Temperature Creep Research Articles

Stress and temperature dependence of irradiation creep in zircaloy-4 studied using proton irradiation

This paper describes a series of experiments conducted to investigate the in-situ irradiation creep behavior of Zircaloy-4 using proton irradiations as a surrogate to neutrons. The first series of experiments investigated the impact of the initial irradiation-induced defect evolution during the 0 – 0.1 dpa regime on the subsequent in-situ steady-state behavior derived from experimentation. These experiments were conducted at a constant applied load of 85 MPa, a constant damage rate of 1.58 × 10−6 dpa/s, and were repeated at both 250 °C and 350 °C. We also examined the use of a ‘conditioning-irradiation’ step prior to the creep tests on the results derived from subsequent in-situ proton irradiation creep experiments. By extension, we aimed to further develop and refine optimum testing procedures when using proton irradiations to investigate the in-situ creep behavior of nuclear materials. The second series of in-situ proton irradiation experiments were conducted on two Recrystallized (RX) Zircaloy-4 samples in order to investigate the temperature and stress dependence of irradiation creep. One sample was held at a constant load while the temperature was varied in the range of 275 – 350 °C, and the other was held at a constant temperature while stress was varied in the range of 65 – 105 MPa. The associated strains and creep rates were measured at each interval and used to determine an activation temperature of 5000 ± 1700 K and a stress sensitivity exponent of 4.3 ± 0.8 for RX Zr-4 over the given temperature and stress ranges. A discussion of potential deformation mechanisms based on competition between bulk diffusion through the lattice and point defect diffusion enabling dislocation climb and glide is given: the relatively high stress dependence suggested the latter is more likely however further investigations will be required to improve the mechanistic understanding. The results presented in this manuscript, including activation temperature, stress sensitivity, and associated creep rates, determined through proton irradiation investigations are closely comparable to those determined from neutron irradiation experiments found in the literature for similar zirconium alloys at similar temperatures and stresses. Coupled with the primary-secondary creep behavior investigations presented, this demonstrates the usefulness of this approach to estimate neutron-irradiation equivalent creep behavior using in-situ proton irradiation.

Lifespan prediction of glass fiber reinforced polymers subjected to flexural creep and elevated temperatures using analytical and numerical analyses

Lifespan prediction of glass fiber reinforced polymers subjected to flexural creep and elevated temperatures using analytical and numerical analyses

Effect of ARB process on the plane stress fracture toughness, creep properties and forming limit diagram of Aluminum/BN/Copper composite strips

Abstract In the present study, Al/BN/Cu bimetal composite strips were fabricated via the accumulative roll bonding (ARB) process. Then, fracture toughness, mechanical properties, bonding strength, forming limit diagram (FLD), and creep properties have been studied via experimental techniques. The study of creep behavior and mechanical properties showed a formation of a 17 μm atomic diffusion layer at the interface during ARB under three creep loading conditions namely 35 MPa at 225 °C, 35 MPa at 275 °C, and 30 MPa at 225 °C. The tensile strength of samples improved to 144 MPa which was about 234% in comparison with the primary Al monolithic sample. It was observed by the SEM that the fracture types changed to shear kind after higher passes including prominent effects on the fracture morphology. FLD curve dropped severely after the first ARB pass and then began to enhance at higher passes. The fracture toughness value of 30 MPa m1/2 was achieved after ten ARB passes. An intermetallic composition was created near Al. The creep failure time did decline by 44% and the stress level decreased by 13% at a constant temperature. It can be concluded that applied temperature and stress effect on creep properties especially leads to increasing the slope of creep curves with higher stresses. This trend was opposite for the creep temperature at higher temperature levels. Moreover, dynamic recrystallization was observed through the crystalline structure of samples.

Challenges related to elevated temperature during creep in a PM-HIP Ti-6Al-4 V alloy: The role of [formula omitted] dislocations and pyramidal [formula omitted] dislocations

In our current study, the impact of pre-creep treatment at 300 °C and 560 MPa for 1000 h on the subsequent creep behavior and properties at an elevated temperature of 400 °C have been examined. The characterization of the resulting creep deformation structure, including twins and dislocation morphology, was conducted using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Additionally, we have focused on the fine crystallization behavior of the α phase during creep, as well as the study and discussion of the 〈a〉 dislocations and pyramidal 〈c+a〉 dislocations. Our findings indicate that the creep life at 400 °C and 560 MPa is significantly lower, by an order of magnitude, compared to samples without pre-creep treatment. During the pre-creep process at 300 °C, 〈a〉 dislocations were found to be active. Furthermore, the creep temperature was observed to reduce the critical resolved shear stress (CRSS) of pyramidal 〈c+a〉 dislocations, leading to increased dislocation activity at 400 °C compared to 300 °C. However, a substantial number of 〈a〉 dislocations activated during the pre-creep process were unable to accommodate the subsequent heating process, resulting in extensive intergranular fractures. Simultaneously, during the creep process, the α phase underwent marginal fine grain formation in the form of continuous dynamic recrystallization (CDRX), with its internal fine grain promoted by pyramidal 〈c+a〉 dislocations assisting 〈a〉 dislocation to form dislocation locks. These findings offer valuable insights into potential deformation mechanisms and failure causes of PM-HIP Ti-6Al-4 V alloy when subjected to elevated temperatures during creep.

Simulation Study of Coal Seam Gas Extraction Characteristics Based on Coal Permeability Evolution Model under Thermal Effect.

The permeability evolution law of high temperature and high stress coal seam is determined by the influence of multiphase coexistence and multifield coupling. In an environment greatly affected by disturbance and high temperature, the coal permeability model under the coupling of thermal and mechanical creep is not only a vital framework from which to examine gas migration law in multiphase and multifield coal seams but also an important theoretical foundation for gas control in coal seams. The influence of high-temperature environment on creep deformation and permeability is analyzed by several creep seepage tests under different temperature conditions.A mathematical model for the evolution of coal permeability considering the influence of temperature is established through the theory of matrix-crack interaction based on gas adsorption and desorption and thermal expansion deformation. Based on the permeability model under the coupling of thermal and mechanical creep, the numerical model of gas migration, seepage field, diffusion field, stress field, and temperature field is constructed, and the law of gas migration in coal seam under multifield coupling is explored. The influence law of thermal effect on gas extraction characteristics is analyzed, in which the time-varying mechanism of temperature field, the relationship between creep deformation and temperature and pressure, the influence of creep deformation on permeability, the dynamic distribution of gas pressure, and the change of gas extraction quantity are described in detail. It is concluded that the influence of temperature on permeability is much greater than that of creep deformation and that a high initial coal seam temperature is beneficial to gas extraction. It provides theoretical basis and technical guidance for the study of multifield coupled gas migration and coal seam gas treatment.

Thermal creep properties of virgin and irradiated cladding tubes made of Ti-stabilised DIN 1.4970 (15-15Ti) austenitic stainless steel

ABSTRACT This paper presents assessments performed on a large database of virgin material and irradiated material thermal creep data from uniaxial and pressurised DIN 1.4970 Ti-stabilised austenitic stainless steel i.e. EN X10NiCrMoTiB15–15 or ‘15-15Ti’ cladding tubes. The data base incorporates multi-heat data from uniaxial and bi-axial (internal pressure) creep tests conducted during the fast reactor R&D program of the DeBeNe (Deutschland-Belgium-Netherlands) consortium between the 1960s to the late 1980s together with more recent data, for example, from the European projects MATTER and PATRICIA and the EERA JPNM pilot project TASTE. The data comprises of a virgin material data base and a data base with irradiated creep data. The virgin material data comprises of time-to-rupture and time-to-0.2% creep strain in a temperature range of 600–800°C and covers a large range of stresses. The irradiated data base has less data and does not cover all material heats tested for the virgin material properties. The irradiation conditions are also different depending on the test reactor specifics and the irradiation campaign targets. The attained ‘irradiation damage’ in displacements per atom (DPA) for the irradiated materials range from 0.1 up to 38.1. Un-irradiated ‘reference material’ models for tensile strength and creep strength are constructed from the current-state of the art literature data and more recent results from the Sandvik 24% cold worked 1515–Ti cladding tube currently used as the main material batch studied at SCK CEN. The tensile strength model is used throughout the paper for normalizing creep strengths by the tensile properties, needed when applying the Wilshire (WE) creep model. Time factors (TF), stress factors (SF) and temperature ratio factors (TRF) are calculated for different material states for describing the impact of heat treatments, (virgin and irradiated) and irradiation conditions. This initial study targets to give estimates for the thermal creep properties of cladding tubes with a cold work range of 16–24% in a non-annealed state, as it is the preferred option for future designs according to the state-of-the art knowledge base. However, the main bulk of the available data on irradiated material is on claddings with a cold work range of 0-16% with and without annealing and at various levels of irradiation damage, thus leading to the need for estimation by interpolation and extrapolation assuming that irradiation damage levels and trends, e.g. time reduction factors found on a lower cold worked material can also be applied on materials with higher cold work levels. The results of the assessments clearly show relative strength differences between chosen material heats and heat treatments and has enabled constructing simple multilinear models for estimating the life of irradiated material. The models show that low creep test temperatures (and irradiation temperatures), low stress levels, low levels of cold work and high irradiation doses are increasing the detrimental difference between the specific material condition and the reference material. Also, it was found that the general level of life reduction (time factor TF and stress factor SF) for irradiation damaged material failure, in relation to the reference material failure, roughly coincides the corresponding values of time to 0.2% creep strain for virgin material. The estimated ‘failure creep temperature limits’ for 30 000 hours of service are studied and compared for both virgin and irradiated materials at a reference stress level of 1/3 of the tensile strength (at temperature).

Investigation of the precipitation behavior and its role on creep deformation and failure mechanisms at high temperatures for a turbine disk alloy

The creep deformation behavior of a turbine disk alloy at high temperatures was studied, and the role of the multi-scale precipitates on the creep deformation and failure mechanisms was revealed. The turbine disk alloy shows a typical microstructure with the micron-scale MC carbides, submicron delta phase, together with nanoscale γ″ and γ' precipitates distributed in the matrix after hot working and aging treatment. The MC carbides are mostly formed from the solidification process due to enrichment of Nb in the inter-dendritic areas. The creep life decreases from 763.5 h to 5.7 h when the creep temperature increases from 953 K to 1023 K at 500 MPa. The fracture surfaces exhibit a transgranular fracture pattern accompanied with the formation of substantial microvoids. Slipping occurs in both unidirectional and bidirectional modes during deformation. The microvoids along the grain boundaries are generated through decohesion of δ precipitates from the matrix or the interaction between slip traces and grain boundaries. When the intragranular MC carbides interact with the slip traces, microcracks can be formed by particle fracturing. The dense distribution of particles along the grain boundaries will result in a significantly higher critical stress required for microvoid nucleation, whereas the non-coherent particles inside the grains, particularly the large size MC carbides, can decrease the critical shear stress for microcrack formation to as low as 88.9 MPa, thereby contributing to alloy rupture. For the coherent precipitates, a comprehensive strengthening effect of 389.12 MPa is achievable by the synergistic precipitation of γ″ and γ', which primarily serve to enhance and coordinate intragranular deformation during creep.

Enhanced Mechanical and Electromechanical Properties of Compositionally Complex Zirconia Zr1-x(Gd1/5Pr1/5Nd1/5Sm1/5Y1/5)xO2-δ Ceramics.

Compositionally complex oxides (CCOs) or high-entropy oxides (HEOs) are new multielement oxides with unexplored physical and functional properties. In this work, we report fluorite structure-derived compositionally complex zirconia with composition Zr1-x(Gd1/5Pr1/5Nd1/5Sm1/5Y1/5)xO2-δ (x = 0.1 and 0.2) synthesized in solid-state reaction route and sintered via hot pressing at 1350 °C. We explore the evolution of these oxides' structural, microstructural, mechanical, electrical, and electromechanical properties regarding phase separation and sintering mechanisms. Highly dense ceramics are achieved by bimodal mass diffusion, composing nanometric tetragonal and micrometric cubic grains microstructure. The material exhibits an anomalously large electrostriction response exceeding the M33 value of 10-17 m2/V2 at room temperature and viscoelastic properties of primary creep in nanoindentation measurement under fast loading. These findings are strikingly similar to those reported for doped ceria and bismuth oxide derivates, highlighting the presence of a large concentration of point defects linked to structural distortion and anelastic behavior, which are characteristics of nonclassical ionic electrostrictors.

Short-time creep behavior of (TiB + TiC + Y2O3) reinforced titanium matrix composite in the range of 600 °C to 700 °C

A titanium matrix composite reinforced by TiB, TiC and Y2O3 was prepared by induction skull melting. The creep behavior of the composite was tested at the temperature range from 600 °C to 700 °C in short time to investigate the deformation mechanism and microstructure involution. The as-cast composite shows a typical basket-weave microstructure. At the stress of 150 MPa, the stress exponents of the composite are 3.48 at 600 °C and 5.33 at 700 °C, corresponding to the deformation mechanism mainly controlled by solute-dragging and dislocation climbing respectively. Reinforcements added hinder dislocations movement effectively, which improves the creep resistance of the composite. As the creep temperature rises, the solubility of β-Ti increases, accompanied by the precipitation of silicides with different morphologies. Silicides play a role in pinning dislocations and hindering interface from migration. The dislocation movement and rearrangement during creep lead to the formation of sub-grains.

An additively manufactured near-eutectic Al-Ce-Ni-Mn-Zr alloy with high creep resistance

A new additively manufactured (AM) Al-7.5Ce-4.5Ni-0.4Mn-0.7Zr (wt.%) near-eutectic alloy is reported, which shows unprecedented creep resistance up to 400 °C (a homologous temperature of 0.72). The eutectic solidification microstructure comprises ∼ 27 vol% of coarsening-resistant second phase network with an ultrafine (<100 nm) inter-phase spacing. Both Mn and Zr contribute to creep resistance of the alloy. Small amount of Mn addition promotes selection of coarsening resistant phases without compromising the alloy processability. Zr not only improves hot-tearing resistance, but further enhances the second phase coarsening resistance resulting in improved creep resistance. Neutron diffraction performed during creep deformation reveals that the underlying mechanism for creep resistance in this alloy is impedance to dislocation motion stemming from the ultrafine eutectic solidification microstructure, whereas load transfer strengthening becomes less effective as the creep temperature increases. The second phase forms a continuous network in the as-fabricated condition, which is maintained during long-term creep at 300 °C. However, this network is fragmented into fine dispersoids at higher temperatures. It is proposed that the rate-limiting deformation mechanism at 300–400 °C is (i) dislocation climb for the alloy with fragmented second phase dispersoids and (ii) Orowan looping for the alloy with a continuous second phase network. The present design of an AM-processable multicomponent eutectic alloy with high creep resistance can be applied to other metallic systems exhibiting eutectic reactions, with expected extreme creep resistance.

A new tensile creep model for predicting long-term creep strengths with short-term test data for creep resistant alloys

The difficulties in using the conventional Norton creep model to rationalise short-term creep data and to subsequently predict long-term creep rupture strengths for creep resistant alloys are presented and analysed. The results of this study show that these difficulties can be resolved if a new tensile creep model that integrates the tensile strength at creep temperature is applied to rationalise the short-term creep data. This is illustrated with the creep and tensile strength data measured for a grade of Ni-based superalloy. Based on this new tensile creep model, the activation energy of creep determined is independent of stress and the stress exponent is not influenced by temperature. Consequently, the model constants obtained from the short-term creep data can be applied together with the Monkman-Grant relationship to make the long-term creep rupture strength predictions at different temperatures. The factors affecting the reliability of the predictions made by this method are also analysed.

Long‐term creep deformation of carbon fiber/epoxy composites with physical aging: Experimental investigation and constitutive modeling

AbstractThe tensile creep tests for two kinds of epoxy resin matrix composite laminates were performed under 25°C and 50°C. Results indicate that these materials deform in a time‐dependent manner, that is, the tensile strain increases within a short time after the tests began, and then decreases during most of the loading period. Such increase and decrease trends reflect the creep characteristic and effect of physical aging, respectively. A linear viscoelastic model is developed to include both the creep temperature effect and physical aging effect, starting from the Boltzmann superposition principle. In modeling, these two effects both contribute to the transient compliance and result in the increase and decrease of the transient compliance. Time–temperature/aging time superposition principle is adopted to deal with the influences of temperature and physical aging. This theoretical model is further implemented numerically within a finite element framework. To verify the newly proposed model, not only the experimental and numerical results of the tensile deformations of laminates are compared, but also the bending deformations of structural components made from the above laminates are analyzed.Highlights Tensile creep tests were done for carbon fiber/epoxy laminates. The strain shows creep temperature and aging effect (TE and AE). New constitutive relation was proposed to describe the TE‐ and AE‐caused strains. Structural‐level flexure creep tests and simulation analyses were conducted.

A Method for Predicting the Creep Rupture Life of Small-Sample Materials Based on Parametric Models and Machine Learning Models.

In view of the differences in the applicability and prediction ability of different creep rupture life prediction models, we propose a creep rupture life prediction method in this paper. Various time-temperature parametric models, machine learning models, and a new method combining time-temperature parametric models with machine learning models are used to predict the creep rupture life of a small-sample material. The prediction accuracy of each model is quantitatively compared using model evaluation indicators (RMSE, MAPE, R2), and the output values of the most accurate model are used as the output values of the prediction method. The prediction method not only improves the applicability and accuracy of creep rupture life predictions but also quantifies the influence of each input variable on creep rupture life through the machine learning model. A new method is proposed in order to effectively take advantage of both advanced machine learning models and classical time-temperature parametric models. Parametric equations of creep rupture life, stress, and temperature are obtained using different time-temperature parametric models; then, creep rupture life data, obtained via equations under other temperature and stress conditions, are used to expand the training set data of different machine learning models. By expanding the data of different intervals, the problem of the low accuracy of the machine learning model for the small-sample material is solved.

Impacts of different designed microstructures on creep performance of a Ni-based wrought superalloy

The creep behavior of a novel Ni-based wrought superalloy with varying aging states, classified as large under-aged (UA), little UA, and little over-aged (OA) states, was investigated at a temperature higher than the intra-grain/grain-boundary iso-strength temperature (750 ​°C/250 ​MPa). In the samples before creep test, there are spherical γ′ precipitates homogeneously dispersed within the γ matrix and carbides precipitated at grain boundaries (GBs). After creep tests, precipitate-free zones (PFZs) appear on grain boundaries, with a large block-like γ′ phase present between carbides and PFZs, potentially serving as initiation sites for cracks. Little UA state samples exhibit the strongest creep resistance and intergranular fracture. The dominant creep deformation mechanism in the intra-grain is a combination of Orowan process involving slip and climb of matrix dislocations and shearing of γ′ precipitates. The results indicate that the optimizing the grain boundary microstructure may be more important than the change of intragranular microstructure when the creep temperature exceeds the iso-strength temperature.

Study on creep behaviors of nickel-based single-crystal alloys considering microstructure evolution

Purpose The purpose of this study is to describe the mechanism of single-crystal high-temperature creep deformation, predict the creep life more accurately and study the creep constitutive and lifetime models with microstructure evolution.Design/methodology/approachThe mechanical properties of nickel-based single-crystal superalloy are closely related to the γ' phase. Creep tests under four different temperature and stress conditions were carried out. The relationship between creep temperature, stress and life is fitted by numerical method, and the creep activation energy is obtained. The creep fracture surface, morphology and evolution of strengthening phase (γ') and matrix phase (γ) during different creep periods were observed by scanning electron microscope. With the increase of creep temperature, the rafting time is advanced. The detailed morphology and evolution of dislocations were observed by transmission electron microscope (TEM).FindingsWith the increase of creep temperature, the rafting time is advanced. The detailed morphology and evolution of dislocations were observed by TEM. Dislocations are mainly concentrated in the γ channel phase, especially at high temperature and low stress.Originality/valueA creep constitutive model based on the evolution of γ' phase size and γ channel width was proposed. Compared with the experimental results, the predicted creep life is within 1.4 times error dispersion band.

Laboratory investigation of the impact of peanut husk ash as an asphalt binder modifier on physical and rheological properties

Researchers propose using agricultural waste into asphalt pavements as a means to raise recycling rate of waste, conserve natural resources, and reduce disposal issues. This paper was set out to explore the ability of peanut husk ash (PHA) as an asphalt binder modifier. To this end, scanning electron microscope (SEM), softening point, penetration, ductility, storage stability, rotational viscosity, multiple stress creep recovery (MSCR), temperature sweep, frequency sweep, and bending beam rheometer (BBR) tests were applied. Besides, the temperature susceptibility was evaluated using the penetration index, while the fatigue parameter was applied to assess the intermediate temperature performance of asphalt binder. Moreover, the response surface methodology (RSM) was performed to assess the interactive impact of load frequency, temperature, and PHA concentrations on the rutting factor and complex modulus, besides the interaction between them. Moreover, models were proposed to fit the findings of experimental work to predict the responses. The findings manifested that asphalt binder can be modified with 8% or less of PHA without experiencing phase separation issues during high temperature storage. Furthermore, the SEM demonstrated that PHA was well dispersed in the modified asphalt. Moreover, the increment of PHA led to increase penetration index, softening point, viscosity, rutting parameter, percent recovery, and complex shear modulus, as well as reducing nonrecoverable creep compliance (Jnr), phase angle, penetration, and ductility. Although the high temperature performance was enhanced, there was a decrease in fatigue and low temperature cracking resistance. The RSM analysis revealed that the input factors significantly affected the asphalt's rutting factor, and complex modulus, and quadratic models were suggested to predict the potential for rutting.

Synergic effects between vacuum residue and polymers for preparing high-performance bitumens

This research presents a laboratory study on a polymer-modified bitumens (PMBs) made with a vacuum residue (VR) as a bitumen extender. Two different PMBs were prepared, one with a plastomeric Elvaloy and the other one with an elastomeric styrene-butadiene-styrene SBS. The mechanical properties of the samples were determined by means Multi Stress Creep Recovery tests (MSCR) and temperature sweep test. The results show that polymers can improve the mechanical properties of bitumen, and the vacuum residue can be used as softening bitumen extender. However, their simultaneous presence highlights non-additive effects giving optimal mechanical properties with a lowered temperature dependence of the viscoelastic properties and a high extension of the usage temperature range. The peculiar effect of the combined action of polymers and vacuum residue was also confirmed by chemical-physical analyses. In fact, Atomic Force Microscopy (AFM) and Nuclear Magnetic Resonance (NMR) relaxometry have shown that vacuum residue and polymers are able to effectively interact with the inner structures of the bitumen to give enhanced performances, with an increased stability upon temperature changes.

Evaluation of Turbine Tip Clearance With Performance Degradation Using Multilayer Perceptron

Abstract This paper developed a process for turbine tip clearance prediction and control considering performance degradation to address the contradiction between computational efficiency and computational accuracy. The developed process consists of an offline high-accuracy database establishment for tip clearance with performance degradation and an online fast tip clearance prediction and control using machine learning. For the former, the steady-state tip clearance is obtained by the calculations for the two-dimensional axisymmetric casing and disk deformations using the finite element method and the one-dimensional blade deformation using the engineering calculation method. The effects of performance degradation, including blade creep and turbine inlet temperature degradation are introduced to update the boundary conditions in gas path and initial clearance. For the latter, the multilayer perceptron is used to realize the fast tip clearance prediction. Considering the independence of component deformations, the tip clearance prediction is achieved by the component deformation predictions, which also reduces the dimension of input parameters for each prediction model and improves the prediction accuracy. Combining the above two parts, the tip clearance with performance degradation can be obtained within 0.00025 s/time, and the maximum absolute error is only 0.012 mm. In addition, with the help of the process, the optimized tip clearance control strategy can be obtained for the performance degradation states, which restores the tip clearance with a 17.66% increment to the initial state without performance degradation. This paper will provide a reference for the tip clearance prediction and control with small computation and high accuracy.

Effect of substituting Gd with 3 wt% Nd on high temperature creep behaviors of peak-aged Mg-10Gd-0.4Zr casting Mg alloy

In this paper, the high temperature creep behaviors of peak-aged Mg-10Gd-0.4Zr and Mg-7Gd-3Nd-0.4Zr casting alloys were investigated. The results showed that substituting Gd with 3 wt% Nd promoted precipitation of the β′ phase with higher number densities and larger aspect ratios, which led to the higher tensile mechanical properties of the peak-aged Mg-7Gd-3Nd-0.4Zr casting alloy at both room and elevated temperatures. The peak-aged Mg-7Gd-3Nd-0.4Zr casting alloy had better tensile creep properties at temperatures of 200 °C–225 °C. However, the peak-aged Mg-10Gd-0.4Zr casting alloy showed better creep resistance when the creep temperature ≥250 °C. The creep mechanism for the peak-aged Mg-10Gd-0.4Zr casting alloy was dislocation climb assisted by grain boundary diffusion under all experimental conditions, while the creep mechanism of the peak-aged Mg-7Gd-3Nd-0.4Zr casting alloy transformed from dislocation glide at 200 °C–225 °C into dislocation cross-slip above 250 °C. Due to the faster transformation from β′ phase into β1 phase in peak-aged Mg-7Gd-3Nd-0.4Zr casting alloy, the blocking effect of precipitation on dislocation movement was weakened. And a larger number of basal < a > dislocations accumulated in the grain boundary region, leading to precipitate free zones (PFZs) widening more obviously in the peak-aged Mg-7Gd-3Nd-0.4Zr casting alloy, and which well explained the reason why the creep resistance of peak-aged Mg-7Gd-3Nd-0.4Zr casting alloy deteriorated rapidly above 250 °C.

Azine-Based dynamic covalent polyurethane elastomers with excellent mechanical and Creep-Resistant properties

Dynamic covalent bonds have received tremendous attention since they endowed classical crosslinked polymers with new properties, including self-healing, stimuli-responsive, anti-bacterial, and recycling properties. The dynamic crosslinked polymers combined the merits of thermoset and thermoplastic elastomers. However, the dynamic nature also compromised the heat resistance of dynamic crosslinked polymers, especially the creep resistance at high temperatures. Herein, a hexahydroxy azine crosslinker was synthesized and used to prepare a range of azine-based polyurethane elastomers (PUEs). The exchange reactions of small molecular model compounds revealed that the azine moieties could exchange fast at 150 °C. The resulting azine-based PUEs showed outstanding thermal stabilities and mechanical properties owing to the conjugated structure and related hydrogen bonds of azine. The obtained azine-based PUEs also exhibited a high creep temperature of 150 °C thanks to the relatively higher exchange temperature of azine moieties and high functionality of the azine crosslinker. The degradation process of azine-based PUEs displayed solvent and acid strength dependence. The excellent reprocessing properties of azine-based PUEs were attributed to the distribution of the azine moieties both in the backbone and crosslinking points. This study provides an alternative way to solve the creep resistance of dynamic crosslinked polyurethanes and a feasible method for the industrialization of the recyclable thermoset polyurethane elastomers.