High Temperature Creep Conditions Research Articles

In-situ synchrotron high energy X-ray diffraction study on the compressive creep behavior of an extruded Ti-45Al-8Nb-0.2C alloy

Wrought high Nb containing (high Nb-TiAl) alloys are potential materials for low pressure turbine blades in aero-engines. The performance and microstructure evolution under high temperature creep condition is of importance to the service stability of these materials. In this study, the internal strain and FWHM evolution of an extruded TNB-based high Nb-TiAl alloy during compressive creep are characterized by in-situ high energy X-ray diffraction (HEXRD) technique for the first time. The compressive creep test was conducted under a constant load of 300 MPa at 900 °C for 10 h in vacuum. The final creep strain is approximately 1.72 %. The microstructure and phase constitution after creep shows little difference from that before creep. Lattice strain analysis shows that γ phase is plastically deformed while the α2 phase deforms elastically due to the low creep strain. The lattice strain of α2 grains is dependent upon the deformation of surrounding γ grains. Creep induces dynamic recovery, exerting a softening effect. A high number of dislocations are visible in the γ lamellae while almost no dislocations exist within the α2 lamellae. α2 lamellae are partially decomposed and refined via the α2→γ transformation.

Description of step loads at high temperature creep of metallic materials

Abstract In metallic materials under high-temperature creep conditions, the damage evolution occurs. To describe it, Kachanov and Rabotnov proposed the concept of damage. In this paper, the damage parameter is defined as the relative change in material density, serving as an integral measure of damage. By incorporating the damage parameter and the mass conservation law, are we formulated interrelated kinetic equations for rate of creep deformation and the damage parameter. The case of variable loads is crucial both theoretically and practically, as most structural components typically operate under such conditions. Therefore, solutions of considered equations are derived for the case of two-stage loading. The received solutions and experimental results on the creep of titanium alloy under variable loads at 500°C were compared. There is a good agreement between the theoretical results and experimental data.

EXPERIMENTAL AND THEORETICAL INVESTIGATIONS OF HIGH-TEMPERATURE CREEP OF ENAW 5251 (AMg2) ALUMINUM ALLOY UNDER STEP LOADING

In conditions of high-temperature creep, the evolution of the damage of metallic materials is observed. To describe this effect, the damage conception of the Kachanov-Rabotnov is used. The damage parameter is defined as the relative changes of the material density, which is an integral characteristic of the damage. Taking into account this parameter and the law of conservation of mass, interrelated kinetic equations for creep deformation and damage parameter are formulated. Analytical solutions of these equations are obtained for the case of two-stage loading. Experimental studies of uniaxial stress state under conditions of high-temperature creep under two-stage loading of aluminum alloy ENAW 5251 (AMg2) at a temperature of 250°C were conducted. A good agreement of the obtained theoretical curves with the experimental results is observed.

Exact and approximate solutions of the system of interrelated equations of the theory of creep and long-term strength

The problem of creep and long-term strength of metallic materials and alloys is considered. Under the long action of high temperatures and relatively small stresses, many metallic alloys and pure metals loose plasticity and undergo brittle fracture. This problem in mechanics of materials is known as the phenomenon of thermal brittleness. To solve this problem Kachanov and Rabotnov introduced the concept of damage into the mechanics of materials. Also, they suggested a system of interrelated kinetic equations for creep deformation and damage parameter. In this work, a modified system of interrelated kinetic equations for creep deformation and damage parameter are formulated. In these equations, the mass conservation law is taking into account and the relative changes of material density are considered as a damage parameter. The results of numerous studies on the changes of porosity and density of various metals and alloys due to the formation and development of micropores and microcracks under conditions of high-temperature creep make it possible to consider density as an integral measure of the accumulation of structural microdefects. Different analytical and numerical solutions of the considered system of kinetic equations are obtained. It is shown, that the character of the curves according to different solutions is identical and agrees with the corresponding experimental results. It was shown, that the numerical solution completely coincides with the case of purely brittle fracture and small deformations. The long-term strength criterion is formulated. Comparison of theoretical creep and long-term strength curves with experimental results for 2.25Cr-1Mo steel is given. A good agreement between the corresponding theoretical and experimental results is observed.

Assessment of High-temperature Component Containing Multiple Flaws

Multiple flaws are often found in many high-temperature components. The interaction of multiple flaws may significantly affects service life of high-temperature components. Flaws re-characterization rules for multiple flaws have been proposed in ASME and BS7910. However, most rules are formulated according to the elastic fracture parameter. Due to the different failure mechanism among linear-elastic stress conditions, elastic-plastic conditions and high-temperature creep conditions, the interacting behavior of components containing multiple flaws is also different. Therefore, it is the important to investigate the interacting behavior of multiple flaws under the high-temperature creep conditions. In the current work, the high-temperature creep fracture parameter is calculated for plate containing multiple flaws by using the finite element analysis (FEA) under the tensile load. Based on these analyses, the new re-characterization rules are proposed to exclude the non-conservativeness caused by the current re-characterization rules in high-temperature creep conditions for heat-resistant steel. The coalescence rule for collinear cracks is S≤2.5a, and the parallel cracks can be treated as collinear cracks for all distance S when the equation h≥a is satisfied.

On the Plastic Strain Accumulation in Notched Bars During High-Temperature Creep Dwell

ABSTRACTStructural integrity plays an important role in any industrial activity, due to its capability of assessing complex systems against sudden and unpredicted failures. The work here presented investigates an unexpected new mechanism occurring in structures subjected to monotonic and cyclic loading at high temperature creep condition. An unexpected accumulation of plastic strain is observed to occur, within the high-temperature creep dwell. This phenomenon has been observed during several full inelastic finite element analyses. In order to understand which parameters make possible such behaviour, an extensive numerical study has been undertaken on two different notched bars. The notched bar has been selected due to its capability of representing a multiaxial stress state, which is a practical situation in real components. Two numerical examples consisting of an axisymmetric v-notch bar and a semi-circular notched bar are considered, in order to investigate different notches severity. Two material models have been considered for the plastic response, which is modelled by both Elastic-Perfectly Plastic and Armstrong-Frederick kinematic hardening material models. The high-temperature creep behaviour is introduced using the time hardening law. To study the problem several results are presented, as the effect of the material model on the plastic strain accumulation, the effect of the notch severity and the mesh element type and sensitivity. All the findings further confirm that the phenomenon observed is not an artefact but a real mechanism, which needs to be considered when assessing off-design condition. Moreover, it might be extremely dangerous if the cyclic loading condition occurs at such a high loading level.

High-temperature creep and damage of metallic materials

The problem of creep and long-term strength of metallic materials and alloys is considered. This problem is demanded in such important fields of modern engineering, as thermal and nuclear power plants, aircraft and spacecraft, etc. The damage conception was introduced in the mechanics of materials to describe long-term strength under conditions of high-temperature creep. In this paper is proposed to determine the damage parameter changes from experimental high-temperature creep curves. Therefore, only one kinetic equation for the creep rate for the case of a compressible medium is formulated. The continuity parameter is determined from this kinetic equation and depends on the creep rate and creep deformation. Similarly, the changes in the continuity parameter are determined by the Rabotnov solution. It was obtained that for the case of a compressible medium, a more intense accumulation of damage and, correspondingly, fracture processes are observed, compared with the Rabotnov solution.

45-degree rafting in Ni-based superalloys: A combined phase-field and strain gradient crystal plasticity study

45° rafting of Ni-based superalloys has been investigated with the help of creep test simulations applying a strain gradient crystal plasticity model coupled to the multi-phase field method. This mode of rafting lies in between P- and N-type rafting modes. The model parameters are calibrated against experimental data for N-type rafting under high temperature and low stress creep condition. By increasing the stress level, the mixed-mode rafting of precipitates with a clear tendency toward formation of 45° rafts is observed. We show that the key factor for the occurrence of this type of rafting is the generation of highly localized creep strain of more than 10% due to non-homogeneous creep deformation in the form of slip bands. We have successfully captured the evolution of microstructure under high stress leading to production of localized shear bands.

CRACK RESISTANCE OF CENTRIFUGAL-CAST STEAM P I P E S MADE OF STEEL 1 5 K h l M l F

Significance of assessing the characteristics of crack resistance of centrifugal-cast (CFC) tubes made of steel 15KhlMlF is substantiated. The characteristic features of the metal under study are considered along with the methodology of crack resistance tests. Three modifications of the metal of the CFC pipes in the initial state, that differ in the degree of developed segregational heterogeneity, and two modifications of the metal with almost the same degree of structural heterogeneity, which differ in the running time as a part of the operating equipment are studied. It is shown that a decrease in the intensity of the segregational damage of the metal structure has almost no effect on the level of the strength characteristics, but is accompanied by an increase in the plastic properties and toughness of the metal. We present the results of studying crack resistance of centrifugal-cast pipes (steel 15KhlMlF) under static, cyclic and prolonged static loading in conditions of high-temperature creep. It is shown that an increase in the degree of structural heterogeneity of the CFC steel adversely affects the characteristics of static crack resistance and cyclic crack resistance as well, but to a far lesser extent and has little if any effect on the crack resistance of the CFC steel at creep. Cyclic crack resistance slightly decreases with increase of the operating time of the metal in functioning pipelines, whereas the characteristics of the prolonged static crack resistance get worse significantly. It is shown that welded joints of the CFC pipelines have a substantially lower crack resistance under creep conditions compared to the base metal. Statistical processing of the experimental data is used to get kinetic diagrams of cyclic and prolonged static crack resistance of 15X1M1F CFC-steel in creep conditions. The results of practical applications of the obtained crack resistance characteristics of the metal of the CFC pipes are demonstrated with a focus on the problems of ensuring the reliable operation of the steam pipelines of thermal power plants at the end of their service lifetime.

ЧИСЛЕННОЕ МОДЕЛИРОВАНИЕ ВЫСОКОТЕМПЕРАТУРНОЙ ПОЛЗУЧЕСТИ ЭЛЕМЕНТОВ КОНСТРУКЦИЙ ИЗ ЖАРОПРОЧНЫХ СПЛАВОВ С УЧЕТОМ ЭФФЕКТОВ НЕЙТРОННОГО ОБЛУЧЕНИЯ

The technique of numerical research on the basis of FEM processes of deformation and damage accumulation in the structural elements of heat-resistant alloys under conditions of high-temperature creep taking into account the influence of neutron irradiation is developed. The description of the mechanical behavior of the material is carried out within the framework of the previously developed general model of the damaged material and the creep model for non-irradiated heat-resistant alloys, supplemented by taking into account the effect of irradiation on the creep rate and the appearance of brittle fracture in a given range of temperature variation and irradiation intensity. The defining relations of the creep model of the irradiated material were obtained by modifying the creep model of the non-irradiated material: a material function was introduced, taking into account the effect of the flux of neutrons on the rate of thermal creep deformation; a material function was introduced that takes into account the effect of the neutron flux on the creep surface radius; A material function was introduced, which takes into account the effect of the neutron flux on the ultimate value of the dissipation energy at full power. To simulate the processes of brittle fracture during creep under neutron irradiation conditions, it is assumed that the destructive values of effective normal stresses are a function of temperature, flux of neutrons and the current value of accumulated creep. The material functions of the model were obtained from the results of basic experiments conducted at the Research Institute of Mechanics for the heat-resistant alloy without irradiation under consideration and the available experimental data on the study of the creep of this alloy during its irradiation. Based on the proposed model, a numerical method for solving problems of high-temperature creep of structures made of heat-resistant alloys under neutron irradiation was developed and implemented within the UPAKS computing complex. To verify and illustrate the capabilities of the developed methodological and software tools, a number of problems of modeling the processes of high-temperature creep and destruction of structural elements made of the high-temperature alloy under consideration are solved.

Creep Property of Boron Added 9Cr Heat Resistant Steels after Welding

Although welding results in premature failure by type IV fracture under high temperature creep conditions, the alloy design of light elements such as boron addition and nitrogen reduction enhances the creep lifetime of 9Cr heat resistant steel. In particular, the simulated heat affected zone (SHAZ) sample of new 9Cr steel (called TA steel) shows about 10 times longer creep lifetime than that of the standard Gr. 91 steel. The welded TA steel is thus expected to exhibit good creep properties because its SHAZ sample has coarser grains and suppresses type IV fracture. The preservation of base metal’s microstructure after welding results from the precipitate morphology, such as high grain boundary coverage by precipitates and low amount of MX being nucleation sites of ferrite grains during the a-g phase transformation. In addition, the increase of stability of M23C6 affects high pinning pressure toward grain boundary migration upon rapid heating during welding. First-principles calculations confirm the increased stability when boron is absorbed by M23C6. Moreover, the calculations reveals that boron decreases the coherency between matrix and M23C6, suppressing grain coarsening during creep tests in TA steel. It is concluded that the increased microstructural stability during welding and long high temperature exposure generates the elongated creep lifetime in welded TA steel including about 0.01 wt% boron and less than 0.01 wt% nitrogen.

Determination of necking time in tensile test specimens, under high-temperature creep conditions, subjected to distribution of stresses over the cross-section

The work describes an experimental research of creep of cylindrical tensile test specimens made of aluminum alloy D16T at a constant temperature of 400°C. The issue to be examined was the necking at different values of initial tensile stresses. The use of a developed noncontacting measuring system allowed us to see variations in the specimen shape and to estimate the true stress in various times. Based on the obtained experimental data, several criteria were proposed for describing the point of time at which the necking occurs (necking point). Calculations were carried out at various values of the parameters in these criteria. The relative interval of deformation time in which the test specimen is uniformly stretched was also determined.

Interfacial failure in dissimilar weld joint of high boron 9% chromium steel and nickel-based alloy under high-temperature creep condition

The advanced ultra-supercritical (A-USC) power generation system is expected to become the next-generation base-load power station in Japan. Dissimilar weld joints between high-Cr heat-resistant steels and nickel-based alloys with a nickel-based filler metal (Alloy 82) will need to be adopted for this purpose. However, interfacial failure between the steels and weld metal has been observed under high-temperature creep conditions. Fractography and microstructure observations showed the failure initiated in a brittle manner by an oxide notch at the bottom of the U-groove. The fracture then proceeded along the bond line in a ductile manner with shallow dimples, where micro-Vickers hardness tests showed remarkable softening in the steel next to the bond line. In addition, the steel showed a much larger total elongation and reduction of area than the weld metal at low stresses under long-term creep conditions, leading to mismatch deformation at the interface. According to the results, it can be concluded that the interfacial failure between the 9Cr steels and Alloy 82 weld metal is initiated by an oxide notch and promoted by softening and the difference in the plasticity of the steels and weld metal.

高温クリープ環境下におけるNi基溶接金属の延性低下

高温クリープ環境下におけるNi基溶接金属の延性低下

Fracture-based correlation of uniaxial and small punch creep data

A critical problem of small punch test is determining the relation between this test and the conventional uniaxial test. In high-temperature creep conditions studied by constant-force variant of small punch test, this problem is often approached empirically by comparing force in small punch test and the applied stress in conventional creep test of the same duration, i.e. of the same time to fracture. A different approach is used in the evaluation of constant-rate small punch test: the force is normalized by the square of specimen thickness or, recently, by the product of the specimen thickness and a relevant deflection. Modifications of this advanced approach in the creep area are examined in the present paper. The deflection observed at fracture of specimen in small punch creep test is taken as a starting normalization quantity. The suggested correlations are illustrated by means of data obtained by creep testing of steels usually employed in power plants (P91, P92) or proposed for heavy-duty service (Eurofer’97).

Creep deformation mechanisms in a γ titanium aluminide

Titanium aluminides (TiAl) are considered as potential alternatives to replace nickel-based alloys of greater density for selected components within future gas turbine aero-engines. This is attributed to the high specific strength as well as the good oxidation resistance at elevated temperatures. The gamma (γ) titanium aluminide system Ti-45Al-2Mn-2Nb has previously demonstrated promising performance in terms of its physical and mechanical properties. The main aim of the current study, which is a continuation of a previously published paper, aims at evaluating the performance of this titanium aluminide system under high temperature creep conditions. Of particular interest, the paper is strongly demonstrating the precise capability of the Wilshire Equations technique in predicting the long-term creep behaviour of this alloy. Moreover, it presents a physically meaningful understanding of the various creep mechanisms expected under various testing conditions. To achieve this, two creep specimens, tested under distinctly different stress levels at 700°C have been extensively examined. Detailed microstructural investigations and supporting transmission electron microscopy (TEM) have explored the differences in creep mechanisms active under the two stress regimes, with the deformation mechanisms correlated to Wilshire creep life prediction curves.

МАТЕМАТИЧЕСКОЕ МОДЕЛИРОВАНИЕ И ЭКСПЕРИМЕНТАЛЬНОЕ ИССЛЕДОВАНИЕ ФОРМИРОВАНИЯ И РЕЛАКСАЦИИ ОСТАТОЧНЫХ НАПРЯЖЕНИЙ В ПЛОСКИХ ОБРАЗЦАХ ИЗ СПЛАВА ЭП742 ПОСЛЕ УЛЬТРАЗВУКОВОГО УПРОЧНЕНИЯ В УСЛОВИЯХ ВЫСОКОТЕМПЕРАТУРНОЙ ПОЛЗУЧЕСТИ

The authors carried out a complex (computational and experimental) research of residual stresses in prismatic samples made of EP742 alloy after ultrasonic hardening and he temperature of 650 °С for 100 hours unloaded. The laws of residual stresses distribution over the thickness of the surface-hardened layer were discovered. The experiments proved that the ultrasonic hardening of the sample caused compression residual stresses in the surface layer. The maximum stresses were observed in the subsurface layer and they decreased on reaching the surface. When the temperature was exposed, the induced compression residual stresses became relaxed, the level of residual stresses decreased by 1.4-1.6 times and its maximum was displaced deep into the sample, but the thickness of the compressed layer was kept to about 200 micron. We developed the mathematical model of residual stresses forming in prismatic samples after surface plastic deformation and relaxation under high-temperature creep conditions. We used the hardened half-space as a model object for the prismatic sample, because the hardened layer was rather thin. We introduced Cartesian coordinate system, where x 0 y plane coincided with the half-space hardened surface, and 0 z axis was directed into the depth of the hardened layer.

Propagation of High-Temperature Creep Cracks in Metals Subjected to Neutron Irradiation (A Survey)

We obtain a new equation for finding the rate of propagation of high-temperature creep cracks with regard for the influence of neutron irradiation. By using this equation, we construct a mathematical model for finding the residual life of a plate weakened by a macrocrack under the conditions of hightemperature creep and neutron irradiation. The application of the model is demonstrated for an analog of the Griffith problem. We establish a good correlation of the obtained analytic results with the available experimental data.

Law of fracture life under creep–fatigue interactive conditions for Ni-base directionally solidified superalloy based on non-equilibrium science (the effect of stress holding time)

Ni-base directionally solidified superalloy are used under complex conditions of high temperature creep and fatigue interaction. Under these conditions, the characteristics of load frequency for fracture life show a nonlinear behavior, and these characteristics show various features depending on material, temperature and stress holding time. In this study, crack growth tests were conducted under creep-fatigue interactive conditions with stress holding time, tH using the in-situ observational system for directionally solidified Ni-base superalloy CM247LC. To characterize the load frequency of fracture life for CM247LC, the separate estimation of cyclic dependent from time dependent mechanisms was conducted. Additionally, a developed representation of the characteristic of fracture life under the conditions of creep fatigue interaction based on non-equilibrium science and chaos theory was proposed. Furthermore, verification of the established law of predicting the fracture life under creep-fatigue interactive conditions was conducted.

On the role of Re in the stress and temperature dependence of creep of Ni-base single crystal superalloys

In the present study we investigate the creep behavior of a Ni-base single crystal superalloy. We evaluate the stress and temperature dependence of the minimum creep rate, which shows a power law type of stress dependence (characterized by a stress exponent n) and an exponential type of temperature dependence (characterized by an apparent activation energy Qapp). Under conditions of high temperature (1323K) and low stress (160MPa) creep, n and Qapp are determined as 5.3 and 529kJ/mol, respectively. For lower temperatures (1123K) and higher stresses (600MPa) the stress exponent n is higher (8.5) while the apparent activation energy of creep is lower (382kJ/mol). We show that there is a general trend: stress exponents n increase with increasing stress and decreasing temperature, while higher apparent activation energies are observed for lower stresses and higher temperatures. We use density functional theory (DFT) to calculate the activation energy of diffusion for Re in a binary Ni–Re alloy with low Re-concentrations. The resulting energy is almost a factor 2 smaller than the apparent activation energy of creep. We explain why it is not straightforward to rationalize the temperature dependence of creep merely on the basis of the diffusion of one alloying element. We show that the evolution of the microstructure also must be considered.