Inconel 939 is a high-temperature nickel superalloy exhibiting an excellent combination of fatigue and creep properties with high oxidation and corrosion resistance. Therefore, it is often used for high-temperature applications, mainly in cast form. Laser powder bed fusion (L-PBF) technology has become a promising manufacturing method for producing power components with complex geometries that would be difficult to manufacture with other conventional technologies. However, the microstructure of the printed superalloy is completely different from the conventional as-cast state due to the high-temperature gradients and solidification rate during the L-PBF process. The article is focused on the creep behaviour and fracture mechanisms of the nickel superalloy IN939 produced by the L-PBF method at three temperatures 700, 800, and 900°C, in the wide range of applied uniaxial tensile stresses. The creep results show that the creep deformation of the INC939 alloy prepared by the L-PBF method is in the region of power-law dislocation creep. The values of the determined stress exponents of the minimum creep rate and time to fracture indicate that the controlling creep deformation mechanism and the fracture mechanism are identical. The power-law creep regime is explained by a dislocation climb-controlled mechanism. Creep strain, however, arises mainly through the glide of mobile dislocations.

Among various degradation processes, thermal creep is considered a life-limiting factor for zirconium nuclear fuel cladding in light water reactors. In this work, the creep behavior of pre-oxidized and pre-hydrided in steam thin-walled Zr1wt.%Nb alloy fuel cladding tube was investigated in alpha-Zr-* (alpha+R)-Zr-* R-Zr phase transit regions to further develop knowledge on acting creep deformation mechanisms and the role of dissolved hydrogen. Short-term constant-stress creep tests in tension were conducted on pre-hydrided tubular segments of the cladding tube over a temperature range of 550-900 degrees C and at applied stress of 5 to 60 MPa. The hydrogen contents in creep specimens before creep tests were 150 and 600 wppm, respectively. Creep tests were followed by microstructural analysis of the specimens using scanning and transmission electron microscopy. The activation analysis of the creep data indicated that the dominant creep deformation mechanism during the phase transition may be dislocation glide, which is not altered by oxygen or hydrogen. However, the oxygen and hydrogen influence its kinetics. The dominant creep-hardening effect in the alpha-Zr-* R-Zr transition provides oxidation hardening. Dissolved hydrogen does not directly contribute to creep hardening.

The creep behavior of the cast nickel-based superalloy EEQ111 was investigated over a range of temperatures and applied stresses relevant to high-temperature turbine applications. Constant-load creep tests were performed to evaluate the evolution of creep strain, minimum creep rate and time to fracture. The dependence of creep resistance on stress and temperature was analyzed to identify the dominant deformation mechanisms governing the creep response.The experimental results reveal a pronounced sensitivity of the minimum creep rate to both applied stress and temperature. The stress exponent and apparent activation energy for creep were determined and discussed in relation to the underlying microstructural features of the alloy. The observed trends indicate that creep deformation in EEQ111 is controlled by thermally activated dislocation processes, strongly influenced by the presence and stability of γ′ precipitates and grain-boundary carbides. At higher temperatures and lower stresses, a transition in the creep mechanism is suggested by changes in the stress exponent and damage evolution behavior.The results contribute to a more detailed understanding of the creep mechanisms operating in cast nickel-based superalloys and provide insight into the role of microstructural stability in controlling long-term creep performance.

The prime objective of any safety action in a nuclear power plant is to maintain the structural integrity of nuclear fuel cladding, which encases the radioactive fuel undergoing nuclear fission. Of different acting degradation processes, creep is regarded as one of the life-limiting factors for zirconium nuclear fuel cladding in all light-water reactor systems. However, deformation, fracture, and structural processes in the thermal creep of zirconium nuclear cladding tubes remain poorly understood. This inconvenient situation is due to the relatively limited number of studies published in the open literature. This work mainly carried out uniaxial constant-stress creep tests of the zirconium Zircaloy-4 (Zr-Sn-Fe-Cr) alloy cladding tubes in the power-law and the power-law breakdown creep regimes. Fractographic and metallographic analyses of the crept specimens followed the creep tests. They were used to explain the observed thermal high-temperature creep behavior and to determine acting deformation and fracture processes. The analysis of the creep data indicates close links between creep deformation and fracture.

The origin of the thermal creep of zirconium cladding tubes in all light water reactors is still the subject of deep confusion and intricate controversies. The reason for this inconvenient situation is obviously that microstructural processes in thermal creep remain poorly understood and this is due to the relatively small number of studies that have been carried out. In this study uniaxial creep tests in tension of zirconium alloy cladding tubes in the as-received and pre-hydrided states are followed by metallographic analysis of the as-received and crept specimens by light microscopy and SEM to explain the observed high-temperature creep behavior of the tubes.

Zirconium alloys are used as a fuel cladding material in light water reactors due to their low neutron cross section, good corrosion resistance, appropriate high-temperature strength, and dimensional stability under radiation. Thermal creep is considered one of the severe degradation factors during the reactor core operation. Therefore, an understanding of the creep flow processes and fracture control is necessary for predicting the lifetime of the cladding component. This paper summarizes the results from an extensive experiment on the characteristics of creep flow of the non-irradiated Zr1

The effect of hydrogen on the thermal creep behaviour of Zr1%Nb alloy fuel cladding tubes for use in light water-cooled nuclear reactors was investigated. The tubes were hydrogen charged using an alternative method with a hydrogen content in the range 371–656 wppm H. Comparative constant-stress creep tests were carried out at 350°C, under stress ranging from 150 to 225 MPa, on both non-hydrided and hydrided tubes. These creep tests were followed by a microstructural analysis of the specimens exposed to creep using SEM and TEM. It was found that these creep tests were conducted in the transition zone between the power-law and power-law breakdown creep regions. Hydrogen, either in solid solution (atomic hydrogen) or as precipitated zirconium hydrides, altered the creep behaviour of the hydrided tubes, and the impact of hydrogen depended on the hydrogen content CH. It was observed that under the creep loading conditions considered here with a hydrogen content CH < 450 wppm H, the presence of hydrogen resulted in an increase in the creep rate; however, a higher concentration of hydrogen caused a decrease in the creep rate. The difference in the effects of hydrogen on the creep rate was explained by the different roles of hydrogen in solid solution and in the form of zirconium hydride precipitates.

For pipeline systems operating for a long time in thermal and/or power plants at high temperature and pressure, the degree of creep degradation is a key factor for their safe operation. The advanced 9% chromium creep-resistant P92 steel is now widely used in modern power plant boiler pipes operated at steam temperatures up to 650°C. In this work, the relationship between creep deformation and damage have been analysed. Constant load tensile creep tests were conducted at 600°C under different applied stresses. The microstructure evolution after creep loading was studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that long-term creep exposition led to coarsening of the size of M23C6 carbides and their precipitation at the grain boundary, which promotes intergranular creep cavitation and thus reduces the creep lifetime of the steel. The acoustic emission (AE) method was applied to monitor the formation and the rate of propagation of cracks arising from creep damage. This nondestructive method is able to detect in advance the degradation processes leading to high temperature component failure. The necessity for nondestructive examination rather than destructive investigation to identify creep damage is emphasized.

The present study was initiated to perform a detailed evaluation of the creep behaviour of a cast GTD 111 nickel-based superalloy. Over the past two decades, experimental studies have been carried out to investigate the creep mechanisms and microstructure changes in this superalloy at testing temperature frequently above 1000°C. However, the creep behaviour at intermediated temperatures has received only a very limited attention, especially for the creep testing temperatures around 900°C, which represent a transition situation between medium-temperatures and high-temperatures. Accordingly, the present work was undertaken to examine the flow characteristics and creep damage evolution in a cast GTD 111 nickel-based superalloy. Constant load creep tests were conducted at 800, 900 and 950°C under tensile stress range from 125 to 700 MPa. Creep tests were followed by metallographic and fractographic investigations using scanning and transmission electron microscopy. The study aims to identify operating creep deformation and damage mechanisms and to clarify the decisive factors governing the creep resistance of the superalloy under investigation.

This work investigates the creep behavior of severely deformed commercial aluminum. The commercial aluminum was processed by helical rolling (HR) and equal-channel angular pressing (ECAP) at room temperature. During these processes, the equivalent strain up to about 4 was imposed into the as-received material. The creep testing at 200 °C revealed that HR and ECAP significantly increased the time to fracture compared to the as-received material. The stress dependences showed that the value of stress exponent n decreased with the value of the imposed strain. The stress-change tests showed that as-received and severely deformed states exhibited different recovery rates after unloading. The microstructure analysis showed that creep behavior was influenced by the microstructure formed during severe plastic deformation. The relationships between creep behavior and microstructure in the investigated states are discussed.

The creep behaviour and properties of nanostructured materials are attributed to their operating deformation mechanisms, which could be different from those in their coarse-grained counterparts. Accordingly, in this review, recent progress on the creep behaviour of nanostructured materials will be described. The results of large sets of tensile creep tests on selected more complex metallic materials are analysed for evaluating the effect of different SPD processing methods on creep resistance at high temperatures. The resultant creep characteristics are compared with those attained in unprocessed conditions of the same materials. By contrast to the creep behaviour of UFG pure metals SPD processing of more complex materials mostly exhibit no essentially improved creep resistance. Evaluated stress dependences of the creep rate and the creep life suggest that creep deformation mechanisms in UFG materials are similar to those operating in coarse-grained materials. However, creep mechanisms in SPD processed materials are not clearly resolved and this is due to complexity of phenomenon and very small number of studies that have been carried out before now.

A segment of tube cut off from the high pressure steam pipeline was subjected to routine basic material tests, which resulted in very low impact energy of the pipe material (low-alloyed Cr steel). A part of the segment (and a comparative non-operated sample) was then subjected to detailed mechanical tests (including creep), metallographic investigations and fractographic analyses with an aim to determine a root cause of the embrittlement. The results showed that grain boundaries of exposed steel are covered by particles of primary and secondary cementite. However, a matrix is weakened more, therefore a fracture of creep, tensile and impact specimens is always transgranular and the cleavage facets are limited to grain size, if present. Thus, the cause of embrittlement was determined not by a rapid ageing, but by an unsuitable initial heat treatment of the pipe material.

Short-term constant stress creep tests in tension were performed on two zirconium Zr1%Nb cladding alloys in the form of thin-walled cladding tubes in the temperature range of 623-1173 K and at the applied tensile stress sigma range of 5 - 225 MPa to provide further information on their creep behaviour in the alpha-Zr and (alpha+beta)-Zr phase regions. In parallel, the evolution of the microstructure of the studied alloys was investigated using SEM and TEM microscopy in the as-received state and after creep exposures. Using the dependence of diffusion coefficientcompensated minimum creep rate vs modulus compensated applied stress the values of the stress rate exponent n = ( partial differential ln epsilon m/ partial differential ln sigma)T were determined ranging from -2.5 up to -16 depending on the applied stress and testing temperature. The stress-dependant activation energy for creep QC=[ partial differential ln epsilon m/ partial differential (-1/kT)]sigma was determined and possible creep deformation mechanisms as well as creep strengthening mechanisms are discussed. Under the loading conditions used in this study three distinct stress regions were identified according to the relevant controlling creep deformation mechanisms.

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) 3 B 2, were identified in the interdendritic region. Creep damage and fracture are closely connected with decohesion of the interface between M 3 B 2 boride and matrix.

Sheets of coarse-grained S304H austenitic steel were processed by high-pressure sliding (HPS) at room temperature and a ultrafine-grained microstructure with a mean grain size of about 0.14 µm was prepared. The microstructure changes and creep behavior of coarse-grained and HPS-processed steel were investigated at 500–700 °C under the application of different loads. It was found that the processing of S304H steel led to a significant improvement in creep strength at 500 °C. However, a further increase in creep temperature to 600 °C and 700 °C led to the deterioration of creep behavior of HPS-processed steel. The microstructure results suggest that the creep behavior of HPS-processed steel is associated with the thermal stability of the SPD-processed microstructure. The recrystallization, grain growth, the coarsening of precipitates led to a reduction in creep strength of the HPS-processed state. It was also observed that in the HPS-processed microstructure the fast formation of σ-phase occurs. The σ-phase was already formed during slight grain coarsening at 600 °C and its formation was enhanced after recrystallization at 700 °C.

The effect of service degradation on the microstructure and creep properties of a 0.5%Cr-0.5%Mo-0.3%V steel steam pipe after an accumulated service time of 77,900 h was investigated and analysed. For comparison pur -poses, additional experiments and analyses were carried out on the same pipe in the as-received (service unused) conditions. The results of the microstructural analysis and the short-term creep testing confirmed that the high creep resistance of the investigated steel is caused predominantly by the dispersion of fine vanadium carbides, providing an effective precipitation hardening of the steel matrix. However, the microstructure is not in an equilibrium state, and as the size of the vanadium carbides and the accompanying mean interparticle spacing increase during the creep, the detrimental effect of vanadium carbide coarsening leads to a significant weakening in the precipitation hardening and, consequently, a decrease in the creep resistance. No creep cavitation was observed in the service-exposed steel. The short-term creep testing of the service-exposed and unused states of the steel was carried out in the power-law (dislocation) creep regime. Analysis of the creep data did not reveal any change in the operating creep deformation mechanisms as a response to the structural processes occurring during the long-term service. Instead, changes in the kinetics of creep flow and fracture were found.

Severe plastic deformation (SPD) is effective in producing bulk ultrafine-grained and nanostructured materials with large densities of lattice defects. This field, also known as NanoSPD, experienced a significant progress within the past two decades. Beside classic SPD methods such as high-pressure torsion, equal-channel angular pressing, accumulative roll-bonding, twist extrusion, and multi-directional forging, various continuous techniques were introduced to produce upscaled samples. Moreover, numerous alloys, glasses, semiconductors, ceramics, polymers, and their composites were processed. The SPD methods were used to synthesize new materials or to stabilize metastable phases with advanced mechanical and functional properties. High strength combined with high ductility, low/room-temperature superplasticity, creep resistance, hydrogen storage, photocatalytic hydrogen production, photocatalytic CO2 conversion, superconductivity, thermoelectric performance, radiation resistance, corrosion resistance, and biocompatibility are some highlighted properties of SPD-processed materials. This article reviews recent advances in the NanoSPD field and provides a brief history regarding its progress from the ancient times to modernity. Abbreviations: ARB: Accumulative Roll-Bonding; BCC: Body-Centered Cubic; DAC: Diamond Anvil Cell; EBSD: Electron Backscatter Diffraction; ECAP: Equal-Channel Angular Pressing (Extrusion); FCC: Face-Centered Cubic; FEM: Finite Element Method; FSP: Friction Stir Processing; HCP: Hexagonal Close-Packed; HPT: High-Pressure Torsion; HPTT: High-Pressure Tube Twisting; MDF: Multi-Directional (-Axial) Forging; NanoSPD: Nanomaterials by Severe Plastic Deformation; SDAC: Shear (Rotational) Diamond Anvil Cell; SEM: Scanning Electron Microscopy; SMAT: Surface Mechanical Attrition Treatment; SPD: Severe Plastic Deformation; TE: Twist Extrusion; TEM: Transmission Electron Microscopy; UFG: Ultrafine Grained

The fatigue properties of a submicrocrystalline titanium are shown to be substantially higher than those of a coarse-grained state. A deposition of an oxide coating leads to insignificant increase in these properties for titanium with a submicrocrystalline and coarse-grained structures. Some peculiarities of the fatigue fracture of submicrocrystalline and coarse-grained titanium are analyzed.