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Damage and fracture processes in high temperature creep of an investment cast B1914 Ni-based superalloy with the increased amount of boron to 0.08wt.% for high temperature applications were analysed. Constant load creep tests in tension were conducted at temperatures from 800 to under applied stress ranging from 150 to 700 MPa. The microstructure of fractured specimens was investigated by scanning electron microscope Tescan equipped with an electron-back scatter diffraction. Microstructure investigation showed that the microstructure of the B1941 superalloy consists of a gamma (γ) phase with a dendritic structure and gamma prime (γ ́) phase with a cuboidal shape. Precipitates of γ ́and a lamellar eutectic, composed of γ/(Mo,Cr,Ni)3B2, were identified in the interdendritic region. Creep damage and fracture are closely connected with decohesion of the interface between M3B2 boride and matrix.

This work deals with the influence of the processing technology of ultrafine-grained titanium by the application of severe plastic deformation (SPD) on its degradation processes in creep at elevated temperatures. Commercial grade titanium heats with different content of carbon were processed by multi-stage rolling. Creep tests under constant tensile stress were performed in a protective atmosphere of argon at a temperature of 673 K with application of different stress levels. The microstructure of the fractures creep specimens were analysed by metallographic and fractographic analyses using scanning and transmission electron microscopy (SEM, TEM). Under the same creep loading conditions the heat with higher content of carbon exhibited better creep resistance due to stronger effect of precipitation hardening by titanium carbides.

Creep is defined as a time dependent component of plastic deformation. Creep tests can be performed either at constant load or at constant applied stress. Engineering creep tests carried out at constant load are aimed at determination of the creep strength or creep fracture strength, i.e. the data needed for design. The constant stress tests are important as a data source for fundamental investigations of creep deformation and fracture mechanisms and for finite element modelling of more complex stress situations. For some materials, the difference between the two type of testing can be very small, while for other materials is large, depending on the creep plasticity of the material under testing. The paper aims to compare the creep results of two different creep-resistant materials: the advanced 9%Cr martensitic steel (ASME Grade P91) and a Zr1%Nb alloy obtained by both testing methods and to clarify the decisive factors causing observed differences in their creep behaviour.

Nickel-based superalloys have already found many different applications in industrial facilities, especially those working at high temperatures, such as turbine blades in industrial and aircraft turbines, but also in the glass industry to construct glass shaping tools. During service, such tools are exposed to high temperature, high pressures and high-temperature corrosion in a severe corrosive medium like molten glass and thereby accentuate more exacting demands on high creep and corrosion resistance of nickel-based superalloys used in the glass industry. At present, the published data on microstructural changes and degradation processes in nickel-based superalloys used in the glass industry are sparse and often controversial. Therefore, the present study was initiated to perform a detailed evaluation of high temperature microstructural processes and damage in a carbide strengthened nickel-based superalloy which has been developed for an investment casting of spinner discs used in the glass industry for producing glass fibers. In this study, the relationship between the type, distribution and morphology of carbides, and the deformation and degradation processes in an alloy under investigation subjected to high temperature exposition was investigated. The paper aims to identify decisive factors governing high-temperature performance of the nickel-based superalloy under investigation.

The cast nickel-base MAR-M247 superalloy has been widely used for high-temperature components. In this work, the creep behaviour of two alternates of MAR-M247 superalloy with different grain size processed at different temperatures of casting are compared. Under the creep testing conditions used in this study, only negligible differences of creep behaviour of the alternate alloys were found and the evaluated creep characteristics correspond to the power-law or dislocation creep. The microstructure of the alloys consists of a 𝛾 matrix with a eutectic, 𝛾' strengthening cubic precipitates, and M6C and M23C6 carbides. Increasing the temperature induces the dissolution of some M23C6 carbides. Fractures of both variants of alloys exhibit a more ductile character at higher temperatures, while at lower temperature a mixture of brittle and ductile fracture modes was observed, which changes the creep fracture ductility.

Cast nickel-based superalloys INC713 LC, B1914 and MAR-M247 are widely used for high temperature components in the aerospace, automotive and power industries due to their good castability, high level of strength properties at high temperature and hot corrosion resistance. The present study is focused on the mutual comparison of the creep properties of the above-mentioned superalloys, their creep and fracture behaviour and the identification of creep deformation mechanism(s). Standard constant load uniaxial creep tests were carried out up to the rupture at applied stress ranging from 150 to 700 MPa and temperatures of 800&ndash, 1000 &deg, C. The experimentally determined values of the stress exponent of the minimum creep rate, n, were rationalized by considering the existence of the threshold stress, &sigma, 0. The corrected values of the stress exponent correspond to the power-law creep regime and suggest dislocation climb and glide as dominating creep deformation mechanisms. Fractographic observations clearly indicate that the creep fracture is a brittle mostly mixed transgranular and intergranular mode, resulting in relatively low values of fracture strain. Determined main creep parameters show that the superalloy MAR-M247 exhibits the best creep properties, followed by B1914 and then the superalloy INC713 LC. However, that each of the investigated superalloys can be successfully used for high temperature components fulfils the required service loading conditions.

Martensitic creep-resistant P92 steel was deformed by different methods of severe plastic deformation such as rotation swaging, high-pressure sliding, and high-pressure torsion at room temperature. These methods imposed significantly different equivalent plastic strains of about 1&ndash, 30. It was found that rotation swaging led to formation of heterogeneous microstructures with elongated grains where low-angle grain boundaries predominated. Other methods led to formation of ultrafine-grained (UFG) microstructures with high frequency of high-angle grain boundaries. Constant load tensile creep tests at 873 K and initial stresses in the range of 50 to 300 MPa revealed that the specimens processed by rotation swaging exhibited one order of magnitude lower minimum creep rate compared to standard P92 steel. By contrast, UFG P92 steel is significantly softer than standard P92 steel, but differences in their strengths decrease with increasing stress. Microstructural results suggest that creep behavior of P92 steel processed by severe plastic deformation is influenced by the frequency of high-angle grain boundaries and dynamic grain coarsening during creep.

The creep resistance of two discontinuously reinforced with 20 vol.% alumina short-fibres magnesium composites were examined by performing a comparison between the creep properties of these composites and their magnesium matrix alloys AZ 91 and QE 22. It was found that the magnesium AZ 91 composite exhibits an improved creep resistance arising mainly from an efficient load transfer effect and the existence of a threshold stress, respectively. By contrast, the beneficial effect of reinforcement and the improved creep resistance of the QE 22 composite is significantly influenced by the creep loading conditions.

Degradation processes in high-temperature creep of two cast high-chromium cobalt-based superalloys strengthened by niobium and tantalum for use in the glass industry were analysed via various experimental techniques. Constant load creep tests were conducted at 900, 950 and 1000 °C in a tensile stress range from 40 to 80 MPa. It was found that the CoNb superalloy possesses considerably longer creep life compared to the CoTa superalloy under the same loading conditions. Conversely, the creep ductility of the fractured specimens shows the opposite order of the creep life. The creep behaviour of both superalloys obeys the Monkman-Grant formula indicating that the creep deformation mechanism and fracture processes are mutually interlinked. The homogeneously distributed creep damage of the CoNb superalloy is closely connected with primary carbides and is predominantly initiated either as interface decohesion between the carbide/matrix and carbide eutectics/matrix or by breakage of bulk M23C6 carbides. The dominant type of creep damage in the CoTa superalloy is localized breakage of M23C6 carbides in close proximity of the fracture path. The final brittle fracture in the CoNb superalloy occurs via relatively fast propagation of the longest cracks after the ultimate state of damage is reached. Due to premature fracture, the inherent creep ductility of the CoNb matrix is not exhausted. The final ductile transgranular creep fracture of the CoTa superalloy is caused by a local strain-induced instability of the dislocation microstructure leading to a loss of an external section of specimen (necking).

The components used in power plants generally operate at elevated and/or high temperature and are subjected to internal pressure. Under such conditions creep is of a great concern and there is an urgent demand for methods which can be used to predict the creep life. In this work, using our earlier published creep data for advanced creep-resistant T23 and P92 steels, the interrelationship between creep deformation and damage have been analysed by linking them to the identified acting mechanisms, in terms of empirical formulas for the fracture time assessment. The validity and the applicability of various formulas are examined with the objective to gain insight into the creep deformation and fracture behaviour of the steels under investigation.

Two cast NbC and TaC- strengthened cobalt-base superalloys have been developed for a precision casting of spinner discs for glass wool industry. In the present study, the relationships between the type and morphology of carbides and the degradation processes in both types of cast cobalt-based superalloys subjected to high temperature creep have been examined. It was found that the nature of carbides within the alloy microstructure plays a critical role in determining the creep damage processes and microstructure stability of the alloy system under high temperature creep. The morphology of the carbides is a strong function of their chemical composition. The interface decohesion between the complex carbides and the matrix and cracking of the brittle carbides homogeneously distributed in the crept NbC - strengthened alloy lead to brittle intergranular and/or interdendritic fracture. By contrast, Ta - strengthened alloy exhibited very small extent of isolated creep damage and the final fracture is ductile transgranular mode.

The influence of the application of severe plastic deformation on microstructure changes and creep deformation behaviour was investigated in P92 creep-resistant pipe steel. P92 steel in its as-received state, with a microstructure typical of tempered martensite, was subjected to high-pressure torsion at room temperature. An investigation of the microstructure using an electron backscatter diffraction technique revealed that severe plastic deformation led to the subsequent formation of an ultrafine-grained microstructure, containing {110} fibre texture and randomly misoriented boundaries. The microstructure characteristics and hardness measurements exhibited saturated values at an equivalent strain of higher than about 20. The creep behaviour of the ultrafine-grained specimen produced in the saturated region exhibited a minimum creep rate that was about two orders of magnitude faster, and a larger creep ductility, in comparison with the as-received state. Creep exposure at 873 K with a time to fracture of about 3 h led to a slight grain coarsening and to the formation of Laves phase.

We have investigated the effect of prior short-term ageing on the mechanical and creep properties of tungsten and boron modified 9%Cr ferritic steel (ASTM Grade P92 steel), to provide material data base for the estimation of material degradation due to occurrence of accidental overheating in the early stage of service loading. The steel was aged at 650 °C for up to 100, 200, 500, 1000, 2000 and 5000 h prior to mechanical and creep testing. Changes in the microstructure during ageing had a slight effect on the reduction of room temperature tensile properties. A sudden decrease on impact properties occurred after ageing for 2000 h, but the occurred impact energy of about 70 J is in accordance with the specified minimum value of 68 J at room temperature for the virgin condition. The creep behaviour of P92 steel in the as-received condition and after different isothermal ageing exposures prior to creep was investigated at 600 and 650 °C in the power-law creep regime. The initial applied tensile stress ranged from 90 to 200 MPa. The results clearly show a detrimental influence of isothermal ageing on creep properties even after a few hundred hours. Based on microstructure analysis, which was done mostly in a qualitative way, a significant decrease in the creep life after ageing resulted from changes of the dislocation substructure and instability of Laves phase and M23C6 particles.

The objective of the present study was to provide new relevant information on creep behaviour and fracture processes in a sponge-based modified Zr1%Nb cladding alloy (modified E110 alloy) in the α-Zr and (α+β)-Zr phase regions. To this end, constant load creep tests were carried out in argon at testing temperature intervals from 350°C to 950°C, and applied tensile stresses ranging from 5 MPa to 210 MPa, corresponding to the power-law breakdown creep regime and/or high testing temperatures, and thus to simulate disaster conditions. Creep tests were followed by metallographic and fractographic analyses of the specimens to explain the observed creep behaviour. It was found that in the power-law region (at 350°C) the values of the stress exponent n of the minimum creep rate e ˙ m (n = ∂ln e ˙ m /∂lnσ)T, and the stress exponent m of the time to fracture tf (m = - ∂lntf /∂lnσ)T, were very high and near to each other, indicating a close relationship between creep deformation and fracture. Further support for the idea that creep deformation and fracture are interconnected can be observed by the validity of the empirical Monkman-Grant relationship. Creep tests in the α-Zr phase region revealed creep cavitation near to, and at, the fracture surface. The final fracture is a ductile dimple mode with the synergistic effect of creep cavitation. By contrast, the final fracture in the (α+β)-Zr phase region is caused by a local strain-induced instability of the matrix leading to a loss of an external section of specimen (necking). There is a marked difference between the values of the strain to fracture ef. In the α-Zr region, typical values of ef ~ 0.3-0.5 were found, whereas for the (α+β)-Zr region, the value ef increases with testing temperature up to 850°C. Following drop of ef at temperatures ≥ 900°C was explain by intensive oxidation of the alloy.

Two commercially pure (CP) Ti heats containing the same total impurity content have been used under investigation. Both materials were subjected to the processing procedure method of severe plastic deformation (SPD). This procedure consists of two steps of hot rolling (at 673 K) and the final rolling step at ambient temperature. The difference in individual chemical contents of impurities resulted in two states, both with ultrafine-grained (UFG) microstructures. For comparison reason, coarse-grained (CG) state was prepared by annealing of UFG state at temperature 823 K/1h. Microstructure investigations were performed using scanning electron microscope equipped with EBSD unit (SEM/EBSD) and transmission electron microscopy (TEM). Constant load tensile creep tests were conducted at temperature 673 K and applied uniaxial stress of 200MPa. Creep tests were run up to the final fracture of the creep specimens. The main objective of this work was to evaluate how the synergistic effect of SPD and the resulting microstructure affects the creep behaviour and properties of the studied materials.

This work investigates creep and microstructure characteristics in P92 martensitic and 304L austenitic stainless steel after the imposing of large value of plastic strain. The as-received coarse-grained steels were strongly deformed at room temperature (RT) before tensile creep testing. The flat specimens were tested at 873 K and 923 K under different applied stresses. Microstructures were analysed by means of scanning electron microscope with EBSD (electron back-scatter diffraction) camera and transmission electron microscope. It was observed that application of severe plastic deformation (SPD) techniques at RT significantly reduces grain size and changes phase structure of 304L austenitic steel. The microstructure investigations revealed that the growth of new phases is enhanced in plastically deformed P92 martensitic and 304L austenitic stainless steels compared to their undeformed states. Creep results showed that the mean grain size of SPD-processed steels is still near ultrafine-grained region even after long-term creep testing and the coarsening of microstructure is predominantly influenced by creep time and strain.

The effect of an application of an external high hydrostatic pressure on creep behaviour, the microstructure, and nanoscale porosity in an ultrafine-grained cladding Zr-2.5 wt%Nb alloy processed by equal channel angular pressing (ECAP) was investigated by comparative creep testing. The results show that, under identical creep loading conditions, ECAP processed specimens subjected to subsequent high hydrostatic pressurization treatment prior to creep exposure exhibited no substantial differences in creep behaviour compared to specimens processed by ECAP only. The selected level of high hydrostatic pressurization treatment used in this study is not indicative of an effective removal of nanoscale porosity resulting from ECAP processing. By comparison to the as-received state, an increasing number of ECAP passes leads to a significant weakening of creep resistence of the alloy. It is suggested that the reasons for the occurrence of creep weakening after ECAP processing are an accumulation of high-angle grain boundaries, multiplication of dislocations, coarsening and/or dissolution of the Nd-rich second phase precipitates, and dislocation/particle interactions.

The microstructure and creep behaviour of the welded joints of P92 steel pipe were investigated in order to determine the influence of orbital heat welding technology on the creep resistance. Creep specimens were machined from the welded joints. Tensile creep tests of welded joints were performed at 873 K using different stresses. The microstructure of tested specimens was investigated by scanning electron microscope Tescan equipped with an electron-back scatter diffraction. The creep results showed that the creep fracture strain of the welded joints decreases with decreasing value of applied stress. Microstructure investigation showed that fracture behaviour of welded joints is influenced by an enhanced cavity formation at grain boundaries in the heat-affected zone causing lower fracture ductility.

The creep damage tolerance factor λ as an important outcome of the continuum damage mechanics approach has been used to asses the creep fracture mode and the susceptibility of material to localized cracking at strain concentrations. In this work, using sets of our earlier published creep data of three advanced ferritic creep-resistant steels (T23 low alloy steel, P91 and P92 chromium steels) are analysed in terms of the creep damage tolerance factor λ. It was found that the value of the creep damage factor λ is not constant and depends on the creep loading conditions. The data analysis is followed by fractographic investigations, which is used to identify the creep fracture mode (s) experimentally.

Tensile creep tests on austenitic stainless HR3C steel pipe in an as-received state and after heat affected zone (HAZ) thermal cycle simulation by means of the GLEEBLE 3800 physical simulator were carried out at 923 and 1023 K and an applied stress range between 100 and 350 MPa to evaluate the effect of welding on its creep properties and behaviour. It was found that creep testing was conducted in a power-law creep regime. The results show a clear detrimental influence of the HAZ simulation on creep properties of HR3C steel at 923 K and for very short creep exposure. However, with increasing time to fracture, the effect of weld simulation becomes less significant. By contrast, no such deterioration of creep properties was detected at 1023 K. The evaluated similar values of the stress exponents of the minimum creep rate and the time to fracture are explained using the assumption that both creep deformation and the fracture process are controlled by the same mechanism. The main creep fracture mode was intergranular brittle fracture due to grain boundary creep damage.

Present work investigates the effect of long-term annealing at low homologous temperature (~0.22 Tm) on creep behaviour of pure copper (99.99%) processed by equal-channel angular pressing (ECAP). Coarse-grained pure copper was processed by 1 and 8 ECAP passes at room temperature. The ECAP-processed specimens were annealed at room temperature for 3 years. Microstructures of ECAP-processed specimens and their room temperature-annealed states were investigated by electron back scatter diffraction (EBSD) technique. Tensile tests were performed at room temperature under constant rate and at 373 K in creep under constant load. It was found that long-term annealing at room temperature led to occurrence of large grains in microstructure which influenced the strength. The influence grew with number of ECAP passes.

Creep processes in MAR-M247 Nickel - Base Superalloy were studied at elevated temperatures. The stress exponents of creep rate n and time to fracture m indicate power-low (dislocation) creep regime and suggest the same controlling mechanism for the creep deformation and fracture. The variation of values of parameter n within the interval of applied stress may indicate changes in the rate-controlling creep deformation mechanism. The possibility of using of Monkman-Grant relationship for creep life prediction was demonstrated.

As candidate materials for high-temperature components, most attention has been paid to improving tempered martensitic creep-resistant 9-12%Cr steels. In this work, creep damage and fracture behaviour of an advanced W-modified P92 steel (ASTM Grade P92) was investigated at 600 and 650°C. Tensile creep tests were followed by fractographic analysis of crept and broken specimens. Besides experimental investigations, the creep damage tolerance parameter λ has been used to assess the creep fracture mode. In accordance with experiments the values of λ indicate variety in the fracture mode and provide some evidence on accelerated degradation of the creep strength. The SEM investigations of creep fracture surface revealed substantial changes in microfractographic features of creep fracture. At high applied stress level, the fracture was frequently transgranular due to local loss of a stability of plastic deformation. The fracture ductility drops with decreasing applied stress, demonstrating ductile dimple (transgranular) to brittle (intergranular cavitation) transition of the fracture mode. It was suggested that both the creep deformation and fracture processes are controlled by the same processes and the rate controlling mechanism is most probably climb of intergranular mobile dislocations.

Cast cobalt-based superalloys have already found applications in the glass industry to construct glass shaping tools. The present study deals with the high temperature characterization of the two cast cobalt-based CoNb and CoTa superalloys which have been developed for precision casting of a rotary fibreglass spinner disc for glass industry. The superalloys have been characterized through constant load creep tests at 1000, 1050 and 1100 °C in a tensile stress range from 20 to 100 MPa, with the aim to simulate the variation of mechanical properties occurring in a rotary fibreglass spinner disc at operational conditions. The results have been compared and discussed, with the aim to link the difference in creep properties to the evolution of creep damage and fracture in superalloys. It was found that the CoNb superalloy possesses longer creep life compared to the CoTa superalloy under the same loading conditions. Fractographic investigations of the creep fractured specimens of the examined superalloys revealed that the dominating creep fracture of the CoTa is of the ductile transgranular dimple fracture mode due to a loss of an external section of specimen (necking). By contrast, the final brittle fracture in the CoNb superalloys occurs via relatively fast propagation of the longest cracks after the ultimate state of creep damage is reach.

The creep behaviour of a welded joint of a real thick-walled P92 steel pipe was investigated at 600 and 650 °C and a stress ranging of 60–250 MPa using uniaxial tensile creep tests. Creep smooth cross-weld specimens were machined from different positions of the weld representing the welding sequence. Following analysis of creep data, all creep tests were performed in the region of power-law creep. The rate-controlling creep deformation mechanism is most probably climb of intergranular mobile dislocations for both the weld and base P92 steel. The creep resistance of the weld joint was lower than that of the base metal. No substantial differences were found in the minimum creep, the time to fracture and creep ductility for creep specimens taken from different locations of the weld. The fracture morphology of the crept welded specimens was found to be a mixture of transgranular ductile and intergranular mode. The fracture locations were found to shift from the weld metal in the higher stress region to the heat affected zone in the lower stress region. Many cavities formed at grain boundaries during creep. It was considered that the coarse precipitates of Laves phase acted as the preferential nucleation sites for cavities.

Advanced tungsten modified 9%Cr ferritic steel (ASTM Grade P92) is a promising material for the next generation of fossil and nuclear power plants. Unfortunately, there are rather few published reports on damage processes in P92 steel during high temperature creep and the effect of damage evolution on the creep strength is not fully understood. In this work, the creep behaviour of P92 steel in as-received condition and after long-term isothermal ageing was investigated at 600 and 650°C using uniaxial tension creep tests. To quantify the effect of each damage process on the loss of creep strength, most of creep tests were followed by microstructural and fractographic investigations. It was found that the large Laves phase particles, which coarsened during creep exposure, served as preferential sites for creep cavity nucleation.

The deformation kinetics of ultrafine-grained Ti-6Al-4V with mean (sub)grain size about 150 nm (produced by isothermal multiaxial forging) and superplastic properties at the relatively low temperature of 873 K was investigated in compression and tension over a large range of strain rates from 10 − 7 to 10 − 2 s − 1 . Electron microscopic observations showed that the grains coarsen during deformation towards the quasi-stationary spacing w qs of strain induced boundaries. In spite of the grain coarsening the grains were generally smaller than w qs allowing high-angle boundaries to dominate the quasi-stationary strength. Texture measurements indicate that dislocation glide plays a large role in deformation. Glide in this alloy is significantly influenced by solid solution strengthening leading to a stress sensitivity of strain rate of n = 3. The present ultrafine-grained Ti alloy displays a stress sensitivity exponent n = 2 over an extended stress range where its superplastic behavior is optimal. While the deformation kinetics of present ultrafine-grained Ti alloy can be roughly explained by the traditional formula for superplastic flow, the significant discrepancy to the measured values suggests that solid solution strengthening must be taken into account to get a complete insight.

The austenitic stainless steel 304 L was subjected to severe plastic deformation using high-pressure torsion technique at room temperature. The severe plastic deformation led not only to the grain size reduction but also to the transformation of austenite into deformation-induced martensite. At high imposed strains the martensitic boundaries disappeared and misorientation distribution exhibited nearly random misorientation distribution. The thermal exposure and tensile creep tests under constant load performed at 923 K and stresses ranging from 50 to 150 MPa revealed that austenitic microstructure contains σ phase. The ultrafine-grained microstructure coarsens during creep testing but the mean grain size is significantly finer than the predicted stationary subgrain. The results demonstrate that during creep testing new grains at precipitates of σ phase are formed and texture changes. The formation of new grains during creep keeps the microstructure of being ultrafine-grained and thus grain boundary mediated processes are enhanced in this steel.

The deformation behavior of an AlTiVNbZr0.25 high entropy alloy at 1073 K was investigated in creep in protective argon atmosphere at stresses of 100–560 MPa with and without changes. In the initial condition after annealing at 1473 K for 24 h the alloys was composed of B2 matrix phase and coarse Zr5Al3-type particles. During creep a limited amount of new Nb2Al-type sigma phase precipitates at the Zr5Al3/B2 hetero-phase interface and a quasi-stationary dislocation density of the expected order of magnitude are developed. The quasi-stationary creep rate obeys a three-power law and the transients after stress changes are inverted. This is interpreted in terms of the class A creep behavior with viscous glide due to cloud drag.

In this study, the creep specimens of an age-hardened IN 740 alloy were subjected to heat affected zone (HAZ) thermal cycle simulations using a Gleeble 380 physical simulator. The short-term uniaxial creep tests in tension were carried out at temperatures of 973 and 1023 K and at applied stresses ranging from 220 to 600 MPa to evaluate the effect of welding simulations on the microstructural evolution, creep properties and behaviour of the IN 740 alloy. It was found that the stress exponent, n, of the minimum creep rate is not constant and the evaluated values of n are of the order of ~3 for lower stresses, with the gradual variation of n with an increase of the applied stress up to ~15 at the highest stresses, indicating that the creep testing was carried out in the power-law (dislocation) creep region. Analysis of the creep data leads to the conclusion that the creep behaviour of the simulated HAZ of the IN 740 alloy arises from a synergism between the deformation process that is based on intragranular dislocation processes, involving the glide and climb of mobile dislocations, and grain boundary mediated processes. Finally, an assumption was proposed that both creep deformation and fracture are controlled by the same mechanism(s). Examination of the creep fracture surfaces revealed that the main fracture mode is mixed-mode fracture with dominating intergranular character.

Advanced creep resistant tungsten modified 9%Cr martensitic steel (ASTM Grade P92) is a promising structural material for the next generation of fossil and nuclear power plant. The P92 steel has been used to construct new coal-fired ultra-supercritical (USC) power plants with higher efficiency. Creep behaviour and fracture processes in creep are phenomena of major practical relevance, often limiting the lives of power plant components and structures designed to operate for long periods under stress at elevated and/or high temperatures. The creep behaviour of P92 steel has widely been reported. Furthermore, in recent years, extensive experimental studies and thermodynamic modelling of the microstructure and its stability during high-temperature creep of P92 steel have been published. Unfortunately, there are rather few published reports on damage processes in P92 steel during high-temperature creep, and the effect of damage evolution on the creep strength is nor fully understood at present. Therefore, it is not surprising that there are different and often controversial opinions about the role of secondary phases resulting from the additions of high concentrations of tungsten and molybdenum in P92 steel. In addition to M23C6 carbides and MX carbonitrides, an intermetallic Laves phase Fe2(W,Mo) is another dominating precipitating phase.

A low-alloy 2.25%Cr1%Mo steel (ASTM Grade 22) has been greatly improved by the substitution of almost all of the 1%Mo by 1.6%W. The improved material has been standardized as P/T23 steel (Fe–2.25Cr–1.6W–0.25V–0.05Nb–0.07C). The present investigation was conducted on T23 steel in an effort to obtain a more complete description and understanding of the role of the microstructural evolution and deformation processes in high-temperature creep. Constant load tensile creep tests were carried out in an argon atmosphere in the temperature range 500–650 °C at stresses ranging from 50 to 400 MPa. It was found that the diffusion in the matrix lattice is the creep-rate controlling process. The results of an extensive transmission electron microscopy (TEM) analysis programme to investigate microstructure evolution as a function of temperature are described and compared with the thermodynamic calculations using the software package Thermo-Calc. The significant creep-strength drop of T23 steel after long-term creep exposures can be explained by the decrease in dislocation hardening, precipitation hardening and solid solution hardening due to the instability of the microstructure at high temperature.

In this work, the creep behaviour of an actual thick-walled P92 steel pipe bent (90°) using local induction heating was investigated at 600 and 650 °C using uniaxial tension creep tests. Creep specimens were machined from the bend at different positions, namely at the intrados and extrados areas and neutral position and from the straight part of the pipe. Creep tests were followed by metallographic and fractographic analyses of the crept specimens to explain the observed creep behaviour. In order to accelerate the microstructural changes and thus to simulate long-term service conditions, isothermal ageing at 650 °C for 10,000 h was applied to selected creep specimens before creep exposures. The results of the creep tests performed on specimens extracted from the intrados and extrados of the thick-walled bent P92 steel pipe fall into the scatter band of the base P92 steel average line. No substantial differences were found at different positions of the pipe in the minimum creep rate, the time to fracture and creep fracture strain. However, significant detrimental effects on the creep resistance of the pipe were found after long-term static thermal ageing due to the microstructural instability of the material. The large Laves phase particles, which coarsened during ageing and creep testing, served as preferential sites for cavity nucleation leading to an accelerated tertiary stage of creep and/or premature creep fracture. Activation analysis of the creep data leads to the conclusions, that the creep tests were undertaken in the region of the power-law creep regime and the rate controlling creep deformation mechanism is most probably the climb of the intragranular dislocations.

The microstructure of P92 steel pipe processed by hot bending was evaluated in order to characterize the influence of hot bending on microstructure changes in extrados and intrados of bend. The straight pipes, with an outer diameter 350 mm and wall thickness of 39 mm (Productos Tubulares, s.a.u. Spain), were normalized at 1050°C/60min/air and tempered at 740°C/140min/air.The thick-walled P92 pipe was hot bended at 920-960 °C using bending rate 7 mm/min. The bends were normalized at 1050°C/60min/air and tempered at 775°C/140min/air after hot bending. The selected specimens, machined from different locations of pipe, were additionally thermal ageing at 650 °C to simulate microstructure degradation typical for long-term service conditions.Microstructure was investigated by scanning electron microscope Tescan Lyra 3 equipped with NordlysNano EBSD detector operating at an accelerating voltage of 20 kV with specimen tilted at 70° and by transmission el. microscope JEM-2100F.Microstructure investigation revealed that different processing history influenced distribution and size of secondary phases and caused changes in interboundary spacing in straight pipe and bends. It was found that microstructure changes induced during hot bending and subsequent post-bending heat treatment caused deterioration in creep properties of bends in comparison with those of straight pipe.

Processing by severe plastic deformation (SPD) may be defined as such metals forming procedure in which a very high strain is imposed on a bulk material. This paper investigates the effect of different equal channel angular pressing (ECAP) routes and number of ECAP passes on resulting microstructure, mechanical properties and creep behaviour of selected materials. The distinction between various ECAP routes (A, B and C) and the difference in number of ECAP passes applied may lead to variations both in the macroscopic distortions of the individual grains and in the capability to develop a reasonably homogeneous and equiaxed ultrafine-grained microstructure. Experimental materials were processed by ECAP at room temperature using a die with an internal angle of 90° between the two parts of the channel. The ECAP pressing was performed by different routes up to 12 ECAP passes. Tensile creep tests were conducted at temperatures 473 - 673 K and at different applied stresses on ECAP materials and, for comparison purposes, on their unpressed states. Microstructure of samples was characterized by scanning electron microscope (SEM) equipped with the electron backscatter diffraction (EBSD) unit. In conclusion, the ECAP processing route and number of applied ECAP passes could play an important role in creep behaviour and their effect may be different for particular materials. The highest differences in processing routes were revealed for materials especially at lower number of ECAP passes. However, a little apparent dependence of the creep properties was observed during subsequent pressing.

The creep behavior of high purity aluminum and copper, Al-0.2wt.%Sc and Cu-0.2wt.%Zr alloys was examined after processing by equal-channel angular pressing (ECAP) with an emphasis on the link between microstructure and creep. The microstructure was revealed by electron backscatter diffraction (EBSD) and analyzed by stereological methods. Representative microstructural parameters were obtained using orientation imaging microscopy and EBSD on the relationship between creep behavior and microstructure.

Experiments were conducted to evaluate creep behavior of conventional and ultrafine-grained metallic materials using nanoindentation technique. The polished surface of samples was loaded up to 5 mN. The load was held constant to examine the creep behavior. Nanoindentation tests were performed at room temperature. Strain rate was evaluated from load and displacement data. The stress exponents of strain rates n were determined from loading stress dependences of creep rate. The values of stress exponents of the indentation strain rate indicate that creep behavior of investigated materials is influenced in particular by slip of intragranular dislocations. By contrast, deformation mechanisms like grain boundary sliding and diffusion processes seem to be improbable.

The applicability of the Monkman-Grant relationship was analyzed and validated for ultrafine-grained metallic materials under investigation. A special attention has been given to the creep damage tolerance factor which is defined as the ratio of the strain to fracture to the Monkman-Grant ductility and which describes the coupling between creep deformation and damage based on continuum creep damage approach. It was found, that ultrafine-grained materials generally obey the Monkman-Grant relationship, however, the relationship is especially suitable for materials exhibiting short secondary creep and long tertiary creep stages when dislocation-controlled creep is dominant.

This paper reports the experimental results on the creep behavior of electrodeposited ultrafine-grained nickel and its particle-reinforced nanocomposite. The objective of this research was to further improve the knowledge of the creep behavior of monolithic nickel and to explore the role of nano-sized SiO2 particles in the potential creep strengthening of electrodeposited Ni nanocomposite. The creep behavior and microstructure of the pure ultrafine-grained nickel and its nanocomposite reinforced by 2 vol% nano-sized SiO2 particles were studied at temperatures in the range from 293 to 573 K and at the applied tensile stresses between 100 and 800 MPa. The results indicate that the creep resistance of the nanocomposite may be noticeably improved compared to the monolithic nickel due to the interaction of the particles with dislocation motion. It was found that the applied stress interval can be divided into lower and higher stress intervals corresponding to dislocation (power-law) and exponential creep regions, respectively. Analysis of the creep data leads to the suggestion that the creep behavior of both electrodeposited nickel and its nanocomposite in power-law region may be grain boundary controlled. However, the mechanism responsible for the observed creep behavior at lower temperatures and the highest stresses is still not well established.