Improved Creep Resistance Research Articles

Effects of Y Additions on the Microstructures and Mechanical Behaviours of as Cast Mg–xY–0.5Zr Alloys

Previous investigations demonstrated that rare‐earth elements (REs) could improve their creep properties effectively. Herein, the influence of Y content on the creep properties of magnesium is investigated systematically with different amount of Y additions. The mechanisms responsible for creep deformation are clarified by the analysis of stress exponent and microstructural characterizations. It is found that the addition of Y in Mg can improve both the ambient strength and high temperature strength owing to its effective solid solution strengthening. At room temperature, the yield strength of Mg–Y alloys has a linear relation with the content of Y. When tested at high temperatures, the yield strength reduces. Compared with pure magnesium, Mg–Y alloys exhibit a high thermal stability even above 200 °C. Small amount of Y addition can improve the creep resistance of Mg largely. With further increasing its content, its contribution to the improvement of creep resistance is weakened for Mg. Under the applied stresses 60–100 MPa and at temperatures of 200–250 °C, the responsible creep mechanism is dislocation controlled. During creep deformation, the Y segregation regions play an important role in hindering the movement of dislocations.

Molecularly engineered dual-crosslinked elastomer vitrimers with superior strength, improved creep resistance, and retained malleability

Preparation of covalently crosslinked elastomers with an integration of high mechanical performance, enhanced creep resistance and retained malleability by incorporating quadruple hydrogen bonds into dynamic boronic ester bonds crosslinked SBR.

Microstructure control and creep resistance improvement of AZ31 extruded rod by precompression and heat treatment

The creep tests were performed at 473 K in the range of 40-80 MPa on the AZ31 alloy extruded rod in which microstructures are controlled by pre-compression and heat treatment. The microstructures were characterized by EBSD. As a result, fraction of grains in which basal planes were almost perpendicular to extruded direction were evaluated after microstructure control by pre-compression and heat treatment. In addition, remarkable coarsening grains were confirmed. In the creep test, it was found that the minimum true strain rate and the rupture life were improved by microstructure control. The coarsened grains and texture control are considered to be the factors for improving the creep resistance.

Improving creep resistance while maintaining reversibility of covalent adaptive networks via constructing reversibly interlocked polymer networks

Thermoresponsive covalent adaptive networks (CANs) have attracted increasing research attention because of their temperature-dependent reversibility, which endows the materials with versatile smart functionalities including intrinsic self-healability not available for traditional thermosets. Nevertheless, the associated reduction in creep resistance of this type of CANs limits their practical application. Here in this work, we demonstrate that the reversibility and dimensional stability can be combined via interlocking the thermoresponsive network with an ultraviolet-responsive network. The two types of networks are, respectively, crosslinked by orthogonal reversible Diels-Alder (DA) bonds and coumarin. Owing to the interlocked architecture, the latter single network can be uniformly distributed in the former, restricting the chain movement and enhancing the creep resistance even when the former is decrosslinked at elevated temperature as a result of retro-DA reaction. Meanwhile, the ultraviolet-responsive network plays the role of a photo-reversible switch. Its decrosslinking on exposure to 254 nm ultraviolet (UV) light and recrosslinking under a UV irradiation of 350 nm lead to repeated releasing and reimposing of the restraints on the neighboring thermoresponsive network. By using this smart habit, the material can either be conditionally self-healed with moderate healing efficiency or completely self-healed depending on whether only DA bonds or both DA bonds and coumarin are triggered. More importantly, the conflicting properties, that is, creep resistance and reversibility of the CANs are thus united. The proposed strategy provides a facile way for overcoming the weaknesses of CANs while maintaining the advantage.

Creep in reactive colloidal gels: A nanomechanical study of cement hydrates

From soft polymeric gels to hardened cement paste, amorphous solids under constant load exhibit a pronounced time-dependent deformation called creep. The microscopic mechanism of such a phenomenon is poorly understood in amorphous materials and constitutes an even greater challenge in densely packed and chemically reactive granular systems. Both features are prominently present in hydrating cement pastes composed of calcium silicate hydrate (C-S-H) nanoparticles, whose packing density increases as a function of time, while cement hydration is taking place. Performing nanoindentation tests and porosity measurements on a large collection of samples at various stages of hydration, we show that the creep response of hydrating cement paste is mainly controlled by the interparticle distance and results from slippage between (C-S-H) nanoparticles. Our findings provide a unique insight into the microscopic mechanism underpinning the creep response in aging granular materials, thus paving the way for the design of concrete with improved creep resistance.

Understanding the High Creep Resistance of MRI 230D Magnesium Alloy through Nanoindentation and Atom Probe Tomography

Due to their low density, magnesium alloys are very appealing for light-weight constructions. However, the use of the most common magnesium alloy, AZ91 (Mg 9 wt.% Al, 1 wt.% Zn), is limited to temperatures below 150 °C due to creep failure. Several alloys with an improved creep resistance have been developed in the past, for example the alloy MRI 230D or Ca-alloyed AZ91 variants. However, there is an ongoing discussion in the literature regarding the mechanisms of the improved creep resistance. One factor claimed to be responsible for the improved creep resistance is the intermetallic phases which form during casting. Another possible explanation is an increased creep resistance due to the formation of precipitates. To gain more insight into the improved creep resistance of MRI 230D, nanoindentation measurements have been performed on the different phases of as-cast, creep-deformed and heat-treated samples of MRI 230D and Ca-alloyed AZ91 variants. These nanoindentation measurements clearly show that the intermetallic phase (IP) of the alloy MRI 230D does not lose strength during creep deformation in contrast to the Ca-alloyed AZ91 variants. High-temperature nanoindentation measurements performed at 200 °C clearly show that the intermetallic phases of the MRI 230D alloy maintain their strength. This is in clear contrast to the Ca-alloyed AZ91 variants, where the IP is significantly softer at 200 °C than at room temperature. Atom probe measurements have been used to gain insight into the differences in terms of chemical composition between the IPs of MRI 230D and the Ca-alloyed AZ91 variants in order to understand the dissimilar behaviour in terms of strength loss with increasing temperature.

Abnormal creep stress exponents in a high-pressure die casting Mg–Al−RE alloy

Abnormal creep stress exponents (n) are frequently reported for heat-resistant magnesium alloys, but hitherto without in-depth elucidation or rationalization from the underlying creep mechanisms. In this work, tensile creep behavior of a high-pressure die casting Mg−3.76Al−3.52RE−0.23Mn (wt.%) alloy at 200 °C were studied, adopting small applied stress intervals of 1–2.5 MPa. The result indicates a roller-coaster n-value evolution of 5 → 23 → 38 → 8 → 45 → 76, as the applied stress increases from 80 MPa to 100 MPa. Microstructural analysis using transmission electron microscopy suggests that basal slip of <a> dislocations is dominant at lower stresses while dislocation climb at higher stresses and, simultaneously, twinning becomes more prominent and other sub-microstructures such as dislocation bands gradually occur as the applied stress increases such as to 87 MPa. By simultaneously introducing a threshold stress and an internal stress, the abnormal n-values are rationalized to 3.0 at relatively lower stresses and gradually to 7.7 at higher stresses, corresponding basal slip of dislocations to dislocation climb. Therefore, we suggest that alloy design aimed to improve creep resistance of magnesium alloys should pay more attention to impeding dynamic twinning during creep.

Microstructure and high-temperature mechanical properties of RAFM steel processed by friction stir processing combined with tempering heat treatment

In this study, the microstructure and high-temperature mechanical properties of a 9Cr-1.5W reduced activation ferritic/martensitic (RAFM) steel processed by friction stir processing (FSP) combined with a tempering treatment were investigated. During the FSP, lath martensite microstructures containing needle-like M3C and nano-sized MX precipitates were formed in the steel owing to the severe plastic deformation and high cooling rate. The steel subjected to FSP and the tempering treatment at 700 °C for 60 min (FSP-T60) had a fine-grained microstructure consisting of ferrite and martensite, and a large number of M23C6 precipitates formed at the grain boundaries and within the grains. The FSP-T60 steel showed an increase of ∼100 MPa in the tensile strength at 600 °C and a high elongation of 26% as compared to the steel prepared via traditional hot rolling, normalizing, and tempering. The FSP-T60 steel also exhibited improved creep resistance with a creep rupture time of 4523 h at 600 °C/210 MPa. The increase in the strength and ductility of the steel can be attributed to its refined grain structure and the high density and dispersion of the M23C6 precipitates. Meanwhile, the homogeneous distribution and refinement of M23C6 carbides could reduce the growth rate of the Laves phase during creep, thus extending the formation time of the creep cavities and cracks and further increasing the creep rupture time of the steel.

High-temperature tensile and creep behaviour of Inconel 625 superalloy sheet and its associated deformation-failure micromechanisms

Thermal tensile and creep properties provide important information for selecting the suitable service conditions for metal materials. In this study, the effects of temperature on the tensile and creep properties, microstructure evolution, and deformation mechanisms of thermal-rolled Inconel 625 sheets were systematically investigated at 650 and 750 °C. The results showed that type B + C serrations dominated the tensile deformation at 650 °C. As the tensile temperature increased to 750 °C, type C serrations occurred in the early stages of the deformation and gradually transformed to type B serrations with the increase of the strain level. The microscopic observations through transmission electron microscopy (TEM) indicated that type B and C serrations were related to the carbon atom–moving dislocation interactions and nucleation–growth process of twin deformation, respectively. Moreover, the creep tests at 650 °C exhibited a better creep resistance than that at 750 °C. The results from the electron backscatter diffraction and TEM indicated combined grain boundary sliding and dislocation slipping as the main creep mechanisms at 650 °C. Moreover, the creep stress can accelerate the precipitation of the γ″ phases at 650 °C, which plays an important role in improving creep resistance, thereby improving the efficiency in strengthening Ni-based alloys. In addition, δ phase precipitation occurred during creep loading at 750 °C. The δ phases/matrix interfaces can act as nucleation sites to initiate dynamic recrystallization, which greatly affects the reduction of the work hardening rate and consequently, decreasing the creep resistance at 750 °C.

Fully recyclable and reprocessable polystyrene-based vitrimers with improved thermal stability and mechanical properties through nitrogen-coordinating cyclic boronic ester bonds

The cross-linked polystyrene (PS) are commonly used plastic materials, however, the greater brittleness and the difficulties in recycling severely limit their applications. In this work, we provide a new strategy for the crosslinking of PS based on the nitrogen-coordinating cyclic boronic ester (NCB) linkages. PS vitrimers are synthesized from PS with hydroxyl groups and isocyanate-terminated NCB oligomer. Incorporating NCB linkages cangreatly improve the solvent resistance, thermal stability and mechanical properties of PS without sacrificing its dielectric properties. PS vitrimers exhibit rapid stress relaxation and improved creep resistance due to the stable NCB linkages. PS vitrimers were able to withstand multiple reprocessing cycles via the exchange reactions of NCB linkages without catalyst. The recycled PS vitrimers were shown to regain the structural integrity, mechanical strength and dielectric properties as the original. The glass fibre reinforced PS vitrimer composites were prepared to explore the application in copper clad manufacture, which showed good mechanical properties and can be conveniently recycled to resin solution and clean glass fibre. This work provides a new strategy for the development of the green dielectric polymer materials for copper clad laminates, and may serve as a guide for rational design of dynamic materials based on general-purpose plastics.

Fabrication and creep properties of eutectic-composition Al2O3/YAG/YSZ sintered composites

Three-phase alumina/YAG/yttria-stabilized cubic zirconia (YSZ) composites were fabricated by a solid-state reaction route starting from commercial powders of Al2O3, Y2O3 and monoclinic ZrO2. The final phases Al2O3, YAG and YSZ were obtained after calcination of the powder mixtures at 1400 °C. Dense bulk composites were obtained after sintering, with a homogeneous microstructure of fine and equiaxed grains with sizes of 1 μm. Compressive mechanical tests were performed at 1300–1450 °C in air at constant load and at constant initial strain rate. A brittle-to-ductile transition was found with increasing temperature. Grain boundary sliding is the main deformation mechanism in the ductile regime, characterized by a stress exponent of 2 and by the absence of dislocation activity and changes in grain morphology. Alumina seems to be the rate-controlling phase owing to the improvement in creep resistance by the presence of yttrium and zirconium of the other two phases.

Improved creep performance of melt‐extruded polycaprolactone/organo‐bentonite nanocomposites

AbstractIn this work, biodegradable nanocomposites based on polycaprolactone reinforced with pristine and organo‐modified bentonites are prepared by melt extrusion. Bentonite is exchanged with benzalkonium chloride (CBK) in a pilot plant scale reactor. The influence of clay type and loading on morphology, rheology, mechanical properties, and creep performance of the resulting materials is analyzed. Besides, several theoretical models then applied to experimental creep data and master curves are used to relate time and temperature with the compliance of the materials. The morphology characterization of the nanocomposites show that the organo‐modification of the clay greatly improves its dispersion in the polymer matrix. As a consequence, it is demonstrated that reinforcement of PCL with 3 wt% loading of organoclay produces the strongest improvement in creep resistance. The instantaneous creep strain and the experimental creep rate decrease more than 9% and 27%, respectively, in the range of temperatures analyzed. Moreover, the experimental values are used to adequately fit theoretical creep models for different clay loadings. On the other hand, the material with optimal creep behavior also shows the greatest improvements in tensile mechanical properties.

Microstructure and Properties of TiAl-Based Alloys Melted in Graphite Crucible

This paper presents the chemical and phase composition, microstructure, and selected properties both at room temperature and at the temperature corresponding to the expected operating conditions of three successive generations of TiAl-based alloys (Ti-47Al-2W-0.5Si, Ti-45Al-8Nb-0.5(B,C), and Ti-45Al-5Nb-2Cr-1Mo-0.5(B,C)-0.2Si) melted in a vacuum induction furnace with high-density isostatic pressed graphite crucibles. The obtained results of mechanical and physical properties of the produced alloys were compared to the properties of reference alloys with similar chemical composition and melted in a cold copper crucible furnace. The effect of increased carbon content in the produced alloys due to the degradation of the graphite crucible during melting is higher strength properties, lower plastic properties, higher coefficient of thermal expansion, and improved creep resistance. It was shown that the proposed technology could be successfully used in the production of different generation TiAl-based intermetallic alloys.

Effect of Si addition on the precipitation and mechanical/electrical properties of dilute Al–Zr-Sc alloys

Microstructural evolution and mechanical/electrical properties of aged Al-0.2 wt%Zr-0.05 wt%Sc alloys with different Si additions (0, 0.05, 0.10 and 0.15 wt%) were comparatively studied by using transmission electron microscopy, electrical conductivity measurements, and microhardness measurements. A significant Si addition effect was demonstrated that the Si addition accelerated the precipitation kinetics and enhanced the heterogeneous precipitation of Al 3 (Sc, Zr) nanoparticles. However, both the peak-aging temperature and peak microhardness during isochronal aging were insensitive to the Si addition. The underlying mechanism is related to a partitioning effect of Si among Sc, Zr and vacancy that produces a saturated concentration of Si-vacancy clusters to serve as heterogeneous nuclei. It was further manifested that dual-scale particles, i.e. , Al 3 Zr dispersoids and Al 3 (Sc 1-x Zr x ) nanoprecipitates in different length scales, could be produced by deliberately designing the heat treatment protocol. A coupling effect of the dual-scale particles led to an enhanced threshold stress in high-temperature creep testing, indicative of an improved creep resistance promoted by the minor Si addition. Moreover, the recrystallization resistance was seldom affected by Si addition, while slightly improved by introducing dual-scale particles.

Improving creep resistance of Al-12 wt.% Ce alloy by microalloying with Sc

The near-eutectic Al-12 wt.% Ce alloy has been alloyed with 0.4 wt.% Sc to improve its creep resistance. A bidirectional influence between Sc and Ce has been identified: (i) the Ce element suppresses the Sc partition between the liquid and solid phases during solidification, as implied by the reduction of Sc microsegregation in the as-cast ingot; (ii) the Sc atoms migrate towards the Al11Ce3 interface to form Al3Sc thin layer or particles upon aging. The former effect promotes the spatial uniformity of Al3Sc precipitate distribution in the Al matrix and the latter one introduces additional lattice misfit strain to the Al11Ce3/Alα phase boundary inhibiting matrix dislocations to climb over the Al11Ce3 lamellae. Both effects can improve the creep resistance of the aged Al-12Ce-0.4Sc alloy showing a high tensile stress threshold for dislocation creep of ∼ 60 MPa at 300 ∘C.

The Possible Detriment of Oxygen in Creep of Alumina and Zirconia Ceramic Composites Reinforced with Graphene.

This paper aims to give an answer to the following question: is the oxidation of graphene a critical issue for high-temperature plasticity in graphene-reinforced ceramics? To give a convincing reply, we will focus on two very different graphene-based ceramic composites: reduced graphene oxide (rGO)-reinforced alumina (α-Al2O3) and reduced graphene oxide (rGO)-reinforced yttria tetragonal zirconia (t-ZrO2). The processing of the powders has been made using a colloidal route, and after that, a spark plasma sintering process was performed in order to densify the samples. Creep tests were performed at temperatures between 1200–1250 °C in an argon atmosphere. The microstructure obtained by SEM of the sintered and tested specimens was characterized quantitatively to elucidate the deformation mechanism. Raman spectroscopy was carried out to check the integrity of the graphene. The average grain size was in the order of 1 µm and the shape factor was 0.7 for all the studied materials. The integrity of the graphene was checked before and after the creep experiments. The careful analysis of the creep tests shows that graphene oxide or its reduced version are not efficient phases for creep resistance improvement in general, contrary to what is reported elsewhere. However, the results permit the suggestion of a creep improvement in nanocomposites at a very high temperature regime due to an enhanced reactivity of oxygen between carbon and alumina interfaces. In the case of zirconia, the results give us the conclusion that the oxidation of graphene is a highly detrimental issue regarding the improvement of high-temperature plasticity.

Introduction to Macroscopic Optimal Design in the Mechanics of Composite Materials and Structures

The main goal of building composite materials and structures is to provide appropriate a priori controlled physico-chemical properties. For this purpose, a strengthening is introduced that can bear loads higher than those borne by isotropic materials, improve creep resistance, etc. Composite materials can be designed in a different fashion to meet specific properties requirements.Nevertheless, it is necessary to be careful about the orientation, placement and sizes of different types of reinforcement. These issues should be solved by optimization, which, however, requires the construction of appropriate models. In the present paper we intend to discuss formulations of kinematic and constitutive relations and the possible application of homogenization methods. Then, 2D relations for multilayered composite plates and cylindrical shells are derived with the use of the Euler–Lagrange equations, through the application of the symbolic package Mathematica. The introduced form of the First-Ply-Failure criteria demonstrates the non-uniqueness in solutions and complications in searching for the global macroscopic optimal solutions. The information presented to readers is enriched by adding selected review papers, surveys and monographs in the area of composite structures.

Enhancing High-Temperature Creep Resistance of &lt;i&gt;In Situ&lt;/i&gt; TiAl-Based Matrix Composite by Low Volume Fraction of Ti&lt;sub&gt;2&lt;/sub&gt;AlC Particles

Samples of TiAl-based matrix in-situ composite with the chemical composition Ti-46.4Al-5.1Nb-1C-0.2B (at.%) reinforced with a low volume fraction of primary Ti2AlC particles were prepared by vacuum induction melting in graphite crucibles and centrifugal casting into graphite moulds. The hot isostatic pressing (HIP) of the as-cast samples and subsequent heat treatments leads to the formation of equiaxed grains with fully lamellar α2(Ti3Al) + γ (TiAl) microstructure and uniformly distributed Ti2AlC and TiB particles. The minimum creep rates of the in-situ composite are significantly lower compared to those measured for the counterpart low carbon benchmark alloy with the chemical composition Ti-47Al-5.2Nb-0.2C-0.2B (at.%) at temperatures ranging from 800 to 900 °C and applied stress of 200 MPa. The studied in-situ composite shows also significantly improved creep resistance compared to that of some TiAl-based alloys with fully lamellar, convoluted and pseudo-duplex microstructures at a temperature of 800 °C and applied stress of 200 MPa.

A novel route to improve the mechanical and rheological properties of HTPE/AP/Al propellant by adding a modified hyperbranched polyester

Both better mechanical and rheological properties are pursued for composite solid propellant. In this work, varying proportions of a modified hyperbranched polyester (MHBPE) were added to HTPE/AP/Al propellant. The static tensile property as one kind of mechanical properties of MHBPE/HTPE/AP/Al propellant were found to be superior to those of blank HTPE/AP/Al propellant as a result of the entanglement and interpenetration of molecular chains caused by the introduction of the hyperbranched structure. Evaluations on the related improved creep resistance and stress relaxation performance further demonstrated the advantages of introduction of MHBPE to HTPE/AP/Al propellant. Rheological properties of HTPE/AP/Al propellant with and without MHBPE during the casting process were investigated and compared and the results confirmed the improvement benefiting from low viscosity and loose void structure. Thus, modified hyperbranched polyester provided a novel route to potentially meet the requirements for propellant manufacturing and applications.

Creep deformation of WC hardmetals with Fe-based binders

Fe is a candidate to replace Co in WC hardmetals, due to its lower cost and toxicity. Here, a WC-FeCr hardmetal was compression tested at 900–1200 °C. Particular attention was paid to the steady-state creep rates and stress-exponents (n) during isostress treatments. Three regimes of n are observed. Two of these were previously reported for WC-Co: power law creep (n ≈ 3) at stresses below ~100 MPa; and grain boundary sliding (n ≈ 1) at higher stresses. A previously unreported regime at very low stresses (<10 MPa), with an exponent of n ≈ 2, is also observed. By combining electron microscopy with X-ray diffraction texture measurements, the low stress regime is attributed to viscous flow of the binder, which is accommodated by diffusional creep in the WC skeleton. The mechanism may be applicable to other hardmetals. Compared to analogous WC-Co materials, WC-FeCr shows improved creep resistance below 1000 °C, which can be explained by its lower self-diffusivity, and a lower solubility for WC than Co. However, at temperatures corresponding to liquid eutectic formation (~1140 °C), its creep resistance becomes inferior. These results indicate FeCr may be a suitable replacement for Co provided the eutectic temperature is not exceeded.