Dominant Creep Mechanism Research Articles

Role of Cerium on the creep properties of the Mg-Sn-Al based alloys

ABSTRACT In the present study, the effect of Ce alloying element with different quantities (1 and 2 wt.%) on the microstructure and mechanical properties of Mg-5Sn-2Al alloy was investigated. Phase analyses and microstructural studies were carried out using XRD and SEM. It was observed that base alloy was composed of the α-Mg and Mg2Sn intermetallic phase, and after Ce addition Ce5Sn4 intermetallic phase was formed. Also, an increase was detected on the microhardness of the base alloy with increasing Ce addition. The test results showed that Ce addition has no notable effect on the tensile properties at room temperature but has a positive effect at elevated temperatures. Furthermore, creep properties of the Mg-5Sn-2Al alloy were obviously improved by adding Ce. The values of stress exponents and activation energy indicated that the dominant creep mechanisms of the alloys were identified dislocation climb controlled creep..

Elevated Temperature Nanoindentation Creep Study of Plastically Deformed and Spark Plasma Sintered UO2

Current challenges with uranium dioxide fuel degradation include fuel swelling, cracking and fission gas release, all of which reduce the operational life span of the fuel in commercial reactors. One important factor in prolonging the life span of uranium dioxide (UO2) fuel is developing a good understanding of the mechanical properties of the high burnup structure that forms at the periphery of the fuel pellet during the fuel cycle. Nanoindentation based testing techniques can probe the mechanical properties of small volumes of material and measure properties including hardness, elastic modulus, and creep. In this work, elevated temperature nanoindentation and nanoindentation creep testing is performed on UO2 samples with different microstructures, including nanocrystalline (NC) grains, as well as microcrystalline samples with different grain sizes and creep prestrains introduced at high temperatures. Tests allowed measuring hardness, elastic modulus and creep stress exponents up to 500°C. The test results indicate that for temperatures and stresses used here, NC UO2 had limited to no creep, with stress exponents greater than 10. For UO2 samples with creep prestrain and spark plasma sintered (SPS) UO2 with a 1.8 μm grain size the dominant creep mechanism at the high stresses and relatively low temperatures used is dislocation glide: the creep stress exponents of 3-4 at 500°C also matched well with recent literature values for macro scale compression creep tests. In addition, it was observed that the UO2 samples containing higher dislocation defect densities had lower creep stress exponents than conventional sintered UO2. The stress exponent measured on the conventional sintered UO2 at 500°C was ~ 7, which suggests that the deformation of UO2 at lower temperatures might be hindered by the energetic barriers to dislocation nucleation.

Micromechanics and indentation creep of magnesium carbon nanotube nanocomposites: 298 K–573 K

In the present study, the time-dependent plastic deformation (creep) response of magnesium/carbon nanotube (CNT) nanocomposites containing 0.25, 0.5, and 0.75 vol% of CNTs is investigated through instrumented indentation tests against monolithic pure magnesium. The Mg-CNT nanocomposites were synthesized via powder metallurgy coupled with microwave sintering followed by hot extrusion. Indentation creep tests were performed at 298, 373, 473, and 573 K with a cubic boron nitride Berkovich indenter. Creep tests are dual-stage, i.e., all tests involve loading to a prescribed peak load (50 mN), holding this peak load constant for a set dwell period (120 s), and finally unloading. Three load rates of 0.5, 5, and 50 mN/s were tested. Results are explained through hardness, microstructure, and CNT volume fraction in each sample. Mg reinforced with 0.25–0.5 vol% CNTs exhibit the best creep resistance. At the ambient temperature of 298 K, dislocation creep is the dominant creep mechanism. At elevated temperatures, multiple creep mechanisms are identified. However, diffusion creep mechanisms are expected to prevail when temperatures exceed 573 K. These materials are promising candidates for aerospace, automotive, military, and many other industrial applications. To this end, the findings of this study can be used for material selection in these industries. This work may also be used as a baseline for future high-temperature nanoindentation creep studies on other nanocomposites.

Enhancing mechanical properties of squeeze-cast AZ91 magnesium alloy by combined additions of Sb and SiC nanoparticles

The microstructure, compression, and creep responses of the squeeze-cast AZ91 alloy; the AZ91 alloy with 0.6Sb (wt.%) addition; and the AZ91 alloy with 0.6Sb (wt.%) + 2.0SiC (wt.%) (1.2 vol%) nanoparticles addition have been evaluated. The additions of 0.6Sb and 0.6Sb+2.0SiC to the AZ91 alloy reduce the grain size and decrease the β-Mg17Al12 phase, and result in improved compression and creep responses. The AZ91 + 0.6Sb+2.0SiCnp nanocomposite exhibits the best compression properties and the highest creep resistance. The dislocation climb controlled by pipe diffusion is the dominant creep mechanism in the alloys and nanocomposite. The superior compression responses are attributed to the reduction in grain size, and the improved creep resistance is attributed to the decrease in the β-Mg17Al12 phase for both the AZ91 + 0.6Sb alloy and AZ91 + 0.6Sb+2.0SiCnp nanocomposite. The strengthening due to CTE mismatch between the alloy and SiC nanoparticles, as well as the Orowan strengthening for the nanocomposite additionally contributed to the improved compression and creep responses.

Effects of twinning time node on the creep properties of dilute Mg–Y alloy

In this study, the effects of twinning time node on the creep resistance of an extruded Mg-2wt%Y sheet were investigated. Pre-compression with a low strain was applied along the normal direction at 298 K before creep, resulting in a large amounts of {101‾2} twins. For comparing, the samples with and without pre-compression are referred to as PC and NPC, respectively. Compressive creep tests were performed along extrusion direction at 473 K and applied stress ranged from 30 to 70 MPa. The results showed that PC samples present a higher creep resistance than NPC samples. Using various characterization methods, different dominant creep mechanisms were found in two types of samples. The dominant creep mechanisms of NPC sample are cross-slip and twinning while pyramidal <c+a> slip dominates PC sample. Furthermore, it was verified that cross-slip containing <a> dislocations can easily penetrate into the twins activated during creep, leading to a lower creep resistance. Inversely, the preset twins can slow down the creep rate by retarding cross-slip.

Tensile and creep properties of Al–7Si–0.3Mg alloy with Zr and Er addition

The combined and singular effects of Zr and Er addition on the microstructure, tensile and impression creep behaviour of the cast A356 alloy were investigated. The Zr and Er refined the α-Al dendrites and secondary dendrite arm spacing (SDAS) and modified the eutectic morphology. The simultaneous addition of Zr and Er exhibited the smallest α-Al dendrites and SDAS with a fibrous eutectic morphology, resulting in the highest yield (∼118 MPa) and tensile (∼190 MPa) strength, and the largest creep resistance. The stress exponent, and the activation energy, were within the range of 6.28–7.22 and between 121.6 and 148.2 kJ mol−1, respectively. The lattice self-diffusion climb-controlled creep was the dominant creep mechanism. The constitutive creep equation was developed for each alloy.

Elevated temperature mechanical properties of Inconel 617 surface oxide using nanoindentation

Inconel 617 is a principal candidate material for helium gas cooled very-high-temperature reactors with outlet temperatures of 700–950 °C. Recent findings showed that this alloy develops unique surface oxide layers especially at high temperature (HT) helium environment with distinctive wear, friction and contact properties. This study investigates the elevated temperature mechanical properties of Inconel 617 top surface layers aged in HT helium environment. Nanoindentation technique is used to obtain load-displacement graphs of the alloy top surface oxide in temperatures ranging from 25 °C up to 600 °C. In addition, using finite element analysis along with an iterative regression technique, a semi-numerical method is developed to further measure and quantify the material parameters and, in particular, time-independent and creep characteristics of the oxide. While Young's modulus of the oxide is found to be relatively close to the bulk for the tested temperatures, the yield strength and hardness, in comparison to the bulk material, increase significantly as the material is oxidized after aging. The oxide exhibits significant softening as the temperature increases to 600 °C. Unlike the bulk material, diffusion through the grains is found to be the dominant creep mechanism for the oxide. Considerable difference between the mechanical properties of the oxide and the bulk material shows the need for accurate measurements of near surface mechanical properties, if reliable predictive contact and tribological models are sought at elevated temperatures.

Microstructural evolution and creep properties of Mg-4Sn alloys by addition of calcium up to 4 wt.%

The effects of addition of calcium up to 4 wt.% on the microstructure and creep properties of Mg-4Sn alloys were investigated by the impression creep test. Impression creep tests were performed in temperature range between 445 and 475 K under normalized stresses σ / G (where σ is the stress; G is the shear modulus) between 0.0225 and 0.035. Optical microscopy and scanning electron microscopy were used to study the microstructure of samples. It is observed that the addition of Ca more than 2 wt.% suppresses less stable MgSn 2 phase, and instead forms more thermally stable phases of Ca-Mg-Sn and Mg 2 Ca at the grain boundaries which improve the creep resistance of Mg-4Sn alloys. According to the stress exponents (6.04< n <6.89) and activation energies (101.37 kJ/mol< Q <113.8 kJ/mol) which were obtained from the impression creep tests, it is concluded that the pipe diffusion climb controlled dislocation creep is the dominant creep mechanism.

Contrasting roles of Laves_Cr2Nb precipitates on the creep properties of novel CuCrNbZr alloys

To enhance the creep resistance at elevated temperatures, a new precipitation-strengthened CuCrNbZr alloy has been designed and fabricated to achieve a target microstructure with coarse Laves_Cr2Nb precipitates at grain boundaries and fine Cr-rich precipitates in the matrix. This work systematically studied the creep property of the CuCrNbZr alloy at 500 °C under 90–140 MPa applied stress and compared to that of a reference commercial CuCrZr alloy without Laves_Cr2Nb precipitates. Microstructures before and after creep testing were investigated by optical and transmission electron microscopy. Based on the creep testing and microstructural characterization results, the dominant creep mechanism in both alloys was grain boundary sliding with a threshold stress of ~80 MPa. The CuCrNbZr alloy has higher creep strength and higher creep fracture ductility, and longer creep life than the CuCrZr alloy. The improved creep property in the CuCrNbZr alloy was due to the presence of Laves_Cr2Nb precipitates that efficiently impede the crack propagation.

Microstructure and Impression Creep Behavior of Al–7Si–0.3Mg Alloy with Zr Addition

The effect of 0.25 wt% and 0.5 wt% Zr addition on the impression creep behavior of the cast A356 alloy was investigated. Optical and scanning electron microscopy (SEM) equipped with energy dispersive spectrometry (EDS) were used for examination of the microstructure. The impression creep test was conducted at the temperature range of 493–553 K under the stress range of 480–680 MPa (corresponding to the normalized stresses between 0.022 and 0.03). Microstructural investigations showed that the Zr addition refined the α-Al grains and modified the eutectic structure. The Zr-containing alloys were characterized by Zr-rich intermetallics, suggesting the creep resistance of the A356 alloy was improved by Zr addition. The creep resistance of the A356–0.25Zr alloy was higher than that of the A356–0.5Zr alloy that was attributed to the morphology and the distribution of intermetallics that are rich in Zr. Results showed that the lattice self-diffusion climb-controlled creep was the dominant creep mechanism in all alloys.

Effects of pre-compression on the three-dimensional creep anisotropy of a hot-rolled Mg-4wt%Y binary alloy sheet

The effects of pre-compression on the creep behaviors along rolling direction (RD), transverse direction (TD) and normal direction (ND) of a hot-rolled Mg-4wt%Y alloy sheet were investigated in this study. A valid and convenient method for enhancing creep resistance and a systematic research in understanding creep anisotropy were provided. The pre-compression was carried out at 298 K along TD and performed to a true strain of 0.07. The samples with pre-compression (PC) and without pre-compression (NPC) were labeled as NPC-RD, NPC-TD, NPC-ND and PC-RD, PC-TD, PC-ND, respectively. Compressive creep tests along different directions were carried out at 523 K under various applied stresses. The results showed that both NPC and PC samples had obvious a three-dimensional creep anisotropy. The sequence of creep resistance of NPC samples was ND > TD > RD. The dominant creep mechanisms of NPC-RD and NPC-TD samples were both cross-slip and twinning. While dislocation climb and pyramidal <c+a> slip dominated the creep of NPC-ND sample, resulting in the highest creep resistance among the three NPC samples. After pre-compression, the resultant sequence of creep resistance of PC samples was RD > ND > TD. Dislocation climb and frequent twin intersections became the dominant creep mechanisms and significantly enhanced the creep resistance of PC-RD sample. Due to detwinning, only a slight enhancement of creep resistance happened in PC-ND sample. The creep resistance in PC-TD sample perceptibly declined, because heavy cross-slip and stepped <c> dislocations accelerated the creep strain. Although the three-dimensional creep anisotropy still insisted in PC samples, pre-compression was manifested to be an excellent method for improving creep properties along the worst direction (RD).

Effect of Rare Earth Er on Microstructure and Creep Behavior of Al–7Si–0.3Mg Alloy

The effect of 0.25 wt% and 0.5 wt% rare earth Er addition on the microstructure and creep behavior of the cast A356 alloy was investigated. The microstructure of the alloys before and after the creep was evaluated by optical and scanning electron microscopy equipped with energy dispersive spectrometry. The creep behavior was examined by the impression creep test at the temperature range of 493 to 553 K under the stresses between 480 and 680 MPa. Results showed that the addition of Er addition caused the α-Al dendrites to refine and the morphology of the eutectic silicon to modify significantly. The microstructure of the A356 alloy with Er addition was characterized by Er-rich intermetallics. The lattice self-diffusion climb-controlled creep was found to be the dominant creep mechanism in all alloys. The addition of Er improved the creep resistance of the A356 alloy and the alloy modified with 0.5 wt% Er exhibited the highest creep resistance.

Effect of SiC Nanofiber Addition on Microstructure, Mechanical Properties and Creep Behaviour of High Pressure Die Cast AZ91-1Sr Alloy

Effects of nanofiber SiC (0.5 and 1 vol%) addition on microstructure, mechanical properties and creep behaviour of cold chamber high pressure die casting AZ91-1Sr alloy have been investigated. Experimental results indicate that the AZ91-1Sr alloy contains of α-Mg, β-Mg17Al12 and Al4Sr intermetallic phases. With addition of nanofiber SiC, the Al4C3 and SiC peaks were detected. Also, the mechanical properties of the matrix alloy were improved. The AZ91-1Sr-1%SiC nanocomposite displays the best yield strength and hardness increment. Furthermore, the creep resistance of the matrix alloy was increased with increasing nanofiber SiC and the best creep resistance was obtained in the AZ91-1Sr-1SiC nanocomposite. The values of stress exponents and the activation energies for experimental specimen were found between 3.17–5.54 and 103.5–155.3 kJ/mol., respectively. When considered the results obtained from the values of stress exponents, the activation energies and SEM micrographs, it was thought that the dominant creep mechanism for the matrix alloy and nanocomposites is dislocation climb controlled. Both of the nano-composites exhibited superior creep resistance compared to the matrix alloy under all the creep test conditions.

Effects of aging on the creep properties of hot-compressed Mg-2.5 wt%Nd binary alloy

Tensile creep tests were performed at 523 K on a hot compressed Mg-2.5 wt%Nd binary alloy with peak aging (AA sample) and without aging (AS sample) in this work. The dominant creep mechanisms of AS sample were dislocation climb and pyramidal slip while AA sample was mainly dominated by cross-slip. During creep testing, only plate-like β1 precipitate was formed in AS sample while both plate-like β1 and lenticular β′ phases were formed in AA sample. Moreover, AA sample had larger scale and higher density of precipitates compared to AS sample. Besides, precipitate-free zones (PFZs) were clearly observed along grain boundaries of both AS and AA samples during their secondary creep stages. And the width of PFZs was narrower in AA sample than that in AS sample. The narrower PFZs, the higher density of β1 and β′ phases, and the strong interactions between cross-slip and precipitates effectively enhanced the creep properties of AA sample. As a result, AA sample behaved higher creep resistance in comparison to AS sample under all applied stresses at 523 K. Thus, this method of pre-aging and hot compression is believed to have deep potential in improving the service performance of Mg–Nd series at medium to high temperatures.

Scaling of heat transfer in stagnant lid convection for the outer shell of icy moons: Influence of rheology

The persistence of subsurface oceans in icy moons primarily depends on the efficiency of heat transfer through the outer ice shell. Scaling laws for heat transfer have already been determined by previous studies. These consider however a simplified rheology assuming a single creep mechanism, often using a linearized version of the temperature-dependent viscosity law (called Frank-Kamenetskii approximation). In the present study, we test the influence of ice rheology on the efficiency of heat transfer by considering a more realistic composite viscosity law, including diffusion creep, grain boundary sliding, basal slip and dislocation creep. We performed a large number of numerical simulations of thermal convection in the stagnant lid regime using a 2-D spherical annulus geometry with different rheology assumptions. Our results show that the dynamics of the ice shell is mainly controlled by the dominant creep mechanism in the rheological sublayer (the unstable lower portion of the cold boundary layer beneath the stagnant lid), which is mostly determined by grain size. For grain sizes lower than 3 mm, diffusion creep is the dominant creep mechanism in the rheological sublayer, while a combination of grain boundary sliding and basal sliding prevails for coarser grains. We derive new simple scaling relationships for the heat transfer suitable for the specific case of thin ice shells that account for the activation energy of the dominant creep mechanisms, thus allowing for a more accurate quantification of the temperature profile in the ice shell and cooling rate of the internal oceans.

Effect of Ca addition on microstructure and impression creep behavior of cast AZ61 magnesium alloy

Abstract Microstructure and creep properties of AZ61 alloy containing 1 and 3 wt.% Ca were investigated. The creep properties were examined using impression method under different stresses between 200 and 500 MPa at the temperature ranging from 423 to 491 K. The microstructure of AZ61 alloy contains α(Mg) matrix and Mg17Al12 intermetallic phases. It is shown that adding Ca to AZ61 alloy reduces the amount of Mg17Al12 phase via forming (Mg, Al)2Ca phase; furthermore, increasing the Ca content to 3 wt.% leads to the formation of (Mg, Al)2Ca phase, as well as the elimination of the Mg17Al12 phase. Creep properties of AZ61 alloy are improved with the Ca addition. The improvement in creep properties is attributed to the reduction in the amount of Mg17Al12 phase and the formation of (Mg, Al)2Ca phase with high thermal stability. According to the obtained creep data, it is concluded that the pipe diffusion−climb controlled dislocation creep is the dominant creep mechanism and Ca addition has no influence on this mechanism. The effect of pre-deformation on the creep properties of AZ61+3%Ca alloy reveals that the creep resistance of the alloy depends on the continuity of (Mg, Al)2Ca phase. It is decreased by reducing the phase continuity.

Effects of T5 Treatment on Microstructure and Mechanical Properties at Elevated Temperature of AZ80-Ag Alloy.

Effects of T5 treatment on microstructure and mechanical properties at elevated temperature of hot-ring-rolled (HRRed) AZ80-Ag magnesium alloy were systematically investigated. Results show that, after aging at 175 °C for 36 h, discontinuous and continuous precipitates form inside grains, with the former one taking up a volume fraction of ~64.9%. T5 treatment improves the tensile strength at ambient temperature of the alloy but weakens its tensile strength and creep resistance at elevated temperatures (120–175 °C), indicating opposite effects of T5 on mechanical properties at ambient and elevated temperatures. During creep at 120–175 °C and under 70–90 MPa, the dynamic precipitation process in HRRed specimen is accelerated with increasing temperature. At 150–175 °C massive nucleation and growth of dynamic discontinuous precipitates could result in an atypical primary creep stage, consisting of deceleration and acceleration creep stages, which is reported in wrought Mg-Al-based alloy for the first time. Such primary creep stage can be eliminated by T5 treatment. Besides, diffusion-controlled dislocation creep is the dominant creep mechanism for both HRRed and T5 specimens.

Microstructure evolution and creep behavior of high-pressure die-cast Mg–9Al–1Zn–1Sr alloy

The creep behavior of high-pressure die-cast Mg–9Al–1Zn–1Sr alloy was investigated in detail under temperatures ranging from 130 to 170 °C and stresses of 30–80 MPa. The studied alloy exhibited a high stress exponent of 8.4 and low activation energy of 102 kJ/mol. The obtained high stress exponent in studied alloy indicated the breakdown of the conventional power law. For rationalization, a threshold stress was introduced in the analysis, and then the high stress exponent was modified to be 5.2. Simultaneously, taking into account the significant influence of tested temperature on the minimum creep rate, a normalized stress exponent of 5 was achieved via the normalization method. Both methods suggested that the creep deformation of the studied alloy was mainly controlled by the dislocation climb. Moreover, transmission electron microscopy observations revealed that the origin of the threshold stress could be associated with the Orowan strengthening of γ-Mg17Al12 platelets that dynamically precipitated in Mg matrix during creep. Microstructure analysis results clarified that dislocation climb is the dominant creep mechanism of AZJ911 alloy, while the reticular eutectic Mg17Al12 phase disintegration, dynamical precipitation of γ-Mg17Al12, and twinning also influence the creep deformation of AJZ911 alloy.

Effects of initial texture on the three-dimensional creep anisotropic behaviors of dilute Mg–Y binary alloy sheets

The effects of initial texture on the three-dimensional creep anisotropic behaviors of dilute Mg–2Y(wt%) binary alloys were investigated in this study. Two kinds of sheets were used: one had a concentrated texture (CT sample) and the other had a split texture (ST sample). According to the compressive directions and conventional orthogonal coordinate system for sheet, the samples were named as CT-RD, CT-TD, CT-ND and ST-RD, ST-TD, ST-ND. Compressive creep tests were conducted at 493 K and applied stresses ranged from 40 to 100 MPa. The results showed that both CT and ST samples had evident three-dimensional creep anisotropy. Specifically, the creep resistance was highest in CT-ND sample, followed by ST-ND, ST-TD, CT-TD, CT-RD samples, and lowest in ST-RD sample. Next, the dominant creep mechanisms in all samples were strictly investigated using various characterization methods. For CT-RD and CT-TD samples, the dominant creep mechanisms were similar, including cross-slip and twinning. For CT-ND sample, the dominant creep mechanisms were dislocation climb and pyramidal <c+a> slip. For ST samples, a variety of creep mechanisms were detected, ranging from heavy basal slip to easy activations of cross-slip and twinning. A little bit dislocation climb and pyramidal <c+a> slip acted as plastic accompanying roles in ST samples. Furthermore, the variation of initial texture types led to the variations of dominant creep mechanisms and the different three-dimensional creep anisotropic behaviors. The rank of deteriorating creep resistance was revealed as basal slip > cross-slip > twinning > dislocation climb > pyramidal slip. Finally, it is proposed that future work on the enhancement of creep resistance of Mg alloys with rare earth addition should focus on the inhibition of basal slip and cross-slip.

Creep behavior of solid solutioned and annealed Al-7075 alloy processed by equal channel angular pressing

Effect of equal channel angular pressing ECAP process on creep properties of solid solutioned and annealed Al-7075 alloy was investigated at different temperatures and applied stresses. The microstructures of treated alloys before ECAP process were analyzed via transmission electron microscopy (TEM) and electron back scatter diffraction (EBSD). Mechanical properties of ECAPed alloy were also characterized. The fracture surface of samples was also observed by scanning electron microscopy (SEM). Subsequently, the creep mechanisms at different temperatures and stresses were discussed. The results showed that applying the initial pass of ECAP can effectively improve the mechanical properties and creep resistance of Al-7075 alloys due to impeding the dislocations movement as a result of dislocation density growth. However, applying multi passes of ECAP on annealed Al-7075 alloy did not have a positive effect on creep strength. Increasing the fraction of HAGBs and grain refinement of multi-pass annealed Al-7075 ECAPed alloy deteriorate the creep characteristic. The maximum creep rate was achieved for annealed sample after 1 pass of ECAP at 503 K under 70 MPa. Furthermore, according to different values of stress exponent, n, creep mechanism of treated samples altered. Dislocation creep as a dominant creep mechanism of annealed Al-7075 ECAPed alloy at both temperatures (503,543 K) was not altered after initial pass of ECAP; however it was replaced by grain boundary sliding after multi passes of process. Core creep mechanism of solid solutioned sample was also changed from Coble diffusion creep to viscos glide dislocation after one pass of ECAP.