Greater Creep Resistance Research Articles

The enhancement effects of multiple hydrogen bonds between bi-terminal groups and penta-alanine assemblies on creep resistance and mechanical strength of polyisoprene rubbers

Along with the continuously advanced requirements for high performance rubbers such as in aircraft tires, natural rubbers show limitations in achieving high temperature creep resistance and high mechanical strength simultaneously. Meanwhile, the bi-terminal interactions of natural rubber (NR) exhibit unique reinforcement effects, which are difficult to realize in synthetic rubbers. In this article, four different types of bi-terminal polyisoprenes with distinct groups were synthesized, and they were assembled with penta-alanine molecule (5 A) to obtain high-performance rubbers by terminal hydrogen bonding reinforcement. Infrared tests indicate that the strong hydrogen bonds can be formed between two terminals and turn stronger after adding 5 A. Thus, the network of rubbers remains stable under different strains and temperatures and the activation energy of networks is elevated. As a result, this biomimetic design endows polyisoprene with high tensile strength compared with Malaysian NR and greater creep resistance. Further Mooney fitting and DMA test suggests that these confined hydrogen bonds not only increased the number of crosslinking points and entanglements of network, but also are beneficial for the segmental chain motion. In contrast to our common sense on hydrogen bonds which present large relaxation and hysteresis, this study proposes a facile method for preparing high-temperature creep-resistant rubbers by introducing confined hydrogen bonds, which represents a great advance in the structural biomimicry of NR.

Effect of solution cooling rate on the microstructure and creep deformation mechanism of a rhenium-free second-generation single crystal superalloy

• The cubic degree and size of γ′ phases were inversely proportional to the solution cooling rate. • Air cooling method significantly improved the creep lifetime at 800 °C/750 MPa and 1100 °C/137 MPa. • Air cooling facilitated the formation of dislocation locks and retarded γ′ phases to coarsen at 800 °C/750 MPa. • The influence mechanism that air cooling facilitated the denser interface dislocation networks was explored in combination with the long-term ageing test. • The nucleation and precipitation of secondary γ' phase might well be influenced by the solution cooling rate. Various cooling scenarios (water, oil, air and furnace) were employed to study the impacts of the solution cooling rate (SCR) on the microstructure and creep behavior of a novel single-crystal (SX) superalloy. The results showed that the cubic degree and size of the γ′ phases were inversely proportional to the SCR. The creep life first increased and then dropped dramatically with a reduction in the SCR. The creep life of the sample cooled with air cooling (AC) was the highest, up to 144.90 h at 800 °C/750 MPa and 160.15 h at 1100 °C/137 MPa. During creep at 800 °C/750 MPa, the improved creep life of the AC sample was mainly attributed to the fine cubic γ′ phases, which decreased the rate of γ′-phase coarsening and favoured plastic deformation by promoting the active movement of dislocations. The AC helped the γ′ phases become rich in Al, Ti and Ta while depleted in Co and Cr, which enhanced its stacking fault energy, thus promoting the formation of dislocation locks. Meanwhile, the largest negative lattice misfit caused by AC induced denser γ/γ′ interface dislocation networks at 1100 °C/137 MPa, which efficiently reduced the minimum creep rate. The calculated average dislocation spacing results indicated that the smallest density of excess dislocations corresponded to the AC sample, proving its greatest creep resistance. Interestingly, the size of the secondary γ′ phases first decreased and then increased sharply with decreasing SCR during creep at 1100 °C/137 MPa, when fine secondary γ′ phases had a positive role in the blockage of dislocation movement in the matrix. Eventually, the comprehensive SCR effect was explored to provide more guidance in the design of Re-free SX superalloys.

High-level hierarchical morphology reinforcing covalent adaptable networks

High-level hierarchical morphology reinforcing covalent adaptable networks

Thermomechanical activation achieving orthogonal working/healing conditions of nanostructured tri-block copolymer thermosets

Conventional thermosets, despite their technological significance in today’s materials economy, present a modern sustainability challenge because of their lack of end-of-life options for recyclability or reprocessability. Emerging covalent adaptable networks (CANs) offer sustainable alternatives to permanently crosslinked materials, but ideal orthogonal working/reprocessing conditions are hardly achievable by the current thermochemical activation mechanism. Here we report a CAN system of additive/catalyst-free, fully reprocessable, crosslinked, tri-block copolymer (tri-BCP) thermoplastic elastomer networks based on acid-anhydride bond exchange operated on a thermomechanical activation mechanism. The unique functionality of the tri-BCP architecture enables self-assembly into inter-linked, hexagonally packed cylinder nanostructures that preclude any productive inter-cylinder bond exchange (and, thus, creep) without cooperative thermal and mechanical (heating and compression) processing conditions. Tri-BCP assembly into additive-free, inter-linked hexagonally packed cylinder networks Healable crosslinked networks via thermomechanical-induced inter-domain bond exchange Working conditions restrict creep, establishing orthogonal working/healing behavior Clarke et al. show crosslinked tri-block copolymers with nanostructured networks, constructed by a catalyst/additive-free self-crosslinking and self-assembly process. Compared with analogous random and di-block copolymers, these crosslinked tri-block copolymers show not only superior mechanical performance but also much greater creep resistance when operated on a morphology-regulated and thermomechanical activation mechanism.

Ambient and non-ambient temperature depth-sensing indentation of Mg-Sm2O3 nanocomposites

Magnesium matrix nanocomposites (Mg-Sm2O3 nanocomposites in the present study), containing nanosize reinforcements within the magnesium or magnesium alloy matrix, are a group of energy-saving novel material with enhanced strength-to-weight ratio. However, in order to scale up the applications of the magnesium nanocomposites toward industrial dimensions, detailed response of the materials at ambient and elevated temperatures must be established. Having said this, the primary objective of this paper is to obtain an in-depth understanding of small-scale property–microstructure-composition correlation at ambient (298 K) temperature up to 473 K using a depth sensing nanoindentation testing approach as well as advanced microstructural characterization. Mg-Sm2O3 nanocomposites with 0.5, 1.0, and 1.5 vol% Sm2O3 were compared against pure Mg. The properties measured are reduced modulus, elastic modulus, hardness, indentation creep rate, indentation creep exponent, thermal activation volume, as well as indentation size effect as a function of temperature. Pure Mg and Mg-1.0 Sm2O3 nanocomposite provided the least and the greatest creep resistance, respectively. This is attributed to the presence of thermally stable Sm2O3 nanoparticles which can effectively produce dislocation pile-ups and dislocation tangling.

Colloidal polymer composites: Are nano-fillers always better for improving mechanical properties?

HypothesisColloidal polymer composites, in which polymer particles are blended with a filler, are widely used in applications including pharmaceuticals, crop protection, inks, and protective coatings. It is generally found that the presence of hard particulate fillers will increase the elastic modulus of a polymer colloid composite. However, the influence of the size of the filler particle on the large-strain deformation and fracture and on the viscoelastic characteristics, including creep, is not well explored. We hypothesize that the size ratio of the filler to the colloidal polymer will play a critical role in determining the properties of the composite. ExperimentsColloidal composites were prepared by blending soft polymer colloids (as a binder) with calcium carbonate fillers having four different sizes, spanning from 70 nm to 4.5 μm. There is no bonding between the filler and matrix in the composites. The large-strain deformation, linear viscoelasticity, and creep were determined for each filler size for increasing the filler volume fractions (ϕCC). Weibull statistics were used to analyze the distributions of strains at failure. FindingsWe find that the inclusion of nano-fillers leads to brittle fracture at a lower ϕCC than when μm-size fillers are used. The data interpretation is supported by Weibull analysis. However, for a given ϕCC, the storage modulus is higher in the rubbery regime, and the creep resistance is higher when nanoparticles are used. Using scanning electron microscopy to support our arguments, we show that the properties of colloidal composites are correlated with their microstructure, which can be altered through control of the filler:polymer particle size ratio. Hard nanoparticles pack efficiently around larger particles to provide reinforcement (manifested as a higher storage modulus and greater creep resistance), but they also introduce weak points that lead to brittleness.

Effects of microstructure and C and Si additions on elevated temperature creep and fatigue of gamma TiAl alloys

The creep properties of K5 (Ti-46Al-3Nb-2Cr-0.2W) based alloys were analyzed in wrought processed microstructure forms. The brittle–ductile-transition-temperature (BDTT) depends distinctly on microstructure as well as strain rate, with the minimum value for each microstructure achieved at ∼10−6/s being about 680 °C and 780 °C, respectively. The greatest creep resistance is achieved in coarse-grained fully lamellar (FL) material and is related to the strong anisotropy of lath structure, large grain size and consequently high BDTT. Additional significant resistance improvement is realized with additions or increases of refractory elements (Nb or W) and decrease in Al content. The most remarkable improvements in primary as well as the minimum creep resistance are realized when small amounts of C or C + Si are added to generate incoherent (to gamma) carbide and silicide particles along γ/γT interfaces. The significance of primary creep is assessed for controlling subsequent creep behavior and discussed for its crucial role in satisfying the stringent design creep requirements for advanced rotational components. The accelerated or tertiary creep is used to explain the high temperature (870 °C) high cycle fatigue deformation that exhibits two-stage SN curves with the rapidly softening second stage.

Characterization of the Microstructure, Tensile and Creep Behavior of Powder Metallurgy Processed and Rolled Ti-6Al-4V-1B Alloy

This work investigated the microstructure and elevated-temperature (400-475oC) tensile and tensile-creep deformation behavior of a powder metallurgy (PM) rolled Ti-6Al-4V-1B(wt.%) alloy. The PM rolled Ti-6Al-4V-1B alloy exhibited a duplex microstructure, and it did not exhibit a strong α-phase texture compared with the PM extruded Ti-6Al-4V-1B alloy. The PM rolled Ti-6Al-4V-1B alloy exhibited greater creep resistance than the PM extruded Ti-6Al-4V-1B alloy as well as the as-cast Ti-6Al-4V-1B alloy.

Anisotropy of creep resistance in extruded magnesium

Creep deformation of extruded magnesium, prepared via a powder metallurgical route with a final hot extrusion, is examined in the temperature range of 473–623 K. The greatest creep resistance is observed in specimens with the stress axis parallel to the extrusion axis and the lowest at declinations between 45° and 90°. Most of this orientation dependence can be accounted for by the variation in the resolved shear stress on the basal slip planes.

The effect of molybdenum on the microstructure and creep behavior of Ti–24Al–17Nb–xMo alloys and Ti–24Al–17Nb–xMo SiC-fiber composites

The effect of molybdenum (Mo) on the microstructure and creep behavior of nominally Ti–24Al–17Nb (at.%) alloys and their continuously reinforced SiC-fiber composites (fiber volume fraction = 0.35) was investigated. Constant-load, tensile-creep experiments were performed in the stress range of 10–275 MPa at 650 °C in air. A Ti–24Al–17Nb–2.3Mo (at.%) alloy exhibited significantly greater creep resistance than a Ti–24Al–17Nb–0.66Mo (at.%) alloy, and correspondingly a 90°-oriented Ultra SCS-6/Ti–24Al–17Nb–2.3Mo metal matrix composite (MMC) exhibited significantly greater creep resistance than an Ultra SCS-6/Ti–24Al–17Nb–0.66Mo MMC. Thus, the addition of 2.3 at.% Mo significantly improved the creep resistance of both the alloy and the MMC. An Ultra SCS-6 Ti–25Al–17Nb–1.1Mo (at.%) MMC exhibited creep resistance similar to that of the Ultra SCS-6/Ti–25Al–17Nb–2.3Mo (at.%). Using a modified Crossman model, the MMC secondary creep rates were predicted from the monolithic matrix alloys’ secondary creep rates. For identical creep temperatures and applied stresses, the 90°-oriented MMCs exhibited greater creep rates than their monolithic matrix alloy counterparts. This was explained to be a result of the low interfacial bond strength between the matrix and the fiber, measured using a cruciform test methodology, and was in agreement with the modified Crossman model. Scanning electron microscopy observations indicated that debonding occurred within the carbon layers of the fiber-matrix interface.

Estimation of anisotropy of mechanical properties in Mg alloys by means of compressive creep tests

A detailed knowledge of dependence of mechanical properties on orientation in materials prepared by directional processes may present an important factor influencing the design of construction parts. Toward this end, the compressive creep testing of short specimens may be useful. Three different magnesium-based materials were subjected to this testing: (i) pure magnesium, (ii) magnesium matrix composite reinforced with 10 vol.% of titanium, and (iii) magnesium alloy WE54. All three materials were prepared through a powder metallurgical route with final hot extrusion. The specimens for creep tests were cut in such a way that their longitudinal axis (i.e., the direction of compressive creep stress) and the axis of extruded bar contained a predestined angle. Two extreme cases can be observed: In pure Mg and in Mg-Ti composites, the dependence of the creep rate is very sensitive to the orientation especially at small inclinations from extrusion axis. The greatest creep resistance is observed in specimens with stress axis parallel to the extrusion axis, the lowest at declinations from 45 to 90°. On the other hand, in WE54 no orientation dependence was observed. Possible explanations of the behavior based on microstructural observations are discussed.

Inelastic deformation and its related life under cyclic/creep loadings in Si 3N 4-monolithic and Si 3N 4/SiCw-composite ceramics at elevated temperature

Push–pull low cycle fatigue and creep tests were performed in monolithic-Si 3N 4 and composite-Si 3N 4/SiCw materials at 1573 K. Then cyclic stress–strain response and creep strain were measured with a newly developed extensometer. The inelastic strain, which is greatly dependent upon stress/strain rate, was detected in both the materials. It was found from the shape of the cyclic stress–strain hysteresis loop that inelastic strain was easier to generate under tensile than under compressive loading; the width of the hysteresis loop is larger on the tensile stress side than on the compressive stress side. Such a characteristic cyclic stress–strain response is much more enhanced in the composite-Si 3N 4/SiCw than in the monolithic-Si 3N 4. The low cycle fatigue life plotted against inelastic strain range is much longer in the former than in the latter, although the difference is reduced with decreasing the controlled strain range. Moreover, greater creep resistance is attained in the former than in the latter at the lower stress level, while the opposite is the case at the higher stress level.

The creep of lead-free solders at elevated temperatures

Full implementation of the new generation of lead-free solders requires a detailed knowledge and understanding of their mechanical behavior. This paper reports an investigation of the creep behavior of three lead-free alloys: Sn-0.5 Cu, Sn-3.5 Ag, and Sn-3.8Ag-0.7Cu, at 75°C, and compares their response to that of Sn-37Pb at the same temperature. In terms of stress and time to rupture, the Sn-0.5Cu alloy behaves similarly to the eutectic Sn-Pb over the range of rupture lives considered (up to 1000 h). The silver-containing alloys exhibit much greater creep resistance, typically a hundred fold and a thousand fold for the binary and tenanary, respectively. These alloys are less ductile but their creep strains to failure are generally above ten percent. Their minimum creep rates are at least 100 times slower. When testing at the same homologous temperature (0.76), the silver-containing alloys retain the substantial superiority. The relationship between applied steady-state (or minimum) creep rate behavior is best described by a power law equation, although the steady state domain generally occupies less than 30 percent of life. The microstructural changes induced by creep are briefly described and used to explain some of the creep characteristics of the alloys.

Amorphous Silicoboron Carbonitride Ceramic with Very High Viscosity at Temperatures above 1500°C

Recently, the viscosity of a predominantly amorphous silicon carbonitride (Si1.7C1.0±0.1N1.5) alloy with an apparent glass‐transition temperature (Tg) of 1400°–1500°C was studied. In this study, the creep behavior of silicoboron carbonitride (Si2B1.0C3.4N2.3), which seems to have a Tg value of >1700°C, was examined. Both materials exhibited a three‐stage creep behavior. In stage I, the creep rate declined, because of densification. In stage II, the strain rate approaches a steady state. In stage III, it resumes a declining strain rate, which ultimately decreased below the measurement limit of the system. At 1550°C in stage II, the viscosity of silicoboron carbonitride was six orders of magnitude higher than that of fused silica. Among the Si‐C‐N ceramics, only chemical‐vapor‐deposited and reaction‐bonded silicon carbides seem to have greater creep resistance than the silicoboron carbonitrides at temperatures >1550°C.

Processability and properties of yarns made from polypropylene containing small amounts of polystyrene

The effect of the addition of 2–8% by weight of commercial atactic polystyrene to fiber-grade polypropylene on the drawability of the multifilament yarn spun from the blends and on the mechanical properties and thermal shrinkage of the drawn yarns was studied. In the as-spun state, polystyrene is present in the form of near-spherical particles and improves the drawability and plastic deformation characteristics of the yarn. In the drawn state, with higher molecular orientation and more compact structure, the intrinsic rigidity of the polystyrene molecule manifests itself in the form of enhanced stiffness and greater creep resistance. © 1996 John Wiley & Sons, Inc.

Processability and properties of yarns made from polypropylene containing small amounts of polystyrene

The effect of the addition of 2–8% by weight of commercial atactic polystyrene to fiber-grade polypropylene on the drawability of the multifilament yarn spun from the blends and on the mechanical properties and thermal shrinkage of the drawn yarns was studied. In the as-spun state, polystyrene is present in the form of near-spherical particles and improves the drawability and plastic deformation characteristics of the yarn. In the drawn state, with higher molecular orientation and more compact structure, the intrinsic rigidity of the polystyrene molecule manifests itself in the form of enhanced stiffness and greater creep resistance. © 1996 John Wiley & Sons, Inc.

Creep damage in SiC/SiC composites

In SiC/SiC composites, the matrix has greater creep resistance than the fibers. The consequent fiber deformation has been shown to transfer sufficient stress onto the matrix that, above a threshold, matrix cracks propagate across the 0 0 plies. This process increases the compliance of the composite and reloads the fibers, resulting in creep rates dominated by the fibers. The extent of the compliance change is limited by the high temperature interface friction stress, τ, through its effect on the crack density. After creep, τ is diminished substantially from its room-temperature magnitude, resulting in low crack densities and extensive fiber pull-out. A matrix-cracking solution for composites with creeping fibers has been used to correlate the experimentally determined rates of compliance change and the creep strain.

Tensile creep behaviour of a silicon carbide-based fibre with a low oxygen content

The high-temperature mechanical behaviour and microstructural evolution of experimental SiC fibres (Hi-Nicalon) with a low oxygen content (<0.5 wt%) have been examined up to 1600 °C. Comparisons have been made with a commercial Si-C-O fibre (Nicalon Ceramic Grade). Their initial microstructure consists of β-SiC crystallites averaging 5–10 nm in diameter, with important amounts of graphitic carbon into wrinkled sheet structures of very small sizes between the SiC grains. The fall in strength above 800 °C in air is related to fibre surface degradation involving free carbon. Crystallization of SiC and carbon further develops in both fibres subject to either creep or heat treatment at ∼1300 °C and above for long periods. The fibres are characterized by steady state creep and greater creep resistance (one order of magnitude) compared to the commercial Nicalon fibre. The experimental fibre has been found to creep above 1280 °C under low applied stresses (0.15 GPa) in air. Significant deformations (up to 14%) have been observed, both in air and argon above 1400 °C. The stress exponents and the apparent activation energies for creep have been found to fall in the range 2–3, both in air and argon, and in the range 200–300 kJ mol−1 in argon and 340–420 kJ mol−1 in air. The dewrinkling of carbon layer packets into a position more nearly aligned with the tensile axis, their sliding, and the collapse of pores have been proposed as the mechanisms which control the fibre creep behaviour.

Tensile-compressive yield asymmetries in high strength wrought magnesium alloys

Owing to magnesium's very low density (of 1.7 g/cm[sup 3]) there is potential for producing Mg alloys with higher specific strengths than those of existing aluminium alloys for applications where weight savings are at a premium. Magnesium alloys can also have greater creep resistance and superior elevated temperature specific tensile properties than those of Al. However, the hexagonal crystal structure of magnesium severely limits its available slip systems and preferred orientations can develop during working processes, slip occurring predominantly on the basal planes which leads to their alignment parallel to the direction of metal flow. As a geometric consequence of the c/a ratio of magnesium being less than ideal, twinning can theoretically only occur in compression, parallel to the basal planes, which would be expected to result in there being large asymmetries between the tensile and compressive yield stresses in textured wrought products. The compressive yield stresses in wrought magnesium alloys have been previously found in forgings to the 0.6--0.7 times those of the tensile. Such an asymmetric yield behavior, resulting in a poor compressive yield stress, would prohibit many potential applications for wrought Mg alloys. The aim of this paper is to investigate how significant the problem of yieldmore » asymmetry can be in two typical high strength wrought Mg alloys, the Mg-Y-Nd mischmetal based alloy, WE54, and the Mg-Zn-Cu based alloy, ZC71.« less

Creep of an electroslag refined aluminium alloy

AbstractThe creep resistance ofelectroslag refined aluminium alloyRR 58 has been studied in the temperature range 450-485°C and compared with that ofpermanent-mould cast, unrefined alloy. The electroslag refined RR 58 had a greater creep resistance than the unrefined alloy; this improvement has been attributed to the reduction in hydrogen content, macroporosity and microporosity, the removal ofoxide inclusions and the fine an uniform dispersion ofsecond phase precipitates in refined alloy. From the estimated activation energy for high temperature creep deformation (155 kJ mol-1), a self-diffusion mechanism has been identified as rate controlling for both refined and unrefinedRR 58. The rate equation έ= A1[sinh (ασ)]nexp(–Q/RT] has been found to be applicable for RR 58 deformed under creep conditions in the range oftemperatures and strain rates studied