Low Creep Resistance Research Articles

Creep behavior and deformation feature of TiAl–Nb alloy with various states at high temperature

By means of creep properties measurement and microstructure observation, an investigation has been made into the creep behaviors and deformation features of TiAl–Nb alloy with various states at high temperature. Results show that the microstructure of as-cast alloys consists mainly of lamellar γ/α2 phases with various orientations, the boundaries located in between the ones consist of single γ phase. The bigger plastic deformation of alloy during forging may refine the grains and increase the volume fraction of boundaries which consists of the fine block-like γ/α2 phases with weaker strength. During creep, the bigger volume fraction of boundaries increase the probability of crack initiated and propagated along boundaries, which is thought to be the main reason of the forged alloy displaying a lower creep resistance. Compared to the forged alloy, the as-cast alloy displays a better creep resistance and longer creep life at high temperature. The deformation mechanism of as cast alloy during creep is significant amount of dislocations shearing into the lamellar γ/α2 phases in the form of dislocations rows and rings. Compared with α2-Ti3Al phase, the γ phase possesses a weaker strength, therefore, the crack is easily initiated and propagated along the inclined boundaries. Wherein, the propagation of crack along the direction parallel to the lamellar γ/α2 phases displays the smooth surface of fracture, while the tearing edges appear on the surface of fracture in one side of the inclined boundaries relative to lamellar γ/α2 phases, which is attributed to the α2-Ti3Al phase with better strength hindering the propagation of crack during creep.

Evaluation of Secondary Hardening and Microstructure in the Real T24 Steel Weld Joint

Development of new steels with higher creep resistance must be based on detailed knowledge of strengthening and degradation processes, structural stability at high temperature exposure and their welding. This paper is concerned with the real weld joint hardness and microstructure of progressive low alloy creep resistance T24 steel. These steel is widely used in many power-plant constructions all over the Europe. Welding the T24 steels components is one of the most important technological procedures. The aim of the performed investigation is to evaluate the hardness and microstructure in the heat affected zone (HAZ) and the weld metal. Results of the secondary hardening in the real weld joints after low temperature post weld heat treatment (460°C/48h) and without post weld heat treatment at service temperature are described in this paper. Obtained results are compared to the hardness values of the weld joint post weld heat treated at recommended temperature (740°C). The microstructure of the real weld joints was analysed by transmission electron microscopy (TEM) in order to identify minority phases - MX nanoparticles and/or coarse (M23C6) particles. Secondary hardening of the not heat treated and low temperature heat treated T24 weld joint was proved.

Creep of mullite zirconia composites

The compressive creep resistance of two zirconia mullite composites and a monolithic mullite was measured. The zirconia additions were either monoclinic zirconia or 5.4 mol% yttria stabilised zirconia. Samples were tested between 1225°C and 1500°C. It was found that the addition of zirconia lowered the creep resistance of monolithic mullite (5.8×10−7s−1 at 1400°C, 13MPa), with yttria stabilised zirconia (3.3×10−5s−1) having a greater effect than monoclinic zirconia (3.4×10−6s−1) under equal conditions. The lower creep resistance of the composites is consistent with creep being controlled by oxygen diffusion as the changes in creep rate can be explained using the effective oxygen diffusion coefficients for the composites calculated from literature values for oxygen diffusion in mullite and zirconia.

Study And Characterization of A Metal Matrix Composite Reinforced with Tantalum Carbide-TaC

Steel is an alloy EUROFER promising for use in nuclear reactors, or in applications where the material is subjected to temperatures up to 550°C due to their lower creep resistance under. One way to increase this property, so that the steel work at higher temperatures it is necessary to prevent sliding of its grain boundaries. Factors that influence this slip contours are the morphology of the grains, the angle and speed of the grain boundaries. This speed can be decreased in the presence of a dispersed phase in the material, provided it is fine and homogeneously distributed. In this context, this paper presents the development of a new material metal matrix composite (MMC) which has as starting materials as stainless steel EUROFER 97, and two different kinds of tantalum carbide-TaC, one with average crystallite sizes 13.78 nm synthesized in UFRN and another with 40.66 nm supplied by Aldrich. In order to improve the mechanical properties of metal matrix was added by powder metallurgy, nano-sized particles of the two types of TaC. This paper discusses the effect of dispersion of carbides in the microstructure of sintered parts. Pure steel powders with the addition of 3% TaC UFRN and 3% TaC commercial respectively were ground for 5 hours in the planetary mill. Each of the resultant particulate samples were cold compacted under a uniaxial pressure of 600MPa in a cylindrical die 5 mm in diameter. Subsequently, the compressed were sintered in a vacuum oven at a temperature of 1250°C with an increment of 20°C and 10°C per minute and maintained at these isotherms for 60 minutes. The distribution, size and dispersion of steel and composite particles were determined by X-ray diffraction, laser particle size and scanning electron microscopy(SEM). The structures of the sintered bodies were observed by optical microscopy(OM) and scanning electron microscopy(SEM).

Hardening and Coherent Precipitates Size Evolution with Aging Fe-12Al-12 V Alloy

Fe-Al based alloys have a remarkable potential for high temperature structural applications, provided that the limitation of their low creep resistance is solved. Third element addition (Nb, Ti, Zr or Ta) has proved to perform the task, at the expense of a low ductility. In previous works we have investigated ferritic alloys in the Fe-Al-V system with coherent precipitation of L21 phase (Fe2AlV) and have identified phases in equilibrium at the vertical section Fe1-2xAlxVx. We have found that a wide two-phase field is present in the composition range 0.10≤x≤0.15 (x: atomic fraction) till 720°C. In this work we study the feasibility of a structural application at high temperatures of the Fe-12Al-12V alloy. We look for ageing conditions leading to a stable coherent precipitation that could strengthen the ferritic matrix. In this way, we measured vickers hardness of coherent L21 precipitates at 600, 650 and 700°C. In all cases hardness has a typical behavior with hardness maximum located at longer times and higher intensities as temperature decreases. We measured size evolution with time through Transmission Electron Microscopy of precipitates at 700°C ageing temperature. As a further result, we could calculate from these measurements the activation energy for precipitate growth.

Biodegradable microfibrillar polymer-polymer composites from poly(L-lactic acid)/poly(glycolic acid)

Biodegradable coronary stents have been under development for several years and a trend in biodegradable stent material development has emerged: reinforcement to enhance mechanical properties and creep resistance to improve vessel support. The aim of this work is to investigate the mechanical and viscoelastic characteristics of poly(L-lactic acid)/poly (glycolic acid) (PLLA/PGA) microfibrillar polymer-polymer composites (MFCs) at 37°C to determine the suitability of PGA fibrils as a reinforcement for polymeric, biodegradable stents. PLLA/PGA MFCs were produced via cold-drawing and subsequent compression moulding of extruded PLLA/PGA blend wires. Scanning electron microscopy revealed excellent fibril formation in the case of a 70/30 wt% PLLA/PGA MFC- the mean fibril diameter being 400 nm and aspect ratios exceeding 250. Tensile tests demonstrate Young's modulus and strength increases of 35 and 84% over neat PLLA in the case of a 70/30 wt% PLLA/PGA MFC. Creep resistance of the PLLA/PGA MFCs is lower than that of neat PLLA, as shown via relaxation. Dynamic mechanical thermal analysis demonstrates that it is the onset of glass transition of PGA that is the underlying cause for low creep resistance of the PLLA/PGA MFCs at 37°C.

Microstructure and Compression Creep Strength of the Newly Developed Magnesium Alloy DieMag422

<p class="TTPAbstract">Magnesium alloys have been finding increasingly more types of application in the automotive and aerospace industries for over twenty years. Despite the fact conventional magnesium alloys have limited high-temperature strength and creep resistance, especially when they contain aluminium as an alloying element. Aluminium is necessary to improve the castability when high-pressure die casting is the favoured process. Applications with higher operating temperatures require additional alloy elements, which form precipitates with the aluminium during solidification and therefore prevent the formation of Mg17Al12, which is responsible for the low creep resistance of magnesium alloys that contain aluminium. The precipitates formed may also strengthen grain boundaries and so improve the creep strength. Barium and calcium were investigated as elements in a magnesium alloy containing aluminium (DieMag422: 4 wt.-% Al, 2 wt.-% Ba, 2 wt.-% Ca). The compression creep strength was compared at 240°C for stresses between 60 and 120 MPa with two commercial creep-resistant magnesium alloys, AE42 and MRI230D. The stress exponents were calculated from the stress dependence of the minimum creep rate. The concept of a threshold stress was applied and true stress exponents ntclose to 5 were found. The new alloy DieMag422 exhibits improved creep strength compared to both commercial alloys and also has proven it is die castable.</p>

In situ small-angle X-ray scattering study of the perovskite-type carbide precipitation behavior in a carbon-containing intermetallic TiAl alloy using synchrotron radiation

Intermetallic γ-TiAl based alloys of the latest generation, e.g. TNM alloys with a nominal composition of Ti–43.5Al–4Nb–1Mo–0.1B (inat.%), exhibit the potential to be used in modern high-performance combustion engines due to their low density, high strength and creep resistance as well as their good oxidation properties at elevated temperatures. Alloying with C can further improve the high-temperature performance by both solid solution hardening and/or carbide formation. In this study, starting from a supersaturated TNM–1C alloy the precipitation behavior and thermal stability of perovskite-type carbides Ti3AlC during isothermal annealing and ensuing re-heating to 1200°C are quantified by means of an in situ small-angle X-ray scattering experiment using synchrotron radiation. Complementarily, the formed hierarchical structures on the nano-scale, i.e. p-type carbide precipitates within ultra-fine γ-lamellae of the α2/γ-colonies, were investigated by means of monochromatic high-energy X-ray diffraction in combination with scanning and transmission electron microscopy. Additionally, an explanation of an obtained diffraction phenomenon is given, i.e. streak formation that is caused by the very small lamellar spacing of the γ-phase within the α2/γ-colonies. It was also found that the geometrically well-defined nanostructure allows a correlation between the γ-lath thickness and a characteristic dimension of p-type carbides.

High temperature compressive creep of spark plasma sintered zirconium (oxy-)carbide

The effect of stoichiometry (i.e. carbon and oxygen contents) and microstructure (i.e. micro-sized grains) on creep mechanism of zirconium oxycarbide was considered. The synthesis of ZrCxOy powder of controlled stoichiometry and without impurity has been performed via the carboreduction route. Compressive creep experiments were conducted at 1500–1600°C under applied stresses ranging from 60 to 140MPa on fully dense zirconium oxycarbide specimens obtained by spark plasma sintering. The higher oxygen content composition (i.e. ZrC0.79O0.13) reveals low creep resistance in contrary to ZrC0.94O0.05 composition. The analysis of the creep data shows the existence of a linear creep limit (σt). At low stress (i.e. σ≤σt=100MPa, n≈1, m≈1), the creep mechanism seems to be independent of the chemical composition, and is governed by zirconium volume diffusion. At high stress (i.e. σ≥σt=100MPa, n≈3, m≈0), a power law regime of creep appears. However, the nature of the rate-determining step of creep process depends on stoichiometry. The limiting species for volume diffusion are (i) the metal atom for ZrC0.94O0.05; (ii) and the carbon atom for ZrC0.79O0.13 linked to a Rowcliffe dislocation diffusion mechanism.

Creep behavior and microstructural evolution of deformed Al–Cu–Mg–Ag heat resistant alloy

The creep behavior of Al–Cu–Mg–Ag alloy and its microstructural evolution during creep deformation were studied at 100–210°C with the external stress of 150–300MPa. The results show that both Ω and θ׳ precipitate secondarily in the under-aged sample during the creeping process, resulting in the excellent creep-resistant property compared to the peak aged sample. The temperatures between 100 and 150°C have little affect on the creep curves of the alloy. The larger external stress led to the growth of the creep holes on the grain boundaries and the higher creep temperature led to the growth of the precipitations in the grains inner. They both result in the lower creep resistance of Al–Cu–Mg–Ag alloy. The steady creep rate increased with increasing creep temperature or external stress, which can be described by a constitutive equation with the creep deformation activation energy 102.00kJ/mol.

Novel Polyurethane Materials and Their Applications in Warship

This comprehensive review summarized the characteristics of high performance polyurethane,including high tensile strength,low creep resistance,stable modules in wide range of temperature,and tunable damping properties.The potentials of this kind of material in real applications,for example,in ship industries,were also presented,with emphasis on polyurethane vibration isolator,anechoic tile,water lubricated bearings,and anti-fouling coatings.

Evolution of microstructure, thermal and creep properties of Ni-doped Sn–0.5Ag–0.7Cu low-Ag solder alloys for electronic applications

For development of lead-free solder for advance electrical components, the correlation of microstructure with thermal and creep properties of novel Ni-doped Sn–0.5Ag–0.7Cu (SAC (0507)) lead free solders has been investigated. Results showed that addition of 0.05Ni into the lead-free SAC (0507) solder led to the microstructural refinement, more uniform distribution of the Ag3Sn, (Cu,Ni)6Sn5 intermetallic compounds (IMCs) and small primary β-Sn grains. However, the SAC (0507)–0.1Ni alloy has relatively high fraction of the primary β-Sn phase and the IMCs appeared coarse within the matrix compared with the other examined alloys. DSC results showed that the addition of Ni did not produce any significant effect on the melting behavior. Interestingly, 0.05wt.% Ni addition exhibited a drastically reduced undercooling to be 6.3°C. In terms of creep behavior, the SAC (0507)–0.05Ni gave the highest creep resistance due to the fine dispersion of IMCs. Furthermore, 0.05wt.% Ni addition can evidently increase the creep–rupture life, about 2.0 times greater than that of the baseline SAC (0507) and approximately 5.0 times better than that of SAC (0507)–0.1Ni solder. Meanwhile, the SAC (0507)–0.1Ni alloy shows lower creep resistance which is mainly attributable to smaller volume fraction of the precipitate phases. Based on the obtained stress exponents and activation energies, it is proposed that the dominant deformation mechanism in SAC (0507) solders is dislocation climb over the whole temperature range investigated.

Microstructure, microstructural stability and mechanical properties of sand-cast Mg–4Al–4RE alloy

This paper presents a methodology for assessing the phase composition and the results of structural stability tests of the sand-cast Mg–4Al–4RE alloy after annealing it at 175 and 250°C for 3000h. The microstructure was analyzed with optical, scanning electron, and transmission electron microscopy. The phase composition was determined with X-ray diffraction. The structure of the Mg–4Al–4RE (AE44) alloy is composed of large grains of α-Mg solid solution, needle-shaped precipitates of the Al11RE3phase, polyhedral precipitates of the Al2RE phase and Al10RE2Mn7 phase. After annealing at 175°C for 3000h, no changes in the alloy structure are observed, whereas after annealing at 250°C the precipitates of the Al11RE3 phase are found to be in the initial stages of spheroidization. The coarse-grained structure and unfavorable morphology of the intermetallic phases in the sand-cast AE44 alloy, which are caused by low solidification rates, result in low creep resistance up to 200°C and low mechanical properties at ambient temperature and at 175°C.

A Comparative Study of UHMWPE Multifilament and Aramid Multifilament

In order to analyze the creep properties of UHMWPE multifilament and aramid multifilament, four-element model and Matlab software were used to get creep fitting curve, the corresponding mechanical fitting equation and microscopic parameters. The creep properties of the two kinds of multifilament were discussed by microscopic parameters. At the same time, the tensile and relaxation performance of UHMWPE multifilament and aramid filament were studied. Compared with aramid multifilament, UHMWPE multifilament has higher strength, better elongation, lower creep resistance and obvious the phenomenon of stress relaxation. This article will provide some basic performance reference to the application of UHMWPE multifilament.

Bulk-fill Resin-based Composites: An In Vitro Assessment of Their Mechanical Performance

The study aimed to assess the mechanical performance of seven bulk-fill RBCs (Venus Bulk Fill, Heraeus Kulzer; SureFil SDR flow, Dentsply Caulk; x-tra base and x-tra fil, VOCO; Filtek Bulk Fill, 3M ESPE; SonicFill, Kerr; Tetric EvoCeram Bulk Fill, Ivoclar Vivadent) by determining their flexural strength (σ), reliability (Weibull parameter, m), flexural modulus (Eflexural), indentation modulus (YHU), Vickers hardness (HV), and creep (Cr). The significant highest flexural strengths were measured for SonicFill, x-tra base, and x-tra fil, while x-tra base, SureFil SDR flow, and Venus Bulk Fill showed the best reliability. The differences among the materials became more evident in terms of Eflexural and YHU, with x-tra fil achieving the highest values, while Filtek Bulk Fill and Venus Bulk Fill achieved the lowest. The enlarged depth of cure in bulk-fill RBCs seems to have been realized by enhancing the materials' translucency through decreasing the filler amount and increasing the filler size. The manufacturer's recommendation to finish a bulk-fill RBC restoration by adding a capping layer made of regular RBCs is an imperative necessity, since the modulus of elasticity and hardness of certain materials (SureFil SDR flow, Venus Bulk Fill, and Filtek Bulk Fill) were considerably below the mean values measured in regular nanohybrid and microhybrid RBCs. The class of bulk-fill RBCs revealed similar flexural strength values as the class of nanohybrid and microhybrid RBCs, and significantly higher values when compared to flowable RBCs. The modulus of elasticity (Eflexural), the indentation modulus (YHU), and the Vickers hardness (HV) classify the bulk-fill RBCs as between the hybrid RBCs and the flowable RBCs; in terms of creep, bulk-fill and the flowable RBCs perform similarly, both showing a significantly lower creep resistance when compared to the nanohybrid and microhybrid RBCs.

Crystallization behavior and mechanical properties of nano-CaCO3/β-nucleated ethylene-propylene random copolymer composites

To provide ethylene-propylene random copolymer (PPR) with balanced mechanical properties, !-nucleating agent and CaCO3 nanoparticles are incorporated into PPR matrix by melt blending. It is found that crystallization rate and relative content of !-crystal increase with the addition of !-nucleating agent together with nanoparticles. Size of PPR spherulite is greatly reduced, and a specific morphology appears, in which #-crystal lamella is grown upon the !-nucleus. The results suggest that both !-nucleating agent and nano-CaCO3 have heterogeneous nucleation and synergistic effects on !-nucleation of PPR. Mechanical characterization shows that mechanical properties of PPR can be tuned by incorporation of !-nucleating agent and nano-CaCO3 particles. Under suitable compositions, low temperature impact strength and high temperature creep resistance of PPR, the bottlenecks of application of such material, can be simultaneously improved with- out sacrificing the Youngs'modulus and tensile strength.

Past research and fabrication conducted at SCK•CEN on ferritic ODS alloys used as cladding for FBR’s fuel pins

In the 1960s in the frame of the sodium-cooled fast breeders, SCK•CEN decided to develop claddings made with ferritic stainless materials because of their specific properties, namely a higher thermal conductivity, a lower thermal expansion, a lower tendency to He-embrittlement, and a lower swelling than the austenitic stainless steels. To enhance their lower creep resistance at 650–700 °C arose the idea to strengthen the microstructure by oxide dispersions. This was the starting point of an ambitious programme where both the matrix and the dispersions were optimized.A purely ferritic 13 wt% Cr matrix was selected and its mechanical strength was improved through addition of ferritizing elements. Results of tensile and stress-rupture tests showed that Ti and Mo were the most beneficial elements, partly because of the chi-phase precipitation. In 1973 the optimized matrix composition was Fe–13Cr–3.5Ti–2Mo.To reach creep properties similar to those of AISI 316, different dispersions and methods were tested: internal oxidation (that was not conclusive), and the direct mixing of metallic and oxide powders (Al2O3, MgO, ZrO2, TiO2, ZrSiO4) followed by pressing, sintering, and extrusion. The compression and extrusion parameters were determined: extrusion as hollow at 1050 °C, solution annealing at 1050 °C/15 min, cleaning, cold drawing to the final dimensions with intermediate annealings at 1050 °C, final annealing at 1050 °C, straightening and final aging at 800 °C. The choice of titania and yttria powders and their concentrations were finalized on the basis of their out-of-pile and in-pile creep and tensile strength. As soon as a resistance butt welding machine was developed and installed in a glove-box, fuel segments with PuO2 were loaded in the Belgian MTR BR2. The fabrication parameters were continuously optimized: milling and beating, lubrication, cold drawing (partial and final reduction rates, temperature, duration, atmosphere and furnace). Specific non-destructive tests (ultrasonic and eddy currents) were also developed. In-pile creep in argon and in liquid sodium was deeply studied on pressurized segments irradiated up to 75 dpaNRT. Finally two fuel assemblies cladded with such ODS alloys were irradiated in Phenix to the max dose of 90 dpa. Creep deformation and swelling were limited but the irradiation-induced embrittlement became acute. The programme was stopped shortly after the Chernobyl disaster, before the embrittlement problem was solved.

The Machinability of MAR-M247 Superalloy

Nickel-base superalloy is a special super heat resistant alloy developed by U.S in 1970s. It is mainly applied to turbine parts as well as high-temperature components. Nickel-base superalloys exhibit an excellent high strength, low thermal conductivity and creep resistance as well as work hardening. It is the most difficult to be machined with high-speed cutting among different sorts of high-temperature superalloys and is a material presenting multifold challenges for machining. The purpose of this study aims at the machinability of Nickel-base alloys. Engineering statistical analysis was employed to observe the cutting speeds, feed rates and surface roughness at first place. The researcher further applied the half-normal probability plot (HNPP), Pareto analysis and ANOVA to identify the cross effects and probed into the characteristics of Nibase alloy.

On Localized Deformation and Recrystallization as Damage Mechanisms during Thermomechanical Fatigue of Single Crystal Nickel-Based Superalloys

Thermomechanical fatigue (TMF) in superalloys is growing in importance due to the introduction of advanced cooling systems but also due to the changes in demand and competition within the power generation market; this is requiring many power plants to operate under cyclic conditions. In this paper the TMF behaviour of three different single crystal nickel-based superalloys are compared. It is demonstrated that the deformation and damage mechanisms occurring during TMF are rather different from those traditionally reported for creep or isothermal fatigue. In all cases examined, the deformation is localized within a rather small number of deformation bands. While these bands were found to consist mainly of micro-twins in some alloys, in others they might be better described as slip or shear bands. Furthermore, in some circumstances these bands are prone to recrystallization. In CMSX-4, the intersection points of twins of different orientation act as initiation sites for this process. In the SCA425 alloy – of smaller gamma’ content, lower creep resistance and less great oxidation resistance – twinning is observed infrequently; however the deformation is still very localized and in the distorted gamma-gamma’ microstructure, along the shear bands, recrystallization is observed. Furthermore the recrystallization is enhanced by oxidation due to the development of a gamma’-depleted zone. In CMSX-4, TCP phases precipitated during long term ageing cause a more dispersed deformation behaviour which prevents recrystallization. Our findings confirm the importance of an inhomogeneous microstructure for good TMF resistance.

Composite Filler Metals for Joining Temperature-Sensitive Aluminium- Matrix Composites

Lightweight composite materials like aluminium-matrix composites (AMC) have been applied in many different sectors such as aerospace industry, automobile production, or power-electronics. As a special group of composites, AMCs produced by ECAE (equal-channel angular extrusion) feature a very high strength due to a very fine-grained structure. But they are very temperature-sensitive. Therefore, an adapted joining technique is required. In this regard, soldering offers some advantages in comparison to other joining processes like welding or bonding. Because of their low melting range below 300 °C, Sn-based filler metals are suitable for this purpose; Ag and Cu are the common alloying elements. The low strength and creep resistance of the joints are disadvantageous features. By the development of Sn-based composite fillers, an improvement of these properties can be achieved due the addition of ceramic reinforcing particles like Al2O3 or SiC. Investigations into the formation of an interfacial reaction layer between reinforcing particles and filler matrix were carried out. Ti as active element was alloyed to improve the bonding between matrix and particles. The microstructure observed by SEM has been correlated with the results of the tensile tests.