Primary Creep Regime Research Articles

Power law creep and delayed failure of gels and fibrous materials under stress.

Motivated by recent experiments studying the creep and breakup of a protein gel under stress, we introduce a simple mesoscopic model for the irreversible failure of gels and fibrous materials, and demonstrate it to capture much of the phenomenology seen experimentally. This includes a primary creep regime in which the shear rate decreases as a power law over several decades of time, a secondary crossover regime in which the shear rate attains a minimum, and a tertiary regime in which the shear rate increases dramatically up to a finite time singularity, signifying irreversible material failure. The model also captures a linear Monkman-Grant scaling of the failure time with the earlier time at which the shear rate attained its minimum, and a Basquin-like power law scaling of the failure time with imposed stress, as seen experimentally. The model furthermore predicts a slow accumulation of low levels of material damage during primary creep, followed by the growth of fractures leading to sudden material failure, as seen experimentally.

Role of Thermo-Mechanical Treatment on Creep Deformation Behaviour of Reduced Activation Ferritic Martensitic Steel

<div class="section abstract"><div class="htmlview paragraph">To enhance the microstructural and mechanical properties of Reduced Activation and Ferritic-Martensitic (RAFM) steel thermo mechanical treatment (TMT) was performed on as received Normalised and Tempered steel (N+T). The RAFM with 9Cr-1W-0.06Ta major alloying elements steel was used in the study in N+T and TMT conditions. The refinement in microstructure and precipitates was observed in TMT steel in comparison to N+T steel. There is a drastic improvement in mechanical properties such as hardness, tensile properties are observed without losing the ductility. The creep deformation behaviour of thermo-mechanical treated steel and N+T steel were studied at different stress levels at 823 K. The comparative creep rupture and strain properties were studied, and various creep deformation regimes were also analysed for both the conditions of steel. The rate of exhaustion of primary creep (r), minimum creep rate and time spent in primary and tertiary creep regime, rate of acceleration of tertiary creep and time to reach the onset of tertiary creep. Microstructural studies on Optical and Scanning electron microscopy and transmission electron microscopy results were correlated with the mechanical properties of steels. On a whole, a comparative investigation was performed on various reasons for drastic improvement in tensile and creep rupture properties of TMT steel than the steel in N+T steel and necessary microstructural investigation support is considered.</div></div>

Creep in high Al ferrite based low density steel

Low density steels have attracted the attention of researchers across the globe as substitute alloys in structural and locomotive applications with promising mechanical properties and reduced density. Most of the research in low density steels is concentrated on room temperature properties after the thermomechanical processing of the same. In the present study, a high Al Fe-11Al-5Mn-0.1C low density steel was melted by vacuum arc melting in argon atmosphere. The as cast alloy was homogenized at 1200 °C and then hot rolled from 20 mm to 2 mm at 1100 °C. Hot rolled strips were annealed at 1000 °C for 1 h followed by furnace cooling. After thermomechanical processing, creep testing of alloy samples was carried out at 600 °C for 50 MPa and 100 MPa stress. The processed alloy has shown a total creep life of 6.21 h and minimum creep rate of 8.5 × 10 −1 h −1 for 100 MPa stress and for a stress of 50 MPa, creep life of 1005.1 h with minimum creep rate of 1.4 × 10 −3 h −1 . A dominating tertiary creep with appreciable secondary and negligible primary creep regime was observed. Fracture occurred due to cavitation at ferrite-kappa carbide interface. Ductile mode of fracture with fine dimples was observed.

Primary creep regeneration in 10%Cr martensitic steel: In-situ and ex-situ microstructure studies

Primary creep regeneration (PCR) is a phenomenon observed during stress-varying/cyclic creep loading conditions where a load reversal might clear the previous strain hardening memory and cause the regeneration of the primary creep regime (i.e. a period of high creep strain rate) upon reloading. In this study, in-situ and ex-situ microstructural examinations, including transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), neutron and synchrotron X-ray diffraction were conducted to better understand the responsible mechanisms of PCR for a 10%Cr martensitic steel at 600 °C. Our experimental evidence indicated that the PCR phenomenon is related to the change of dislocation density due to activation of dislocation generation and recovery mechanisms, formation and relaxation of dislocation pile-ups, as well as bowing/unbowing of dislocation-lines during stress-varying creep loading conditions. These mechanisms could explain the observed creep strain accumulation in the steel during the examined stress-varying creep loading conditions reported in the current and previous studies. The presented mechanistic description of the PCR phenomenon and the reported experimental observations for the microstructural and mechanical parameters can provide a basis for the formulation of physically-based models to describe the creep behaviour of the steel under high-temperature stress-varying creep loading conditions.

Microstructural evolution during high-temperature tensile creep at 1,500°C of a MoSiBTiC alloy

AbstractMicrostructural evolution in the TiC-reinforced Mo–Si–B-based alloy during tensile creep deformation at 1,500°C and 137 MPa was investigated via scanning electron microscope-backscattered electron diffraction (SEM-EBSD) observations. The creep curve of this alloy displayed no clear steady state but was dominated by the tertiary creep regime. The grain size of the Moss phase increased in the primary creep regime. However, the grain size of the Moss phase was found to remarkably decrease to <10 µm with increasing creep strain in the tertiary creep regime. The EBSD observations revealed that the refinement of the Moss phase occurred by continuous dynamic recrystallization including the transformation of low-angle grain boundaries to high-angle grain boundaries. Accordingly, the deformation of this alloy is most likely to be governed by the grain boundary sliding and the rearrangement of Moss grains such as superplasticity in the tertiary creep regime. In addition, the refinement of the Moss grains surrounding large plate-like T2 grains caused the rotation of their surfaces parallel to the loading axis and consequently the cavitation preferentially occurred at the interphases between the end of the rotated T2 grains and the Moss grains.

Compressive creep behavior of a γ-TiAl based Ti–45Al–8Nb–2Cr-0.2B alloy: The role of β(B2)-phase and concurrent phase transformations

The primary and steady-state compressive creep deformation behavior of an as-cast high Nb containing γ-TiA1 alloy having remnant β(B2)-phase has been studied over the temperature range of 750–850 °C at constant initial applied stress levels between 75-200 MPa. The creep curves derived over the entire creep testing conditions show a prominent primary creep regime which is attributed to the presence of dislocation debris of the coarse γ-grains at the colony boundaries. The stress and temperature dependence of the steady-state creep rate follows the Norton-Bailey power law and the corresponding average value of the stress exponent and apparent activation energy are estimated to be 3.6 and 375 kJ/mol, respectively. The average value of the stress exponent in conjunction with the dislocation substructure of the crept samples suggests that the kinetics of creep deformation within the studied experimental conditions is controlled by the non-conservative motion (climb) of dislocations. Further misorientation analyses of the phase-resolved EBSD maps imply that for most of the conditions creep strain is carried by the γ-TiAl phase. Consequently, contrary to the commonly believed idea, the β(B2)-phase does not appear to deteriorate the creep resistance of the present alloy below suitable combinations of temperature and stress. Beyond these conditions, the concurrent deformation of γ-TiAl and β(B2)-phases is found to enhance the steady-state creep rate. In addition, characteristics of the dynamic transformation of β(B2) phase into γ and α2 phases during creep and its effect on the steady-state creep rate are further considered. Finally, the application potential of the present alloy is compared through a composite plot of Larson-Miller parameter with several other potential alloys reported in the literature.

Creep Behavior of As-Cast Mg-10 wt.%Sn and Mg-10 wt.%Sn-3 wt.%Al-1 wt.%Zn Alloys: A Comparative Study

The present investigation highlights the creep deformation and rupture behavior of indigenously produced as-cast Mg-10 wt.%Sn-3 wt.%Al-1 wt.%Zn (TAZ1031) and Mg-10 wt.%Sn (T10) alloys. Conventional creep tests were conducted at 423 K (150 °C) in the stress range of 30-45 MPa and at 45 MPa in the temperature range of 423-453 K (150-180 °C) for both the alloys. The nature of creep curves was found to be identical for both the alloys with short primary and secondary creep regime, followed by extended tertiary creep region. The creep resistance of the TAZ1031 alloy was better than the T10 alloy. The former exhibited reduced minimum creep rate and enhanced creep rupture life under identical creep test conditions with respect to the latter. The change of minimum creep rate (έmin) followed a power-law relationship with a change in applied stress (σ) for both the alloys with a significant change in stress exponent (n) value. Activation energy for creep deformation of TAZ1031 alloy (~ 117 kJ/mol) was found to be higher than T10 alloy (~ 105 kJ/mol). Fine secondary Mg2Sn precipitates within the α-Mg matrix and primary Mg2Sn, Mg17Al12 and MgZn2 precipitates along the grain boundaries improved the creep properties of TAZ1031 alloy. On the contrary, the threshold stress required for creep deformation was found to be significantly low in TAZ1031 alloy due to the presence of Mg17Al12. Both creep damage analysis and dimpled fracture surfaces revealed necking dominated creep deformation for the alloys. SEM studies revealed the absence of creep cavities along grain/dendritic boundaries for the investigated alloys, which substantiated the formation of dimpled fracture surfaces during creep deformation.

Self-consistent modelling of cyclic loading and relaxation in austenitic 316H stainless steel

ABSTRACTIn the present study the Hu-Cocks micromechanical model [1,2] for dislocation-obstacle interactions, implemented in a crystal plasticity self-consistent model, is employed to simulate thermo-mechanical histories typical for AGR nuclear plants in order to assess the implications of creep-fatigue interactions in 316H stainless steel. Their physical model is enhanced by including the effect of dynamic recovery, which introduces a new material parameter – the annihilated segment length . The full model contains five independent material parameters; other parameters are prescribed by the fundamental physics of inelastic deformation processes. Having calibrated the model, we explore its ability to predict material response under complex loading histories to provide insight into the physical phenomena controlling cyclic-creep interactions. Introduction of strain dwells during cyclic loading results in an increase of the extent of relaxation with an increasing number of cycles, but histories with dwells at different strain levels indicate that relaxation is strongly dependent on initial stress and level of constant strain. Predictions of history-dependent relaxation demonstrate that the least stress relaxation results after creep into the secondary regime and the largest stress drop results during hold-dwells after monotonic elastic-plastic loading, with the cyclic-dwell history behaviour laying in between these two. Both prior cycling and the generated residual stress field are found to affect the primary creep regime under hold-stress dwells. These results are consistent with experimental observations; this demonstrates that deformation response is dependent on both the evolution of microstructural state and redistribution of stress between the grains of the polycrystalline aggregate.

The influence of niobium, tantalum and zirconium on the microstructure and creep strength of fully lamellar γ/α2 titanium aluminides

Titanium aluminides are excellent materials for light-weight applications at high temperatures. To enhance their creep strength, various alloying elements are added. In this work the effect of 5 at% Niobium (Nb), Tantalum (Ta) and Zirconium (Zr) on the creep properties of ternary Ti-44Al-5X (X = Nb, Ta, Zr) with a fully lamellar microstructure, is systematically studied. These alloying elements have a high solubility in titanium aluminides, and show a different α2/γ partitioning behavior as well as a different influence on the lattice parameters of both phases. The average interface distance between the α2- and γ-phase is adjusted to about 300 nm by the cooling rate to evaluate the influence of the three alloying elements. The creep tests reveal that all three alloying elements increase the creep resistance of titanium aluminides, particularly Ta and Zr. Ta seems to extend the pronounced primary creep regime, whereas Zr seems to reduce it. The alloying elements decrease also the lamellar spacing compared to the binary alloy. Microstructural investigations after the creep tests show a beneficial effect of the alloying elements on the microstructural stability during creep exposure, since they retard the discontinuous coarsening rate.

Creep behaviour of AISI 316H stainless steel under stress-varying creep loading conditions: primary creep regeneration

ABSTRACTPrimary creep regeneration (PCR) is an important reported observation from creep under stress-varying conditions for several alloys. For a specimen deforming in the secondary creep regime, a stress reversal leads to an enhanced creep rate upon reloading due to reactivation of the primary creep regime (i.e. PCR). This paper focuses on an investigation of the PCR phenomenon during stress-varying creep loading for AISI 316H stainless steel at 650°C. The experimental observations clarify the influence of different parameters (e.g. forward creep stress level, reverse stress magnitude and forward and reverse accumulated inelastic strain) on the extent of PCR activation. In addition, a correlation between the extent of PCR activation and inelastic strain accumulation during the reverse loading period was found, which was employed to develop an empirical–phenomenological model for prediction of the creep behaviour of the alloy after stress transients (e.g. stress reversals).

Rejuvenation of Single-Crystal Ni-Base Superalloy Turbine Blades: Unlimited Service Life?

Rejuvenation of the initially hot isostatic pressing (HIP) heat-treated single-crystal Ni-base superalloy (SX) ERBO/1 was examined experimentally and via phase field simulation to establish rejuvenation treatments as a cost-effective alternative for another interval of service life. Creep was performed at 950 °C and 350 MPa, and the specimens were crept to 0.6 pct (creep rate minimum) or 2 pct strain, respectively. A slight coarsening of the γ/γ′ microstructure was observed experimentally and via simulation at 0.6 pct and rafting at 2 pct strain. The damaged microstructures were rejuvenated in a novel hot isostatic press that provides fast quenching rates before the same specimens were recrept under the same initial creep conditions. High-resolution microscopy proves that the rejuvenation re-establishes the original γ/γ′ microstructure in the dendrite core of the precrept specimens (0.6 and 2 pct). However, the interdendritic areas of the 2 pct precrept and rejuvenated specimen still contain elongated γ′ particles enwrapped by interfacial dislocation networks that survived the applied rejuvenation. The subsequent experimental and simulated creep tests after rejuvenation demonstrated that the creep behavior is only reproducible by the proposed rejuvenation for specimens that had crept until the end of the primary creep regime.

Recrystallisation-assisted creep of an austenitic Fe-Ni alloy under low stresses after hot deformation

Static and meta-dynamic recrystallisation are known to take place in some steels under low tensile stresses during thermo-mechanical processes, e.g. between rolling stands in a hot strip finishing mill. The main objective of the present work is to understand the mechanisms responsible for the recrystallisation-assisted visco-plastic deformation in a hot worked iron-nickel austenitic alloy subjected to low stress levels at high temperatures. To that purpose, the recrystallisation kinetics of the alloy was first quantified from stress relaxation, interrupted creep and double-hit compression tests, followed by a microstructural study. The evolutions of strain both in the recrystallising and in the fully recrystallised microstructures were quantified. One of the most original aspects of this work is the experimental verification that the acceleration of the viscoplastic deformation of a recrystallising material is unequivocally and intrinsically associated with the on-going recrystallisation phenomenon itself. For the austenitic alloy of interest, the most important contribution to this deformation enhancement was found to arise from the deformation of newly recrystallised grains in the primary creep regime. Finally, it is also shown that this recrystallisation-assisted deformation can be qualitatively predicted with acceptable accuracy using a relatively simple, physics-inspired constitutive model.

Effect of temperature and stress on creep behavior of ultrafine grained nanocrystalline Ni-3 at% Zr alloy

In this paper, molecular dynamics (MD) simulation based study of creep behavior for nanocrystalline (NC) Ni-3 at% Zr alloy having grain size ~ 6 nm has been performed using embedded atom method (EAM) potential to study the influence of variation of temperature (1220-1450 K) as well as change in stress (0.5-1.5 GPa) on creep behavior. All the simulated creep curves for this ultra-fine grained NC Ni-Zr alloy has extensive tertiary creep regime. Primary creep regime is very short and steady state creep part is almost absent. The effect of temperatures and stress is prominent on the nature of the simulated creep curves and corresponding atomic configurations. Additionally, mean square displacement calculation has been performed at 1220 K, 1250 K, 1350 K, and 1450 K temperatures to correlate the activation energy of atomic diffusion and creep. The activation energy of creep process found to be less compared to activation energies of self-diffusion for Ni and Zr in NC Ni-3 at% Zr alloy. Formation of martensite is identified during creep process by common neighbour analysis. Presence of dislocations is observed only in primary regime of creep curve up till 20 ps, as evident from calculated dislocation density through MD simulations. Coble creep is found to be main operative mechanism for creep deformation of ultrafine grained NC Ni-3 at% Zr alloy.

An assessment of creep deformation and rupture behaviour of 9Cr–1.8W–0.5Mo–VNb (ASME grade 92) steel

Creep deformation and rupture behaviour of 9Cr–1.8W–0.5Mo–VNb steel have been investigated at 873K, 923K and 973K over a stress range of 80–220MPa. The absence of clear primary creep regime and prolonged secondary stage of creep deformation have been noticed under lower stress level at 973K. The variation of minimum creep rate with applied stress obeyed Norton's power law of creep. The apparent stress exponents of 15.2, 12.3 and 5.8, and apparent activation energy of 619kJ/mole have been estimated for creep deformation of the steel. The apparent stress exponents and activation energy have been rationalised on the basis of threshold stress. The threshold stress values of 137.5MPa, 83.3MPa and 29.7MPa were obtained at 873K, 923K and 973K respectively. The threshold stress compensated true stress exponent of 4 and true activation energy of 244kJ/mole, and threshold stress normalised by Orowan stress confirms that the lattice diffusion assisted localised climb of dislocation is the rate controlling of creep deformation in the steel. The steel obeyed Monkman and modified Monkman–Grant relationships. Damage tolerance factor of 6 in the steel demonstrates that the microstructural degradation such as coarsening of precipitates and subgrain structure is the dominant creep damaging mechanism in the steel.

Influence of fiber orientation and matrix processing on the tensile and creep performance of Nextel 610 reinforced polymer derived ceramic matrix composites

The tensile and creep performance of two Nextel® 610 (N610) reinforced polymer derived ceramic composites (UMOX™, OXIPOL) is studied up to 1200°C. Independent of the fiber orientation (±45° or 0°/90°) all samples exhibit a segment where the strain rate was constant. The creep performance in ±45° is matrix dominated and shows a more pronounced primary creep regime, due to changes within the matrix. The following creep regime with a constant strain rate might be attributed to viscous flow of the SiOC within the matrix based on activation energy (283kJ/mol) and stress exponent (0.6). In 0°/90° orientation the creep and tensile performance is independent of oxidation, but directly influenced by grain structure of the fiber. The coarser and non-uniform microstructure of the fibers in UMOX™ decreases the stationary creep rates and changes the diffusional creep mechanism. The possibility to modify the microstructure of the fiber during the manufacturing process might be used to adjust e.g. the strength and creep stability of these materials related to the desired applications.

Creep and fracture of a protein gel under stress.

Biomaterials such as protein or polysaccharide gels are known to behave qualitatively as soft solids and to rupture under an external load. Combining optical and ultrasonic imaging to shear rheology we show that the failure scenario of a protein gel is reminiscent of brittle solids: after a primary creep regime characterized by a power-law behavior whose exponent is fully accounted for by linear viscoelasticity, fractures nucleate and grow logarithmically perpendicularly to shear, up to the sudden rupture of the gel. A single equation accounting for those two successive processes nicely captures the full rheological response. The failure time follows a decreasing power law with the applied shear stress, similar to the Basquin law of fatigue for solids. These results are in excellent agreement with recent fiber-bundle models that include damage accumulation on elastic fibers and exemplify protein gels as model, brittlelike soft solids.

High-resolution electron microscopy of dislocation ribbons in a CMSX-4 superalloy single crystal

High-resolution scanning transmission electron microscopy (STEM) has been used to study the structure of dislocations in single crystal superalloy samples that have been subjected to conditions that favour the primary creep regime. The study has revealed the detailed structure of extended a2〈112〉 dislocations as they shear the γ′ precipitates during creep. These dislocations dissociate in a manner that is consistent with predictions made using the phase-field model of dislocations and also suggests the importance of the reordering process during their movement. The shearing done by the a〈112〉 dislocations was also found to distort the γ/γ′ interface, changing its appearance from linear to a “saw tooth” pattern. Another important observation was the segregation of alloying elements with a high atomic mass to the stacking faults, presumably to reduce their energies during shear. Numerous a2〈110〉 dissociated dislocations were also observed in the γ channels of the superalloy. The high resolution provided by the STEM imaging enables one to study the high-energy faults that are usually difficult to observe in conventional weak-beam TEM, such as complex intrinsic and extrinsic stacking faults in the γ′ and intrinsic stacking faults in the γ, and to make estimates of their energies.

Lattice strain evolution during creep in single-crystal superalloys

In situ neutron diffraction studies are carried out to characterize the micromechanical deformation occurring during tensile creep of a typical single-crystal nickel-based superalloy, CMSX-4. The loading responses of the matrix γ phase and the precipitate γ′ are distinct. Moreover, the behaviour in the tertiary creep regime (in which the γ′ phase remains intact) is qualitatively different from that in the primary creep regime (when γ′ is sheared). In tertiary creep, initial deformation of the matrix leads to a release of misfit between the phases in the (100), resulting in elastic compression of the γ in the loading direction. The load state then remains fairly constant during creep. During the initial stages of primary creep, elastic compression of the γ phase is observed until at around 2–4% creep strain this compression stabilizes as the (100) misfit is released. This is the point at which γ′ shearing is thought to begin. Subsequently, the load in the γ increases by around 200MPa until a maximum is reached at around 8% creep strain. This load is then suddenly released, which may be due to the release of back-stress.

In Situ Neutron-Diffraction Studies on the Creep Behavior of a Ferritic Superalloy

Precipitate strengthening effects toward the improved creep behavior have been investigated in a ferritic superalloy with B2-type (Ni,Fe)Al precipitates. In situ neutron diffraction has been employed to study the evolution of the average phase strains, (hkl) plane-specific lattice strains, interphase lattice misfit, and grain-orientation texture during creep deformation of the ferritic superalloy at 973 K (700 °C). The creep mechanisms and particle-dislocation interactions have been studied from the macroscopic creep behavior. At a low stress level of 107 MPa, the dislocation-climb-controlled power-law creep is dominant in the matrix phase, and the load partition between the matrix and the precipitate phases remains constant. However, intergranular stresses develop progressively during the primary creep regime with the load transferred to 200 and 310 oriented grains along the axial loading direction. At a high stress level of 150 MPa, deformation is governed by the thermally activated dislocation glide (power-law breakdown) accompanied by the accelerated texture evolution. Furthermore, an increase in stress level also leads to load transfer from the plastically deformed matrix to the elastically deformed precipitates in the axial direction, along with an increase in the lattice misfit between the matrix and the precipitate phases.

Plastic behavior of some yield stress fluids: from creep to long-time yield

The yielding of several reversible yield stress fluids is studied during scissometric-like creep experiments. The temporal evolution of the apparent deformation is recorded for applied stresses close and below the usual yield stress. Similarly to solids, three main creep regimes are observed. First, a primary creep regime displaying a temporal power law evolution of the deformation rate occurs, followed by a temporal minimum, which leads to an apparent flow of the material. This local minimum, defined as the “transition time,” and the subsequent fluidization can be observed at long times. The evolution of this time as a function of the applied stress appears to follow a universal law reminiscent of fracture behavior in hard solids.