respectively. It is indicated that the intergranular cavities are formed by the continuous fracture of filaments along grain boundaries and the filaments are not formed by liquid phase but by severe elongation of O-shaped cavities under tensile stress. Finally, a filament-induced cavitation model was proposed.