Creep Aging Research Articles

Regulating effect of pre-stretching degree on the creep aging process of Al-Cu-Li alloy

Regulating the deformation and property synchronously is a key point of creep aging process. And pre-deformation has been considered to be a good regulating method. In this paper, the laws of the influence of pre-stretching degree on creep strain, strength property, precipitate during creep aging process of Al–Cu–Li alloy were studied, and the pre-stretching design methods suitable for creep aging process were explored. The results indicate that the introduction of different pre-stretching degrees can have various influences on the creep strain of Al–Cu–Li alloy and the nucleation, growth and composition of the precipitates, so as to possess the capacity to regulate deformation and property. The introduction of proper pre-stretching can significantly prolong the duration of the initial creep stage, therefore, the total creep strain in 24 h are 2 times greater than that of the un-pre-stretched specimen. Besides, appropriate pre-stretching can effectively inhibit the orientation precipitation effect of T1 phase and promote a large deal of nucleation and dense distribution. However, when the pre-stretching degree is too large, the composition of the precipitates will be changed, so that the number of θ′ precipitates increases and T1 precipitates decreases, and the alloy strengthening effect is obviously reduced.

Effect of Different Aging System on Microstructure and Mechanical Properties of X2A66 Alloy

Through a series of mechanical properties tests and microstructure observation, the effects of artificial heat treatment (AA), pre-stretching aging (PA) and pre-stretching creep aging (PCA) treatment on the precipitation phase type and mechanical properties of X2A66 aluminum-lithium alloy were studied. The results show that compared with AA, PA treatment promotes the uniform precipitation of high-density T1 phase, so that the yield strength, tensile strength and elongation of the alloy reach 593.4Mpa, 610.8Mpa, 10.7%. Further, the creep aging treatment on the basis of the pre-deformation can further increase the strength (613.6Mpa, 628.9Mpa) of the alloy with a slight decrease in the elongation (10.3%).

Microstructure Variation of Ultrasuper Critical 11Cr Ferritic Steel Under Long-Term Thermal Ageing and Creep

The microstructure variation of ultrasuper critical 11Cr ferritic steel was quantitatively investigated under long-term thermal ageing and creep, with the emphasis on the size of precipitates, martensite lath width, and dislocation density. The tests were interrupted at different deformation stages. The precipitates on prior austenite grain and martensite lath boundaries became obscure due to coarsening of secondary particles. The Laves phase grew faster than M23C6 carbides in both creep and thermal ageing. The dislocation density rapidly decreased at the initial stage, while the martensite lath width increased with ageing and creep duration. The microstructure variations under creep were more pronounced than under thermal ageing, indicating that stress exerts an important effect on microstructure degradation.

Tracking Supervision of Service Performance and Life Assessment of Defective 15Cr1Mo1V Steel Pipeline

A large number of inclusion defects were found during the metal supervision process of 15Cr1Mo1V steel main steam pipeline of the Russian-made thermal power unit. In this paper, the tracking supervision research of 15Cr1Mo1V steel pipe elbow with inclusion defects is carried out. The variation law of strength, toughness, metallographic structure and creep rupture strength with inclusion defects at different operating time is studied. Type and composition of inclusions are analysed and creep fatigue crack evaluation of the most serious inclusion defects discovered is performed. The results show that with the increase of service time, the room temperature and high temperature strength of the material gradually decreased, the impact toughness deteriorated, the ductile-brittle transition temperature increased and greater than room temperature, the metallurgical organization aging grade rose from 3 to 4, creep rupture strength decreased, creep aging was increasingly serious, and creep residual life reduced. The main inclusions are plastic MnS、SiO2 and severe inclusion levels up to 3. Longitudinal inclusions are mostly long-chain features, and the ends are sharp but no sharp cracks are found at the sharp ends; the transverse inclusions are granular. Creep fatigue crack evaluation show that there was no obvious growth of material inclusion defects with the increase of service time, it is necessary to strengthen the supervision and inspection of brittle inclusions in the follow-up operation.

Effects of Pre-Stretching on Creep Behavior, Mechanical Property and Microstructure in Creep Aging of Al-Cu-Li Alloy

The effects of pre-stretching on creep behavior, mechanical properties and microstructure during the creep aging process of Al-Cu-Li alloy were investigated. AA2195 was taken as the representative of Al-Cu-Li alloys. It is found that the total creep strain and strength property of creep aged AA2195 specimens can be improved through effective pre-stretching. Unlike with artificial aging, yield strength increased increasing by 47%. The TEM images show that the constitution of aging precipitates in the creep-aged specimens are obviously changed by pre-stretching. Precipitates in the 2% pre-stretched specimen are mainly composed of T1 phase, while a great amount of θ’ phase accompanied with a few T1 phase were found in the non-pre-stretched specimen. Moreover, pre-stretching introduces many dislocations which benefit the creep deformation, but the increasing dislocation density also accelerates the nucleation and growth of the precipitates as well. The premature T1 phase has a great blocking effect to the dislocation motion, creating a lower decrease rate but a longer duration in the early creep stage. Except for the initial dislocations, the dislocation motion in the creep aging process is also a favorable factor to precipitate the T1 phase.

Dynamic interplay between dislocations and precipitates in creep aging of an Al-Zn-Mg-Cu alloy

Creep age forming (CAF) is an advanced forming technology used for manufacturing large complex integrated panel components. However, in creep aging (CA), unlike in sole creep or aging procedure, the dislocation movement and the precipitation process occur simultaneously, leading to difficulty in understanding of the dynamic interplay between these two phenomena. In this work, taking 7050 Al alloy, a typical Al-Zn-Mg-Cu alloy, as the test material, an experimental scheme combining pre-deformation, artificial aging (AA), and tensile/compressive CA is designed to decouple and reveal the dynamic interaction mechanism of both phenomena. From AA experiments, the static interaction between dislocations and precipitates is studied, and then their dynamic interactions in CA and each evolution are comparatively investigated. The research shows that both total strain and strain rate increase with the increase in pre-deformation in tensile and compressive CA. However, the total creep strain in compressive CA is larger than that in tensile CA. In additional, the more the dislocations are induced, the sparser and more heterogeneous the overall distribution of precipitates becomes. For dynamic interplay, in the first stage of CA (I), under thermal-mechanical loading, the GP zones and η′ phases gradually nucleate and grow, while the effect of dislocation multiplication is dominant compared with dislocation annihilation, leading to an increase in total dislocation density. Soon, the dislocation movement is gradually hindered by tangling, pile-up, and the precipitates that have grown on the dislocation lines, this decreases the mobile dislocation density and results in a significant decrease in creep rate. In the second stage (II), the precipitates grow further, especially those lying on the dislocation lines; the effects of pinning and hindrance are enhanced until the dislocation multiplication and annihilation reach a dynamic equilibrium, and the total and mobile dislocation densities tend to be roughly unchanged, thus, the creep rate remains relatively constant in this stage.

Precipitate Evolution and Fatigue Crack Growth in Creep and Artificially Aged Aluminum Alloy

The fatigue performance of high-strength Al-Cu-Mg alloys is generally influenced by the process of creep age formation when applied to acquire higher strength. The results show that creep aging accelerates the precipitation process, leading to a more uniform precipitation of strengthening phases in grains, as well as narrowed precipitation-free zones (PFZ). Compared with the artificially aged alloy, the yield strength and hardness of the creep aged alloy increased, but the fatigue resistance decreased. In the low stress intensity factor region (ΔK ≤ 7 MPa·m1/2), the fatigue crack propagation (FCP) rate was mainly affected by the characteristics of precipitates, and the fatigue resistance noticeably decreased with the increased creep time. In a 4 h creep aged alloy, the microstructure was dominated by Cu-Mg clusters and Guinier-Preston (GP) zones, while S″ phases began to precipitate in the matrix, showing better fatigue resistance. After aging for 24 h, the needle-shaped S’ phases were largely precipitated and coarsened, which changed the mode of dislocation slip, reduced the reversibility of slip, and accelerated the accumulation of fatigue damage. In stable and rapid crack propagation regions, the influence of precipitates on the FCP rate was negligible.

Effects of creep ageing on the mechanical properties and micro precipitates of X2A66 alloy

In the present work, effects of creep ageing on the mechanical properties and micro precipitates of X2A66 alloy are analysed herein. Compared with artificial ageing, creep ageing controls the formation process of fine T1 nano precipitates. During CA treatment and thus facilitate the development of the mechanical properties of X2A66 alloy including its hardness and strength. However, the elongation of the alloy is slightly reduced.

Timeliness of Creep Deformation in the Whole Visco-Elasto-Plastic Process of Surrounding Rocks of the Tunnel

Nishihara model is modified by using unsteady viscous body. By combining with Lade–Duncan criterion, this study analyzes the whole deformation process of surrounding rocks of tunnels at visco-elastic and plastic stages and calculated visco-elastic–plastic solutions for surrounding rocks of tunnels. By taking an expressway tunnel as the research object, the displacement of surrounding rocks in creep aging process is calculated and the creep deformation of surrounding rocks of the tunnel is measured by monitoring vault settlement on site. By comparing theoretical calculation value with field measured value, it is verified that the theoretical calculation is correct. The theoretical calculation shows that when t = 0, creep phenomenon occurs to surrounding rocks and the displacement of surrounding rocks is 17.890 mm. When t is a constant, displacement of surrounding rocks gradually increases with time. While t = ∞, surrounding rocks are completely crept and the ultimate displacement of surrounding rocks reaches 26.451 mm. The monitoring results on site demonstrated that the first stable accumulative deformation of vault of surrounding rocks was 19.1 mm. After entering into creep stage, the second stable accumulative deformation of vault was 29.3 mm. The error between theoretical calculation and actual monitoring result of initial creep deformation is 6.76%, while 10.77% of error is found between theoretical calculation and actual monitoring results of final creep deformation. The small errors between them reveal that theoretical calculation shows a high accuracy. Theoretically, deformation of surrounding rocks increases by 47.85% after considering creep effects. Actually, the deformation of surrounding rocks rises by 53.4% after taking creep effects into account. This reflects that creep effects play an important role. Therefore, as for the support design of tunnels in soft rocks, creep effects are bound to be taken into consideration to avoid failure of primary supports caused by creep effects of surrounding rocks at later stage and then influences on overall stability of the tunnel.

Non-isothermal creep aging behaviors of an Al-Zn-Mg-Cu alloy

To shorten production period and reduce manufacturing cost of high strength aluminum alloy integral panel components, a novel non-isothermal creep age forming (CAF) technology was developed. Taking an Al-Zn-Mg-Cu alloy as the case material, the non-isothermal creep aging (NICA) behavior and the conventional isothermal creep aging (ICA) behavior were comparatively investigated. The corresponding creep deformations, material performance variations and precipitate microstructure evolutions were studied by creep aging tests, mechanical property and corrosion resistance tests, and transmission electron microscope (TEM) and scanning electron microscope (SEM) characterizations. Unlike the ICA which contains short heating and cooling stages and long holding stage, the NICA only has heating and cooling stages, viz., the alloy is heated slowly and the cooling stage begins immediately when reaching the peak temperature. In the NICA process, the precipitates nucleate and grow up in the first half of heating stage, resulting in the reduction of creep rate and enhancement of mechanical strength. The coarsening of precipitates occurs in the second half of heating stage, elevating the creep rate and significantly improving the corrosion resistance. Nearby the peak temperature, the primary precipitates partially dissolve, and the creep rate, mechanical strength and corrosion resistance decrease. Distinctive from the ICA process, a secondary precipitation phenomenon takes place in the cooling stage of NICA, leading to the recovery of high mechanical strength. The higher heating-cooling rate (1 °C/min) is not beneficial to accumulating creep deformation in the shorter creep aging period, while the lower one (0.25 °C/min) causes the loss of mechanical strength due to the excessive dissolution of precipitates. The moderate heating-cooling rate (0.5 °C/min) can lead to larger creep deformation, better mechanical properties and more satisfactory corrosion resistances in the NICA treated alloy, reaching 87.0% of creep strain, 105.7% of yield strength and 97.9% of electric conductivity in the ICA treated alloy. It is worth noting that the time consumption of the optimal NICA process is only 52.9% of the ICA process.

Stress-level-dependency and bimodal precipitation behaviors during creep ageing of Al-Cu alloy: Experiments and modeling

Understanding the deformation and strengthening mechanism at different stress levels is crucial for accurate prediction of creep ageing behaviors, especially for alloys subjected to a complex stress-state. A comprehensive investigation has been performed on the creep deformation and age hardening during creep ageing of an aluminum-copper alloy AA2219-T4 under various stress levels that lead to elastic or initial plastic deformation. Microstructural variables including dislocations and precipitates have been characterized in detail by X-ray diffractometer, scanning/transmission electron microscopy and differential scanning calorimetry. The creep mechanism is found to be dominated by diffusion creep under small stress level and then by dislocation climb with increasing stress. The high-stress-induced dislocations raise the creep strain considerably and cause a notable extension of the primary creep stage. The existence of preferential alignment of θ" precipitates (stress-orienting effect) in creep-aged samples results in an evident decline in the age hardening potential. A typical bimodal distribution of precipitates comprised of θ" homogeneously dispersed in the matrix and relatively large stable θ′ heterogeneously nucleated at dislocations forms during high-stress ageing. The θ′ precipitates exhibit no stress-orienting effect and lead to a considerable enhancement of hardening response. Based on detailed microscopic observations, a unified constitutive model is developed and calibrated by experimental data to predict the observed stress-level dependency of creep ageing behaviors. The dislocation evolution during both the loading stage and the creep stage as well as the bimodal precipitation of shearable and nonshearable phases are quantified and incorporated in a dislocation-based internal variable creep model. The proposed model is able to well describe the concurrent evolution of dislocations and precipitation, whose interplay gives rise to a characteristic upturn behavior of stress effects on creep strain and age hardening response. The model can be applied to simulate the creep age forming of complex aluminum alloy component structures.

Effect of pre-strain on creep behaviour and constitutive modelling of titanium alloy sheets

This paper discusses the effect of pre-strain on the creep behaviour and constitutive modelling of titanium alloy Ti-3Al-5Mo-5V-8Cr sheets. A set of uniaxial tensile creep tests at different pre-strains and different stresses were carried out to analyse the creep behaviour of titanium alloy sheets. The results show that pre-strains have significant influence on the creep behaviour which was opposite to the effect of stress. And based on the experiment results, a modified Garofalo creep constitutive model was established which can illustrate the creep aging behaviour of titanium alloy Ti-3Al-5Mo-5V-8Cr sheet at 500°C under different stresses and different pre-strains very well.

Effects of creep aging upon Al-Cu-Li alloy: Strength, toughness and microstructure

Effects of creep aging on strength and toughness and microstructure evolution of Al-Cu-Li alloy were investigated by tensile test and Kahn tear test at room temperature, combined with fractograph and transmission microstructure analysis. The specimens of pre-strain creep aging state (CA) have been studied and compared with artificial aging state (AA) and pre-strain artificial aging state (PA). It is demonstrated that pre-strain before artificial aging process can improve the strength property and the fracture toughness of Al-Cu-Li alloy. On top of that, both of them can be further improved by creep aging process: the yield strength was increased by 8.7% and the unit initiation energy of Kahn tear was increased by 11.7%. The increasing of fracture toughness has also been verified by Kahn tear fractographs that the dominating fracture type changes from the ductile intergranular fracture to the ductile transgranular fracture. Transmission electron microscopy revealed that creep aging promotes the nucleation of T1 phase, resulting in a fine and dense distribution of precipitates in the matrix, and the enrichment of subgrain boundary T1 precipitates was greatly reduced.

Effect of Process Parameters on Fatigue and Fracture Behavior of Al-Cu-Mg Alloy after Creep Aging

A set of creep aging tests at different aging temperatures and stress levels were carried out for Al-Cu-Mg alloy, and the effects of creep aging on strength and fatigue fracture behavior were studied through tensile tests and fatigue crack propagation tests. The microstructures were further analyzed by using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that temperature and stress can obviously affect the creep behavior, mechanical properties, and fatigue life of Al-Cu-Mg alloy. As the aging temperature increases, the fatigue life of alloy first increases, and then decreases. The microstructure also displays a transition from the Guinier-Preston-Bagaryatsky (GPB) zones to the precipitation of S phase in the grain interior. However, the precipitation phases grow up and become coarse at excessive temperatures. Increasing stress can narrow the precipitation-free zone (PFZ) at the grain boundary and improve the fatigue life, but overhigh stress can produce the opposite result. In summary, the fatigue life of Al-Cu-Mg alloy can be improved by fine-dispersive precipitation phases and a narrow PFZ in a suitable creep aging process.

Damage in Creep Aging Process of an Al-Zn-Mg-Cu Alloy: Experiments and Modeling

In creep age forming (CAF), large integral panel components of high-strength aluminum alloy can be shaped and strengthened under external elastic loading at an elevated temperature through creep deformation and age hardening, simultaneously. However, the high ribbed structure on panel may induce stress concentration, inhomogeneous plastic deformation and even damage evolution on the bending rib, leading to the difficulty in controlling forming precision and material properties. Therefore, the generation and evolution of damage are necessary to be considered in the design of CAF. Taking 7050 aluminum alloy as the case material, the continuous and interrupted creep aging tests at 165 °C and three stress levels (300, 325, and 350 MPa) were conducted, and the corresponding material properties, precipitate, and damage microstructures were studied by mechanical properties tests, transmission electron microscope (TEM) and scanning electron microscope (SEM) characterizations. With the increase of stress level, the creep deformation occurs easier, the precipitates grow up faster, the creep damage occurs earlier, the growth rate and the size of microvoids increase, the mechanical properties decrease more rapidly, and the dominant mechanism of creep fracture changes from shear to microvoid coalescence. To simulate creep aging behavior with damage, a continuum damage mechanics (CDM) based model is calibrated and numerically implemented into ABAQUS solver via CREEP subroutine. The CAF of 7050 aluminum alloy panels with different height ribs were conducted by experiment and FE simulation. The forming process presents a typical stress relaxation phenomenon. The creep damage mainly occurs on the bending rib due to the severe stress concentration. With the increase of rib height, the creep strain and damage degree increase, but the springback decreases.

Effect of creep aging forming on the fatigue crack growth of an AA2524 alloy

Effects of creep aging forming on the microstructural evolution, conventional mechanical properties and fatigue crack propagation (FCP) behavior for 2524 aluminum alloy were studied by tensile and hardness testing, the FCP behavior testing and transmission electron microscopy (TEM) observation. The results show that in the low stress intensity range, the characteristics of precipitates have a significant effect on fatigue performance of the creep-aged aluminum alloy, and the mainly precipitated S″ phases in the alloy creep-aged for 4 h is beneficial to the reversibility of coplanar slip and dislocation slip, and the alloy presents better FCP resistance and relatively lower FCP rate, while the FCP rate increases with increasing aging time. In the Paris regime, effects of microstructure on the FCP rate are no longer obvious, and the FCP rates are almost consistent. Meanwhile, the presence of creep stress significantly accelerates the aging precipitation process. In terms of yield strength σ0.2, hardness and FCP resistance, the 9 h artificial aged alloy is analogous to the 4 h creep aged alloy. The peak strength of the alloy creep-aged for 24 h is the highest, but the fatigue crack growth resistance degraded obviously. It is not comprehensive to determine the optimal process of creep aging only in accordance with the peak strength.

Anisotropy in creep-ageing behavior of textured Al-Cu-Mg alloy

Creep age forming (CAF) has been developed to manufacture large complex components in aerospace industry. However, crystalline textures in initial material for CAF process may influence the creep ageing behavior, leading to low precision of formed component. In-planer anisotropy in creep-ageing behavior of a textured Al-Cu-Mg alloy was investigated. The total creep deformation decreased with the increase of the angles between the applied stress and rolling direction. The creep deformation of samples with loading direction parallel to rolling direction is 2.33, 2.07 and 1.62 times greater than that perpendicular to the rolling direction, under 120 MPa, 150 MPa and 180 MPa, respectively. A strong dependence of the anisotropy in creep deformation on the initial textures was suggested, according to the equivalent slid system numbers. With the rise of the external stress, the creep deformation of three different orientation samples increases but the anisotropy of those declines. However, the stress exponent difference among the three orientation samples may be related to the equivalent Schmid factor changes. Although the anisotropy of the mechanical properties in creep aged samples with various orientations is less than 5%, the in-planar anisotropy values of creep deformation under different creep age forming conditions are larger than 30%. It indicated that creep-ageing anisotropy of the studied alloy sheet cannot be ignored in their CAF applications.

Modeling of the Temperature-Related Two-Stage Creep Aging for Al-Zn-Mg-Cu Aluminum Alloy

In order to model the creep aging process of Al-Zn-Mg-Cu aluminum alloy using common temperature-related two-stage aging, a constitutive framework was proposed considering the creep strain and yield strength. The material parameters involved in the model were estimated using a simple fitting method from the experimental data. With a simple expression, this model is capable of handling stress relaxation, hardening response, and variable aging temperature in the creep aging process. It could also be implemented using finite element analysis software to simulate the creep strain, yield strength, and springback of the component. The modeling results are consistent with the creep strain curves measured under different applied stresses. The numerical simulations are compared with the applied experimental data and excellent consistency is observed between them.

Effects of Mechanical Vibration on Double Curvature Creep Aging Forming of 2124 Aluminum Alloy

This Paper studied the precipitation behaviour and creep deformation of 2124 aluminum alloy based on the concept of complex field. A mechanical vibration field was introduced into the creep aging forming process of 2124 aluminum alloy, and its effects on creep deformation, precipitations behaviour and mechanical properties under the condition of double curvature loading and aging temperature were investigated by three-dimensional scanning technique, TEM and tensile test, respectively. The results showed that the spring back value along the rolling and transverse direction presented after creep aging forming were reduced by 25% and 15% respectively. The volume fraction of precipitates increased and distributed more densely and uniformly. Meanwhile, the yield stress improved by 15MPa and the degree of anisotropy decreased by 17% with mechanical vibration field applied to the manufacturing process.

Role of thermal-mechanical loading sequence on creep aging behaviors of 5A90 Al-Li alloy

Creep age forming (CAF), under thermal-mechanical environment, may be sensitive to the thermal and mechanical loading conditions. Unfortunately, the different creep aging (CA) behaviors under two inverse thermal-mechanical loading sequences, viz. loading-heating (LH) and heating-loading (HL), are almost neglected in experiments or modeling of CAF. In this work, taking solution treated 5A90 Al-Li alloy as a case material, the loading sequence related CA behaviors of the alloy are investigated using continuous/interrupted tensile CA tests at 130 °C/175 MPa combined with high-resolution Transmission Electron Microscope (HRTEM) and mechanical properties tests. The results show that, the inverse loading sequences result in different initial tempers for CA process, viz. solution temper under LH and stress-free aged temper under HL. The solution temper for CA process under LH can lead to a more stable and repeatable CA process than HL does, which causes a stress-free aged temper with randomly and discretely distributed precipitates for CA process. After two inverse loadings and 18 h of isothermal CA, the total creep deformation for LH (0.1475%) is twice larger than that for HL (0.0720%), thanks to the remarkable creep strain in the LH non-isothermal CA process with 49.2% of total creep strain. The samples under LH present a lower strength and a higher elongation than those under HL during the whole process, and the gap of the mechanical properties of samples under the two loading sequences reaches its maximum (15 MPa for yield strength and 8.45% for elongation) after 0.5 h of isothermal CA, and then gradually decreases in subsequent process. After 0.5 h of isothermal CA, though the volume fraction of the δ′ precipitates (Al3Li) under LH is larger than that under HL, the dislocations density under LH is less than that under HL, which makes a lower material strength under LH. It is noted that, after 18 h of isothermal CA, the microstructures and macroscopic mechanical properties of the samples have less disparity for the two inverse loading sequences.