Transient behavior of diffusion-induced creep and creep rupture

Steady state solutions to three types of diffusion problems: creep, grain boundary sliding and intergranular crack growth, have been published in the literature. This paper considers, in detail, the events which occur between the time when the external stress is applied and the time when the steady state is eventually reached. The time constant of the transient has been calculated. It is shown how the grain boundary tractions change with time from the initial “elastic” configuration (when sliding has been elastically accommodated) to the steady state “diffusional” configuration (when the sliding rate is diffusionally accommodated). This requires infusion of excess grain boundary dislocations; the distribution of these dislocations is calculated. The results are applied to problems of diffusional creep, grain boundary sliding and intergranular crack growth.     

Transient behavior of diffusion-induced creep and creep rupture

Steady state solutions to three types of diffusion problems: creep, grain boundary sliding and intergranular crack growth, have been published in the literature. This paper considers, in detail, the events which occur between the time when the external stress is applied and the time when the steady state is eventually reached. The time constant of the transient has been calculated. It is shown how the grain boundary tractions change with time from the initial “elastic” configuration (when sliding has been elastically accommodated) to the steady state “diffusional” configuration (when the sliding rate is diffusionally accommodated). This requires infusion of excess grain boundary dislocations; the distribution of these dislocations is calculated. The results are applied to problems of diffusional creep, grain boundary sliding and intergranular crack growth.     

Effect of low-pressure oxygen on the creep properties of W-25 pct Re

The creep properties of W-25 pct Re were measured in low-pressure oxygen to simulate its interaction with oxygen derived from the decomposition of oxides at high temperatures. The results are compared with the creep properties of the alloy in a vacuum of 10⁻⁷ torr. Between 1650° and 2000°C and stresses of 1000 and 2000 psi, the creep rates were lower in 10⁻⁵ torr O₂ than in vacuum by factors as high as 250; however, rupture strains were only 2 to 7 pct in the O₂ environment. Specimens exposed to oxygen before creep testing in vacuum were also stronger than specimens tested in vacuum. The oxygen content of specimens tested in oxygen was generally lower than in the as-received material. The lower creep rates of W-25 pct Re in oxygen are attributed to the presence of a surface layer of σ phase. This reaction layer results from the formation of volatile tungsten oxides at a higher rate than rhenium oxides. Formerly with the Oak Ridge National Laboratory     

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 high-temperature creep behavior of nickel-rich Ni-W solid solutions

The steady-state creep behavior of four nickel-rich Ni-W solid solutions (1, 2, 4, and 6 wt pct W) was investigated in the temperature range 850° to 1050°C. Constant stress tensile creep tests were performed in vacuum in the stress range 3000 to 7000 psi. Activation energies for creep were observed to be 71.4 ± 2.0, 74.4 ± 3.0, and 75.8 ±2.0 kcal per mole, after correcting temperature dependence of the elastic modulus, for alloys containing 2,4, and 6 pct W respectively. These values closely approximate the activation energies for the weighted diffusion coefficient, =D _Ni D _W/(X _W D _Ni) whereX _Ni andX _w are the atom fractions of nickel and tungsten respectively, andD _Ni andD _w are the diffusion coefficients of nickel and tungsten in the alloy. The steady-state creep rates exhibit a power law stress dependence with an exponent,n, equal to 4.8 ±0.2 for all of the alloys studied. For tests conducted at temperatures and stresses such that both the diffusivity, , and the ratio of the applied stress to the elastic modulus, σ/E, are. held constant, the steady-state creep rate, , was found to vary with the stacking fault energy, γ, according to the empirical relation ∼ γ^{4 2±4} over the range of creep rates studied. W. R. Johnson, formerly Graduate Student, Stanford Univeristy, Stanford, Calif.     

Solid solution creep behavior of Sn-xBi alloys

This study details the steady-state creep properties of Sn-1 wt pct Bi, Sn-2 wt pct Bi, and Sn-5 wt pct Bi as a function of stress and temperature. All data, including previous work on pure Sn, are described by the following empirical equation:

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 high temperature creep and fracture behavior of 70-30 alpha-brass

Constant stress creep tests have been carried out over a range of stresses at 324°, 413°, 503°, and 550°C. In all cases, over almost the entire creep curve, the time dependence of the true creep strain, ∈, can be described accurately as where ∈₀ is the instantaneous strain on loading, ∈_T the transient creep strain,m a parameter relating to the rate of exhaustion of transient creep, the steady creep rate, ∈_L andP are tertiary creep parameters, andt _t is the time to the onset of tertiary creep. A number of relationships exist between the transient and tertiary parameters in this equation and the steady creep rate, which suggests that the same basic deformation process operates throughout almost the entire creep life. Transient creep ends after a time,t _s, such thatmt _s is a constant (≃4). Similarly, the duration of tertiary creep (t _f – t_t), wheret _f is the time to fracture, depends on the parameter,p, asp (t_f − t_t) is a constant (≃4.5). The time to fracture is found to be related to the parametersm andp asmt _s +pt _f − t_s) =constant (−12) independent of stress and temperature. Formerly Research Student, University College     

Compressive creep behavior of spray-formed gamma titanium aluminide

The creep behavior of spray-formed γ-TiAl with a fine, equiaxed fully lamellar (FL) microstructure was studied in a temperature-stress regime of 780 °C to 850 °C and 180 to 320 MPa. An apparent stress exponent of 4.3 and an activation energy of 342 kJ/mol were observed in the high-temperature high-stress regime. Compared with the FL γ-TiAl which was obtained through conventional casting+heat treatment processes, the spray-formed γ-TiAl exhibited higher creep resistance. The higher creep resistance observed in the present study was discussed in light of the interstitial level, the chemical composition, the grain size, and the interlocking of lamellae at the grain boundary, which in turn may be a function of interlamellar spacing and the step height of the serrated grain boundaries. It was suggested that the small interlamellar spacing and possibly larger step height may contribute to the higher creep resistance observed in the present study.     

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.     

W-34Ni-5Fe合金的流动温压成形

流动温压工艺是一项以短流程低成本制造复杂几何形状零件的粉末冶金新工艺.该文作者自行设计并制造了一套流动温压成形横向流动能力及横向压力测量装置,用于研究流动温压专用混合粉末的制备,考察压制速度、温度及轴向压力对W-34Ni-5Fe混合粉末横向成形能力的影响.结果表明,该混合粉末在一定的温度下具有横向流动的能力,横向压力随压制速度和轴向压力增大而增大;横向距离愈长,横向压力愈小,将混合粉末加热到粘结剂的熔点温度96℃时横向压力最大.实验初步证明了流动温压成形十字形粉末冶金零件的可行性.     

Part II. The creep behavior of Ti-Al-Nb O+bcc orthorhombic alloys

The intermediate-temperature (650 °C to 760 °C) creep behavior of orthorhombic (O)+bcc alloys containing 50 at. pct Ti was studied. Ti-25A1-25Nb, Ti-23Al-27Nb, and Ti-12Al-38Nb ingots were processed and heat treated to obtain a wide variety of microstructures. Creep deformation mechanisms and the effects of grain size, phase volume fraction, tension vs compression and aging on creep rates were examined. Unaged microstructures, which transformed during the creep experiments, exhibited larger primary creep strains than transformed microstructures, which were crept after long-term aging. The deformation observations and calculated creep exponents and activation energies suggested that separate creep mechanisms, dependent on the applied stress level, were dominating the secondary creep behavior. Coble creep characteristics, including relatively low activation energies and dislocation densities as well as stress exponents close to unity, were exhibited at low applied stresses. Experiments on fiducially marked specimens indicated that grain-boundary sliding was occurring for intermediate applied stresses. In this regime, the minimum creep rates were proportional to the applied stress squared and inversely proportional to the grain size. Dislocation-controlled creep characteristics, including stress exponents of greater than or equal to 3.5 and relatively high activation energies and dislocation densities, were exhibited at high stresses. Overall, the minimum creep rates were dependent on microstructure and stress. Within the low-to-intermediate stress regimes, subtransus processed and heat-treated microstructures, which contained much finer grain sizes than supertransus microstructures, exhibited the poorest creep resistance. The influence of grain size was not as significant within the high-stress regime. It is shown that for low-to-intermediate stress levels, grain size is the dominant microstructural feature influencing the creep behavior of O+bcc alloys.     

喷雾干燥-氢还原制备超细/纳米晶W-10Cu粉末及其烧结行为

采用喷雾干燥-氢还原法制备超细/纳米晶W-10Cu(质量分数,%)复合粉末,并经过压制和烧结制备W-Cu复合材料,系统研究烧结温度和保温时间对该材料性能和组织的影响,以及在1 100～1 300℃温度范围内的烧结激活能。结果表明,W-10Cu还原粉末晶粒度仅为30～60 nm;在1 200℃烧结时开始发生明显的致密化行为;随烧结温度升高相对密度增大,当烧结温度升高到1 300℃时W-10Cu复合材料的相对密度为90%,但当温度达到1 460℃时有所降低。1 420℃保温90 min时材料相对密度高达99.1%,且此时晶粒度仅为1.8μm。W晶粒尺寸为30～60 nm的W-10Cu复合粉末在1 100～1 300℃烧结的平均激活能为129.14 kJ/mol。烧结温度为1 420℃时W-10Cu的电导率随保温时间延长先增大后减小,保温90 min时最大达到19 MS/m,超过国标有关规定。     

W-4.9Ni-2.1Fe 高比重合金的摩擦磨损行为

采用高能球磨和放电等离子体烧结技术制备W-4.9Ni-2.1Fe高比重合金,研究不同球磨时间对合金显微组织结构和摩擦磨损行为的影响. 结果表明:当球磨时间较短（2 h）时,合金粉末中Ni、Fe元素仍以单质的形式存在;随着球磨时间的延长,Ni(Fe)溶入W晶格中形成W的过饱和固溶体,W衍射峰强度逐渐变弱,峰形明显宽化,合金试样的相对密度呈下降趋势;适量的球磨时间（24 h）,既可以保证合金中黏结相的含量和均匀分布,又不至于引入过量的杂质元素而引起合金成分改变,合金拥有最优的耐摩擦磨损性能.      

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.