Rate-controlling Creep Mechanism Research Articles

The synergistic effects of nano-sized α-Al(Mn,Cr,Fe)Si and Al–Si–Zr dispersoids on the creep behavior Al–Si–Mg–Cu (Mn, Cr, Zr) diesel engine alloy

The influence of Mn, Cr and Zr on the creep behavior of an Al–Si–Mg–Cu alloy intended for Diesel engine applications was studied. Two alloys, base alloy Al–7Si-0.3Mg-0.5Cu (MG1) and Al–7Si-0.3Mg-0.5Cu-0.15Mn-0.25Cr-0.15Zr (MG2-1) were creep tested at 300–350 °C and 28 MPa in the T7 heat-treated and soaked (100 h at 300 °C) condition. The apparent activation energy for creep for MG1 was 184 kJ mol−1 and for the MG2-1 was 234 kJ mol−1. The two alloys exhibited differing rate controlling creep mechanisms. The minimum creep rates of the MG2-1 alloy were 3–5 times lower than the MG1 alloy at 300 °C and 325 °C, respectively and only 1.3 times lower at 350 °C. The TEM investigation indicated creep deformation in the MG2-1 alloy was due to thermally activated restoration processes where grain boundary migration experienced Zener pinning by coherent and ∼5 nm αMG2-1-Al(Mn,Cr,Fe)Si. In the MG1 alloy, the creep mechanism was identified as climb over nano-sized (∼50 nm) non-coherent α-Al(Mn,Fe) dispersoids. EDS analysis showed that semi coherent Q’ phase (near equilibrium version of Q phase) existed in the MG1 alloy that had a nucleating role for nano-sized non-coherent α-Al(Mn,Fe) dispersoids.

Creep behaviour of alloy 690 in the temperature range 800–1000 °C

The creep behaviour of Ni-base superalloy 690 has been studied in the temperature range of 800–1000 °C, and stress range of 25–105 MPa under constant load creep conditions. The alloy follows a power-law creep behaviour in the tested conditions and obeys a Monkman-Grant relationship. All the creep data points lay within a ±5% scatter band of the Larson-Miller parameter vs. log stress plot. The alloy spent more than 50% of its life in the tertiary regime and failed predominantly by cavitation. Creep parameters, like apparent activation energy and stress exponent, were relatively high (∼363–498 kJ/mol and 5.5–7, respectively). A high value of the apparent activation energy has been attributed to the resistance of fine carbide particles that form during creep. The microstructural sub-structure of creep deformed samples shows subgrains separated by low angle grain boundaries and randomly oriented dislocations suggesting dislocation climb as the rate-controlling creep mechanism.

Characterization of stress-rupture behavior of nuclear-grade C26M2 FeCrAl alloy for accident-tolerant fuel cladding via burst testing

FeCrAl alloys demonstrate excellent high-temperature oxidation resistance compared to zirconium alloys used in LWRs. FeCrAl C26M2 has been down-selected as one of the candidate ATF cladding materials for Gen II LWRs. This study investigates the stress-rupture behavior of the nuclear-grade FeCrAl C26M2 tubing in the temperature range of 753-923 K. Rupture time and uniform strain at rupture were determined as a function of the temperature and applied stress using burst tests. The rupture data was found to obey the Larson-Miller Parameter and the Monkman-Grant relationship. Furthermore, the stress exponent (n) and the activation energy (QC) at lower stresses were determined as 4.4 ± 0.3 and 289 ± 25 kJ/mol, respectively, confirming power-law behavior. Above the normalized stress (σ/E) of 2 × 10−3, the power-law creep transited into the power-law breakdown regime. The steady-state deformation microstructure was examined using transmission electron microscopy (TEM) to detect the rate-controlling creep mechanism(s). Observation of sub-grains with dislocations at the boundaries in the power-law regime along with a stress exponent of 4.4 suggests dislocation climb as the rate-controlling mechanism. Two different fracture modes were observed: the tubing failed either by direct open-up or small crack and pinhole formation. The outcome of this study would provide essential data for the development of informative mesoscale deformation models paving way for discovery of new creep resistant alloys.

Influence of phase transformation on the creep deformation mechanism of SA508 Gr.3 steel for nuclear reactor pressure vessels

This study analyzed the effect of phase transformation on the creep behavior and deformation mechanism of SA508 Gr.3 steel used for nuclear reactor pressure vessels (RPVs). The creep tests were conducted on the steel at the temperatures from 650 to 800 °C at a stress ranging from 15 to 80 MPa. Detailed microstructural examination and theoretical analysis were conducted on the specimens to investigate the creep mechanism in each testing condition. The results showed that phase transformation induced an increase in the minimum creep rate and a decline in creep life. Based on the calculations of stress exponents, activation energies, Larson-Miller and Orr-Sherby-Dorn parameters, the creep deformation and fracture behavior were deemed to be different before and after phase transformation. Before the phase transformation, the predominant creep-deformation mechanism was dislocation gliding whereas it was converted to dislocation climbing after the phase transformation. During the phase transformation, the dislocation gliding coupled with grain-boundary sliding became the rate-controlling creep mechanism. This study provides a comprehensive insight into understanding the creep deformation of the RPV steel, giving a direct guideline to optimize the parameters for in-vessel retention strategies.

Investigation of short-term creep deformation mechanisms in MarBN steel at elevated temperatures

This paper reports the short-term creep behaviour at elevated temperatures of a MarBN steel variant. Creep tests were performed at three different temperatures (625°C, 650°C and 675°C) with applied stresses ranging from 160 MPa to 300 MPa, and failure times from 1 to 350 h. Analysis of the macroscopic creep data indicates that the steady-state creep exhibits a power-law stress dependence with an exponent of 7 and an activation energy of 307 kJ mol−1, suggesting that dislocation climb is the dominant rate-controlling creep mechanism for MarBN steel. Macroscopic plastic instability has also been observed, highlighted by an obvious necking at the rupture region. All the macroscopic predictions have been combined with microstructural data, inferred from an examination of creep ruptured samples, to build up relations between macroscopic features (necking, damage, etc.), and underlying microstructural mechanisms. Analysis of the rupture surfaces has revealed a ductile fracture mode. Electron Backscatter Diffraction (EBSD) analysis near to the rupture surface has indicated significant distortion and refinement of the original martensitic substructure, which is evidence of long-range plastic flow. Dislocation pile-ups and tangles from TEM were also observed near substructure boundaries and precipitate particles. All of these microstructural observations suggest that creep is influenced by a complex interaction between several elements of the microstructure, such as dislocations, precipitates and structure boundaries. The calculated stress exponent and activation energy have been found to agree quantitatively with the highlighted microstructural features, bearing some relationships to the true observed creep microstructures.

Creep behavior of high-pressure die-cast Mg-4Al-4La-0.4Mn alloy under medium stresses and at intermediate temperatures

Tensile creep behavior of the high-pressure die-cast Mg-4Al-4La-0.4Mn alloy under stresses of 50–100MPa and at 150–225°C was thoroughly investigated. This alloy exhibits a stress exponent of approximately 6 and the activation energy Q falls within the range of 80–120kJ/mol at 150–200°C. From fitting results, the commonly adopted threshold stress approach is not suitable for ALa44 alloy, and the normalized stress exponent is approximately 7, suggesting that creep deformation of ALa44 alloy was controlled by low-temperature dislocation climb (core diffusion). Furthermore, the microstructural analysis results indicate that the rate-controlling creep mechanism of ALa44 alloy is dislocation climb, but the cell boundary disintegration and the dislocation cross-slip also assisted deformation during creep processes.

Creep Behaviors of Stress-Relieved and Annealed Zr-1Nb-0.7Sn-0.1Fe Nuclear Cladding Tubes at Intermediate Temperatures

In order to explore the possibility of enhanced creep resistance by grain size control, creep tests of stress-relieved and annealed Zr-1Nb-0.7Sn-0.1Fe cladding tubes were carried out at intermediate temperatures. The creep rate decreased significantly with increase of grain size from ~1.2 μm to 4 μm with annealing. The stress exponent was observed to be in the range of 5.0~6.5 for both stress-relieved and annealed Zr-1Nb-0.7Sn- 0.1Fe, suggesting that the dislocation climb is the rate controlling creep mechanism. It was shown that not only the creep rate decreased but also creep life increased in the annealed Zr-1Nb-0.7Sn-0.1Fe, suggesting the grain size control may provide an effective way to enhance the creep resistance in Zr alloys for nuclear application.

High temperature bending creep behavior of a multi-cation doped α/β-SiAlON composite

SiAlON ceramics with high hardness and high toughness can be made through designing α/β-SiAlON composites. An important advantage of α-SiAlON phase is that the amount of intergranular phase is reduced by the transient liquid phase being absorbed into the matrix of α-SiAlON phase during sintering. But, the thermal stability of the α-SiAlON phase is an important concern for α/β-SiAlON composites especially at high temperatures. The use of different types of single or multiple cations during fabrication directly affects resultant microstructures and mechanical behavior of α/β-SiAlON composites. In this study, the creep behavior of a multi-cation (Y, Sm and Ca) doped α/β-SiAlON composite, in which aluminum-containing nitrogen melilite solid solution phase was designed as intergranular phase, was investigated by four-point bending creep tests under stresses from 50 to 150MPa and at temperatures from 1300°C to 1400°C in air. The stress exponent was determined to be 1.6±0.13 at 1400°C and the creep activation energy was calculated to be 692±37kJ/mol−1. Grain boundary sliding coupled with diffusion was identified as the rate-controlling creep mechanism for the α/β-SiAlON composite.

Creep strength of a binary Al62Ti38 alloy

Abstract Al-rich Ti – Al alloys, as compared to Ti-rich -TiAl-based alloys, offer an additional reduction in density of 20 %, better oxidation resistance and sufficient strength at high temperatures. High temperature creep of a binary Al62Ti28 alloy was studied in compression in the temperature range between 1 173 K and 1 323 K in air. It is shown that the alloy exhibits quite reasonable creep resistance at 1 173 K, especially in view of its low density of around 3.8 g cm– 3. Stress exponents calculated as the slope n = log (strain rate)/ log (stress) = 4 were found to be relatively constant for the temperature and stress regime investigated. This indicates that dislocation climb may be the rate controlling creep mechanism. The values of the activation energies for creep for the as-cast and the annealed Al62Ti38 material coincides well with those found in the literature for interdiffusion of Al in -TiAl.

Creep strength of centrifugally cast Al-rich TiAl alloys

High-temperature creep of a binary Al 60Ti 40 (at.%) alloy in the as-cast state and after annealing at 1223 K for 200 h which produced nearly lamellar γ-TiAl + r-Al 2Ti microstructure was studied utilizing creep compression tests in a temperature range between 1173 and 1323 K in air. The material was manufactured by centrifugal casting. Microstructural characterization was carried out employing light-optical scanning (SEM) and transmission electron microscopy (TEM) as well as X-ray diffraction (XRD) analyses. It is shown that the alloy exhibits reasonable creep resistance at 1173 K, especially in relation to its low density of around 3.8 g/cm 3. Stress exponents calculated as n = Δlog (strain rate)/Δlog (stress) = 4 were found to be relatively constant for the temperature and stress regime investigated. This indicates that dislocation climb may be the rate controlling creep mechanism. The assessment of creep tests conducted at identical stress levels and varying temperatures yielded activation energies for creep of around Q = 457 kJ/mol in the as-cast condition. This value is significantly higher than those found in literature for interdiffusion of Al or Ti in γ-TiAl. It is concluded that the difference is a due to the instability of the microstructure of the as-cast multi-phase alloy.

PHENOMENOLOGICAL MODEL FOR CREEP BEHAVIOR INCu–8.5 at.%AlALLOY

Creep experiments were conducted on Cu –8.5 at.% Al alloy in the intermediate temperature range, 673–873 K, corresponding to 0.46–0.72 Tm, where Tmis the absolute melting temperature. The present analysis reveals the presence of two distinct deformation regions (climb and viscous glide) in the plot of log [Formula: see text] versus log σ. The implications of these results on the transition from power-law to exponential creep regime are examined. The results indicated that the rate-controlling mechanism for creep is the obstacle-controlled dislocation glide. A phenomenological model is proposed, which assumes that cell boundaries with sub-grains act as sources and obstacles to gliding dislocations.

Creep mechanism and microstructure evolution in silicon nitride ceramics

Tensile creep processes in a commercial grade of silicon nitride, SN 88, are reviewed with the focus on cavitation and microstructure changes. Experimental measurements showed that cavitation at multigrain junctions is the rate controlling creep mechanism. The cavitation creep model of Luecke and Wiederhorn was extended to explain non-power law behaviour and strong creep asymmetry. Secondary phase composition changes after prolonged creep tests resulted in the formation of Yb-disilicate phase at the expense of Yb-oxynitride due to oxidation. Such secondary phase changes affect residual glass composition, which has crucial effect on the processes involved in cavitation and resulting creep behaviour.

The effect of sheet processing on the microstructure, tensile, and creep behavior of INCONEL alloy 718

The grain size, grain boundary character distribution (GBCD), creep, and tensile behavior of INCONEL alloy 718 (IN 718) were characterized to identify processing-microstructure-property relationships. The alloy was sequentially cold rolled (CR) to 0, 10, 20, 30, 40, 60, and 80 pct followed by annealing at temperatures between 954 °C and 1050 °C and the traditional aging schedule used for this alloy. In addition, this alloy can be superplastically formed (IN 718SPF) to a significantly finer grain size and the corresponding microstructure and mechanical behavior were evaluated. The creep behavior was evaluated in the applied stress (σa) range of 300 to 758 MPa and the temperature range of 638 °C to 670 °C. Constant-load tensile creep experiments were used to measure the values of the steady-state creep rate and the consecutive load reduction method was used to determine the values of backstress (σ0). The values for the effective stress exponent and activation energy suggested that the transition between the rate-controlling creep mechanisms was dependent on effective stresses (σe=σa σ0) and the transition occurred at σe ≅ 135 MPa. The 10 to 40 pct CR samples exhibited the greatest 650 °C strength, while IN 718SPF exhibited the greatest room-temperature (RT) tensile strength (>1550 MPa) and ductility (ef >16 pct). After the 954 °C annealing treatment, the 20 pct CR and 30 pct CR microstructures exhibited the most attractive combination of elevated-temperature tensile and creep strength, while the most severely cold-rolled materials exhibited the poorest elevated-temperature properties. After the 1050 °C annealing treatment, the IN 718SPF material exhibited the greatest backstress and best creep resistance. Electron backscattered diffraction was performed to identify the GBCD as a function of CR and annealing. The data indicated that annealing above 1010 °C increased the grain size and resulted in a greater fraction of twin boundaries, which in turn increased the fraction of coincident site lattice boundaries. This result is discussed in light of the potential to grain boundary engineer this alloy.

An Analysis of Flow Mechanisms in High Temperature Creep and Superplasticity

The flow of crystalline solids at elevated temperatures is dependent upon creep mechanisms associated with the movement of dislocations, the relative displacements of adjacent grains and the stress-directed flow of vacancies. It is convenient to express the steady-state creep rate in terms of the dependences of these various creep mechanisms on the applied stress, the testing temperature and the grain size of the material. In practice, however, there are similarities in some of the predicted dependences for different creep processes and this may lead to experimental difficulties in unambiguously identifying the rate-controlling creep mechanism. The difficulties in identifying the creep mechanism become especially significant at low stresses. This paper reviews the characteristics of the various flow processes occurring in simple metallic systems such as pure metals and metallic alloys and describes procedures that may be adopted to provide an unambiguous identification of the rate-controlling flow mechanism.

Indentation creep studies of iron aluminide intermetallic alloy

Indentation creep behaviour of iron aluminide intermetallic (Fe–28Al–3Cr) was studied in the temperature range of 843–963 K. The hardness and the plasticity parameter were also calculated from room temperature to 1273 K. The variation of hardness with time showed that the stress exponent ( n) was weakly dependent on temperature. Thermal activation parameter-activation area and energy were also calculated to identify the creep mechanism. The activation energy for high temperature creep was found to be in good agreement with that for self-diffusion of pure iron and in excellent agreement with previous studies. These values of n, activation area and activation energy were found to be consistent with dislocation climb as the rate controlling creep mechanism. Conventional creep curves were also derived from hardness values.

Effect of microstructural parameters and Al alloying on creep behavior, threshold stress and activation volumes of molybdenum disilicides

Creep behavior of several molybdenum disilicides, with varying grain sizes, silica content, SiC reinforcement, or Al additions has been studied at 1200 °C, and compared with the deformation behavior at constant strain rates. The effect of grain size on creep rate supercedes all other factors. A high silica content is less deleterious at coarser grain sizes in the dislocation creep range. In addition, 20 vol% SiC reinforcement of MoSi 2 can enhance creep strength, if grain size remains coarse. Furthermore, Al addition has been found to be beneficial up to a critical concentration, which helps in the removal of silica at grain boundaries, while retaining the tetragonal structure of MoSi 2. A threshold stress for creep can arise due to the build up of internal stresses at second phase particles or dislocation networks. These stresses inhibit further movement of dislocations. The activation volume was found to vary between 27 b 3 and 2 b 3 with applied stress, where b is the Burgers vector. This implies that the rate controlling creep mechanism can be overcoming glide type of barrier (say, Peierls stress) at low stresses, and dislocation climb at higher stresses.

Physically-based model for creep in nickel-base superalloy C263 both above and below the gamma solvus

The polycrystalline nickel-base superalloy (C263) is used for stationary components in aero-engines such as combustion chambers, casings, liners, exhaust ducting and bearing housings. It is a fine-precipitate strengthened alloy at 800°C, with a precipitate solvus temperature of 925°C. Below the solvus, the precipitate coarsens at elevated temperature. A critical precipitate size exists below which particle cutting is the rate-controlling creep mechanism. Above the critical size, it is dislocation pinning and climb. Once the solvus temperature has been exceeded, however, the rate-controlling mechanism becomes pinning and climb within a dislocation network. This paper presents physically-based constitutive equations for creep deformation in C263. In the presence of the γ′ precipitate, populations of climbing and gliding dislocations are assumed to exist in a similar way to that proposed by Dyson and Osgerby [3]. Climbing dislocations are assumed to be pinned at precipitates. The dependence of steady state glide dislocation flux can then be obtained as a function of precipitate volume fraction. Above the γ′ solvus, the pinning process is different and results from the establishment of a dislocation network, the size of which determines the pinning distance. The physical constants arising in the equations have been determined by the conventional minimisation of errors between experimental and calculated creep curves. They have also been determined quite independently using fundamental data and experiments. Remarkable agreement between the two sets of physical constants is achieved. The constitutive equations have been shown to capture the material’s creep deformation characteristics over a broad range of temperature, both below and above the γ′ solvus. In addition, the effect of precipitate coarsening on creep rate is correctly captured, together with the effect of prior ageing on subsequent creep rate. The damage processes of cavitation and multiplication of mobile dislocation density have been coupled with the constitutive equations and used to predict failure in constant and variable stress creep.

Steady-state creep of α-zirconium at temperatures up to 850 °C

Cumulative zirconium creep data over a broad range of stresses (0.1 to 115 MPa) and temperatures (300 °C to 850 °C) were analyzed based on an extensive literature review. Zirconium obeys traditional power-law creep with a stress exponent of approximately 6.4 over stain rates and temperatures usually associated with the conventional “five-power-law” regime. Thus, dislocation climb, rather than the often assumed glide mechanism, may be rate controlling. Power-law breakdown occurs at values of \(\dot \varepsilon _{ss} /D\) greater than approximately 109 cm−2, consistent with most traditional five-power-law materials. The creep rate of zirconium at low values of σ/G varies proportionally to the applied stress. The rate-controlling mechanism(s) for creep within this regime is unclear. A grain-size dependency may exist, particularly at small (<90 µm) sizes, suggesting a diffusional mechanism. A grain-size independence at larger grain sizes supports a Harper-Dorn mechanism, but the low observed activation energy (∼90 kJ/mol) is not consistent with those observed at similar temperatures at higher stresses in the five-power-law regime (270 kJ/mol) where creep is also believed to be lattice self-diffusion controlled. The stress dependence in this regime is not consistent with traditional grain-boundary sliding mechanisms.

Microstructure and creep behavior of an Y-α-β sialon composite

Abstract Flexural creep behavior of a Y-α-β sialon composite was studied in air at temperatures of 1250–1350°C and stresses of 110–290 MPa. The composite in which YAG was designed as intergranular phase has an original α/β ratio of 65/35. The stress exponents in Norton equation were determined to be 1.31±0.12, 1.49±0.15 and 1.62±0.10 at 1250, 1300 and 1350°C, respectively, and creep activation energy was 677±25 kJ mol −1 . Combining the microstructure observations, grain boundary diffusion accommodated by grain boundary sliding was identified as the rate-controlling creep mechanism for the composite. The creep rate exponent p in Monkman–Grant relation was found to be 1.6, and the nucleation and growth of cavities in triple grain pockets was responsible for the creep rupture.

Hot hardness and indentation creep studies of a Fe–28Al–3Cr–0.2C alloy

The hot hardness behaviour of a Fe–28Al–3Cr–0.2C (at.%) alloy was evaluated from room temperature to 1273 K. Indentation creep measurements were also carried out in the temperature range of 843–963 K. The hardness–temperature plot of this alloy showed five distinct regions. The mechanism of deformation operating in each of these regions has been suggested. Indentation creep measurements showed that the stress exponent ( n) obtained from the hardness–time plots was weakly temperature dependent. The activation energy for high temperature creep was found to be in good agreement with that for self-diffusion of pure iron and in excellent agreement with previous studies. These values of n and activation energy were found to be consistent with dislocation climb as the rate controlling creep mechanism.