Creep Of Alloys Research Articles

Effect of solute segregation on thermal creep in dilute nanocyrstalline Cu alloys

The effect of solute segregation on thermal creep in dilute nanocrystalline alloys (Cu–Nb, Cu–Fe, Cu–Zr) was studied at elevated temperatures using molecular dynamics simulations. A combined Monte-Carlo and molecular dynamics simulation technique was first used to equilibrate the distribution of segregating solutes. Then the creep rates of the diluted Cu samples were measured as functions of temperature, composition, load and accumulated strain. In Cu–Nb samples, the creep rates were observed to increase initially with strain, but then saturate at a value close to that obtained for alloys prepared by randomly locating the solute in the grain boundaries. This behavior is attributed to an increase in grain boundary volume and energy with added chemical disorder. At high temperatures, the apparent activation energy for creep was anomalously high, 3 eV, but only 0.3 eV at lower temperatures. This temperature dependence is found to correlate with atomic mobilities in bulk Cu–Nb glasses. Calculations of creep in nanocrystalline Cu alloys containing other solutes, Fe and Zr, show that the suppression of creep rate scales with their atomic volumes when dissolved in Cu.

Stochastic model of nonisothermal creep and long-term strength of materials

A stochastic model of nonisothermal creep and long-term strength of metallic materials is proposed. Experimental data on the creep of the ZhS6KP alloy at temperatures equal to 900, 950, and 1000°C are stochastically analyzed. These experimental data are used to substantiate the hypotheses applied in constructing the model. The stochastic model is checked for adequacy to the experimental data on the creep of the ZhS6KP alloy under stationary and nonstationary loading conditions. It is shown that the calculated and experimental data are in satisfactory agreement.

Analysis of creep–fatigue life prediction models for nickel-based super alloys

Creep fatigue is the main damage form of engine parts operating under both cyclic mechanical and thermal loading conditions. In the present work time-dependent creep and cycle-dependent fatigue of nickel-based super alloys is discussed. Three different kinds of creep–fatigue life models are investigated for the nickel-based super alloys. Effects of temperature, loading ratio and loading hold time to material failure are considered. Comparison with experiments reveals that the linear accumulation concept based on Forman model provides reasonable results on the creep–fatigue interaction if the life is dominated by mechanical fatigue damage. For higher loading ratios, however, deviations become significant. In the lower frequency region the modified FCE models and Saxena models give more reasonable predictions, but show large discrepancy to tests with hold time. The modified Saxena model suggested in the present work contains a separate term for hold time effects and agrees with the experimental observations more accurately.

Evolution of Precipitate Microstructure During Creep of an AA7449 T7651 Aluminum Alloy

Creep forming is a process where plastic deformation is applied at the material’s aging temperature. It enables to obtain parts of complex shape with reduced internal stresses and finds applications, for instance, in the aerospace industry. In this article, we report in-situ small-angle X-ray scattering measurements during creep experiments carried out on an AA7449 Al-Zn-Mg-Cu alloy in the T7651 temper. In the range of temperatures of 413 K to 453 K (140 °C to 180 °C), we show that the initial microstructure is not stable with respect to the applied stress/strain. Accelerated precipitation coarsening is shown to occur, clearly related to the plastic deformation. This strain-induced microstructure evolution is shown to happen even at temperatures well below the aging temperature that has led to the initial temper.

Microstructure Evolution and Analysis of a Single Crystal Nickel-Based Superalloy during Tensile Creep

By means of the finite element method (FEM) for calculating the von Mises stress and strain energy density in the cubic γ/γ′ phases, the regularity of γ′ phase directional growth is investigated. Results show that the change of the strain energy density in the different planes of the cubical γ′ phase occurs during tensile creep of alloy, the cubical γ′ phase is directionally grown, along the crystal plane with bigger strain energy, to transform into the mesh-like rafted structure along the direction perpendicular to the applied stress axis. The change of the atomic potential energy, interfacial energy and lattice misfit stress is thought to be the driving force for promoting the elements diffusion and directional growth of γ′ phase.

Irradiation-induced creep in nanostructured Cu alloys

Irradiation-induced creep measurements were performed on pure Cu and Cu93.5W6.5. By using 1.8 MeV Kr+ irradiation of free-standing 300 nm films, nearly homogeneous damage in excess of 100 dpa could be obtained, yielding transient and steady-state creep regimes. The steady-state creep rate was proportional to the applied stress, yielding irradiation creep compliance of 1.2 and 0.32 × 10−4 MPa−1 dpa−1 in Cu and Cu93.5W6.5, respectively. The microstructure of the two-phase alloy was highly stable but that of pure Cu was not.

Experimental analysis of creep and fatigue of light multifunctional aluminium alloys

The paper presents experimental investigations of two light aluminium alloys (AlSi8Cu3 and AlSi7MgCu0·5) under loading conditions leading to the creep or fatigue processes. During the creep process, the strains were measured under selected tensile stresses and temperatures. An influence of a step loading during creep, at elevated temperature, on steady creep rate and a course of creep curve, was investigated. Moreover, an effect of porosity degree on creep of both aluminium alloys was also analysed. An influence of aging process at the elevated temperature was experimentally evaluated. Fatigue behaviour of materials was checked under strain control using cylindrical samples. The results of LCF tests were analysed for the materials subjected to three‐stage sequence of loading at elevated temperature. A resistance to creep and fatigue processes of the AlSi8Cu3 was weaker than that obtained in the case of the AlSi7MgCu0·5.

粉末圧延法で作製した鉛蓄電池用Pb基合金格子材料のクリープ挙動

For extending the life of a lead-acid battery, the control of the creep of a Pb-base alloy as the grid material is of great importance. In this study, Pb-1.50 mass%Sn alloys were produced by two different processes, namely the cast-rolling process and the powder-rolling one, in order to study the influence of the crystallographic structure on the creep behavior of the Pb-base alloy. The structure produced by the cast-rolling, which is the commonly used process in the lead-acid battery industry, consists of relatively large grains. On the other hand, the powder-rolling is a new process for the preparation of the grid material, which uses air-atomized powders as the raw materials. The mean size of crystal grains of the powder-rolled Pb-base alloy is extremely small in comparison with that of the cast-rolled one. The tensile strength and the Vickers hardness of a Pb-1.50 mass%Sn powder-rolled alloy are 23 MPa and ca. Hv 9.0, respectively. Although these values are twice larger than those of a Pb-1.50 mass%Sn cast-rolled alloy, they are lower than the values of the Pb-base alloy in common use of the VRLA battery, or a Pb-0.08 mass%Ca-1.20 mass%Sn cast-rolled alloy. However, the corrosion creep test showed that the steady creep rate of a Pb-1.50 mass%Sn powder-rolled alloy was extremely smaller than those of all the Pb-base cast-rolled alloys. This finding suggests that the powder rolling process is very effective to improve the creep resistance of a Pb-base alloy as a grid material in a lead-acid battery. It can be also presumed that fine grain size in the powder-rolled alloy of Pb-1.50 mass%Sn function as an obstacle to the movement of dislocation, resulting in remarkably the suppression of the creep rate.

Effect of minor elements on the creep resistance of Mg-Zn-Mn-(Ca) alloy

Along with alumium, titanium and composite alloys, magnesium alloys have been given much attention by industry for applications such as lightweight automobiles and electronics because of their high strength, low specific density and good damping characteristics. In this paper, creep tests were done with magnesium alloys (Mg-3% Zn-1% Mn, Mg-1.2% Zn-1% Mn, and Mg-3% Zn-1% Mn-0.3% Ca) containing different amounts of Zn to investigate the effect of Zn and Ca on the deformation behavior and the rupture time for Mg alloy creep under elevated temperatures. The alloys were obtained as follows: (1) pure magnesium (9.7 kg) was melted at 720°C in an SF6 atmosphere; (2) the temperature was increased up to 800–820°C after adding 0.3 kg of pure Mn to make the Mg-1% Mn master alloy; (3) the minor element (Zn, Ca) was added to the master alloy; and (4) the magnesium alloy melts were cast into a metallic mold preheated to 150°C. The creep tests were executed under a constant load and temperature to measure the steady-state rate and rupture time of creep. Based on the experimental results, the creep behavior of the alloys seemed to be controlled by dislocation climb at around 0.5∼0.55Tm (Tm; melting temperature). In addition, the results showed that the addition of Ca was effective for increasing the creep resistance of a Mg alloy: the more Zn present in the alloy, the stronger the creep strength of the alloy.

Theory of wave propagation in marine sediments.

Saturated, unconsolidated marine sediments support sound waves and shear waves, each of which is dispersive in that its phase speed varies with frequency and the corresponding attenuation scales as the first power of frequency over an extended frequency range. A theory of sound wave and shear wave propagation in unconsolidated marine sediments has recently been developed in which the dissipation is based on a particular form of intergranular interaction involving the molecularly thin layer of pore fluid at grain contacts. As grain-to-grain sliding progresses, the contact is postulated to become progressively stiffer, a phenomenon known as strain-hardening, adapted from the literature on creep in metals and metal alloys. The theory returns algebraic expressions for the phase speed and attenuation of both types of wave, and these expressions depend on frequency as well as the geoacoustic parameters of the sediment. A comparison of the theoretical dispersion relations with data obtained during the ONR-supported SAX99 experiment shows good agreement over the frequency range range from 2–400 kHz. The theoretical expressions also fit measurements, at a fixed frequency, of phase speeds and attenuations versus porosity, overburden pressure, and mean grain diameter. [Research supported by ONR.]

A first study of the high-temperature plasticity of ceria-doped zirconia polycrystals

Ceria–zirconia ceramic alloys were sintered by high-temperature annealing, considering several synthesis temperatures to obtain a full-dense ceria–zirconia ceramic material using a temperature as low as possible. It was found that fully density is achieved at temperatures of 1450 °C. Monolithic specimens were crept under compression at high temperatures. The creep results fitted an empirical constitutive equation consistent with a classical Ratchinger mechanism for grain switching. This result was confirmed through microstructural characterization of as-received and post-mortem specimens. Since the conventional Ashby–Verrall model is contrary to the mechanism controlling creep in other zirconia alloys, the results are considered in the framework of a new grain boundary sliding model, with particular discussion of the validity of that model for the ceria–zirconia case.

Room temperature creep in amorphous alloys: Influence of initial strain and free volume

The role of imposed strain on the room temperature time-dependent deformation behavior of bulk metallic glasses (BMGs) was systematically investigated through spherical nanoindentation creep experiments. The results show that creep occurred even at very low strains within elastic regimes and, interestingly, a precipitous increase in creep rate was found in plastic regimes, with BMG that had a higher free volume exhibiting greater creep rates. The results are discussed in terms of prevailing mechanisms of elastic/plastic deformation of amorphous alloys .

Creep and Low Cycle Fatigue Investigations of Light Aluminium Alloys for Engine Cylinder Heads

Abstract: An influence of the chemical composition, porosity and ageing on mechanical behaviour of light, multifunctional aluminium alloys (AlSi8Cu3 and AlSi7MgCu0.5) subjected to creep and low cycle fatigue (LCF) was investigated. The materials were tested to verify their applicability as the cylinder heads in car engines. During creep tests, a strain response of the materials was observed under a range of the step‐increased stresses and different temperatures. The LCF tests were carried out under strain control in three blocks of 100 cycles each with a constant strain amplitude. The results of creep and LCF tests were analysed with regard to chemical composition, type of porosity and ageing of the materials tested. An influence of porosity on the creep resistance and lifetime was considered. The results of the LCF tests were compared for the materials in the as‐received state and after ageing. An experimental evaluation of cyclic behaviour because of the LCF was carried out to check whether the hardening or softening effects can be observed in the materials. Taking into account the various history of loading, a stress response of the materials was investigated.

Microstructure and impression creep of age hardenable AA2219 aluminium alloy modified by Sc, Mg and Zr additions

The present work pertains to impression creep of age hardenable AA2219 Aluminium-copper (6.3%) alloy correlated with microstructure with the addition of scandium, magnesium and zirconium. These additions were systematically varied by preparing alloys of different composition using gas tungsten arc melting. Impression creep technique was used to study the high temperature stability of the alloys. Modified compositions of the alloy resulted in fine equiaxed grains, refined eutectics, large number of high temperature stable and fine precipitates. Among all the compositions, 0.8%Sc+0.45%Mg+0.2 %Zr addition was found to be significant in improving the high temperature stability of AA2219 alloy. From the calculated values of stress exponent (n) and activation energy (Q), mechanisms of dislocation-climb and glide interaction with the precipitate particle are proposed.

Constitutive equations in creep of 7B04 aluminum alloys

A unified ageing–creep constitutive model has been developed for the analysis and simulation of the correlation between the mechanical properties and strain and age creep parameters of aluminum alloy. The input parameters of the equation consist of initial stress level and heat treatment parameters. The outputs of this model include property parameters namely: creep strain, yield strength, precipitates relative volume fraction, and mean radius of precipitates. The model can be used to describe the development of microstructure and yield strength following creep time. The current study achieved a good performance and the results are in agreement with experimental knowledge in different stress levels. The development of the yield strength from the precipitates hardening of view is analyzed and discussed in detail.

A comparative study of room-temperature creep in lead-free tin-based solder alloys

Abstract Creep behavior of the tin-based lead-free Sn-0.7Cu, Sn-3.8Ag, Sn-9Zn, Sn-5Sb and Sn-2Bi binary alloys together with pure Sn, as the material for comparison, was studied by impression and indentation creep testing at room temperature. Stress exponent values in the range 3.5 to 14.7, obtained by the two methods, were in good agreement with each other and with those determined by room-temperature creep testing of the same materials reported in the literature. All of the tin-based alloys showed creep resistances higher than pure Sn. Among all tested materials, as indicated by their steady-state creep rates, Sn-2Bi showed the highest creep resistance followed by Sn-5Sb, Sn-9Zn, Sn-3.8Ag, Sn-0.7Cu and pure Sn. Different creep behaviors of the materials were discussed in terms of their microstructural features.

Effects of Stacking Faults on High Temperature Creep Behavior in Mg-Y-Zn Based Alloys

Compressive creep behavior of hot-rolled (40%) Mg-Y and Mg-Y-Zn alloys are investigated at 480 ~ 650 K. Creep strength is substantially improved by the simultaneous addition of yttrium and zinc. The minimum creep rate of Mg-0.9mol%Y-0.04mol%Zn (WZ301) alloy decreases to 1/10 lower than that of Mg-1.1mol%Y (W4) alloy at 650 K. Activation energy for creep in W4 and WZ301 alloys are more than 200 kJ/mol at the temperature range of 480 ~ 550 K. These values are higher than the activation energy for self-diffusion coefficient in magnesium (135 kJ/mol). Many stacking faults (planar defects, PDs) are only observed on the basal planes of the matrix in Mg-Y-Zn ternary alloys. Stacking fault energy is considered to decrease by the multiple-addition of yttrium and zinc. The size and density of these planar defects depend on solute content, aging condition. TEM observation has been revealed that the decreasing of the stacking fault energy affects the distribution of dislocations during creep. Many a-dislocations on basal planes are extended significantly. Dislocation motion is restricted significantly by both of these two types of stacking faults (planar type and extended dislocations).

High-temperature deformation and creep in Mg wrought alloys

The high-temperature deformation data obtained for a number of different Mg alloys by torsion and tension creep are discussed. ZM21 alloy exhibited peak flow stresses comparable to that observed in AZ31, and the equivalent strain to fracture in torsion for the two alloys was similar. In the cases of ZEK200 and ZM21, creep was found to obey the same constitutive equations as hot working, suggesting a continuity of mechanical parameters and substructure in the two regimes.

On the High-Temperature Plasticity of Ceria-Doped Zirconia Nanostructured Polycrystals

Ceria-zirconia ceramic alloys with the following molar composition: 0.12CeO2-0.88ZrO2 have been sintered by high-temperature annealing. Monolithic specimens haven been crept in compression at high temperatures. Creep experiments have been rationalized to an empirical constitutive equation which is consistent with a classical Ashby-Verrall creep regime. This result has been assessed through microstructural characterization of as-received and post-mortem specimens. A pure Ashby-Verrall creep is contrary to the conventional mechanism controlling creep in other zirconia alloys. A discussion on the explanation for such mechanism is outlined.

Using the results of creep crack incubation tests on CrMoV steel for predicting long time creep rupture properties

The results of a series of creep crack incubation (CCI) tests have been used to examine the effectiveness of the LICON methodology for predicting long duration uniaxial rupture strength of a CrMoV steel at 550° C. The study has revealed that effective long time predictions can be made, but only with the availability of additional information including: an awareness of the short and long time rupture mechanisms and the associated multi-axial rupture criteria obeyed by the material, the results of uniaxial and multi-axial creep tests, and a knowledge of the steady state creep stress conditions existing in the testpieces (structures) forming part of the evaluation. In order to apply the creep damage enhancement approach to the low alloy creep resistant steel, it has been necessary to establish the steady-state creep stress state in the compact tension testpiece geometry used in the investigation. The new evidence is reviewed.