Creep Mechanism Research Articles

INFLUENCE OF MINOR ALUMINUM ADDITION ON THE SUPERPLASTIC DEFORMATION OF A MICRODUPLEX CU-ZN ALLOY

High residual porosity in superplastically deformed brass carries the risk of reducing the mechanical properties. Multicomponent brasses demonstrate lower residual porosity, associated with a lower grain size and more effective accommodation mechanisms of grain boundary sliding. In this paper, a comparison is made of microstructural evolution in the prepolished surface and in the bulk of duplex brass samples and brass samples with minor aluminum addition during steady-state superplastic deformation. After superplastic deformation, dislocation clusters and dislocation walls are revealed in the α-grains of both alloys, indicating the activation of the dislocation creep mechanism. It is shown that aluminum reduces the contribution of grain boundary sliding along the phase boundaries from ~75 to ~30% and causes strain localization in the ß-phase region with the formation of less than ~300-nm grains as a result of dynamic recrystallization. Alloying with 0.4% Al reduces the flow stress by 20%, increases the relative elongation by a factor of 1.5, and decreases the amount of residual porosity by a factor of 3. This leads to a much lower loss of strength in superplastically deformed specimens at room temperature.

Unraveling the individual effect of yttrium and neodymium on the precipitate-free zone development and the dislocation creep mechanism of Mg-Y-Nd alloy

Unraveling the individual effect of yttrium and neodymium on the precipitate-free zone development and the dislocation creep mechanism of Mg-Y-Nd alloy

Influence of ultrasonic shot peening on the microstructure and impression creep performance of squeeze-cast AZ91 alloy reinforced with graphene nanoplatelets

The present investigation explores the influence of graphene nanoplatelets (GNPs) (0.5, 1.0, and 2.0 wt %) addition as well as ultrasonic shot peening (USP) for 10, 20, and 30 s on the microstructure, hardness, and impression creep response of AZ91 alloy fabricated by squeeze-casting for transmission housing applications. The as-cast AZ91-xGNPs nanocomposites displays lower grain size, lesser Mg17Al12 phase fraction, and higher dislocation density. The USP technique induces compressive residual stress (CRS) and high strain hardening at the surface followed by high dislocation density that increases with time. Both surface roughness as well as the dimple size and thickness after USP treatment are lower for the nanocomposites than for the AZ91. The hardness and creep resistance of the as-cast AZ91-xGNPs nanocomposites are superior to the monolithic alloy, which further improved after USP treatment. In contrast to the base alloy, the GNC3 nanocomposite displays the best hardness and impression creep resistance both in the as-cast and USP-treated condition. Dislocation climb regulated by pipe diffusion is the dominating creep mechanism in the as-cast and USP-treated materials. The improvement of impression creep performance for as-cast nanocomposites is attributed to the reduced Mg17Al12 phase content, high dislocation density, and strengthening from the GNPs. In contrast, the superior creep performance of USP-treated nanocomposites is attributed to the presence of CRS along with high dislocation density and work hardening on the surface.

Effects of lattice distortion and chemical short-range order on creep behavior of medium-entropy alloy CoCrNi

Creep behavior of multi-principal element alloys (MPEAs) is an intriguing topic to explore for their potential high-temperature applications. The challenge on this topic is to elucidate what effect the local chemical fluctuation has on the creep behavior of MPEAs. By using large-scale molecular dynamics (MD) simulations, we investigate the creep performances of CoCrNi medium-entropy alloys (MEAs) with and without chemical short-range order (CSRO) as well as the Average-atom (A-atom) counterpart (no lattice distortion (LD)) under different uniaxial tensile stresses at various elevated temperatures. A power-law model is adopted to analyze the implicit stress exponent and activation energy, which are associated with the creep mechanism and creep resistance, respectively. The results reveal CSRO rather than LD plays an important role in creep performance. Specifically, with the introduction of CSRO, the activation energy for creep in CoCrNi MEA is significantly increased and finally close to the activation energy for the diffusion of the Cr element that is the highest among the three elements (Co, Cr, and Ni). The CoCr clusters in the MEA with CSRO make it difficult to increase the shear strain in the creep process, resulting in a much lower creep rate than that of the CoCrNi MEA without CSRO. For the CoCrNi MEA without CSRO, grain boundary diffusion is the main creep mechanism. For the MEAs with CSRO, the predominant creep mechanism depends on the applied stress. At low stress, grain boundary diffusion still prevails and there is no obvious dislocation glide or grain boundary sliding. When the applied stress is high enough, grain boundary diffusion and dislocation slip dominate the creep behavior, accompanied by the occurrence of grain boundary sliding.

Creep behaviour of ozone treated jute fabric/epoxy composites filled with ozonized and pulverized corn husk particles

In present work, the creep performance of ozone treated jute fabric/epoxy composites was studied at different environment temperatures after incorporation of 1, 3 and 5 wt % of ozonized and pulverized corn husk particles. The ozone treatment of jute fabric as well as corn husk fibres was found successful in removal of non-cellulosic contents. Furthermore, the ozone treatment of corn husk fibres was observed beneficial during ball milling as it easily pulverized them into particles due to decrease in strength. When the creep behaviour of composites was investigated using several creep models (i.e., Findley's power law model, Burger's model, and Coupling Model), the incorporation 3 wt % corn husk particles was found to reduce the creep deformation and strain rate of composites to greater extent especially at higher temperatures. The 3 wt % corn husk particles were found to restrict the segmental mobility of epoxy matrix in extended temperature range of 100 °C–140 °C. Nevertheless, the microbead pull out and nanoindentation tests indicated that the interfacial shear strength properties of composites increased after the incorporation of 5 wt % of corn husk particles. Among various creep models, the coupling model predicted the creep behaviour of composites more accurately in addition to explanation on the creep mechanism.

Effects and mechanisms of constraint on creep crack growth behavior of GH3535 alloy

To clarify the influence of constraint effects on the creep crack growth behavior and mechanism of GH3535 alloy, creep crack growth tests on compact tension (CT) specimens of various sizes were performed at 650 °C. To reduce dispersion and more clearly compare the crack growth rate and crack initiation time under different constraints, an incremental polynomial was used to obtain the fracture parameter C* and the crack growth rate. Then the fracture parameter C* were used to correlate the crack growth rate and crack initiation time, respectively, and their relationships in double logarithmic coordinates were obtained. The results show that both the crack initiation time and the crack growth rate are affected by the constraint at the crack tip but remain linearly related to the fracture parameter C* in log–log scales. The essence of the constraint effect is that the structure hinders the plastic deformation of the crack tip. If the constraint is large, the plastic deformation corresponding to the crack tip will be small. However, the deformation accompanying the crack propagation of the alloy in this study will increase with increasing constraint. The crack growth rate at the middle thickness of the plain specimen without side grooves is greater than that at the near surface, and the crack profile is inverse parabolic shape, whereas the crack profile of the side-grooved specimen is relatively uniform.

Flexural creep in plain concrete: State of the Art

Concrete, being a viscoelastic material, creeps with time causing additional strains and deflections in flexural members. Most of the studies in creep of concrete are based on long-term tests on plain concrete standard cylinders under axial compression, very limited studies have been reported on creep of concrete in flexure and direct tension. Whether the mechanism of creep in tension is different from creep in compression is still being debated. Numerous studies have been reported in the literature showing varied results with regard to the effects of creep in direct compression, direct tension and flexure. This paper provides a brief review of various creep tests on plain concrete reported and attempts to understand the effects of various parameters on the creep behaviour of concrete. Some studies reported that the creep in tension is higher than that in compression, whereas a few others obtained contradictory results from creep tests. This may be attributed to the fact that the relative creep behaviour in tension and compression is highly sensitive to the material composition, exposure conditions and loading level. The studies by various researchers on flexural creep in plain concrete are also discussed.

About preliminary statistical processing of information on the study of fatigue strength of machine parts

A generalized approach to statistical processing of the results is proposed fatigue parameters of static strength using methods of mathematical statistics and conventional statistics using the following probabilistic parameters: the mean square deviation, the initial moment, the central moment of dispersion and the coefficient of variation of the distribution series of physical and mechanical characteristics of the part material. Further, the analysis of fatigue strength research at the initial stage is carried out according to statistical information processing, graphical design of a number of stress distributions, statistical stress analysis, a graphical approach is applied in the form of a histogram of a number of distributions, a polygon of frequencies, a polygon of accumulated frequencies, the selection of theoretical stress distribution laws with their empirical confirmation is compared with a more rigorous assessment of the conformity of the distribution laws, which is performed using special consent criteria, for example, the Pearson criterion, a parallel verification of the correctness of the chosen approach using classical dependences of the resistance of materials is proposed. the total error is estimated in the form of a methodological error and a direct discrepancy between the theoretical and experimental values of stresses and temperatures of the physico-mechanical process according to experimental and theoretical normal and tangential stresses arising during the operation of the part as a result of the application of external force factors, a temperature-time superposition is used, in the form of a function of the predicted durability of the fluctuating kinetic theory of strength, in which the temperature, as a linear function, it can be replaced by any energy or force criterion, in particular: specific energy, relative deformation, normal or tangential stresses. The proposed approach requires substantial experimental study on a basic batch of the same type of samples and coordination according to schematized diagrams of the limiting amplitudes of Goodman, Sorensen - Kinasoshvili, Kogaev, subject to the conditions of safe operation of parts in the field of low-cycle fatigue. The proposed probabilistic model of statistical processing of fatigue strength can be recommended for solving applied problems of the theories of mechanics of materials, elasticity, plasticity and creep, resistance of materials, structural mechanics

Fluid-Structure Coupling Creep Characteristics of Red-Bed Soft Rock in South China

In order to study the creep characteristics and mechanism of red-bed soft rock under the water–rock interaction, fluid–structure coupling triaxial compression tests and creep tests under stepwise loading were carried out. Furthermore, the influences of seepage pressure and stress on creep deformation, long-term strength, Poisson’s ratio, and seepage velocity were analyzed. According to the experimental results, the influence of seepage on the creep of soft rock cannot be ignored. The results show that the seepage leads to a decrease in triaxial strength and long-term strength, and an increase in instantaneous deformation and creep deformation. The failure mode of triaxial compression changes from shear failure to tension-shear conjugate failure, whereas the long-term strength of fluid–structure coupling creep is 60%~70% of the triaxial strength. When the stress level and seepage pressure are relatively small, the Poisson’s ratio of creep increases with the increase of seepage pressure, and the radial creep deformation response lags behind the axial creep deformation. However, at a high stress level and osmotic pressure, the Poisson’s ratio and seepage velocity increase rapidly, and the duration of the accelerated creep is obviously shortened. Through the analysis of the influence of seepage pressure on the seepage velocity, with the increase in the seepage velocity, the seepage velocity changes and fluctuations are more obvious, which further confirms the damaging effect of seepage pressure and erosion on the internal structure. In the field monitoring of actual engineering, the rapid change of seepage velocity can be used as a precursor signal to predict the instability. Therefore, the water–rock interaction cannot be ignored in the analysis of mechanical properties and long-term stability of red-bed soft rocks.

Revealing the tensile creep behavior and mechanism of SiC particles reinforced AZ31 composite fabricated by liquid metallurgy

Constant engineering stress tensile creep behavior of AZ31 magnesium matrix composites with SiC particles in 20% volume fraction and a mean diameter of 8.6 μm, fabricated by liquid metallurgy, was performed at the temperature range of 473–623 K, and the stress level range of 7–70 MPa. The minimum creep rates of the as-cast composites reveal better creep resistance at high temperatures and low stresses compared with those of unreinforced AZ31 alloy. The high apparent stress exponents and apparent activation energies of the composites imply the presence of threshold stress in the creep behavior. The true stress exponent of the composite was calculated to be ∼5, indicating that the possible creep mechanism is the climb-controlled creep model controlled by lattice diffusion.

Rubber Creep Model and Its Influence on Mounting Stiffness

The mount of the engine will creep under the action of long-term load. Creep will change its original structure, resulting in changes in static and dynamic characteristics and fatigue life. In order to solve the problem of mounting rubber creep, the creep characteristics were studied in this paper. In order to study the influence of creep characteristics on engine mounting stiffness, a theoretical model of mount creep was established based on the creep mechanism of rubber. The effects of different loads on the creep characteristics of engine mount were studied. The static viscoelastic parameters and creep analysis of rubber mounting were analyzed numerically. The creep variation law of suspension under different loads is obtained. By analyzing the static and dynamic characteristics of no creep, 2.98 mm creep and 3.83 mm creep of engine mount, the creep characteristics and the variation law of mount stiffness of rubber mount were revealed. The results show that the static stiffness of suspension increases with the increase of creep. When the frequency is constant and the creep increases, the dynamic stiffness of the suspension increases obviously. In this paper, the creep characteristics of rubber mount are analyzed, and the results of the analysis provide a design method for rubber mount design.

Evaluation of tensile creep behavior of spray formed and extruded 7075 aluminum alloy by equivalent stress

This work attempted to investigate the creep behavior of a 7075 aluminum alloy in spray formed and extruded (SFEed) and peak aged (PAed) states by equivalent stress. Arrhenius model and Mukherjee-Bird-Dorn (MBD) model were used to calculate creep parameters. The effects of applied stress or equivalent stress on the creep deformation of SFEed and PAed alloys were discussed and compared. The results showed that the equivalent stress could reasonably evaluate the creep resistance of 7075 Al alloys from the aspects of the steady-state creep rate, creep mechanism and dominate deformation texture, except creep rupture life. Under close equivalent stress, the PAed alloys exhibited superior steady-state creep rates and inferior creep rupture life to the SFEed alloys. The Arrhenius model and MBD model had appropriate ranges of temperature and applied stress to estimate and predict steady-state creep rates, and the accuracy of the MBD model was significantly superior. The stress exponents are computed as 7.03–7.24 for SFEed alloy and 8.11–8.62 for PAed alloy, and the tensile creep mechanism of SFEed and PAed alloys was both dislocation creep. The creep plastic deformation at 0.45–0.65Tm is processed along Cu texture orientation by dislocation climbing rather than S texture. The higher temperature and equivalent stress promote the Cu texture intensity. The applied stress or equivalent stress initiates the slipping and climbing of dislocations, and facilitates the transformation of dominant deformation texture. The main factor that affects grain boundary migration on creep deformation is grain boundary energy rather than stress or equivalent stress.

Advanced heat-resistant martensitic steels: Long-term creep deformation and fracture mechanisms

Creep behavior at 650 °C of the 9% Cr martensitic steels with different W, N and B contents was investigated using the threshold stress concept. Creep strength breakdown and the change in the “apparent” stress exponent were revealed at the applied stresses ranging from 140 to 120 MPa in both steels. Moreover, the threshold stresses, which were estimated by extrapolating the two parts of curves “Minimum creep rate vs. Applied stress” corresponding to the high and low stresses at a creep rate of 10−11 s−1, decreased from 94 … 98 MPa for both steels to 30 and 65 MPa in the 9Cr3W and 9Cr1.5WBN steels, respectively, at the transition from the high stress region to the low stress region. The sources of these threshold stresses are considered to be dislocations, the M23C6 carbides and Laves phase particles located at the low-angle lath boundaries as well as MX carbonitrides. Analysis of true stress exponent revealed no evidence for the change in creep mechanism at the transition from the high stresses to low stresses. This was creep controlled by high temperature climb of dislocations due to lattice diffusion. On the other hand, the creep fracture mechanism in the 9Cr3W steel was changed from the transgranular fracture at the high stresses to the intergranular fracture at the low stresses. This was accompanied with the formation of the high fraction of the coarse particles of Laves phase.

Dislocation reconfiguration during creep deformation of an Al-Cu-Li alloy via electropulsing

• Our findings proved another dislocation reconfiguration responsible for the electroplastic behaviour. • Synergy of the climb force F c and electron wind force F ew on the reconfiguration of helical dislocations. • Athermal component of electropulsing is proven to be more important during the dislocation reconfiguration. Creep mechanism was well-known to be mainly dominated by the dislocation sliding and climbing during creep deformation. Here we study the creep deformation of an Al-Cu-Li alloy with the assistance of electropulsing and subsequent microstructural observations. We find that creep strain increased drastically under electropulsing and was almost twelve times as much as that of the non-pulsed sample. Microstructural observations confirmed that dislocation reconfiguration happens via electropulsing, namely helical dislocations being opened rapidly. This opened dislocation structure can possess a much higher mobility than the initial helical dislocation, which mostly responsible for the greatly increased creep strain. Our results revealed a new mechanism accountable for the distinctly electroplastic creep deformation.

Enhancing the high-temperature creep properties of Mo alloys via nanosized La2O3 particle addition

Molybdenum (Mo) alloys with different La 2 O 3 particle additions (0.6, 0.9, 1.5 wt.%) were prepared by powder metallurgy to investigate the effect of La 2 O 3 particles on microstructural evolution and creep behavior of the alloy. Pure Mo, annealed at 1500 °C for 1 h, presented a fully recrystallized microstructure characterized by equiaxed grains. The alloys doped with La 2 O 3 particles (Mo-La 2 O 3 alloys), on the other hand, exhibited fibrous grains elongated in the rolling direction of the plate. In contrast to the shape of the grains, the average grain size of the alloys was found to be insensitive to the addition of La 2 O 3 particles. Nanosized La 2 O 3 particles with diameters ranging from 65 nm to 75 nm were distributed within the grain interior. Tensile creep tests showed that dislocation creep was the predominant deformation mode at intermediate creep rate (10 −7 s −1 -10 −4 s −1 ) in the present alloys. The creep stress exponent and activation energy were found to decrease with increasing temperature, particularly within the low creep rate regime (<10 −7 s −1 ). The Mo-La 2 O 3 alloys exhibited remarkably greater apparent stress exponent and activation energy than pure Mo. A creep constitutive model based on the interaction between particles and dislocations was utilized to rationalize the nanoparticle-improved creep behavior. It was demonstrated that low relaxed efficiency of dislocation line energy, which is responsible for an enhanced climb resistance of dislocations, is the major creep strengthening mechanism in the Mo-La 2 O 3 alloys. In addition, the area reduction and creep fracture mode of the Mo-La 2 O 3 alloys were found to be a function of the creep rate and temperature, which can be explained by the effect of the two parameters on the creep and fracture mechanisms.

Orientation dependence of plastic deformation of sintered Nd-Fe-B magnets at high temperature

While the microstructure at the grain boundary of hot-deformed nanocrystalline Nd-Fe-B magnets is intensively studied, limited works have been devoted to the deformation behavior of the Nd2Fe14B matrix grain, especially for micrometer scale grains and at high temperatures. In this study, the plastic deformation behavior of the Nd2Fe14B matrix grains was studied by uniaxial compression from 1050°C to 1120°C using both anisotropic and isotropic sintered Nd-Fe-B magnets. The anisotropic magnets were orientated with the c-axis 0°, 45° and 90° to the loading axis. According to the Arrhenius-type power law relationship, the stress exponent n increased from 1.5 to 2.7 when the c-axis changed from 0° to 90° to the loading direction and the corresponding apparent activation energies Q increased from 384 to 437 kJ/mol. Although the observed n > 1 coincides with the interface-reaction controlled solution-precipitation creep mechanism, the determined Q is much larger than those reported in the nanocrystalline magnet. Our study conferred the first demonstration of short straight dislocations array on the (001) plane in the 45°-orientated anisotropic specimen after 10% height reduction. Grain elongation and local misorientation inside the grain were most obvious in the 45°-oriented specimen after compression. The extent of the local misorientation agreed well with the spacing corresponding to geometrically necessary dislocation. The results reveal that grain rotation via grain boundary sliding and dislocation-mediated deformation were operable depending on the loading direction when the micro-sized magnets were deformed above 1000°C.

Comparison of Creep Mechanisms Between Co-Al-W- and CoNi-Based Single Crystal Superalloys at Low Temperature and High Stresses

Comparison of Creep Mechanisms Between Co-Al-W- and CoNi-Based Single Crystal Superalloys at Low Temperature and High Stresses

Improving the microstructure and mechanical properties of laser powder bed fusion-fabricated tantalum by high laser energy density

The effect of laser energy density on the microstructural evolution and the mechanical properties, including tensile property and nanoindentation creep behavior of Ta parts were investigated. A high laser power strategy (600 W) was used to obtain Ta part with high density. The microstructure of Ta fabricated with 200 W comprised columnar grains with strong 〈111〉 texture, while equiaxed grains with random orientation were found in Ta fabricated with 600 W. The elongation to failure of the specimen fabricated at 600 W was higher than that of the specimen fabricated at 200 W, which stemmed from pore elimination and equiaxed grains. Nanoindentation creep results showed that the movement of dislocations was the main creep mechanism and the influence of laser energy density was discussed.

Anisotropy and variability in thermal creep behaviour of Zr-2.5Nb pressure tube

The Zr-2.5Nb alloy pressure tubes of Indian Pressurized Heavy Water Reactors were manufactured from double melted ingots initially and subsequently from quadruple melted ingots to reduce impurities such as chlorine, phosphorus and carbon. In the current investigation, creep tests were carried out in the stress range of 0.7–0.9 times the yield strength and the temperature range of 350 °C–450 °C to characterize the thermal creep behaviour of Zr-2.5Nb alloy pressure tubes fabricated from double and quadruple melted ingots. The rupture time, minimum creep rate, stress exponent, activation energy and threshold stresses were obtained by carrying out the creep tests of the samples fabricated with their axis parallel to the axial and transverse directions of the pressure tubes. Experimental analysis revealed that the rupture time for double melted tube was 2–3 times lower than that of quadruple melted tubes, whereas no significant change in minimum creep rate was observed. Stress exponent values were obtained in the range of 3–5 signifying dislocation creep as the dominant creep mechanism. To predict the long-term creep properties, the master curves employing the Larson-Miller and Monkman-Grant methods were also evaluated. The effect of microstructural anisotropy, alloying elements and impurities on the thermal creep behaviour of Zr-2.5Nb pressure tube has been investigated.

Creep-fatigue of P92 in service-like tests with combined stress- and strain-controlled dwell times

Complex service-like relaxation- and creep-fatigue tests with strain- and stress-controlled dwells and fatigue cycle durations of approx. 2200 s were performed exemplarily on a grade P92 steel at 620 °C in this study. The results indicate deviations in the prevailing creep mechanisms of long-term relaxation and creep dwells, affecting subsequent dwells, load shifts, and the macroscopic softening behavior quite differently. In addition, fracture surfaces and longitudinal metallographic sections reveal intergranular crack growth for complex loading with stress-controlled dwells, whereas complex strain-controlled tests enhance oxidation and transgranular crack propagation. These findings substantiate the limited transferability of relaxation-fatigue to creep-fatigue conditions.