The high temperature creep behavior of doped tungsten wire

The creep behavior of 0.018 cm diam doped tungsten wire has been studied over a range of stress from 30 to 90 MPa and temperature from 2400 to 2800 K. Grain aspect ratio (gar) had a strong influence on creep and rupture of the recrystallized wires, and separated the creep behavior into two regimes with a transitional gar of about 11 between the two. The low gar regime showed lower strength and characteristics typical of grain boundary sliding. In the high gar regime, properties were independent of gar, and evi-dence is presented to show that creep is governed by dislocation-bubble dispersion strengthening. Formerly with the Lamp Business Division of General Electric Company, E. Cleveland, Ohio     

The effect of deformation on abnormal grain growth in tungsten ingots

This paper reports a study of abnormal grain growth in tungsten ingots that had been deformed in compression at elevated temperatures. The results show that very large grains form in these samples directly from the polygonized structure when the deformed samples are annealed. A critical amount of deformation is required to begin this process. Beyond that point, the resulting grain size decreases with increasing amounts of deformation and decreasing test temperature. Abnormal grain growth occurs first in the regions of the sample that have undergone the most strain. Two factors appear to provide the driving force for the formation of these large grains. One is the elimination of grain boundary area. This effect would be present in any system undergoing regular or abnormal grain growth. The other is the elimination of grains that are more highly strained by grains that are less strained. In this way, this process is similar to the large grain growth resulting from strain annealing.     

The effect of substrate on the microstructure and creep of eutectic In-Sn

This study was conducted in order to determine and understand the effect of substrate on the behavior of eutectic In-Sn. Samples for mechanical testing were produced with either bare Cu or Ni on Cu substrates. Both the microstructure and the mechanical behavior are strongly dependent on substrate. When eutectic In-Sn is joined to bare Cu, Cu diffusion into the joint causes the alloy to become off-eutectic, giving a nonuniform and irregular microstructure. The addition of a layer of Ni acts as a diffusion barrier, preventing Cu diffusion sufficiently such that a uniform, normal colony-based eutectic forms. Deformation is more uniform in the In-Sn on Ni, while it is concentrated along the length of the joint in the In-Sn on Cu. This distinction is reflected in the different shapes of shear stress-strain curves between In-Sn on Cu and In-Sn on Ni. The stress exponents and activation energies for creep also vary with substrate. Creep deformation is governed by the In-rich β phase for In-Sn on Cu and by the Sn-rich γ phase for In-Sn on Ni. If In-Sn on Ni samples are aged, the microstructure coarsens and the mechanical behavior changes to resemble that of the as-cast In-Sn on Cu. Formerly with the Department of Materials Science and Mineral Engineering, University of California at Berkeley.     

The effect of grain boundary deformation on the creep micro-deformation of copper

Grain boundary deformation measured using the electron lithographic microgrid technique has been found to give rise to textured regions of compressive longitudinal strain and transverse tensile strain, as well as strain concentrations in a uniaxial tensile creep test. The complexity of the experimental strain distribution is better explained by the use of a multi-grained finite element model of the creep process; that can allow for variations in the grain boundary viscosity.     

The effect of particle reinforcement on the creep behavior of single-phase aluminum

The effect of TiC particle reinforcement on the creep behavior of Al (99.8) and Al-1.5Mg is investigated in the temperature range of 150 °C to 250 °C. The dislocation structure developed during creep is characterized in these materials. The addition of TiC increases creep resistance in both alloys. In pure aluminum, the presence of 15 vol pct TiC leads to a factor of 400 to 40,000 increase in creep resistance. The creep strengthening observed in Al/TiC/15_p is substantially greater than the direct strengthening predicted by continuum models. Traditional methods for explaining creep strengthening in particle-reinforced materials(e.g., threshold stress, constant structure, and dislocation density) are unable to account for the increase in creep resistance. The creep hardening rate(h) is found to be 100 times higher in Al/TiC/15_p, than in unreinforced Al. When incorporated into a recovery creep model, this increase inh can explain the reduction in creep rate in Al/TiC/15_p. Particle reinforcement affects creep hardening, and thus creep rate, by altering the equilibrium dislocation substructure that forms during steady-state creep. The nonequilibrium structure generates internal stresses which lower the rate of dislocation glide. The strengthening observed by adding TiC to Al-1.5Mg is much smaller than that found in the pure aluminum materials and is consistent with the amount of strengthening predicted by continuum models. These results show that while both direct (continuum) and indirect strengthening occur in particle-reinforced aluminum alloys, the ratio of indirect to direct strengthening is strongly influenced by the operative matrix strengthening mechanisms. This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

The stress-dependence of high temperature creep of tungsten and a tungsten-2 Wt pct ThO2alloy

Constant-load creep tests were conducted with pure tungsten and a W-2 wt pct ThO₂ alloy at temperatures between 1600° and 2200°C and at strain rates of about 1 × 10^-8 to 4 × 10^-5 sec^-1. The results were evaluated by the empirical correlations of Robinson and Sherby and also Mukherjeeet al. which describe the stress dependence of the creep of metals and alloys. The agreement of the present experimental data with these correlations was found to be poor. However, when the following empirical relationship was used: •ε _c =A’(σ _c /σ _f ) ⁿ the present creep data for tungsten and the tungsten alloy at various temperatures were much better correlated. Here, •ε _c is the experimental creep rate, σ_c is the applied stress for creep, σf is the flow stress of the material at the same temperature in a constant strain rate tensile test, andA’ is function of temperature, structure, and strain rate.     

Grain-shape parameters for high-temperature creep resistance in powder metallurgy tungsten fine wires

Three grain-shape parameters, f ₁, f ₂, and f ₃, are defined to clarify the morphological effect of grains on the high-temperature creep resistance under the condition that no grain boundary cavitation occurs. The parameter f ₁ is characteristic of complicated grain shapes, suggesting that it can be a measure of the interlocking grain structure. The parameter f ₂ is an important parameter when torsional stresses are imposed on coiled wires, and f ₃ is characteristic of the short-range roughness of grain boundaries only when f ₁ is not greatly changed. The minimum creep rate decreases as the grain aspect ratio, R, increases for R <30, and the creep rate increases as R increases for R >30. The parameter f ₁, as well as Raj and Ashby’s model, gives a satisfactory explanation for the former behavior. On the other hand, it is proven that their model must be modified using f ₃ to explain the latter behavior because of the highly elongated grain configuration that is associated with R >30.     

The effect of rapid thermal cycling on the creep properties of alpha iron

The effect of rapid, low amplitude thermal cycling on the creep properties of alpha iron was investigated. It was found that creep rates under thermal cycling conditions are lower than the creep rates calculated from steady-state isothermal creep experiments. This relative decrease of the creep rate was dependent on the dwell time and the applied stress. The maximum decrease in creep rate occurred when the time at the lower temperature was very short. In single cycle creep experiments it was found that an abrupt temperature decrease was followed by a delay period of zero creep rate. When the temperature was again increased, a period of inverse transient creep was observed. It is concluded that the inverse transient creep is responsible for the relatively lower creep rates observed under thermal cycling conditions. Investigations of the dislocation substructure with the electron microscope did not show any significant changes attributable to thermal cycling. The lowering of the creep rate is tentatively explained on the basis of dislocation pinning by point defects during the cooling part of the cycle which inhibits subsequent dislocation motion during the rest of the cycle and hence decreases the overall creep rate. D. EYLON, formerly with the Department of Materials Engineering, Technion     

The effect of low gold concentrations on the creep of eutectic tin-lead joints

The effects of low Au concentrations on the creep properties of a eutectic Sn/Pb alloy were investigated. Creep testing was performed on double-shear specimens of fine-grained, eutectic Sn/Pb joints with Au concentrations of 0, 0.2, 1.0, and 1.5 wt pct Au at 90 °C, 0, 0.2, and 1.0 wt pct Au at 65°C, and 0.2 wt pct Au at 25 °C. In the absence of Au, the creep of finegrained eutectic Sn/Pb is dominated by grain-boundary sliding at high homologous temperature and intermediate stress. The addition of 0.2 wt pct Au or more suppressed this mechanism; the high-stress, bulk-creep mechanism was dominant at all stresses tested. Higher concentrations of Au increased porosity within the joints. The porosity decreased joint strength. During failure, the crack path followed softer regions of the joint; cracks propagated through Pb-rich islands or along Sn/Sn grain boundaries.     

The effect of SiC particle reinforcement on the creep behavior of 2080 aluminum

The influence of SiC particle reinforcement on the creep behavior of 2080 aluminum is investigated between 150 °C and 350 °C. The effect of particle size (F-280, F-600, and F-1000), volume fraction (10, 20, and 30 vol pct), and heat treatment (T6 and T8) on creep behavior is studied. In both the T6 and T8 conditions all composites are less creep resistant than similarly heat-treated monolithic materials when crept at 150 °C. These results contradict continuum mechanics predictions for steady-state creep rate, which predict composite strengthening. A high dislocation density is observed near SiC particles. It is proposed that strain localization near the reinforcements leads to microstructural breakdown and the subsequent reduction in creep resistance. When both materials are severely overaged or when they are tested at very high temperatures (350 °C), composite materials exhibit improved creep resistance relative to monolithic material. In these cases, the strengthening is consistent with continuum predictions for direct composite strengthening.     

The effect of solute additions on the steady-state creep behavior of dispersion-strengthened aluminum

The effect of solute additions on the steady-state creep behavior of coarse-grained dispersionstrengthened aluminum alloys was studied. Recrystallized dispersion-strengthened solid solutions were found to have stress and temperature sensitivities quite unlike those observed in single-phase solid solutions having the same composition and grain size. The addition of magnesium or copper to the matrix of a recrystallized dispersion-strengthened aluminum causes a decrease in the steady-state creep rate which is much smaller than that caused by similar amounts of solute in single-phase solid solutions. All alloys exhibited essentially a 4.0 power stress exponent in agreement with the model of Ansell and Weertman. This was observed even in alloys whose matrix shows a much lower stress exponent when tested as a single-phase solid solution. The activation energy for steady-state creep in dispersion-strengthened Al−Mg alloys, as well as the stress dependence, was in agreement with the physical model of dislocation climb over the dispersed particles. For the dispersion-strengthened Al−Cu alloys, the activation energy for steady-state creep suggested that the kinetics of dislocation climb may be related to the mobility of copper'atoms in the matrix. Electron microscopic examination of the dislocation structure present after steady-state creep had been reached suggested that few mobile dislocations are involved in the steady-state creep process at the stress levels employed in this investigation. This paper is based on a thesis submitted by G. H. REYNOLDS in partial fulfillment of the requirements of the degree of Doctor of Philosophy at Rensslaer Polytechnic Institute.     

The effect of interstitial sinks on creep of a dispersion strengthened columbium-base alloy

The effects of interstitial sinks on the structure and creep behavior of the dispersion-strengthened columbium base alloy., D43 were examined. Interstitial sinks change the structure of the alloy and render it less creep resistant by reducing the carbon concentration and, therefore, the volume fraction of the dispersed phase. Where the interparticle spacing is small (<≈1 μ) the alloy is strengthened by the direct interaction of dislocations with particles. Where the interparticle spacing is large, subgrains are probably a more important structural feature strengthening the alloy. The creep rate can be described by the empirical expression \(\dot \varepsilon = A\sigma ^n e^{ - H/RT} \) , whereH (106 to 112 kcal per mole) andn (7.7 to 8.3) are independent of the amount of dispersed phase, andA is proportional to the reciprocal carbon concentrations in the range 100 to 800 ppm.     

The effect of rapid thermal fluctuations on the creep rate in Inconel 718

The creep properties of Inconel 718 under thermal cycling conditions were investigated. It was found that the creep rates under thermal cycling conditions are lower than the creep rates calculated from results of isothermal creep experiments. It was also found that an abrupt temperature decrease during steady-state creep is followed by a certain period of zero creep rate. It was shown that the decrease of strain rate during creep under cycling conditions is due to this stopping effect.     

添加钴对W-Ni-Fe高密度合金性能的影响

在原料粉末中加入微量的Co元素,用粉末冶金液相烧结法制备了W-Ni-Fe高密度合金;采用金相显微镜、SEM等仪器对合金组织和杂质分布进行了分析。研究结果表明:添加钴元素后,增强了基体相对钨颗粒的润湿性,使钨颗粒表面更加圆滑,更加有利于塑性变形;提高了合金的钨颗粒与基体相之间的界面结合强度,从而提高了合金的强度和延伸率。     

The effect of carbon content on the mechanical properties of tungsten carbide-cobalt hard alloys

Summary A study was made of the effect of graphite and-phase contents on the properties of VK8 alloy. It was found that, in the presence of the-phase in the alloy in the form of lakes or dendrites (lace), indicating carbon deficiency, the strength of the alloy is greatly reduced. Slight decarburization, taking the form of-phase formation along the interphase boundaries, has practically no effect on the strength of VK8 alloy. The presence of graphite in an amount of less than 0.5 vol. % has little effect on the wear resistance of VK8 alloy, and slightly raises its strength. A high graphite content substantially lowers both strength and wear resistance.     

The effect of cerium on high temperature tensile and creep behavior of a superalloy

The presence of trace impurities such as S and O can cause embrittlement during elevated and high temperature deformation of iron or nickel-base alloys. In this study various amounts of Ce, ranging from 0 to 0.24 wt pct, have been added to an iron-nickel base superalloy, Udimet 901, in order to study its role in the refining process of S and O in the melt, hot workability, creep and stress rupture, and microstructure. It is found that Ce addition decreases the 0 and S content in the melt and improves both the hot workability and creep ductility. An optimum residual Ce content of 30 ppm was found for which ductilities are maximum. Higher residual cerium contents result in deleterious hot embrittlement. SEM as well as TEM/STEM microscopy combined with X-ray EDS spectroscopy were used to determine the inclusion content present in the alloy as well as the fine spatial microchemistries, especially at grain boundaries.