Creep Strengthening Mechanisms Research Articles

Investigation of the thermal creep behaviour of non-irradiated Zr1%Nb cladding alloys between 623 and 1223K

Short-term constant stress creep tests in tension were performed on two zirconium Zr1%Nb cladding alloys in the form of thin-walled cladding tubes in the temperature range of 623–1173 K and at the applied tensile stress σ range of 5 - 225 MPa to provide further information on their creep behaviour in the α-Zr and (α+β)-Zr phase regions. In parallel, the evolution of the microstructure of the studied alloys was investigated using SEM and TEM microscopy in the as-received state and after creep exposures. Using the dependence of diffusion coefficient-compensated minimum creep rate vs modulus compensated applied stress the values of the stress rate exponent n = (∂lnε˙m/∂lnσ)T were determined ranging from ∼2.5 up to ∼16 depending on the applied stress and testing temperature. The stress-dependant activation energy for creep QC=[∂lnε˙m/∂(-1/kT)]σ was determined and possible creep deformation mechanisms as well as creep strengthening mechanisms are discussed. Under the loading conditions used in this study three distinct stress regions were identified according to the relevant controlling creep deformation mechanisms.

Creep strengthening mechanisms of a superlattice γ′-strengthened Co–Al–W–Ta–Ti single crystal superalloy at steady-state conditions under compressive stresses

The strengthening mechanism of steady-state creep is a critical factor in developing advanced superalloys. In this study, the steady-state creep mechanisms of a superlattice γ′-strengthened Co–Al–W–Ta–Ti single crystal superalloy are investigated with the creep conditions of 750 °C/800 MPa, 900 °C/420 MPa and 1000 °C/137 MPa. Results suggest that the microstructures of steady-state creep samples are significantly different under the above three experimental conditions. A detailed TEM study shows that, Lomer-Cottrell locks and stacking faults interactions are responsible for the low creep rate at low temperature and high stress creep regime, dislocations cross-slip and the dislocation tangles contribute the main creep resistance at intermediate temperatures and moderate stress creep regime, interactions of stacking faults and dislocation networks are the strengthening mechanism for the high temperature and low stress creep regime. The steady-state creep rate of investigated alloy follows the classical power law equation, and the creep activation energy calculated by the power law equation is 378.46 kJ/mol. The results of this study provide insights into the effects of temperature and stress on steady-state creep mechanisms, and high activation energy makes the γ′-strengthened Co–Al–W–Ta–Ti single crystal superalloy an attractive candidate for further study as a high temperature structural material.

Enhancing the high-temperature creep properties of Mo alloys via nanosized La2O3 particle addition

Molybdenum (Mo) alloys with different La 2 O 3 particle additions (0.6, 0.9, 1.5 wt.%) were prepared by powder metallurgy to investigate the effect of La 2 O 3 particles on microstructural evolution and creep behavior of the alloy. Pure Mo, annealed at 1500 °C for 1 h, presented a fully recrystallized microstructure characterized by equiaxed grains. The alloys doped with La 2 O 3 particles (Mo-La 2 O 3 alloys), on the other hand, exhibited fibrous grains elongated in the rolling direction of the plate. In contrast to the shape of the grains, the average grain size of the alloys was found to be insensitive to the addition of La 2 O 3 particles. Nanosized La 2 O 3 particles with diameters ranging from 65 nm to 75 nm were distributed within the grain interior. Tensile creep tests showed that dislocation creep was the predominant deformation mode at intermediate creep rate (10 −7 s −1 -10 −4 s −1 ) in the present alloys. The creep stress exponent and activation energy were found to decrease with increasing temperature, particularly within the low creep rate regime (<10 −7 s −1 ). The Mo-La 2 O 3 alloys exhibited remarkably greater apparent stress exponent and activation energy than pure Mo. A creep constitutive model based on the interaction between particles and dislocations was utilized to rationalize the nanoparticle-improved creep behavior. It was demonstrated that low relaxed efficiency of dislocation line energy, which is responsible for an enhanced climb resistance of dislocations, is the major creep strengthening mechanism in the Mo-La 2 O 3 alloys. In addition, the area reduction and creep fracture mode of the Mo-La 2 O 3 alloys were found to be a function of the creep rate and temperature, which can be explained by the effect of the two parameters on the creep and fracture mechanisms.

High-temperature strengthening mechanism and thermal stability of Laves phase in ferritic matrix

The high-temperature creep strengthening mechanism and thermal stability of Laves phase are the key factors that affect its strengthening property in ferritic matrix. Therefore, in this study, two types of C14 Laves phase alloys, namely, Fe2Nb Laves phase Alloy S1 (Fe–9Cr–4Al–2Nb) (wt.%) and Fe2W Laves phase Alloy S2 (Fe–9Cr–4Al–2W) (wt.%) were prepared to investigate the strengthening mechanism and strengthening effect of Laves phase with different compositions in ferritic matrix. The creep properties and microstructure characteristics of the two alloys, in particular, the interfacial structure characteristics between Laves phases (Fe2Nb and Fe2W) and ferritic matrix were analyzed. The true stress exponent n for both Alloys S1 and S2 is 5, and the true activation energies Q of Alloys S1 and S2 are 372.47 and 365.82 kJ mol−1, respectively. The creep mechanisms of Alloys S1 and Alloy S2 are both dislocation climbing-dominated processes reinforced with precipitated phases. Alloy S1 exhibits higher high-temperature creep resistance than that of Alloy S2. The grain size of Alloy S1 is smaller than that of Alloy S2. Both the precipitates Fe2Nb in Alloy S1 and Fe2W in Alloy S2 are mostly located at the grain boundary. The density of dislocation in Alloy S1 is higher than that in Alloy S2, and the bending degree of dislocation lines caused by precipitate hindrance is also higher than that in Alloy S2. The orientation relationships between the Laves phase Fe2Nb and ferritic matrix in Alloy S1 are [011]α-Fe//[5 1‾9‾4‾]Fe2Nb, (011‾)α-Fe//(03 3‾ 1)Fe2Nb, with the atomic interface mismatch of 1.6%; the orientation relationships between the Laves phase Fe2W and ferritic matrix in Alloy S2 are [001]α-Fe//[01 1‾ 2]Fe2W, (1 1‾ 0)α-Fe//(01 1‾1‾) Fe2W, with the atomic interface mismatch of 1.9%. The results indicate that the Fe2Nb-type Laves phase may be more suitable to candidate for the strengthener in the ferritic matrix compared with Fe2W-type.

Design for 1200 °C creep properties of Ni-based single crystal superalloys: Effect of γ′-forming elements and its microscopic mechanism

Enhancing ultra-high temperature mechanical properties of turbine blade-manufacturing Ni-based single-crystal superalloys is crucial to advanced aero-engines. However, the service temperature still does not rise to 1200 °C, at which severe microstructure degradation and accelerated plastic deformation due to thermal activation deteriorate creep resistance. This work investigates the effect of γ′-forming elements, including Al and Ta, on 1200 °C/80 MPa creep properties and its corresponding mechanism. The creep property is sensitive to Al and Ta addition, reaching its peak in our designed composition region. The microstructure evolution during creep is also sensitive to Al and Ta; particularly, adding γ′-forming elements facilitates abnormal rattan-shaped γ′ phase formation. Evidenced by typical three-stage creep behavior, “cubic-raft-collapse” microstructural evolution, and dislocation configuration analysis, 1200 °C creep resistance would originate primarily from classical 1100 °C creep strengthening mechanisms. Besides, the detrimental impact of the rattan-shaped γ′ phase is revealed, which should also be considered. Further research indicates that Al addition produces a lower degree of γ′-solubility, more negative lattice misfit, higher γ′-fraction, and more rattan-shaped γ′ phase, while Ta addition produces lower γ′-solubility, more positive misfit, higher γ′-fraction and more rattan-shaped γ′ phase. The alloying composition-phase/interface-properties relationship has been established for 1200 °C creep, which explains the peak performance of SCA4 alloy. These results provide new opportunities to design superalloys that resist ultra-high temperature service and elevate the service temperature limit.

Analysis of the Combined Strengthening Effect of Solute Atoms and Precipitates on Creep of Aluminum Alloys

The creep strengthening mechanisms in (age‐hardenable) aluminum alloys are analyzed on the basis of a new microstructural study of powder samples, an analysis of a comprehensive revision of creep data from the literature, and a new modeling approach. A strategy based on the strength difference (SD) method to separate the contributions of solid solution atoms and precipitates to creep strengthening is proposed. The new methodology considers the combination of the two contributions avoiding the need of a threshold stress term in the creep equation. The contribution of both precipitates and solid solution is taken into account by means of the analysis of the lattice parameter variation with aging time. For this study, powders of two commercial AA2xxx alloys have been analyzed using diffraction methods. The experimental results are modeled using Lubarda's approach combined with the SD method.

Individual/synergistic effects of Al and AlN on the microstructural evolution and creep resistance of Elektron21 alloy

The creep properties of Mg-2.85Nd-0.92Gd-0.41Zr-0.29Zn (El21) alloys with additions of 0.25 wt% Al, 0.75 wt% AlN and 1 wt% AlN/Al nanoparticles (NPs) were studied over a stress range from 80 to 140 MPa at 240 °C, respectively. The individual/synergistic roles of Al and AlN in the El21 alloy were investigated systematically to reveal their creep strengthening mechanisms. Creep results show that individually all three additions of 0.25 wt% Al, 0.75 wt% AlN and 1 wt% AlN/Al could increase the creep resistance of El21 alloy apparently. However, the addition of mixed 1 wt% AlN/Al NPs shows the best strengthening effect on creep properties in El21 alloy. Microstructural characterizations reveal that the additions of Al and AlN increased the area fraction of intermetallic particles obviously. Blocky Al2Zr, Al2Zr3 particles and Al2(Nd, Gd) (Al2RE) particulates were observed in both El21 + 0.25% Al and El21 + 0.75%AlN. Nevertheless, when Al and AlN were simultaneously added into El21 alloy the formation of these blocky phases Al2Zr/Al2Zr3 was suppressed, and a larger amount of Al2RE phase was observed. This is attributed to the preferential reaction between AlN and Zr, which restricted the formation of Al–Zr phase and subsequently promoted the reaction of Al-RE phase. The dominant mechanism during creep at 240 °C was calculated to be viscous glide of dislocation. The simultaneous additions of Al and AlN NPs could lead to a more homogeneous distribution of intermetallic particles and increase the amount of Al2RE phase. Such kind of microstructures is beneficial for hindering the dislocation movement, transfer the load from matrix and alleviate the local stress concentration. Consequently, El21 + 1% AlN/Al exhibits the best creep properties among four alloys.

Strengthening of 2D-C/SiC composite through creep with low stress and high temperature

ABSTRACTTo improve the resistance to matrix cracking of 2D-C/SiC composite, the composite was treated at certain creep conditions, which involve relative lower stress, higher temperature. The creep strain and matrix cracking stress (σmc) were used to determine the creep strengthening effect. Moreover, the stress redistribution model was applied to explain the stress transfer from the matrix to fibre, and therefore the creep strengthening mechanisms. The results indicate that 2D-C/SiC can be strengthened by creep at certain conditions. The ability to resist the creep strain and σmc can be improved by certain creep treatments with creep temperature of 1400°C, lower stress and longer time. The strengthening effect is resulted from the stress redistribution during the creep. The tensile residual stress is relaxed in the matrix and transferred to the fibre. The stress redistribution leads to compressive stress in the matrix and takes the advantage of superior creep resistance of carbon fibres.

Effect of cold forming strain on creep rupture properties of 47Ni-23Cr-23Fe-7W alloy

47Ni-23Cr-23Fe-7W (HR6W) is a candidate material for high temperature areas in the Advanced Ultra-Supercritical (A-USC) boilers. In this study, creep rupture properties of cold-formed materials were investigated in comparison to the solution annealed materials in order to clarify the creep strengthening mechanism of the cold-formed HR6W. It was found that the creep strength and rupture life of the cold formed HR6W increased substantially as compared with solution annealed material and creep ductility decreased. According to investigation results of microstructure and hardness changes in the crept specimens, grain boundaries in cold-formed HR6W were covered with M23C6 precipitates from the early stages of creep and the fine M23C6 particles in the grains were very stable for long period of time as compared to the lower grain boundary coverage in the solution annealed materials. These results suggested that a high density of dislocation is maintained during the test and hence it exceeded 60,000 h. This was postulated to be reason why creep rupture strength of cold formed HR6W enhanced so remarkably in comparison to the solution annealed material.

Effect of temperature on creep strengthening mechanisms in Mg-Y based alloys

Effect of temperature on creep strengthening mechanisms in Mg-Y based alloys

Atomic-scale investigation of creep behavior in nanocrystalline Mg and Mg–Y alloys

Magnesium (Mg) and its alloys offer great potential for reducing vehicular mass and energy consumption due to their inherently low densities. Historically, widespread applicability has been limited by low strength properties compared to other structural Al-, Ti- and Fe-based alloys. However, recent studies have demonstrated high-specific-strength in a number of nanocrystalline Mg-alloys. Even so, applications of these alloys would be restricted to low-temperature automotive components due to microstructural instability under high temperature creep loading. Hence, this work aims to gain a better understanding of creep and associated deformation behavior of columnar nanocrystalline Mg and Mg–yttrium (Y) (up to 3at.% Y (10wt.% Y)) with a grain size of 5nm and 10nm. Using molecular dynamics (MD) simulations, nanocrystalline magnesium with and without local concentrations of yttrium is subjected to constant-stress loading ranging from 0 to 500MPa at different initial temperatures ranging from 473 to 723K.In pure Mg, the analyses of the diffusion coefficient and energy barrier reveal that at lower temperatures (i.e., T<∼423K) the contribution of grain boundary diffusion to the overall creep deformation is stronger that the contribution of lattice diffusion. However, at higher temperatures (T>∼573K) lattice diffusion dominates the overall creep behavior. Further, we observe a negligible change (within the fitting error) in the overall secondary creep rate with creep activation energy changing from 1.128 to 1.154eV for 0 to 3at.% Y, respectively, indicating that stage two creep activity is insensitive to Y for a given grain size. We also present novel results showing that the (101¯1),(101¯2), (101¯3) and (101¯6) boundary sliding energies are reduced with the addition of yttrium. This softening effect in the presence of yttrium suggests that the experimentally observed high temperature beneficial effects of yttrium addition is likely to be attributed to some combination of other reported creep strengthening mechanisms or phenomena such as formation of stable yttrium oxides at grain boundaries or increased forest dislocation-based hardening.

The Microstructure and Creep Strength of 403Nb Martensitic Heat-Resistant Steel

The microstructure and creep strength of 403Nb martensitic heat-resistant steel after tempering at 650°C, 680°C and 730°C were analyzed by SEM and TEM. Microstructural observation shows that 403Nb steel tempered at 650°C provides a finer lath structure and a smaller size of precipitates than that tempered at 680°C or 730°C. The dislocation density is also higher in 403Nb steel tempered at 650°C. The creep rupture strength of 403Nb steel tempered at 650°C is the highest among those tempered. The main creep strengthening mechanisms in 403Nb steel usually include precipitates hardening, dislocation hardening and sub-grain boundary hardening.

Novel Concept of Creep Strengthening Mechanism using Grain Boundary Fe2Nb Laves Phase in Austenitic Heat Resistant Steel

ABSTRACTThe creep behavior of a new type of austenitic heat-resistant steel Fe-20Cr-30Ni-2Nb (at.%), strengthened by intermetallic Fe2Nb Laves phase, has been examined. Particular attention has been given to the role of grain boundary Laves phase in the strengthening mechanism during long-term creep. The creep resistance increases with increasing area fraction (ρ) of grain boundary Laves phase according to equation ε/ε = (1−ρ), where ε0 is the creep rate at ρ = 0. In addition, the creep rupture life is also extended with increasing ρ without ductility loss, which can yield up to 77% of elongation even at ρ = 89%. Microstructure analysis revealed local deformation and well-developed subgrains formation near the grain boundary free from precipitates, while dislocation pile-ups were observed near the grain boundary Laves phase. Thus, the grain boundary Laves phase is effective in suppressing the local deformation by preventing dislocation motion, and thereby increases the long-term creep rupture strength. This novel creep strengthening mechanism was proposed as “grain boundary precipitation strengthening mechanism” (GBPS).

Type IV Cracking Susceptibility in Weld Joints of Different Grades of Cr-Mo Ferritic Steel

Relative type-IV cracking susceptibility in 2.25Cr-1Mo, 9Cr-1Mo, and 9Cr-1MoVNb ferritic steel weld joint has been assessed. The type-IV cracking was manifested as preferential accumulation of creep deformation and cavitation in the relatively soft intercritical region of heat affected zone of the weld joint. The type-IV cracking susceptibility has been defined as the reduction in creep-rupture strength of weld joint compared to its base metal. The 2.25Cr-1Mo steel exhibited more susceptibility to type-IV cracking at relatively lower temperatures; whereas, at higher temperatures, 9Cr-1MoVNb steel was more susceptible. The relative susceptibility to type-IV cracking in the weld joint of the Cr-Mo steels has been rationalized on the basis of creep-strengthening mechanisms operating in the steels and their venerability to change on intercritical heating during weld thermal cycle, subsequent postweld heat treatment, and creep exposure.

V-Mod. 2. 25Cr-1Mo鋼の500°Cクリープ強度の支配因子

V-Mod. 2. 25Cr-1Mo鋼の500°Cクリープ強度の支配因子

Creep Strengthening Mechanism of Mo and W in 9% Cr Heat Resistant Steels

Creep Strengthening Mechanism of Mo and W in 9% Cr Heat Resistant Steels

Precipitation and Creep Strengthening Mechanisms in K5 Series γ-TiAl Alloys

ABSTRACTA new generation of wrought γ-TiAl alloys having a good balance of properties at both low and high temperatures has recently been developed. Addition of light elements (C, Si) to the base alloy of this system (K5 material) has been shown to increase the creep strength of the material significantly. The microstructure of the materials with superior creep performance was found to consist of wellaligned rows of fine precipitates, delineating partially dissolved α2 laths. These precipitates were found to interact strongly with dislocation structures. This paper studies the mechanisms for α2 dissolution and fine particle precipitation in annealed samples of the K5 series alloys. Microstructural design issues that would allow for optimization of the particular strengthening mechanism will also be discussed.

Metallurgy and creep properties of new 9-12%Cr steels

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Transient Creep Strain of a Fiber-Reinforced Metal-Matrix Composite Under Transverse Loading

Based on the combination of Mori-Tanaka’s mean-field concept and Luo-Weng’s solution of a three-phase cylindrically concentric solid, a local-field theory is developed to study the evolution of stress distribution in the ductile matrix and the time-dependent creep strain of a fiber-reinforced metal-matrix composite. Due to the nonlinear stress dependence in the creep rate and the highly heterogeneous nature of the transverse deformation, this local theory is shown to provide a more accurate estimate for the overall transverse creep of the composite than the simpler meanfield theory (the difference between the two however is not significant in the axial tensile creep). The disclosed transverse stress field in the matrix is truly heterogeneous, with the 45 deg region exhibiting a substantially higher effective stress than the 0 deg and 90 deg regions. The stress in the higher stress region is found to decrease continuously; it is passed on to the fibers and thereby serves as an important creep-strengthening mechanism for the composite. The interfacial tensile stress at the pole, which is the highest stress point around the interface, is seen to grow continuously and becomes a potential site for a later creep debonding. The developed micromechanical theory is finally applied to predict the transverse tensile creepstrain of a Borsic/aluminum composite, and the result is found to be in close agreement with the test data.

Creep Deformation of Ta Modified Gamma Prime Single Crystals

AbstractThe role of substitutional element alloying of single phase γ' has become of primary interest to alloy designers who would like to exploit its low density and excellent oxidation resistance. Current γ' alloys have not shown sufficient strength to be useful in a creep limited environment. In order to maximize the potential of single phase γ' alloys and to more fully understand the creep strengthening mechanisms in two phase Ni-base superalloys, it has become necessary to clarify the role of Al-substitution elements. Ta is a potent strengthening element in γ' as well as imparting beneficial surface stability to superalloys; its effect on the creep properties of Ni3Al is the subject of this paper. The 1300°C isotherm of the Ni-Al-Ta system was determined in order to establish the γ' single phase field. Comrpositions were fabricated having chemistries which systematically varied both the Al:Ta ratio at Ni=75% and Ni:(AI+Ta) ratio at Ta=6%. Creep tests were conducted on &lt;001&gt; oriented single crystals at 760, 871 and 982°C. Electron microscopy was used to characterize the nature of slip deformation, confirm phase purity and to determine the existence of tetragonal distortions in these crystals. In this manner the strengthening due to Ta was examined in the absence of grain boundary effects. These γ' mono—crystals did not display classical creep response. Incubation creep was observed in all of the specimens tested. Surprisingly, the maximum incubation time was found to occur in the high ratio Ni:(Al+Ta) compounds, where less than 0.5% creep strain was obtained after 200 hours at stress. After incubation, either tertiary creep leading to failure, or apparently classic primary, secondary and tertiary creep ensued. In addition extremely long elongations, to 85%, were measured.