Decrease In Creep Rate Research Articles

Ligament rotation-dominated creep in stochastic bicontinuous nanoporous metallic glass

Bicontinuous nanoporous metallic glasses (BNPMGs) are promising candidates in functional applications such as energy storage, sensors, and fuel cells. The time-dependent deformation strongly affects their long-term in-service performance. This study investigates the tensile creep behavior and its underlying deformation mechanisms of Cu50Zr50 BNPMGs through molecular dynamics simulations. The creep-recovery fatigue behavior is revealed. Our results manifest that both the transient and steady creep strengths increase with the solid fraction. The transient strength is larger for smaller ligament sizes, but the steady creep strength has very mild sensitivity to the ligament size. The generalized Kelvin model, consisting of two Kelvin units and one Maxwell unit connected in series, can accurately predict the creep and recovery curves. The rotation of individual ligaments, rather than atomic diffusion, is the dominant creep mechanism. During creep, the shear transformation zones are mainly developed on the ligament surface, and the strain in the ligament core is relatively smaller. The creep stress exponent has no definite relation with the creep mechanism and decreases with the increasing solid fraction. With the increase in the creep-recovery cycle number, the creep rate decreases, and the recovery procedure makes a milder impact on the creep response.

Creep properties and constitutive model for salt rock subjected to uniaxial trapezoidal cyclic loading

Uniaxial trapezoidal cyclic loading creep tests with different loading and unloading rates are performed on salt rock to study its creep properties. The results indicate that the axial strain produced by salt rock under trapezoidal cyclic loading occurs mainly in the 1st cycle, and other cycles generate less axial strain. The existence of constant high pressure stage leads to an obvious horizontal segment in the stress-strain curve, while constant low pressure stage has little effect on its stress-strain curve. The stress-strain curve in the 1st cycle includes elastic stage, plastic stage, creep stage and deformation recovery stage. The axial strain generated at plastic deformation stage gradually tends to 0 with increasing cycle numbers, then the curve in a single cycle includes only elastic stage, creep stage and deformation recovery stage. When the cycle numbers is small, the irreversible deformation produced by salt rock in a single cycle mainly consists of plastic deformation produced at the loading stage and creep deformation generated at the constant high pressure stage. After several cycles, the irreversible deformation in a single cycle mainly consists of creep deformation generated at the constant high pressure stage. With increasing cycle numbers, the minimum value of creep rate in a single cycle basically does not change, and the maximum value of creep rate decreases firstly and then keeps constant. Moreover, with increasing loading rate, the mean value of minimum creep rate basically remains unchanged and the mean value of maximum creep rate gradually increases. With increasing unloading rate, both the mean values of minimum and maximum creep rate decrease gradually. Finally, the creep equation for the Burgers model subjected to trapezoidal cyclic loading is deduced, and the reasonableness of which is checked.

Study on the Effect of Nanoindentation Test Method on Micromechanical Properties of Granite Minerals

The study on microscopic mechanical properties of mineral is of great significance to reveal the damage mechanism of rock and can provide important basis for nondestructive testing of building materials and assisted rock breaking technology. As the main test method for rock and mineral microscopic mechanical properties, nanoindentation test has several parameters such as loading time, indentation depth, and loading rate, which have great influence on the test results. Three schemes are designed as follows: (1) The loading and unloading rates remain unchanged, and the holding time is 0 s, 5 s, 10 s, 20 s, 50 s, and 100 s, respectively. (2) Ten cycles of loading and unloading tests are used to calculate micromechanical indexes at different indentation depth. (3) The maximum load and holding time remain unchanged, and the loading rate is 5 mN/s, 10 mN/s, 20 mN/s, 30 mN/s, 40 mN/s, and 50 mN/s, respectively. The results show that (1) with the increase of holding time, the elastic modulus and hardness of feldspar decrease sharply between 0 s and 5 s, decrease slightly between 5 s and 10 s, and stabilize after 10 s. The creep displacement of feldspar is positively correlated with holding time. The holding time does not change the development path of creep displacement. (2) With the increase of indentation depth, the elastic modulus and hardness of feldspar decrease gradually and become stable when the indentation depth is more than 350 nm. (3) The elastic modulus and hardness of quartz increase with loading rate. The maximum indentation depth and residual indentation depth decrease with the increase of loading rate. The creep displacement increases with loading rate, while the creep compliance decreases with the increase of loading rate. With the increase of holding time, the creep rate decreases and tends to be stable. The results provide theoretical basis for obtaining more accurate rock nanoindentation mechanical properties and revealing rock damage mechanism.

Effect of prior austenitisation temperature on creep rupture in Grade 22 steel

ABSTRACT The uniaxial tensile creep resistance at 600°C for Grade 22 bainitic steel was measured as function of prior normalisation heat treatment which was carried out in the range 900–1100°C. A decrease in creep rate is favoured by larger prior austenite grain size, but creep ductility is substantially reduced. Of particular interest is the significant difference in creep performance for the 900°C and 1000°C heat treatment conditions. Although the prior austenite grain size is only marginally different for the two conditions, the creep rupture time is substantially longer for the 1000°C condition. Dilatometry tests, which simulate the normalisation thermal cycles, indicate differences in phase dissolution during normalisation and hence it is proposed that variations in solute distribution account for the differences in creep behaviour.

Creep Behavior ofγ-TiAl-Based Alloys with Fully Lamellar Microstore

AbstractRecent investigations have demonstrated that the interface spacing in fully lamellar microstructures has a major influence on the creep behavior of γ-TiAl-based alloys. In this paper, the dependence of the creep properties on interface spacing in fully lamellar Ti-46.5 at.% Al-4 at.% (Cr, Nb,Ta, B) sheet material was studied. Creep tests were conducted over a temperature range of 700 to 800 °C and at stresses between 100 and 260 MPa. The results indicate that the primary creep strain as well as the minimum creep rate decrease with decreasing interface spacing. A model which considers the limitation of the free dislocation path by stored dislocations as well as by geometrical obstacles was applied to explain the role of the interface spacing on both the primary creep strain and secondary creep rate. The applicability of the model has been verified by comparing calculated creep curves with experimentally obtained ones.

Improved creep performance of melt‐extruded polycaprolactone/organo‐bentonite nanocomposites

AbstractIn this work, biodegradable nanocomposites based on polycaprolactone reinforced with pristine and organo‐modified bentonites are prepared by melt extrusion. Bentonite is exchanged with benzalkonium chloride (CBK) in a pilot plant scale reactor. The influence of clay type and loading on morphology, rheology, mechanical properties, and creep performance of the resulting materials is analyzed. Besides, several theoretical models then applied to experimental creep data and master curves are used to relate time and temperature with the compliance of the materials. The morphology characterization of the nanocomposites show that the organo‐modification of the clay greatly improves its dispersion in the polymer matrix. As a consequence, it is demonstrated that reinforcement of PCL with 3 wt% loading of organoclay produces the strongest improvement in creep resistance. The instantaneous creep strain and the experimental creep rate decrease more than 9% and 27%, respectively, in the range of temperatures analyzed. Moreover, the experimental values are used to adequately fit theoretical creep models for different clay loadings. On the other hand, the material with optimal creep behavior also shows the greatest improvements in tensile mechanical properties.

Compressive creep behavior of extruded Mg-4Sm-2Yb-0.6Zn-0.4Zr alloy

Abstract A high-strength Mg-4Sm-2Yb-0.6Zn-0.4Zr extruded alloy with bimodal microstructure was creep tested in compression at temperatures of 200 °C and 225 °C under applied stress in the range of 120–200 MPa. The creep curves of the alloy are dominated by a steady-state creep stage and a transient primary creep stage. An abnormal decrease of creep rate is observed in the later period of the steady-state creep stage, particularly the creep at high stress levels more significant, and the underlying reasons are proposed. Moreover, the alloy exhibits high stress exponents (n = 8.7 at 200 °C and n = 7.4 at 225 °C) and activation energies ranging from 225 kJ/mol to 310 kJ/mol. It is shown that the high stress exponents can be well rationalized by the commonly adopted threshold stress method in this work, which results in the modified stress exponents near 5, suggesting five power law, with some indicator of dislocation creep. Transmission electron microscopic observations revealed that cross-slip of dislocations is the dominant creep mechanism in recrystallized grains, while cross-slip of dislocations is operative in hot-worked grains. Besides, a fraction of γ phase in hot-worked regions was transformed to γ′ phase during creep while not in recrystallized regions, thus coexistence of γ and γ’ in hot-worked regions. Also, these γ-typed phases play an important role in creep of the studied alloy. Finally, the studied alloy owns better creep resistance than those of benchmark Mg alloys such as AE and AX series alloys.

Isothermal and anisothermal creep behavior of the nickel base Alloy 602 CA

AbstractComponents in industrial furnaces and apparatus construction are usually exposed to temperatures up to 1,200 °C as well as mechanical loading and corrosive attack of the environmental atmosphere. Furthermore, they undergo numerous temperature cycles in service. To describe the characteristics of the nickel base Alloy 602 CA, which is often used in this industrial sector, the long term creep behavior of this alloy was studied at temperatures from 800 °C to 1,200 °C with and without thermal cycling. During these tests unexpected effects could be noticed. At load levels representing typical component loads and isothermal temperature control an anomalous decrease in creep rate in the third creep stage was observed. Experiments under thermal cycling showed a considerably higher creep rate in the whole range than in the isothermal case. Within the present article, these findings are outlined in detail together with a brief discussion of causes and consequences based on extensive microstructural examinations.

Understanding raft formation and precipitate shearing during double minimum creep in a γ′-strengthened single crystalline Co-base superalloy

ABSTRACT The compressive creep deformation behavior of a Co-base single-crystalline superalloy is studied in the high temperature / low stress regime at 950°C/150 MPa. Emphasis is placed on the mechanisms causing the double minimum creep behavior consisting of a local and a global creep rate minimum. The local minimum occurs at ∼0.2% strain with dislocation accumulation at γ/γ′ interfaces after bowing out of dislocations in horizontal matrix channels. Plate-like rafting occurring normal to the applied stress axis in the early creep stages is regarded as the primary reason for the decreasing creep rate until the global minimum takes place at ∼0.8% strain. In the final creep stage following the global minimum the creep rate accelerates due to coarsening and extensive precipitate shearing. The shearing by partial dislocations leads to the formation of numerous stacking faults, nano/microtwins, and occasionally to the formation of small amounts of ordered D019-Co3W precipitates. The reasons for twinning and second phase formation as well as differences of the deformation mechanisms between Co- and Ni-base superalloys during double minimum creep are discussed in detail in this work.

Study on creep behavior and microstructure of a Fe–Cr–Al–Nb–W ferrtic heat-resistant steel based on C, N and Al element control

In this work, the effects of C, N, and Al on the microstructures and creep properties of Fe–Cr–Al–Nb–W ferritic alloys were investigated through scanning electron microscopy, x-ray diffractometry, as well as uniaxial creep testing and hardness testing. The results demonstrated that the matrix of the Fe–Cr–Al–Nb–W heat-resistant steel was ferrite, while the precipitation phases were Laves phases, M23C6 carbides and MX nitride phases. M23C6 and MX precipitated at grain interior, Laves phases precipitated at grain interior, grain boundary and around MX phases. C, N and Al affected microstructure and creep of heat-resistant steel. As the Al content increased or as N content decreased along with the C increase, the average diameter of the Laves phases, along with M23C6 and MX phase grain interior decrease. Moreover, the unit density increased and the phase spacing decreased, which led to the minimum creep rate decrease and creep rupture time increase. Compared to M23C6, Laves and MX phases mainly affected the alloy creep strength. The decrease of Laves phase continuity coefficient (ratio of Laves phase particle spacing and size) on the grain boundary was conducive to the plasticity improvement of heat-resistant steel.

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.

Effect of micro-additions of Ge, In or Sn on precipitation in dilute Al-Sc-Zr alloys

Trace amounts of inoculants - germanium (0.02 at%), indium (0.02 at%) or tin (0.01 at%) - were added to a ternary Al-0.06Sc-0.06Zr at% alloy to study their effects on Al3(Sc,Zr) precipitation. During isothermal aging at 300 °C, the In- and Sn-containing alloys exhibit an early microhardness increases, which is attributed to In or Sn clustering. Peak isothermal microhardness due to Al3(Sc,Zr) precipitation is achieved simultaneously (~4 h) for the three inoculated alloys and an inoculant-free alloy. Isochronal aging produces a low-temperature microhardening response for the In- and Sn-containing alloys, followed by two microhardness peaks at 350 and 450 °C for all four alloys, associated with precipitation of Sc and Zr, respectively. Atom-probe tomography demonstrates partitioning of Ge and Sn (up to 0.5 at%) from the matrix to the Al3(Sc,Zr) precipitates, with partitioning to Sc-rich precipitate cores (for isothermal aging) or to both Sc-rich cores and Zr-rich shells (for isochronal aging). Inoculant partitioning at precipitate cores and early low-temperature microhardness increases are consistent with inoculant clustering to form nucleation sites for Sc and Zr precipitation. Inoculant segregation at the precipitate shells can be explained by co-precipitation of the fast-diffusing inoculant element and the slow-diffusing Zr, between which an attractive binding energy exists. When compared to the inoculant-free control alloy with a somewhat higher Sc content, the three inoculated alloys exhibit a 2- to 10-fold decrease in creep rates at 300 °C for stresses above ~18 MPa, but a small reduction in creep threshold stresses from 12 to ~10 MPa.

Double minimum creep in the rafting regime of a single-crystal Co-base superalloy

To investigate whether the formation of rafts in Co-base superalloys has a hardening or softening effect, the single crystal deformation behavior was studied in the high temperature/low stress creep regime. The large positive lattice misfit of the investigated Co-base superalloy led to a pronounced directional coarsening of the microstructure normal to the stress axis during compressive creep at 950°C/150MPa. Two creep minima were observed similar to Ni-base superalloys under tensile creep. Rafting was accompanied by a decreasing creep rate indicating a hardening effect, although cutting of γ′ rafts by partial dislocations under the formation of stacking faults occurred.

Creep Behaviors of CrMnFeCoNi High Entropy Alloy at Intermediate Temperatures

In this study, creep properties and fracture behavior of CrMnFeCoNi high entropy alloy (HEA) were studied at intermediate temperatures. The invert-type transient primary creep behaviors were observed in CrMnFeCoNi high entropy alloy. Creep behaviors of HEA are similar to those of class I solid solution alloys. The transient creep curves upon increase of stress by 5MPa in the steady state creep region did not change much except the sudden strain increase. And, no decrease of creep rate was observed upon increase of stress. Instead, the slightly invert transient creep or almost straight creep curves were observed, supporting the high friction stress. CrMnFeCoNi high entropy alloy has a stress exponent of 3.75 and the creep activation energy was calculated to be 278KJ/mole. The fracture strain increased from 1.3 to 1.6 with the decrease of stress from 96 MPa to 48MPa. The lower stress exponent along with the invert type primary creep curves strongly suggest that the creep of CrMnFeCoNi high entropy alloy at 600°C~650°C occurs by a glide controlled process.

Numerical Simulation on Creep Process of Surrounding Rock in Soft Rock Tunnel

It is obvious that creep is one of important characteristics of surrounding rock in soft rock tunnel. There was less study on creep process of surrounding rock in the past which affected the engineer's judgment to long term stability of tunnel seriously. In this paper, the Cvsic model is selected and its parameters are determined based on soft rock creep test results, furthermore, the software FLAC3D was used to analyze the viscoelastic plastic creep process of surrounding rock in soft rock roadway. The results show that: the maximum creep rate occurs at the moment when the soft rock tunnel is excavated , then the creep rate decreases and finally evolves into steady creep; it will initially form a creep zone in the tunnel surface and then the deep surrounding rock will also form a damage zone by stress adjustment, forming a larger creep area; hoop stress decreases while the radial stress increases gradually from the surface to the deep rock mass of roadway, and eventually both will approach to the initial in-situ stress ; The plastic zone will gradually expand after the excavation of viscoelastic-plastic tunnel and tend to be stable when reaches to the steady creep stage. The hoop stress on the border between elastic and plastic zone of the surrounding rock reaches to peak stress; the deformation of surrounding rock changes suddenly and the deformation rate, the area of plastic zone increase with the increase of moisture content in the process of creep.

High Temperature Creep Properties of Al-Al4C3-Al2O3 Alloy by Mechanical Alloying

Tensile tests and creep tests were carried out at high temperatures on an Al-Al4C3 alloy prepared by mechanical alloying technique. The material contains about 2.0 % carbon and 0.9 % oxygen in mass percent, and the volume fractions of Al4C3 and Al2O3 particles are estimated at 7.4 and 1.4 %, respectively, from the chemical composition. Minimum creep rate decreased steeply near two critical stresses, σcl (the lower critical stress) and σcu (the upper critical stress), with decreasing applied stress at temperatures below 723 K. Instantaneous plastic strain was observed in creep tests above a critical stress, σci, at each test temperature. σcu and σci were fairly close to the 0.2% proof stress obtained by tensile tests at each test temperature. It is thought that σcl and σcu correspond to the microscopic yield stress and the macroscopic yield stress, respectively. The lower critical stress corresponds to the local yield stress needed for dislocations to move in the soft region within subgrains. The creep strain in the low stress range below 723 K arises mainly from the local deformation of the soft region. The upper critical stress is equivalent to the macroscopic yield stress necessary for dislocations within subgrains or in subboundaries; this stress can extensively move beyond subboundaries under a stress above the critical point to yield a macroscopic deformation. At higher temperatures above 773 K, the influence of the diffusional creep increases and the stress exponent of the creep rate decreases.

Constitutive description of primary and steady-state creep deformation behaviour of tempered martensitic 9Cr–1Mo steel

The model based on the coupled sine hyperbolic creep rate relation with the evolution of internal stress as a function of strain provides better understanding of primary and secondary creep behaviour of tempered martensitic 9Cr–1Mo steel. The predicted evolution of internal stress as an increase in the internal stress value (or, decrease in effective stress) with strain/time appropriately described the observed decrease in creep rate during primary creep in the steel. The applicability of the model has been demonstrated by comparing experimental and predicted creep strain–time and creep rate–strain/time data of 9Cr–1Mo steel at 793 and 873 K for quenched and tempered and simulated post-weld heat treatment conditions. Irrespective of prior heat treatment and test temperature, the optimised parameters associated with the internal stress values exhibited linear variations with applied stress. The influence of prior heat treatment on primary and secondary creep characteristics of the steel is reflected on the rate constant values associated with the model. At all temperatures and heat treatment conditions, good agreement between the experimental and predicted steady-state creep rates demonstrate the further applicability of the model.

Influence of microstructure modification on the circumferential creep of Zr–Nb–Sn–Fe cladding tubes

Out-of-reactor, non-irradiated thermal creep performances and lives of annealed and stress-relieved Zr-1.02Nb-0.69Sn-0.12Fe cladding tubes were studied and compared. The creep rates of annealed Zr-1.02Nb-0.69Sn-0.12Fe cladding tubes were appreciably slower than those of stress-relieved annealed counterpart. The stress exponent increased slightly from 5.1 to 6.1 in the stress-relieved cladding to 5.3–6.3 in the annealed cladding. The creep activation energy of the annealed Zr-1.02Nb-0.69Sn-0.12Fe alloy (300–330 kJ/mol) was larger compared to that of the stress-relieved alloy (210–260 kJ/mol). The creep activation energy of annealed alloy is close to that of self-diffusion in α-Zr (336 kJ/mol). The smaller activation energy in the stress-relieved alloy is attributed to the increasing contribution of faster diffusion path such as grain boundaries and dislocations. The presence of dislocation arrays with higher dislocation density and smaller grain size in the stress-relived alloy was confirmed by TEM analysis. The creep rupture time increased dramatically in the annealed Zr–1Nb- 0.7Sn-0.1Fe alloy compared to that of stress-relieved alloy, supporting the decrease of creep rate by annealing. The creep life of Zr-1.02Nb-0.69Sn-0.12Fe claddings can be extended through microstructure modification by annealing at intermediate temperatures in which dislocation creep dominates.

Effect of Tantalum Content on Creep Properties of CLAM Steel

This paper presents the results of an experimental study about the effect of tantalum content on the creep properties of China low activation martensitic steel. Four heats of the steel with tantalum content ranging from 0.027 to 0.18 wt% have been fabricated and the creep tests have been carried out at 550 °C over a stress range of 230–240 MPa. The results indicated that the elevation of tantalum content caused an obvious increase in the rupture time of one order of magnitude and a dramatically decrease in creep rate for one order of magnitude. Both the creep rupture time and minimum creep rate revealed good linear relationships versus tantalum content on log–log scale, and empirical models were built up. The improvement of creep properties could be attribute to the change of microstructure caused by the increase of tantalum content. The microstructure observation indicated that the average grain size of the prior austenite decreased and the precipitation of Ta-riched MX was promoted with the addition of tantalum.

Effect of the surface charge density on the creep of copper

The creep of polycrystalline copper under the action of high and low electric potentials is studied. At potentials of ±4 kV and ±5 V, the steady-state creep rate decreases, and the effect in the former case is weaker than in the latter by a factor of 2.5. This difference is caused by the fact that the charge density in the sample-capacitor bank system at the high electric potentials is lower than at the low potentials.