Tensile Creep Rate Research Articles

Cavitation-resistant intergranular precipitates enhance creep performance of θ′-strengthened Al-Cu based alloys

Tensile and compressive creep properties of a quaternary Al-Cu-Mn-Zr (ACMZ) alloy and its commercial counterpart (Al-Cu-Mn-Zr with Ni, Co and Sb additions, RR350) are investigated at 300°C. At low stresses up to 30 MPa where diffusional creep dominates, creep resistance is the same in tension and compression and RR350 deforms more slowly than ACMZ, consistent with RR350 alloy's larger linear fraction of intergranular precipitates (Al7Cu2(NiFe) and Al9FeNi for RR350 vs. θ-Al2Cu for ACMZ) and a reduced fraction of precipitate-free zones near grain boundaries. At stresses between 30 and 80 MPa, dislocation creep with a stress exponent n ∼ 3 becomes rate-limiting in compression, which is expected to be controlled by θ′ precipitates within the grain bulk. By contrast, in tension, enhanced creep rate and higher apparent stress exponents are measured, consistent with cavitation at intergranular precipitates becoming increasingly dominant as the stress increases. In the dislocation creep regime, RR350 alloy is again more creep resistant than ACMZ alloy, which is related to three mechanisms (i) a reduced fraction of softer precipitate-free zones, (ii) more effective load transfer to intergranular precipitates, and (iii) reduced cavitation. A model for cavitation is applied to calculate tensile creep rates from compressive creep rates and the model successfully predicts the improved tensile creep resistance of the RR350 alloy. The present investigation underscores the importance of intergranular grain boundary precipitates, in addition to strengthening θ′ precipitates, in enhancing the creep resistance of Al-Cu alloys.

Investigation of tensile creep for Ultra-High-Performance Fiber Reinforced Concrete (UHPFRC) for the long-term

The research presented in this paper is associated with the time-dependent tensile behavior of UHPFRC, especially on tensile creep for the long-term. This study outlines the experimental program for the implication of the newly developed testing rig, installation and preparation of test specimens, and studies of the research findings. The time-dependent tensile behavior was evaluated in terms of tensile creep under different sustained tensile loads. The sustained tensile loads were harnessed following the different percentages (50% and 75%) of cracking loads. The cracking loads were determined from the instantaneous tensile tests of unreinforced UHPFRC prism specimens at 28-day. A newly designed test rig was introduced to perform the tensile creep test for the long-term, and the test rig was designed in a way to overcome limitations found throughout the critical literature review. This study demonstrates that the tensile creep of UHPFRC is significantly influenced by the shrinkage strain. The experimental results demonstrate that under higher sustained tensile stress directs to a higher tensile creep rate for the first 13 days. Afterward, the total tensile creep strain was dominated by the shrinkage strain.

Heterogeneous [formula omitted] microstructures in nickel-base superalloys and their influence on tensile and creep performance

Mechanical properties of nickel-base superalloys can be greatly influenced by spatial variation of γ′ microstructure, calling for models at mesoscale to study additional complexity in the underlying microstructure/property relationship. We combine a FFT-based elasto-viscoplasticity (FFT-EVP) model with a phase-field (PF) model to study plastic deformation of γ/γ′ superalloys. The model is applied to Haynes 282 (H282), where the low volume fraction (<20%) of dispersed spherical γ′ particles results in a dislocation-particle interaction of either Orowan looping at tensile conditions or climb-bypass at creep conditions. The incorporation of these mechanisms is achieved through the framework of a dislocation density based constitutive model in FFT-EVP, together with the introduction of a location-dependent inter-particle spacing based on the k-nearest neighbor algorithm. Features of non-uniform γ′ microstructures, including γ′ volume fraction variation and non-uniform γ′ distribution due to element microsegregation observed in welded H282 samples, are modeled using the PF method and then passed to FFT-EVP to explore their influence on the tensile and creep properties in a parametric manner. It is found that if the particle distribution is uniform, γ′ volume fraction variation can exhibit a nonlinear effect on both the tensile strength and creep rate; for a given overall γ′ volume fraction, the degree of particle distribution non-uniformity can also exhibit nonlinear influence on the tensile strength and creep rate. It is also shown that the location-dependent inter-particle spacing serves as a better microstructure descriptor than the conventional analytical expression of the average inter-particle spacing in terms of establishing a homogenized but microstructure-sensitive constitutive microstructure-property relationship for fast-acting engineering applications.

Study on the Tensile Creep Behavior of Carbon Nanotubes-Reinforced Sn-58Bi Solder Joints

The microstructure and tensile creep behavior of plain Sn-58Bi solder and carbon nanotubes (CNTs)-reinforced composite solder joints were investigated. The stress exponent n under different stresses and the creep activation energy Q c under different temperatures of solder joints were obtained by an empirical equation. The results reveal that the microstructure of the composite solder joint is refined and the tensile creep resistance is improved by CNTs. The improvement of creep behavior is due to the microstructural change of the composite solder joints, since the CNTs could provide more obstacles for dislocation pile-up, which enhances the values of the stress exponent and the creep activation energy. The steady-state tensile creep rates of plain solder and composite solder joints are increased with increasing temperature and applied stress. The tensile creep constitutive equations of plain solder and composite solder joints are written as $$ \dot{\varepsilon }_{s1} = 14.94\left( {\sigma /G} \right)^{3.7} \exp \left( { - \frac{81444}{RT}} \right) $$ and $$ \dot{\varepsilon }_{s2} = 2.5\left( {\sigma /G} \right)^{4.38} \exp \left( { - \frac{101582}{RT}} \right) $$ , respectively. The tensile creep mechanism of the solder joints is the effects of lattice diffusion determined by dislocation climbing.

Correct Interpretation of Creep Rates: A Case Study of Cu

Traditionally the deformation resistance in creep is characterized by the minimum creep rate ε˙min and its sensitivity to stress (stress exponent n) and temperature (activation energy Q). Various values of constant n have been reported in the literature and interpreted in terms of specific mechanisms. The present case study of coarse-grained Cu at 573 K yields a stress exponent n = 9 for ε˙min in tension and a relatively low activation energy. The evolution of the deformation resistance with strain at constant tensile creep load and comparison with creep in compression without fracture indicates that the tensile ε˙min result from transition from uniform deformation to strain localization during fracture. This is confirmed by the results of creep in compression where fracture is suppressed. Both the tensile ε˙min and the compressive creep rate at strains around 0.3 can be described using existing equations for quasi-stationary deformation containing the subgrain boundary misorientation θ as structure parameter. While in the latter case constant θ leads to monotonic increase of n with stress, the tensile nine-power-law results from variable θ, and has no simple meaning. The result of this case study means that uncritical interpretation of minimum tensile creep rates as stationary ones bears a high risk of systematic errors in the determination of creep parameters and identification of creep mechanisms.

Investigation of mechanical and structural characteristics of platinum and palladium at high temperatures

In order to broaden future application of products based on platinum and palladium a comparative analysis of their high-temperature mechanical properties was performed. Platinum and palladium are of great importance and are widely used in chemical industry, electronics, for making laboratory dishes, to name a few. Mechanical properties of pure metals, such as: tensile strength, creep rate and rupture time were investigated using universal testing machine for tensile testing of materials. Microstructure of samples was investigated by optical microscopy. Based on obtained results it can be concluded that the platinum, compared to palladium, is superior for high-temperature applications.

Evaluation and mathematical modeling of asymmetric tensile and compressive creep in aluminum alloy ZL109

This study investigates the effects of tension/compression asymmetry during creep deformation under different conditions. The asymmetry is found to be dependent on stress and temperature. At high temperatures (350°C, 70MPa) or high levels of stress (250°C, 130MPa), the ratio between the tensile and compressive creep rates can be as large as 10. This ratio is smaller at lower temperatures (200°C, 90MPa) and lower levels of stress (300°C, 30MPa). Scanning electron microscopy (SEM) visualization of different microdefects indicates that the size and volume of microcavities are dependent on the level of stress applied. Similarly, transmission electron microscopy (TEM) is used to visualize dislocations and twinning. The differences in microcavity size and volume in tensile and compressive creep appear to be larger under higher temperature and stress, but no difference in dislocation is observed and no twinning crystals are found. Cavity nucleation appears to be the cause of the asymmetry in creep behavior, which is determined by temperature and stress. A new mathematical model for creep is constructed and validated considering the different asymmetric mechanisms of tensile and compressive creep.

Comparison of the Characteristics of LDPE : PP and HDPE : PP Polymer Blends

Comparative studies have been made on the mechanical properties of High density polyethylene/polypropylene (HDPE : PP) and Low density polyethylene/polypropylene (LDPE : PP) binary blends. Morphological analysis has been also performed using SEM. Blends have been prepared by melt mixing in an extruder. Mechanical tests were performed on the two groups of binary blends. Binary blends (HDPE : PP) gave higher values of tensile strength, fracture strength, young modulus, hardness, creep rate and creep modulus than LDPE : PP. The blend of ratio 20%HDPE : 80%PP shows superior mechanical properties, this blend could bear a load of 846.9 N with an extension of 3.94 mm. SEM results indicated that 20HDPE : 80PP and 20LDPE : 80PP are immiscible blends.

Tensile Creep Behavior of HPC at Early Ages under Different Curing Temperatures

This paper presents an experimental investigation on tensile basic creep behavior of HPC at early ages by using a uniaxial tensile creep testing apparatus. Concrete specimens of 100×100×400mm with compressive strength class 60MPa was used, sealed and loaded at different curing temperature. The effects of the curing temperature and the age at loading on creep behavior are discussed. The results show that tensile specific creep and creep rate of HPC at early ages were governed by the age at loading. The specific creep, creep coefficient and creep rate were larger at earlier loading ages, and decreased exponentially with age at loading. The tensile specific creep decreased with curing temperature, but the difference in creep due to different curing temperatures decreased with the age at loading, and could be ignored while concrete specimen being loaded after the age of 7 days.

Application of time‐temperature superposition to energy limit of linear viscoelastic behavior

AbstractThe energy approach for evaluation of the limits of linear viscoelastic (LVE) behavior is considered. The approach of Foux and Bruller based on the Reiner‐Weissenberg dynamic theory of strength is developed for the temperature effect. Value of the stored energy at the limit of LVE is considered as the material characteristic independent on loading conditions and temperature. Time–temperature superposition principle is extended for the energy calculations. Curves of the stored energy calculated for different temperatures are shifted to each other in the logarithmic time axes similarly as creep compliance and relaxation modulus curves in creep and tension tests, respectively. Temperature is considered as a factor that accelerates transition form linear to non‐LVE at the same stored energy threshold. This is proved by example of polyvinylchloride by comparing temperature dependences of the stress limits of LVE determined in two independent test series: tensile creep and constant strain rate tests. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Anomalously high tensile creep rates from thin cast Sn3.9Ag0.6Cu lead-free solder

The present paper compares the creep and microstructural changes during creep behavior of bulk and thin cast forms of Sn3.9Ag0.6Cu. The processing parameters of the thin cast material were selected to result in a very fine microstructure analogous to what occurs in very small size solder electronic interconnections. We found that the thin cast material is less creep resistant than the bulk material. A comparison of Ag element maps between as-crept bulk and thin cast material shows that the relevant climb process occurs in a very different environment in the bulk material as compared to the thin cast material. In the bulk material, the relevant climb process occurs within a finely dispersed intermetallic compound (IMC) eutectic, which covers broad areas within the material. In the thin cast material, the relevant climb process occurs primarily in the beta-Sn grains that continuously surround isolated, coarse IMC particles. This resulted in the activation energy of the bulk material being larger than that for the thin cast material. Finally, it is important to note that the strength deficiency of the thin cast material is persistent—once the material is cast in thin cast form, it will remain weak in comparison to the bulk material. Therefore, using data obtained from bulk material samples for the construction of thermomechanical models of very small scale solder interconnections is likely to result in significant, intrinsic errors.

Influence of Grain Size on the Tensile Creep Behavior of Ytterbium-Containing Silicon Nitride

The effect of grain size on the tensile creep of silicon nitride was investigated on two materials, one containing 5% by volume Yb2O3, the other containing 5% by volume Yb2O3 and 0.5% by mass Al2O3. Annealing of the Al2O3-free silicon nitride for a longer period during processing increased the grain size by a factor of 2. This increase did not affect the tensile creep rate; the grain size exponent of the creep rate differed little from zero, p=−0.20 ± 1.37 (95% confidence level). This finding supports the more recent theories of tensile creep for which p= 0 or −1 and rejects the more classical theory of solution-precipitation. In compression, a more limited data set showed p=−1.89 ± 1.72 (95% confidence level). In contrast to the Al2O3-free material, a longer term anneal of the Al2O3-containing material during processing did not increase its grain size. Despite this, the longer-annealed Al2O3-containing material crept 10 to 100 times slower than the short-annealed material. The enhancement of creep resistance may be a consequence of SiAlON formation during the additional annealing, which reduces the Al content in the amorphous phase and increases its viscosity. Such changes in chemical composition of the grain boundaries are more effective in controlling tensile creep rate than is the grain size.

A study of grain-boundary structure in rare-earth doped aluminas using an EBSD technique

Oversized rare-earth dopant ions such as Y3+, Nd3+, and La3+ segregate to grain boundaries and reduce the tensile creep rate of α-Al2O3 by 2-3 orders of magnitude. It has been speculated that these dopant ions can modify the grain boundary structure in alumina by promoting the formation of special grain boundaries. If this were indeed the case, it would provide a possible explanation for the aforementioned creep rate retardation. In order to test this hypothesis, electron backscatter diffraction (EBSD) has been used to assess both the proportion of coincidence-site lattice boundaries, and the grain boundary misorientation distribution, in aluminas doped with various ions (Zr, Y, Nd, La, Nd/Zr). The results show that the grain boundary structure in alumina is not significantly altered by the addition of the above dopants, implying that the change in grain boundary chemistry is primarily responsible for the observed creep behavior.

Comparison of Tensile and Compressive Creep Behavior in Silicon Nitride

The creep behavior of a commercial grade of Si3N4 was studied at 1350° and 1400°C. Stresses ranged from 10 to 200 MPa in tension and from 30 to 300 MPa in compression. In tension, the creep rate increased linearly with stress at low stresses and exponentially at high stresses. By contrast, the creep rate in compression increased linearly with stress over the entire stress range. Although compressive and tensile data exhibited an Arrhenius dependence on temperature, the activation energies for creep in tension, 715.3 ± 22.9 kJ/mol, and compression, 489.2 ± 62.0 kJ/mol, were not the same. These differences in creep behavior suggests that mechanisms of creep in tension and compression are different. Creep in tension is controlled by the formation of cavities. The cavity volume fraction increased linearly with increased tensile creep strain with a slope of unity. A cavitation model of creep, developed for materials that contain a triple‐junction network of second phase, rationalizes the observed creep behavior at high and low stresses. In compression, cavitation plays a less important role in the creep process. The volume fraction of cavities in compression was ∼18% of that in tension at 1.8% axial strain and approached zero at strains &lt;1%. The linear dependence of creep rate on applied stress is consistent with a model for compressive creep involving solution–precipitation of Si3N4. Although the tensile and compressive creep rates overlapped at the lowest stresses, cavity volume fraction measurements showed that solution–precipitation creep of Si3N4 did not contribute substantially to the tensile creep rate. Instead, cavitation creep dominated at high and low stresses.

Modeling of Grain‐Boundary Segregation Behavior in Aluminum Oxide

It is believed that the segregation of oversized dopant ions to grain boundaries in Al2O3 hinders grain‐boundary diffusion, thereby reducing the tensile creep rate in this system by ∼2–3 orders of magnitude. In order to explain this improvement in creep behavior, it is helpful to characterize both the effective cation and interstitial volumes at grain boundaries, because the relative openness of some boundary structures suggests a great accommodation of oversized ions. In this study, the boundary volume is determined by a spatially local Voronoi construction, which highlights cation (Al3+) substitutional sites as well as large interstitial voids. In particular, we examine the spatial distribution of free volume near grain boundaries and, in addition, the dependence of the driving force for segregation on misfit strain in doped Al2O3. We interpret our results in light of recent evidence that selective codoping can provide a more efficient means of filling available space near boundaries, thereby further enhancing creep resistance.

A New Model for Tensile Creep of Silicon Nitride

The tensile creep rate of most commercial grades of Si3N4 increases strongly with stress. Although the usual power‐law functions can represent the creep data, the data often show curvature and systematic variations of slope with temperature and stress. In this article, we present a new approach to understanding the creep of ceramics, such as Si3N4, where a deformable second phase bonds a deformation‐resistant major phase. A review of experimental data suggests that the rate of formation and growth of cavities in the second phase controls creep in these materials. The critical step for deformation is the redistribution of the second phase away from the cavitation site to the surrounding volume. The effective viscosity of the second phase and the density of active cavities determine the creep rate. Assuming that the hydrostatic stresses in pockets of the second phase are normally distributed leads to a model that accurately describes the tensile creep rate of grades of Si3N4. In this model, the creep rate increases exponentially with the applied stress, is independent of Si3N4 grain size, is inversely proportional to the effective viscosity of the deformable phase, and is proportional to the cube of the volume fraction of the deformable phase.

Asymmetric tensile and compressive creep deformation of hot-isostatically-pressed Y 2O 3-doped -Si 3N 4

The uniaxial tensile and compressive creep rates of an yttria-containing hot-isostatically-pressed silicon nitride were examined at several temperatures between 1316 and 1399°C and found to have different stress dependencies. Minimum creep rates were always faster in tension than compression for an equal magnitude of stress. An empirical model was formulated which represented the minimum creep rate as a function of temperature for both tensile and compressive stresses. The model also depicted the asymmetric creep deformation using exponential and linear dependence on tensile and compressive stress, respectively. Unlike other models which represent either tensile or compressive creep deformation as a respective function of tensile or compressive stress, the model in the present study predicted creep deformation rate for both tensile and compressive stresses without conditional or a priori knowledge of the sign of stress. A statistical weight function was introduced to improve the correlation of the model’s regressed fit to the experimental data. Post-testing TEM microstructural analysis revealed that differences in the amount of tensile- and compressive-stress-induced cavitation accounted for the creep strain asymmetry between them, and that cavitation initiated in tensile and compressively crept specimens for magnitudes of creep strain in excess of 0·1%.

Influence of Yttrium Doping on Grain Misorientation in Aluminum Oxide

Oversized dopant ions such as yttrium and lanthanum segregate to grain boundaries and reduce the tensile creep rate of α‐Al2 O3 by 2‐3 orders of magnitude. One explanation for this behavior is that the oversized segregants give rise to a “site‐blocking” effect for grain boundary diffusion. It has also been speculated that the dopant ions modify the grain boundary structure in alumina and reduce the creep rate by promoting the formation of special (e.g., coincidence site lattice (CSL)) grain boundaries. In order to test the latter hypothesis, we have used electron backscattered Kikuchi diffraction to characterize the misorientation and special grain boundary distribution for undoped and 1000‐ppm‐yttrium‐doped alumina. The results show that the grain boundary structure in alumina (as characterized by the frequency of selected CSLs and misorientation distribution) was not significantly changed by the addition of yttrium, indicating that creep retardation results mainly from site‐blocking.

The Wear of a Composite of High-Modulus HDPE Fibers in a Matrix of LDPE in Unidirectional Motion against 316L Steel

Injection molding has been used to produce a composite of short, randomly dispersed, high-modulus HDPE fibers in an LDPE matrix. With a fiber loading of 10 wt% the wear coefficient of the composite rubbing against smooth 316L steel on a tri-pin-on-disc machine is lower by nearly an order of magnitude than that of LDPE. There is a reduction in the tensile creep rate of the composite compared with LDPE and modest improvements in strength and modulus.

Viscoelasticity of rubber‐toughened poly(methyl methacrylate). Part I: Deformational behavior

AbstractViscoelasticity of a rubber toughened poly(methyl methacrylate) has been investigated both in the linear and the non‐linear range by means of tensile creep and tensile constant strain rate tests. An empirical time‐temperature‐strain reduction scheme is proposed to analyze the modulus data obtained at different temperatures and strain levels: a single master curve and relevant time‐temperature and time‐strain shift factors have been obtained. Two different thermal histories were considered for testing. It has been shown that, once aging effects can be considered under conditions where aging time elapsed before testing is the same, modulus master curves obtained from linear and from non‐linear data substantially coincide. This shows that both temperature and strain level determine the modulus master curve via their influence on a single variable: the free volume of the material.