High temperature creep of WC-Co alloys

The high temperature deformation of tungsten carbide-cobalt composites has been performed in the 1050 to 1350° C temperature range either in compression or in three point bending. The creep behaviour of these materials exhibits a sigmoidal log plot with three domains of steady state creep rate possessing a different value of the stress exponent. From recent microscopical and analytical results a microstructural model of the composite is proposed. This model assumes the existence of carbide chains formed of crystals linked with coincidence grain boundaries and takes the number of coincidence grain boundaries into account. It has been used to interpret the creep results. The controlling component of the composite is the tungsten carbide and two deformation processes, grain boundary sliding and intragranular deformation, are the main mechanisms which are involved in the explanation of the creep behaviour. The results of the creep tests are discussed in correlation with transmission electron microscope (TEM) investigations on annealed then deformed samples. TEM microstructural studies of the carbide phase after deformation show an extensive intragranular deformation which has been carefully analysed. Attention has also been paid to the defect structure of grain boundaries.     

Effect of temperature on fatigue crack growth in unplasticized polyvinyl chloride

The effect of temperature on the fatigue crack growth in a 150 mm class 12 unplasticized polyvinyl chloride (uPVC) pipe-grade material overa temperature range –30–60°C was studied. The Arrhenius relationship between fatigue crack growth rate, da/dN, and absolute temperature, T, was found to describe the experimental data very well independent of the applied stress intensity factor range, K. In the temperature range –30–10°C the activation energy was 8.8 kJ mol^–1 and between 26 and 60°C this was 30 kJ mol^–1. The two activation energies were found to be associated with two distinctly different crack growth mechanisms. In the low-temperature range there was a predominant shear mechanism, but in the high-temperature range multiple crazing was the major fatigue mechanism. Finally, a stress intensity factor—biased Arrhenius equation for fatigue crack growth was suggested and found to predict rather accurately the data of uPVC, as well as those of other polymeric materials at different temperatures.     

High temperature creep in polycrystalline AIN-SiC ceramics

Polycrystalline dense ceramic specimens containing 75 mol % AIN-25 mol % SiC and 60 mol % AIN-40 mol % SiC were subjected to creep deformation in bending at elevated temperatures. Over the range of temperatures and stresses investigated, the creep rate was found to vary linearly with stress indicative of diffusional creep. Creep was found to be thermally activated with activation energy in the range from 175 kcal mol⁻¹ to 219 kcal mol⁻¹. Electron microscopic observation indicated that crack like cavities formed near the tensile surfaces during creep.     

High temperature creep behaviour of single crystal oxides

The creep resistance of several single crystal oxides is evaluated on the basis of creep data from different sources using a Larson-Miller (L-M) method. The possible creep mechanisms involved in high temperature creep deformation of single crystal oxides are discussed by comparing the collected creep data with theoretical creep models. The high temperature creep of single crystal oxides is generally considered as a diffusion-controlled process: dislocation climb controlled by the lattice diffusion of the slowest moving species (power law) at moderately high stresses, Harper-Dorn creep at low stresses, and power law breakdown at high stresses. The relative comparison of the creep data from different sources using the L-M method and the general analysis about the high temperature creep behaviour indicate that single crystal oxides with a precise stoichiometric composition, complex crystal structure and selected orientation such as [111] oriented YAG (Y₃Al₅O₁₂),c-axis Al₂O₃, [110] oriented MgAl₂O₄ are potential candidates as reinforcements for very high temperature structural applications.     

High temperature creep behaviour of single crystal superalloy

Abstract

Creep behaviour of a newly developed single crystal nickel base superalloy DMSX-l (Ni-7.7Co-6.4Cr-8.4W-0.3Ti-S.S7 Al-7.6Ta-Nf-Y-Nb-Re, wt-%) with 〈001〉 orientation has been analysed and compared with the reported data ofSRR99. It was concluded that the shear stress based model developed for SRR99 is also applicable for the new alloy. Although the material constants for octahedral slip for the new alloy are not exactly the same as those of SRR99, the estimated values of initial creep rate and softening coefficients are nearly of the same order. Therefore it is concluded that in the absence of a material database for cube slip for this new alloy, those available for SRR99 could be used to predict the orientation dependence of its creep behaviour.     

High temperature creep behaviour of an Ni-Cr-W-B alloy

The high-temperature creep behaviour of a solid-solution strengthened Ni-Cr-W-B alloy was studied, with emphasis on microstructural parameters. Creep strength was determined from tests conducted at 925°C/40 MPa. Various techniques of analytical electron microscopy were used to characterize the microstructure and microchemical composition. A number of microstructural parameters which promote creep strength, including (1) pinning of grain boundaries by tungsten-rich M₆C carbide, (2) relatively low stacking-fault energy, and (3) boron segregation to M₂₃C₆ carbide, were identified. However, their beneficial effects were suppressed by the initial presence of discontinuously precipitated M₂₃C₆ carbide at grain boundaries which accelerated intergranular cracking. Suppression of the discontinuous grainboundary reaction and a significant improvement in creep strength could be achieved by a proper heat treatment which appeared to induce a sufficiently high defect density promoting intragranular carbide precipitation. Competition between intergranular and intragranular precipitation was found to be influenced by an external stress. Strengthening by intragranular carbide precipitates appeared to occur by an attractive interaction with dislocations. Dislocations bowing out at subboundaries, cross-slip, motion of jogged screw dislocations and generation of dislocations at high-angle grain boundaries appeared to operate simultaneously as strain-producing mechanisms during steady-state creep.     

High temperature creep resistance of metal matrix composites

Abstract

A new approach to the effective creep resistance of two phase composites at high temperature has been developed through direct analogy between creep resistance and field properties (or transport properties). The new approach can take into account implicitly the effects of size, shape, orientation, and distribution of the reinforcement through the topological parameters. Therefore, it can be applied to a two phase composite containing particles with any size, shape, orientation, and distribution. Compared with Saltzer and Schulz's model, which can only be applied to composites with low volume fraction of the reinforcement, the present approach can predict the variation of creep resistance of two phase composites over the whole range of microstructures (from completely discontinuous to completely continuous) and volume fractions, and more importantly is in better agreement with experimental results. In addition, the effect of particle distribution on the effective creep resistance of two phase composites has been demonstrated quantitatively. The effective creep resistance increases with increasing contiguity of the reinforcing phase.     

High temperature creep of ultrafine-grained Fe-doped MgO polycrystals

Creep deformation in ultrafine-grained (0.1 to 1μm) Fe-doped magnesia polycrystals is studied in compression, at temperatures of 700 to 1050° C, and constant loads of 50 to 140 MPa. The stress exponent observed to be nearly unity and the strong grain size sensitivity (ė∼d ^−2.85) suggest that diffusional creep mechanisms dominate the deformation. In the grain size range of the present study the grain boundary diffusion contribution is significantly more important than lattice diffusion. Magnesium is tentatively identified as the rate-controlling species along grain boundaries from an analysis of the diffusivities inferred from the present work and from other authors for Fe-doped magnesia. Associated with the CNRS.     

High temperature creep of lithium zinc silicate glass-ceramics

The creep and recovery behaviour in compression of two lithium zinc silicate glassceramics is established over the temperature range 590 to 750° C at stresses up to 91.4 MN m^–2. It is shown that the transient creep obtained is linearly viscoelastic and obeys the Boltzmann superposition principle. The activation energy of the rate-controlling process is the same as that found for secondary creep and is due to viscous flow of the residual glass phase. A simple method of analysis of the strain-time curves is presented, which can be modified to apply to stress relaxation tests.     

High temperature creep behaviour of nearly stoichiometric alumina spinel

Creep behaviour of nearly stoichiometric spinel, (Al₂O₃)_n MgO, n=1.1, is investigated for compression axis [001], at temperatures 0.77 to 0.83 T _m and constant loads 88.2 to 117.6 MPa. Experimental observations, including the mechanical creep law and the dislocation substructures as imaged by TEM and Berg-Barrett X-ray topography support the following picture: {100} 110 slip is activated in the very early creep stage, while no evidence for {111} is found; on the other hand {111} slip planes are observed for stress orientation [110], in agreement with Schmid's law. (ii) When in edge orientation slip dislocations become sessile by pure climb splitting. Their dissociation plane has been determinated unambiguously and observed to be perpendicular to their Burgers vector. As a result, it is suggested that slip should be inhibited and further creep should occur by pure climb strain only. This expected climb-creep accounts for experimental rates and, tentatively, for their dependence on stoichiometry n since the latter is observed to change only pre-exponential terms, the creep energies being much the same whatever the value of n.This paper is part of the work done by RD for a Thèse d'Etat dissertation.Associated with CNRS.     

High temperature mechanical spectroscopy and creep of calcium hexaluminate

Samples of calcium hexaluminate (CA₆) were studied by four-point bending creep tests and mechanical spectroscopy at temperatures between 1300 and 1600 K. By using the temperature-compensated time concept, proposed by Dorn (1954, 1956), activation enthalpies of the order of 620 kJ/mol were deduced from both the isothermal creep and the internal friction measurements. A generic curve, “ master curve”, is obtained by a superposition of the isothermal mechanical loss spectrum along the temperature-compensated frequency axis. The master curve is composed of two components: a high-frequency part (peak) and a low-frequency part (exponential background). Both the peak and the background decrease after performing torsional creep. Additionally, the peak shifts towards higher frequency after annealing. The high temperature mechanical loss behavior of CA₆ is discussed in terms of a dislocation model invoking anelastic and viscoplastic relaxation phenomena.     

Effects of processing on fatigue crack growth and creep rupture in unplasticized polyvinyl chloride (uPVC)

The effects of processing of unplasticized polyvinyl chloride (uPVC) pipes on fatigue-crack growth in terms of mean stress, frequency and fatigue-crack initiation, as well as on creep rupture, were investigated. No significant processing effect was observed on fatigue crack growth rates and fatigue crack initiation. However, creep rupture with three-point bending tests was significantly affected by the processing level. Two orders of magnitude difference in time to failure were found between well and poorly processed pipes caused by the large difference in the stress intensity factor at fracture instability between these pipes.     

Fracture of slightly plasticized polyvinyl chloride

Impact tests and fracture toughness tests using compact tension specimens were carried out on a number of slightly plasticized PVC compositions. These measurements, together with calculations from the craze thickness profile were used to determine the fracture mechanisms operating in the various tests. The marked decrease in the impact strength of PVC on addition of small amounts of a conventional plasticizer was found to be due to the plasticizer decreasing the stress intensity necessary to nucleate a craze at the notch tip of the impact specimen. The fracture toughness of the compact tension specimens which failed by a crazing mechanism increased with increasing plasticizer content. It is thought that the fracture of these specimens is controlled by the stress intensity necessary to propagate the pre-existing crack/craze system through the material.     

Elastic behaviour of polyvinyl chloride melts

A study has been made of the elastic behaviour of Polyvinylchloride commercial samples at high shear stresses, under similar conditions to those used in the processing industry, by means of an extrusion capillary rheometer.The influence in the die swell of variables as molecular weight, temperature and shear rate has been studied. Results are in agreement with general literature data.Smooth extrudates are obtained only in the zone of intermediate shear rates. In this zone it is observed that the die swell increases with shear rate.     

High temperature stability of 2.25Cr-1Mo steel during creep

The creep rapture behaviour of 2.25Cr—1Mo steel in air and in a salt mixture was studied. The salt coating, which can form a liquid phase at the test temperatures, increased the creep rate and reduced the rupture life of the material. The coating reduced the available cross-section of the material by removing the surface layers, thereby resulting in a reduction of the rupture life. Cross-sections of coated samples showed an outer oxide layer comprising oxide of the metal and precipitates of sulphide at the metal/oxide interface. This subsurface penetration of the corrodants was responsible for the early failure of the coated samples. This is typical of hot corrosion mechanisms. The formation of various carbides like M₂₃C₆ and M₆C, as observed by transmission electron microscopy, during creep reduced the creep strength of the material both in air and in the coated state. Increasing temperature enhanced the formation of these carbides with a consequent decrease in creep strength. Applied stress did not seem to play much of a role in the degree of carbide precipitation.     

Performance properties of pigmented polyvinyl chloride nanocomposites

This work highlighted the role of blue CoO·MgO·Al₂O₃ pigments in changing some properties of PVC composites such as electrical, mechanical and thermal properties. The pigments were prepared by doping different ratios of magnesium in cobalt aluminate crystals using solid–solid interaction. The prepared pigments were characterized by different instrumental analysis (e.g. XRD, SEM and TEM). The influences of different concentrations of nanosized CoO·MgO·Al₂O₃ pigment on PVC prepared by solution blending were studied. The obtained data revealed that PVC nanocomposites containing 1CoO·1MgO·Al₂O₃ pigments show the most promising results. The composites containing 5 wt% of the three compositions of pigments exhibit the optimum electrical, mechanical as well as thermal properties.