Showing papers on "Creep published in 1990"

Development of Superplastic Structural Ceramics

Abstract: Superplastic structural ceramics (Y-TZP, A1203, Si3N4, and their composites) that can withstand biaxial stretching to large strains have been developed recently. Microstructural design of these ceramics first requires an ultrafine grain size that is stable against coarsening during sintering and deformation. A low sintering temperature is a necessary, but not a sufficient, condition for achieving the required microstructure. In many cases, the selection of an appropriate phase, such as tetragonal phase in zirconia or a phase in silicon nitride, which is resistant to grain growth, is crucial. The use of sintering aids and grain-growth inhibitors, particularly those that segregate to the grain boundaries, can be beneficial. Second-phase particles are especially effective in suppressing static and dynamic grain growth. Another major concern is to maintain an adequate grain-boundary cohesive strength, relative to the flow stress, to mitigate cavitation or grain-boundary cracking during large strain deformation. Existing evidence suggests that a lower grainboundary energy is instrumental in achieving this objective. The selection of an appropriate phase and the tailoring of the grain boundary or liquid-phase composition can sometimes drastically alter the cavitation resistance. Related observations on forming methods, forming characteristics, and sheet formability are also reviewed. The basic deformation characteristics are similar to diffusional creep and are dominated by R. E. Newnham-contributing editor

Creep of Polycrystalline Ice

Abstract: This paper discusses the extension of creep models for polycrystalline ice to include the effects of a non-monotonic loading history. Attention is focussed on cyclic loading since it is of direct relevance to the field problem of ice sheets subjected to oscillatory loads. The modeling approach is based on the internal variable concept in which the change in the underlying microstructure is described by the evolution of structure variables. Significant amount of work has been done using the internal variable concept for the cyclic loading of metallic materials. This serves as a basis for the development of cyclic constitutive models for ice, and is discussed here in some detail. The current paucity of test data essential for theoretical model development and verification underlines the need for systematic experimental research in the non-monotonic loading of ice.

A new model-based creep equation for dispersion strengthened materials

Abstract: The strongly stress-sensitive and temperature-dependent creep behaviour of dispersion strengthened materials cannot be described satisfactorily by current creep laws. In this paper a new creep equation is developed which considers as the rate-controlling event the thermally activated detachment of dislocations from dispersoid particles exerting an attractive force. The approach is motivated by recent TEM observations and theoretical calculations which strongly suggest that the “classical” view, according to which particles merely force dislocations to climb around them, is inadequate. The creep equation is applied to a dispersion-strengthened superalloy, two aluminium alloys and bubble-strengthened tungsten. Practical conclusions, regarding the optimum dispersoid size and alloy development, are drawn.

Flux creep characteristics in high‐temperature superconductors

Abstract: We describe the voltage‐current characteristics of YBa2Cu3O7−δ epitaxial films within the flux creep model in a manner consistent with the resistive transition behavior. The magnitude of the activation energy, and its temperature and magnetic field dependences, are readily derived from the experimentally observed power law characteristics and show a (1−T/Tc)3/2 type of behavior near Tc. The activation energy is a nonlinear function of the current density and it enables the determination of the shape of the flux line potential well.

Experimental determination of constitutive parameters governing creep of rocksalt by pressure solution

Abstract: Abstract Theoretical models for compaction creep of porous aggregates, and for conventional creep of dense aggregates, by grain boundary diffusion controlled pressure solution are examined. In both models, the absolute rate of creep is determined by the phenomenological coefficient Z* = Z0exp (−ΔH/RT), a thermally activated term representing effective diffusivity along grain boundaries. With the aim of determining Z0, ΔH and hence Z* for pressure solution creep in rocksalt, compaction creep experiments have been performed on wet granular salt. Compaction experiments were chosen since theory indicates that pressure solution creep is accelerated in this mode. The tests were performed on brine-saturated NaCl powder (grainsize 100–275 μm) at temperatures of 20–90°C and applied stresses of 0.5–2.2 MPa. The mechanical data obtained show excellent agreement with the theoretical equation for compaction creep. In addition, all samples exhibited well-developed indentation, truncation and overgrowth microstructures. We infer that compaction did indeed occur by diffusion controlled pressure solution, and best fitting of our data to the theoretical equation yields Z0 = (2.79 ± 1.40) × 10−15 m3s−1, ΔH = 24.53 kJ mol−1. Insertion of these values into the theoretical model for conventional creep by pressure solution leads to a preliminary constitutive law for pressure solution in dense salt. Incorporation of this creep law into a deformation map suggests that flow of rocksalt in nature will tend to occur in the transition between the dislocation-dominated and pressure solution fields.

Cone-and-plate rheometry of yield stress fluids. Study of an aqueous gel

Abstract: This work particularly focuses on the rheometric study of a physical gel exhibiting a yield stress. The measurements were carried out in a cone—plate configuration using two different types of rheometer working under controlled torque or under controlled velocity. Shear creep, constant shear rate, and stress relaxation tests have been performed. Measurements of apparent viscometric properties were conducted at the same time as observation of the strain field in the sample. Observing the strain field enables us to confirm the reliability of the interpretation of the results and also to estimate the true shear rate in the fluid. It is shown how the determination of shear rheological properties can be affected by anomalous phenomena such as fracture and slip at the wall. The influence of roughness of the tool surfaces and of evaporation shows up. The results presented in this study show how some rheometrical measurements of the yeild stress and the microstructural interpretations given, may be erroneous. Some recommendations are made in order to improve current rheometrical tests and their interpretation. A log—log graph with typical shear stress-shear rate measurements and their corresponding strain fields is given: it should be used as a guideline in yield stress fluids rheometry. In addition it is made clear that visual observation of the sheared sample is a key technique. A protection which completely eliminates evaporation is suggested. It is shown that the measurement of residual stress in stress relaxation tests may be a convenient means of determining the value of the yield stress.

Geometric factors affecting the internal stress distribution and high temperature creep rate of discontinuous fiber reinforced metals

Abstract: The creep deformation behavior of metal-matrix composites has been studied by a continuum mechanics treatment utilizing finite element techniques. The objective of the work has been to understand the underlying mechanisms of fiber reinforcement at high temperatures and to quantify the importance of reinforcement phase geometry on the overall deformation rate. Internal stress distributions are presented for a material that consists of stiff elastic fibers in an elastic, power law creeping matrix. Results indicate that large triaxial stresses develop in the matrix, and that these stresses have a strong effect on reducing the creep rate of the composite. Reinforcement phase geometry, as measured by the fiber volume fraction, aspect ratio, separation, and overlap, greatly influences the degree of constraint on the flowing matrix material and the overall deformation rate. Theoretical predictions from this modeling are compared to experimental results of creep deformation in metal-matrix composite systems with varying degrees of agreement.

High temperature creep of silicon carbide particulate reinforced aluminum

Abstract: The effect of stress on the creep properties of 30 vol.% silicon carbide particulate reinforced 6061 aluminum (SiC p -6061 Al), produced by powder metallurgy, has been studied in the temperature range of 618–678 K. The experimental data, which extend over seven orders of magnitude of strain rate, show that the creep curve exhibits a very short steady-state stage; that the stress exponent, n , is high ( n > 7.4) and increases with decreasing the applied stress; and that the apparent activation energy for creep, Q a , is much higher than the activation energy for self-diffusion in aluminum. The above creep characteristics of SiC p -6061 Al are similar to those reported for dispersion strengthened (DS) alloys, where the high stress exponent for creep and its variation with stress are explained in terms of a threshold stress for creep that is introduced by the dispersoid particles. Analysis of the creep data of SiC p -6061 Al using the various threshold stress models proposed for DS alloys indicates that the threshold stresses introduced by the SiC particulates are too small to account for the observed creep behavior of the composite. By considering an alternate approach for the source of the threshold stress in SiC p -6061 Al, an explanation for the asymptotic behavior of the creep data of the composite is offered. The approach is based on the idea that the oxide particles present in the Al matrix, as a result of manufacturing the composite by powder metallurgy, serve as effective barriers to dislocation motion and give rise to the existence of a threshold stress for creep.

Constitutive relation and creep-fatigue life model for eutectic tin-lead solder

Abstract: A constitutive equation for a typical SnPb eutectic or near-eutectic solder joint is developed, based on empirical data in shear and generalized to three dimensions. Three strain components are considered: elastic, time independent plastic, and steady-state creep. A continuum mechanics rather than a metallurgical approach is taken with emphasis on the formulation of an equation useful for predicting solder behavior under a variety of conditions. Solutions of the constitutive equation predict the experimental hysteresis data for the loading histories available. Solutions of the equation for the test conditions of three independent sets of solder fatigue data show that the equation, together with the matrix creep failure indicator, can give an estimate of fatigue life. >

Densification of crystalline aggregates by fluid-phase diffusional creep

Abstract: It is widely accepted in the literature that polycrystalline minerals, ceramics, and rocks containing subcontinuous intergranular films of solvent or melt can deform by diffusive mass transfer through the grain-boundary liquid phase (Durney 1972, Robin 1978, Raj 1982, Rutter 1983, Cooper & Kohlstedt 1984). This type of mechanism is generally referred to as fluid-phase diffusional creep (Stocker & Ashby 1973), solution-precipitation creep (Raj 1982), or pressure solution (e.g. Rutter 1983), and is of well known interest both in materials science (see Lange et al. 1980, Raj & Chyung 1981, Raj 1982) and in the Earth sciences (see Robin 1978, Rutter & Mainprice 1978, Rutter 1983, Green 1984, Urai et al. 1986). In the present chapter we shall use the term ‘fluid-phase diffusional creep’ (FPDC) for the process. For mechanisms involving coupled solid-state flow plus dissolution at grain-contact margins (Pharr & Ashby 1983, Green 1984) we shall use the term ‘dissolution-coupled creep’.

Frequency dependence of AC susceptibility in high-temperature superconductors Flux creep and critical state at grain boundaries

Abstract: The loss peak of the AC susceptibility in polycrystalline high- T c superconductors shifts slightly to higher temperatures with increasing frequency of the applied AC magnetic field It is shown that a flux creep term, added to the current density term in the critical state equation, can account for the observed frequency dependence The magnitude of the peak shift is predicted to increase with decreasing average grain size and decreasing grain boundary junction current density The model predictions are compared with the experimental data of Nikolo et al Some of the parameters used in the calculation are determined by fitting data for χ′ and χ″ over the full temperature range using a recently developed model for granular superconductors In addition, the relation between the intergranular pinning potential and the activation energy, which is extracted from log-frequency versus inverse χ″-peak temperature data, is clarified

Effect of a liquid Phase on Superplasticity of 2‐moI%‐Y203‐StabiIlzed Tetragonal Zirconla Polycrystals

Abstract: .. . A small addition of CuO to 2-mol%-Yz03-stabilized tetragonal zirconia polycrystals significantly enhances superplasticity by forming an amor hous grain-boundary phase containing primarily Cu', Y", Zr", and 0'-. This phase apparently melts at around 1130°C, but it already provides a fast diffusion path even below the melting temperature. There are abrupt changes in stress exponent, activation energy, and grain size exponent across the melting temperature. Superplasticity is diffusion-controlled below the melting temperature and is interfaced-controlled above that. [Key words: yttria-stabilized tetragonal zirconia polycrystals, plasticity, liquid phase, creep, grain boundaries.]

The role of strain energy in creep graphitization of anthracite

Abstract: RECENT research on ceramics and natural minerals has demonstrated that non-hydrostatic stress can affect some polymorphic transitions and can increase reaction rates1,2. One such example is the graphitization of anthracite. Under natural conditions graphite forms at temperatures of 300–500° C and confining pressures of ∼500 MPa (refs 3–9). But in simple heating experiments at ambient pressure and high confining pressure (up to 1 GPa), temperatures of ∼ 2,000 °C are required for graphite formation10–13. Here we report creep experiments on natural anthracite at temperatures of 300–600 °C, using variable strains and strain rates and a constant confining pressure of 500 MPa. The experiments yield an apparent activation energy of 68.6 kJ mol-1 for the steady-state process(es) leading to graphite formation. This value is in marked contrast with simple heating experiments, which require an activation energy of 1,000 kJ mol-1 (ref. 10). We suggest that in our experiments, and also under natural conditions, graphitization is facilitated by available strain energy associated with non-hydrostatic stress; such stresses typify conditions of natural graphitization.

Creep deformation of single crystal superalloys—modelling the crystallographic anisotropy

Abstract: A generalised model of creep deformation in cubic single crystals is developed that considers the combined effects of viscous glide on two (or more) slip systems and accounts for tertiary creep by the accumulation of mobile dislocations with plastic strain. The model is applied to analyse a database of creep curves for the nickel-base single crystal superalloy SRR99 with the assumption that creep deformation occurs by glide on both the {111}〈 1 01〉 and {001}〈110〉 systems. A procedure for the calculation of creep curves and associated crystal rotations for arbitrary crystal orientations is described. The model predicts changes in the anisotropy of creep behaviour with stress and temperature that is in general agreement with the limited available experimental data. It also includes the contributions of all possible slip vectors to predict crystal rotations that are consistent with observations.

Influence of molybdenum on the creep properties of nickel-base superalloy single crystals

Abstract: The Mo content of an alloy series based on Ni-6 wt pct Al-6 wt pct Ta was systematically varied from 9.8 to 14.6 wt pct, in order to ascertain the influence of Mo on the creep properties of single crystals. The optimum initial gamma-gamma prime microstructure for raft development and creep strength was established in each alloy before testing. It was found that, as the Mo content increased from 9.8 to 14.0 percent, the magnitude of the lattice mismatch increased; upon reaching 14.6 percent, a degradation of mechanical properties occurred due to the precipitation of a third phase. These results suggest that small refractory metal content and initial gamma-prime variations can profoundly affect mechanical properties.

Flux creep in Bi2Sr2CaCu2O8 epitaxial films

Abstract: We incorporate the experimentally deduced flux line potential well structure into the flux creep model. Application of this approach to the resistive transition in Bi2Sr2CaCu2O8 epitaxial films explains the power law voltage‐current characteristics and the nonlinear current dependence of the activation energy. The results cannot be accounted for by a transition into a superconducting vortex‐glass phase.

Effects of cooling rate and γ′ morphology on creep and stress-rupture properties of a powder metallurgy superalloy

Abstract: The 650 °C creep and stress-rupture (S/R) behavior of powder metallurgy (PM) RENE 95 alloy was investigated under various cooling rates from two subsolvus solution temperatures. The cooling conditions were chosen to alter the morphology of the cooling γ′ (γ′c), which constituted a high volume fraction (≃0.85) of all γ′ precipitates. The dispersion characteristics of γc′ changed from very fine at the highest cooling rate to progressively coarser with reduced cooling rate. At small γc′ (≃0.05 μm), a marked increase in creep and S/R resistance was observed, whereas at larger γc′ (≃0.1 μm), a marked decrease was observed. Also, for a givenγc′ size, both properties benefited from a higher solution temperature. The change in properties with the size of γc′ was associated with a change in deformation mechanism. The operative mechanism was essentially determined by the mean surface-to-surface spacing(ls) of the γc′ precipitates. Atls> 0.05 μm, the Orowan mechanism of dislocation expansion and looping became favorable, which led to higher primary creep strain and steady-state rate and also reduced S/R life. Atls< 0.05 μm, dislocation motion was significantly restricted, and the low dislocation density and inadequate dislocation sources were responsible for greatly suppressed primary creep strain and steady-state rate, and in some cases, for an incubation period in the creep curve. In addition, the S/R life increased significantly at lowls. Atls ≃ 0.05 μm, a mixed mode of deformation was observed due to inherent distribution of particle spacing in the structure, and this gave rise to intermediate creep and S/R resistance.

Creep Behavior of a SiC-Whisker-Reinforced Alumina

Abstract: Creep tests in four-point flexure loading configuration in air employing applied stresses of 37 to 300 MPa at temperatures of 1200°, 1300°, and 1400°C were performed on 20-vol%-SiC-whisker-reinforced alumina and unreinforced single-phase polycrystalline alumina. The creep rate of polycrystalline alumina was significantly reduced through the addition of SiC whiskers, although strain to failure was lower. Transmission and scanning electron microscopy results suggest that substantial increase in the creep resistance in flexure of alumina composites originates from the retardation of grain-boundary sliding by the SiC whiskers.

ALSPEN-A mathematical model for thermal stresses in direct chill casting of aluminum billets

Abstract: This paper presents the mathematical model ALSPEN, in which the thermally induced strains and stresses which develop during direct chill (DC) semicontinuous casting of aluminum billets are calculated by a finite-element method. The metal is assumed to be an isotropic elasticviscoplastic material with strongly temperature-dependent properties. In the material description, the viscoplastic strain is treated in a “unified” manner, in which low-temperature (approximately) time-independent plasticity and creep at high temperatures occur as special cases. Furthermore, in the numerical time stepping procedure, all of these plastic material properties, which are present simultaneously in the solution domain as a result of the large temperature differences, are treated in a similar way. To demonstrate some of the capabilities of ALSPEN, we have modeled the casting of an AlMgSi alloy, AA6063. The material properties of this alloy have been studied in parallel with the development of the mathematical model.

Crack tip fields under non‐steady creep conditions—i. estimates of the amplitude of the fields

Abstract: — Under non-steady creep conditions, the stress and strain rate fields near the tip of a stationary crack can be described by the singular fields of Hutchinson, Rice and Rosengren for power-law creeping materials. Estimation formulae are presented for describing the amplitude of these fields under load and displacement controlled boundary conditions. For constant loading, the formulae reduce to the result of Riedel and Rice for short times after load application and to the steady state line integral C* for long times. At intermediate times, the estimate is validated by detailed finite-element computation. For displacement-controlled loading, the amplitude of the near-tip fields is shown to fall rapidly, consistent with finite-element analysis. The implications of the results for data collection and defect assessments are discussed in a companion paper.

Fracture mechanism maps for advanced structural ceramics

Abstract: The static fatigue behaviour of advanced structural ceramics can be controlled by a variety of failure mechanisms. A fracture mechanism map can define the stress-temperature regimes where the different mechanisms are dominant. The static fatigue resistance of a hot-pressed silicon nitride with magnesia sintering aid is limited by slow crack growth or creep fracture depending upon the specific stress-temperature conditions. The flexural fracture map is considerably refined relative to earlier versions, and in conjunction with available tension data, was used to create a tension fracture map. The fracture map brings together the findings of a number of studies and can be appreciated by materials scientists and engineers.

Micromechanical Analysis of the Creep Response of Unidirectional Composites

Abstract: The paper outlines the use of the micromechanics model proposed by Aboudi in predicting the creep response of unidirectional composites consisting of linearly viscoelastic matrices and elastic fibers. The closed-form expressions for the effective elastic moduli given in terms of the phase moduli and volume fractions provided by the micromechanics model facilitate a straightforward application of the viscoelastic Correspondence Principle. The inversion of the effective moduli in the Laplace transform domain to the time domain is subsequently accomplished using the Bellman method. The predictions of the model are compared with the creep response of T300/934 graphite/epoxy unidirectional coupons at two different temperatures. Very good correlation between theory and experiment is illustrated for the linearly viscoelastic response characterized by relatively small creep strains.