The effect of test temperature on the intergranular cracking of Nb-A286 alloy in low cycle fatigue

The continuous low cycle fatigue behaviors of a Fe-base superalloy, Nb-modified A286 alloy, have been evaluated at the test temperatures of 650°C and 350°C under various total strain ranges. It was found that the change of the slope in the Coffin–Manson plot was closely related to the fatigue cracking with the test temperature. In the high temperature low cycle fatigue (HTLCF) of Nb-A286 alloy, the fatigue cracking exhibited the intergranular mode at 650°C and the transgranular mode at 350°C. The intergranular fatigue cracking at 650°C was due to the precipitation of the phase at the grain boundary assisted by the applied stress during low cycle fatigue. It is investigated whether the precipitate at the grain boundary provides the site for the grain boundary cavitation, which induces the intergranular cracking in low cycle fatigue. This is confirmed by the results of low cycle fatigue at 25°C after heat treatment which forms the phase at the grain boundary.     

Cavitation and fracture of mechanically alloyed aluminium at high homologous temperatures

The tensile behaviour of mechanically alloyed (dispersion strengthened) IN90211 was characterized at strain rates between 0.0001 and 340 sec^–1 at temperatures between 425 and 475 ° C, At strain rates above 0.1 sec^–1, superplastic elongations were obtained (maximum elongation 525% at 475 ° C, 2.5sec^–1. Large elongations were possible due to the lack of cavitation, even though the strain-rate sensitivity was lower (m 0.25) than usually found in superplasticity. Cavitation was precluded by the morphology of the platelet-shaped grains in which low-angle subgrain boundaries were predominantly perpendicular to the tensile axis. Grain-boundary sliding was observed along high-angle grain boundaries which were generally parallel to the tensile axis. At the high homologous testing temperatures (0.76 to 0.81), concurrent grain-boundary sliding and lattice slip was made possible by the rapid lattice diffusivity and easy climb of lattice dislocations over dispersions in the matrix and grain boundaries.     

Mechanical behaviour,percolation and damage of materials with viscous solid grain boundaries

The mechanical behaviour, percolation and damage mechanism of a aluminium alloy with viscous solid grain boundaries (GBs) at 465 °C have been characterized in experiments performed in tension or compression in the strain rate range of 10^–5-10^–2s^–1. It was found that grain-boundary sliding (GBS) occurs as strain rates below 10^–4s^–1. It was shown that the viscous solid interphase migrates during the process of deformation. In the case of tension, it was squeezed out of GBs parallel with the tension axis into GBs perpendicular to the axis and vice versa in the case of compression. This local percolation is discussed in terms of the viscosity of the interphase, gradient of local stresses and percolation time. The viscosity of the solid interphase is estimated. It was also found that cavitation depends on the type of stress (tension or compression) and the strain rate. Cavity nucleation occurs at multiple points when GBS happens or along G B facets in the absence of GBS. Cavity growth takes place along GBs at high normal stresses and the cavity coalsescence leads to saw-tooth fracture.     

Effect of phase proportions on deformation and cavitation of superplastic α/β brass

Studies have been made, using metallographic and precision density techniques, of the deformation and cavitation behaviour during superplastic tensile straining at 873 K of three microduplex/ brasses which, as a consequence of varying composition, contained varying proportions of and phases. It was observed that both strain-rate sensitivity and elongation-to-failure passed through a maximum when approximately equivolume proportions of the two phases were present. Cavitation, on the other hand, decreased rapidly as the volume fraction of phase was increased. The cavitation behaviour was attributed to the relative abilities of the phases to accommodate grain boundary sliding. When a high proportion of phase is present accommodation is minimal and cavity nucleation. occurs readily. Evidence is presented to show that grain-boundary sliding plays a predominant role in cavity growth. When a high proportion of phase is present accommodation is almost complete and cavity nucleation is minimal.     

On the role of grain-boundary migration during the creep of zinc

Constant engineering strain-rate tensile tests have been carried out in the temperature range 20 to 150C on high purity Zn, Zn-0.14 at. % Cu (alloy C) and Zn-0.16 at. % Al (alloy A) alloys. Measurements of angular distribution of orientations of grain boundaries have been used to study grain-boundary migration during deformation. Significant cavitation, with increasing propensity at higher test temperatures, occurred in the two alloys but not in pure Zn. A striking feature of the observations in pure Zn and alloy C, as the test temperature was raised, was the formation and subsequent decay of a diamond pattern of uncavitated grain boundaries, a majority of which were preferentially aligned at 45 to the stress axis. By comparison the changes in the angular distribution of grain boundaries was least marked in alloy A. Cavitation was observed in alloy C to maintain grain boundaries in the 45 orientation. At the test temperature of 150C alloy C, which was prone to the formation of diamond grain-boundary configuration, developed much larger volume fraction of cavities than alloy A. These results are discussed in terms of the different distribution coefficients of Cu and Al in Zn, the different rates of grain-boundary migration in pure Zn and the two alloys and the differences in the substructural features (cells) formed during high-temperature deformation.     

Yttria-zirconia: Effect of microstructure on conductivity

Complex impedance measurements and detailed analysis of the grain-boundary microstructure have been made on fully stabilized yttria-zirconia sintered bodies as a function of grain size. The prereacted yttria-zirconia powder used in this study was obtained from a commercial source. The powder has very high reactivity and starts sintering around 1200° C. The densification process is complete around 1350° C but the grain growth continues almost linearly with sintering temperature. The grain size variation obtained was between 1 and 30 m. The grain-boundary resistivity when plotted against grain size showed an inflection in the vicinity of 1500° C sintering temperature. These results have been explained in terms of the grain-boundary microstructure changing with the sintering temperature. The thickness of the grain-boundary layer determined from impedance data and transmission electron micrographs are in reasonably good agreement. The activation energy for the grain-boundary resistivity was only slightly higher than that for the lattice resistivity.     

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.     

Grain-boundary slide-hardening in zinc bicrystals

Zinc bicrystals containing high-angle tilt boundaries, and others containing twist boundaries were subjected to constant shear stresses parallel to the boundary at temperatures in the range 20 to 300° C. At stresses both above and below that necessary to cause macroscopic slip in the constituent crystals, the grain-boundary sliding rate decreased with time. This slide-hardening at the grain-boundary was identified with the formation of asperities in the grain-boundary which were generated first near the intersection of boundary and free surface. Removal of the layer containing these asperities restored the boundary behaviour to that of a virgin bicrystal in certain cases. This leads to the conclusion that when slide-hardening occurs, the rate-limiting process for grain-boundary sliding is that associated with the deformation of the crystal regions which have been introduced into the original boundary plane by grain-boundary migration.     

Creep in pure and two phase nickel-doped alumina

Creep in pure and two phase nickel-doped alumina has been investigated in the stress range 0.70 to 4.57 kgf mm^–2 (1000 to 6500 psi), and temperature range 1450 to 1800° C, for grain sizes from 15 to 45 m (pure alumina) and 15 to 30 um, (nickel-doped alumina). The effect of stress, grain size and temperature on the creep rate suggests that diffusion controlled grain-boundary sliding is the predominant creep mechanism at low stresses and small grain sizes. However, the stress exponents show that some non-viscous boundary sliding occurs even at the lowest stresses investigated. This mechanism is confirmed by metallographic evidence, which shows considerable boundary corrugation in the deformed aluminas. At higher stresses and larger grain sizes the localized propagation of microcracks leads to high stress exponents in the creep rate equation. The nickel dopant, which introduces an evenly distributed spinel second phase into the alumina matrix, increases the creep rate and enhances boundary sliding and localized crack propagation. The weakening effect of the second phase increases with grain size, and tertiary creep occurs at strains of 0.5% and below in large grained material.     

Wedge crack nucleation in Type 316 stainless steel

Type 316 austenitic steel has been heat-treated to produce a range of grain sizes and then creep-tested at 625° C at various stresses so as to examine the nucleation and the factors which effect the nucleation of grain-boundary triple point or wedge cracks. An internal marker technique was used to evaluate the extent of the grain-boundary sliding in relation to the total creep strain. Triple point crack nucleation occurred over the entire range of grain sizes and stresses examined when the product of the stress and grain-boundary displacement reached a critical value; the effective surface energy for grain boundary fracture, estimated using an expression derived by Stroh, was in approximate agreement with the surface free energy value indicating that only limited relaxation occurred by plastic deformation. The first cracks were observed to form along grain boundary facets perpendicular to the applied stress direction and with the sliding grain boundaries at high angles (60 to 80°) to the crack growth direction. Subsequent cracking occurred under conditions which deviated slightly from this initial condition, and the increase in crack density with strain was expressed in terms of geometrical factors which take account of the orientation effects.     

Magnetic Co2Y ferrite, Ba2Co2Fe12O22 fibres produced by a blow spun process

Gel fibres of Co2Y,Ba2Co2Fe12O22, were blow spun from an aqueous inorganic sol and calcined at temperatures of up to 1200°C. The ceramic fibres were shown by X-ray diffraction to form crystalline Co2Y at 1000°C, and surface area and porosity measurements indicated an unusually high degree of sintering at this temperature. The fibres also demonstrated a small grain size of 1–3 m across the hexagonal plane and 0.1–0.3 m thickness at 1000°C. This only increased to 3 m in diameter and 1 m thickness even at temperature up to 1200°C. The fibrous nature combined with the improved microstructures could be an important factor in improving the magnetic properties of this material.     

Formation of submicrocrystalline structure in TiAl intermetallic compound

The TiAl intermetallic compound was used to illustrate an approach which enables the creation of a submicrocrystalline structure (d0.1 m) in massive semifinished products made of hard-to-deform materials by means of their deformation at elevated temperatures. Tensile mechanical properties of the TiAl intermetallic compound with a mean grain size of 0.4 m were tested. In this state, the lower temperature limit of superplasticity in TiAl was found to be 800°C. At this temperature and at an initial strain rate of 8.3×10^–4s^–1, the relative elongation to rupture attains 225%.     

Mechanical properties of precipitation-strengthened Ni-Al-Cr alloy based on an NiAl intermetallic compound

Mechanical properties of a ternary alloy Ni-30.3 at. % Al-6.6 at. % Cr have been studied in the temperature range 25 to 1100° C. The material was heat-treated to produce a stable dispersion of incoherent rod-shaped Ni₃Al precipitates, 1m in diameter and 20m long. The tensile properties were found to be temperature dependent. Below 750° C the material had high strength, low ductility and low strain-rate sensitivity, whilst above 750° C the strength fell, ductility increased and the material became strain-rate sensitive. The room temperature fracture toughness of the single-phase material was 6 MN m^–3/2 and increased to 50 MN m^–3/2 in the two-phase material. This can be attributed to the effect of Ni₃Al on crack nucleation and propagation.     

Microstructural evolution during superplastic deformation of a 7475 Al alloy

Grain refinement of a superplastic 7475 Al alloy is observed at strain rates of 10-2s-1 or higher. Metallographic observation shows that the average grain size is changed from 14 m to 10 m after 100% elongation. Two-stage strain-rate tests were performed on the 7475 Al alloy to correlate grain refinement with an improvement of superplasticity. The optimum first strain rate and strain in the first stage were determined through tensile superplastic tests. Superplasticity was improved significantly through two-stage strain-rate testing. This is believed to be related to the refinement of the initial grains at high strain rate. The specimen tested at a strain rate of 2.1×10-4s-1 revealed dispersoid-free zones (DFZs) near grain boundaries normal to the stress axis. When a higher strain rate was applied to the specimens with DFZs, no grain refinement was observed. The absence of grain refinement is due to the concentration of plastic deformation in the weak DFZs. © 1998 Kluwer Academic Publishers     

Effects of grain-boundary configuration on the high-temperature creep strength of cobalt-base L-605 alloys

The effects of grain-boundary configuration on the high-temperature creep strength are investigated using commercial cobalt-base L-605 alloys with low carbon content in the temperature range 816 to 1038° C (1500 to 1900° F). Serrated grain boundaries are formed principally by the precipitation of tungsten-rich b c c phase (the same as ₂ phase found in Ni-20Cr-20W alloys) on grain boundaries by a relatively simple heat treatment in these alloys. The creep rupture properties are improved by strengthening of grain boundaries by the precipitation of tungsten-rich bcc (₂) phase. The specimens with serrated grain boundaries have longer rupture lives and higher ductility than those with normal straight grain boundaries under low stress and high-temperature creep conditions, while the rupture lives and the creep ductility of both specimens are almost the same under high stresses below 927° C. The matrix of the alloys is strengthened by the precipitation of carbides at temperatures below 927° C and by the precipitation of tungsten-rich ₂ phase at 1038° C during creep. It is found that there is an orientation relationship between tungsten-rich a2 phase particles and-Co matrix, such that (0 1 1)₂ ¶ (1 1 1) _-Co and [1 1]₂ ¶ [1 0] _-Co. The fracture surface of specimens with serrated grain boundaries is a ductile grain-boundary fracture surface, while typical grain-boundary facets prevailed in specimens with straight grain boundaries.     

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.     

Characterization of the transformation from calcium-deficient apatite to β-tricalcium phosphate

The structural changes that occur during the transformation of a Ca-deficient apatite, prepared by a wet chemical method, to -TCP were investigated. X-ray diffraction (XRD) analysis of as-prepared samples and samples calcined at temperatures between 500 and 1100 °C showed that the transformation occurs over the temperature range 710–740 °C, under non-equilibrium conditions. The change in crystallite size with increasing calcination/sintering temperature was studied by XRD using the Scherrer formula. Fourier transform infra-red (FTIR) analysis indicated considerable structural change in samples above and below this temperature range. Changes were observed in the hydroxyl, carbonate and phosphate bands as the calcination temperature was increased from 500 to 1100 °C. Even once a single -TCP phase is obtained at 740 °C there remains a considerable amount of structural change at temperatures between 740 and 1100 °C. This effect was illustrated by an unusual change in the lattice parameters of the -TCP structure and significant changes in the phosphate bands of the FTIR spectra as the calcination temperature was increased. The results obtained in this study show that the combined experimental techniques of XRD and FTIR are excellent complimentary methods for characterizing structural changes that occur during phase transformations.     

Abnormal grain growth and liquid-phase sintering in Sr0.6Ba0.4Nb2O6 (SBN40) ceramics

Ceramics of Sr0.6Ba0.4Nb2O6 (SBN40) were prepared by the conventional mixed oxide route. Sintering at temperatures 1260 °C led to rapid, non-uniform grain growth and a duplex microstructure. Presintering at 1250 °C followed by higher temperature sintering (1350–1450 °C) controlled grain growth. Rapid cooling from 1450 °C froze-in second phases at grain boundaries. Scanning electron microscopy and transmission electron microscopy showed that the resulting grain-boundary phases were Nb2O5-rich and BaO-deficient, having low liquid-formation temperatures. In contrast, SBN40 ceramics prepared with excess BaO and a deficiency of Nb2O5 showed no enhancement of grain growth at the highest temperature. Sintering behaviour and microstructural development provide evidence for the existence of a liquid phase which assists abnormal grain growth. The effect of presintering in controlling grain growth is discussed, and a mechanism for abnormal grain growth in Sr0.6Ba0.4Nb2O6 (SBN40) ceramics is proposed. © 1998 Chapman & Hall     

Determination of the elastic moduli of a machinable ceramic over the range from room temperature to 800°C

The ultrasonic velocities of a machinable ceramic were measured using the pulse echo overlap technique. The machinable ceramic consists of 5- to 10-m crystallite blocks of mica in a boroaluminosilicate glass matrix. The elastic moduli are deduced from the sound velocities over the temperature range from room temperature to 800°C. Their temperature change is well described by a fourth-degree polynomial. Although the moduli decrease with increasing temperature, a plateau region appears at about 450°C. This anomalous behavior is explained by applying the simple rule of mixtures to constituent materials, the mica crystallites, and the glass matrix.     

Effects of temperature and grain size on deformation and fracture in recrystallized Ni3Al doped with boron

The effects of temperature and grain size on the deformation and fracture behaviour of recrystallized Ni₃Al doped with boron were investigated by tensile tests at temperatures up to 973 K as a function of grain sizes from 1.6 to 105m. The yield stress showed a positive temperature dependence to a peak temperature in somewhat different manners depending on the grain size. For coarse-grained specimens, a rapid drop in elongation was observed with increasing temperature. The predominant fracture mode changed with temperature from the transgranular fracture of {1 1 1} cracking to brittle intergranular fracture. This embrittlement at elevated temperatures was considered to occur by a high stress concentration at grain boundaries arising from increased flow stress level and the occurrence of grain boundary sliding (GBS). In contrast, the elongation was not so markedly decreased with temperature for intermediate- and fine-grained specimens which exhibited ductile intergranular fracture and cavitation fracture, respectively, at elevated temperatures, and a slant-type fracture and cup-cone fracture, respectively, at low temperatures. The suppression of serious high-temperature embrittlement for intermediate-grained specimens was explained in terms of the slow propagation of a crack formed by GBS, owing to stress relaxation by dynamic recrystallization (DR) and plastic deformation. In the case of ultra-fine-grained specimens a large elongation was developed at elevated temperatures, which was interpreted as that the further occurrence of DR with increasing volume fraction of grain boundaries reduces the cavitation promoted by GBS, and that the limited sliding length due to extremely small grain diameter raises the stress for cavity formation.