High Temperature Creep Conditions Research Articles

The Effect of Heat Treatment on Creep Resistance of Castings from the Modified Superalloy MAR-247

mpact of heat treatment on durability has been evaluated in this study in conditions of high-temperature creep of castings made from nickel superalloy MAR-247 post production rejects. Castings made in the processes of modification and filtration when pouring into moulds were subject to solubilization (1185°C/2 h) with subsequent ageing (870°C/20 h). It has been found on the basis of performed tests that in conditions of high-temperature creep at temperature 980°C with stress σ = 150 MPa the resistance of specimens of coarse-grain structure was significantly higher when compared to fine-grain material. The conditions of initiation and propagation of cracks in these specimens were analysed with consideration of morphological properties of material macro-, micro-and substructure. Moreover, it has been proven that creep resistance of heat treated MAR-247 superalloy significantly improves when compared to its condition after the process of modification only. Macrostructural changes in the MAR-247 superalloy that determine the increase of superalloy creep resistance after solubilization and ageing, as observed in the tests, were also examined.

Analysis of Cracking Process in Conditions of High-Temperature Creep of Castings Form the Modified Superalloy IN-713C

mpact of complex modification and filtration during pouring into moulds on durability has been evaluated in this study in conditions of high-temperature creep of castings made from nickel superalloy IN-713C post production rejects. The conditions of initiation and propagation of cracks in the specimens were analysed with consideration of morphological properties of material macro-, micro-and substructure. It has been demonstrated that in conditions of high-temperature creep at temperature 980°C with stress σ =150 MPa creep resistance of the IN-713C superalloy increases significantly with the increase of macrograin size. Creep resistance of specimens made of coarse grain material was significantly higher than the resistance of fine grain material.

High temperature creep behaviour in the γ titanium aluminide Ti–45Al–2Mn–2Nb

The fact that Gamma titanium aluminides (γ-TiAl) offer strong potential for replacing conventional titanium and nickel-base alloys in future gas turbine engine designs has resulted in the requirement to accurately describe the creep behaviour of these complex alloys. Consequently, the system Ti–45Al–2Mn–2Nb has been extensively studied under high temperature creep conditions where creep ductilities are shown to far exceed the relatively low tensile ductilities characteristic of the alloy at room temperature. Modern lifing approaches have been applied to the alloy, helping to develop understanding of the deformation behaviour, and are shown to accurately describe the stress-rupture and minimum creep rate behaviour of the alloy, promoting confidence for the derivation of computational models which are used to describe the behaviour of the alloy for in service conditions. When compared to an existing, conventional, high temperature titanium alloy, Timetal 834, significant improvements in creep life were demonstrated for equivalent stress conditions with a comparable level of creep ductility between the two alloys. In addition, the gamma titanium aluminide showed a superior resistance to surface oxidation and associated cracking with notable α case formation in Timetal 834 leading to premature failure.

Impact of Surface and Volume Modification of Nickel Superalloys IN-713C and MAR-247 on High Temperature Creep Resistance

Abstract Impact of surface and volume modification and double filtration during pouring the moulds on basic mechanical properties and creep resistance of nickel superalloys IN-713C and MAR-247 in conditions of accelerated creep of castings made of post-production scrap of these alloys is evaluated in this paper. The conditions of initiation and propagation of cracks in the specimens were analysed with consideration of stereological properties of material macro- and microstructure. It has been proven that in the conditions of hightemperature creep at 980°C and at stress σ = 150 MPa, creep resistance of superalloy MAR-247 is more than 10 times higher than the creep resistance of IN-713C alloy. In case of IN-713C alloy, the creep resistance negligibly depends on macrograin sizes. But, the macrograin size considerably affects the time to failure of specimens made of alloy MAR-247. Creep resistance of specimens made of coarse grain material was 20% higher than the resistance of fine grain materials.

Effect of stress orientation on microstructural evolution during creep of near-lamellar Ti–47Al–2Cr–2Nb

Microstructural evolution was studied in a near-lamellar two phase (α2+γ) Ti–47Al–2Cr–2Nb alloy under high temperature creep and exposure conditions. The aim of this study was to probe the role of stress orientation, with respect to lamellar plates, on microstructural changes during primary creep. Creep testing was complemented with SEM and TEM based microstructural characterization. It was observed that retention of excess α2 resulted in an unstable microstructure. Under stress and temperature, excess α2 was lost and Cr-rich precipitates formed. Depending on stress orientation, the sequence of precipitates formed was different. α2 loss was accompanied by formation of the non-equilibrium C14 Laves phase when lamellar plates were oriented parallel to the stress axis. In contrast, α2 loss did not result in formation of the C14 phase in perpendicular samples. It was concluded that C14 formed preferentially in certain test orientations because of its effectiveness in relieving residual stresses in α2 that arose from lattice misfit and modulus mismatch.

144 高温クリープ条件下におけるP91およびP92鋼の初期き裂成長速度特性(材料力学II,一般講演)

144 高温クリープ条件下におけるP91およびP92鋼の初期き裂成長速度特性(材料力学II,一般講演)

In Situ Observations of the Deformation Behavior and Fracture Mechanisms of Ti-45Al-2Nb-2Mn + 0.8 vol pct TiB2

The deformation and fracture mechanisms of a nearly lamellar Ti-45Al-2Nb-2Mn (at. pct) + 0.8 vol pct TiB2 intermetallic, processed into an actual low-pressure turbine blade, were examined by means of in situ tensile and tensile-creep experiments performed inside a scanning electron microscope (SEM). Low elongation-to-failure and brittle fracture were observed at room temperature, while the larger elongations-to-failure at high temperature facilitated the observation of the onset and propagation of damage. It was found that the dominant damage mechanisms at high temperature depended on the applied stress level. Interlamellar cracking was observed only above 390 MPa, which suggests that there is a threshold below which this mechanism is inhibited. Failure during creep tests at 250 MPa was controlled by intercolony cracking. The in situ observations demonstrated that the colony boundaries are damage nucleation and propagation sites during tensile creep, and they seem to be the weakest link in the microstructure for the tertiary creep stage. Therefore, it is proposed that interlamellar areas are critical zones for fracture at higher stresses, whereas lower stress, high-temperature creep conditions lead to intercolony cracking and fracture.

Prediction of the corrosion cracking of structures under the conditions of high-temperature creep

On the basis of the continual model of corrosion crack growth proposed earlier and the well-known incremental-type creep theory, we make an attempt to predict the corrosion cracking of structures under the conditions of high-temperature creep. We propose the mathematical statement of the problem taking into account the influence on corrosion cracking of the properties of corrosive media and the redistribution of stresses in time caused by creep. The Bubnov–Galerkin method is applied for the solution of this problem. An example of prediction of the phenomenon of corrosion cracking in the case of creep of a pipe under the action of internal pressure is analyzed.

The application of the analysis of potential driven particle diffusion to the strength of materials

The problems of diffusion in solid are widely treated as those of numerical analyses such as mechanisms of hydrogen diffusion which causes embrittlement, boundary micro crack initiation and growth under high temperature creep condition and stress and electro-migration due to vacancy diffusion in LSI circuit. In this paper, the significance of α multiplication method which we proposed to analyze the particle diffusion problems under potential gradient and the results obtained from this method were discussed for the problems of hydrogen embrittlement, vacancy diffusion which results in creep crack damage and deformation, and line failure in LSI circuit. Furthermore, temperature characteristic of the sensitivity of particle diffusion and concentration such as hydrogen and vacancy were discussed based on this proposed analysis.

P94 How do vascular smooth muscle cell-derived chemokines affect trophoblast cell function?

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).

Effect of Mo concentration on creep properties of a single crystal nickel-base superalloy

The effect of Mo concentration on creep properties of a single crystal nickel-base superalloy has been studied at 1010 °C and 800 °C by using three kinds of alloys with 1%, 2% and 4% of Mo (wt.). The creep properties of alloys were measured and microstructure evolution was analyzed. The results showed that after heat treatment, all the three alloys were composed of γ and γ′ phase without TCP phase precipitation. The increase of Mo concentration from 1 wt.% to 2 wt.% pronouncedly enhanced the creep properties. Compared with 1 wt.% Mo content, 2 wt.% Mo addition can generate denser γ/γ′ interfacial dislocation networks during high temperature creep test, thus lower minimum creep rate. At medium temperature creep condition, Mo addition enhanced creep properties by decreasing the stacking fault energy. When the Mo content reached to 4%, the over-saturation of Mo content in γ phase led to the formation of μ phase precipitation which degraded the creep properties at both high and medium temperature creep conditions.

Characteristics of High Temperature Creep in Pure Aluminum Processed by Equal-Channel Angular Pressing

High purity aluminum was processed by equal-channel angular pressing (ECAP) to reduce the grain size to ~1.3 m. Tensile specimens were cut from the as-pressed billets and these specimens were tested under conditions of high temperature creep. The results show excellent creep properties with a well-defined region of steady-state flow. The flow behavior is analyzed by comparing the creep data with the predicted behavior for different fundamental creep mechanisms and by plotting a deformation mechanism map to provide a visual representation of the creep properties.

CURRENT CAPABILITIES OF THE THERMO-MECHANICAL MODELLING OF WELDING PROCESSES

Some fundamental principles and advanced applications of thermo-mechanical modelling techniques used for industrial welding processes are described. The paper covers a range of modelling procedures comprising welding simulation and residual stress analysis of multi-pass, thick-walled ferritic steel pipes; welding simulation and distortion analysis of nickel-based superalloy thin plates; and inertia friction welding of dual alloy drive shafts. A number of pertinent mechanical and metallurgical concepts are discussed, including material behaviour models and material properties, microstructural evolution, solid state phase transformation and post-weld heat treatment. The paper emphasises the general methodologies of thermal modelling procedures and their role in the holistic process of the life assessment and design improvement of power plant piping systems and aeroengine casings and shaft components, operating under high temperature creep and fatigue service conditions.

Fracture mechanism of full lamellar TiAl under high-temperature creep and fatigue conditions based on the in situ observational tests

In this paper, crack growth mechanism of full lamellar TiAl intermetallic compound under high temperature creep and fatigue conditions were investigated using in situ observational high temperature creep-fatigue testing machine. Under high temperature creep condition, inter-granular voids were found to be originated and to coalesce, which results in inter-granular cracking. Furthermore, since the grain size is large (1.5-3.0 mm), inter-granular cracks distribute not only around a notch but also the sites apart from the notch. Since the creep crack mechanism is coalescence of distributed inter-granular cracks, the resistance against creep crack growth becomes higher. Under high temperature fatigue condition, a trans-granular crack was found to initially grow along the direction of lamellar structure, which is different from that under creep condition. After that, the crack incubates and the crack opening displacement increases until it takes some critical value. After that, final unstable crack growth occurs along the perpendicular direction of applied load due to the delamination between matrix and lamellar structure. The characteristic of load frequency for fatigue fracture life is found to be dominated by cyclic dependent mechanism and it is not affected by time dependent mechanism even under low frequency.

Subcritical growth of a plane crack in a three-dimensional body under the conditions of high-temperature creep

On the basis of the mathematical model developed by the authors, we deduce equations for the investigation of the kinetics and evaluation of the period of subcritical growth of plane cracks in a three-dimensional body under the conditions of high-temperature creep. A method of equivalent areas is proposed for the approximate solution of problems of this sort.

2333 一方向凝固Ni基超合金の高温クリープ・疲労相互作用条件下でのき裂成長寿命則(S07-4 疲労き裂進展,S07 構造材料の疲労強度とき裂進展問題)

The Ni base directionally solidified super-alloy has been developed as a gas turbine blade material for high electrical efficiency power plant. Since turbine blades are faced with the centrifugal stress from the high-speed rotation, it is used under the high temperature creep condition. On the other hand, it is used under periodical condition caused by combustion gas. Therefore, the Ni base directionally solidified super-alloys are operated under the condition of high temperature creep-fatigue interaction. In this paper, high temperature crack growth tests were conducted under the condition of creep-fatigue interaction for the Ni base directionally solidified super-alloy, CM247LC. Based on the non-equilibrium science, crack growth mechanisms were investigated by analyzing the effects of cycle dependent and time dependent mechanisms on the crack growth behaviors under the condition of creep-fatigue interaction. Furthermore, the law-of crack growth life under the conditions of creep-fatigue interaction was derived.

ペナルティ関数を用いた有限要素法による高温先進耐熱材料のクリープき裂成長挙動に及ぼすき裂の分岐効果

The Nickel based directionally solidified superalloy has been developed as a gas turbine blade material for high electrical efficiency power plant. Turbine blades are faced with the centrifugal stress from the high-speed rotation under high temperature creep conditions. In the present study, in order to clarify the effect of an oblique crack on the resistant of creep deformation and crack growth, analyses of elastic-plastic creep FEM using a penalty function were conducted. The model of analysis used is a plate with double edge V notches and the type of elements used for analysis is 6-nodes isoparametric elements. Two type models for crack extension were considered such as straight-cracking and an oblique crack. The method of a crack extension is based on the increment application of inverse reaction and release of penalty function at the release nodes. On the basis of the elastic-plastic-creep FEM analysis using a penalty function, it was found that creep crack growth behavior and deformation were characterized by the toughening effects induced by the constitution law of stress-strain rate for creep deformation and crack branch such as an oblique crack. The crack growth behavior derived by this analysis were found to be in good agreement with the corresponding experimental characteristics.

Fracture mechanics of metallic plates under the conditions of high-temperature creep

We propose a mathematical model for the analysis of subcritical growth of high-temperature creep cracks in metallic plates. The model is based on the first law of thermodynamics on the energy balance and the balance of rates of changes in the energies in a metallic plate containing a macrocrack stretched by a long-term static load under the action of a high-temperature field, which creates favorable conditions for steady-state creep in the process zone near the crack tip. We deduce an equation for the description of the process of propagation of high-temperature creep cracks. This equation equipped with the corresponding initial and final conditions is regarded as a mathematical model for the evaluation of the period of subcritical growth of hightemperature creep cracks in metallic plates. The validity of the model is experimentally checked.

第4世代Ni基単結晶超合金のクリープ強度に及ぼす微量添加Yの影響

The effects of yttrium addition on high temperature properties of the 4th generation nickel-base superalloy were studied. Nickel-base superalloys, which possess both creep strength and oxidation resistance, are being required to satisfy high efficiency and reliability of jet engines. 4th generation superalloys contain platinum groups metals such as ruthenium. In order to satisfy creep strength and phase stability, however ruthenium worsens oxidation resistance. Yttrium doping was found to increase the oxidation resistance due to the improved adhesive strength of Al2O3 oxide, and the effects of yttrium doping on creep strength are established in this paper. Yttrium-added TMS-138 alloy (TMS-138Y) showed high creep strength of 4th generation superalloys, however the creep strength was slightly worse than the alloy without yttrium in the high-temperature low-stress creep condition. Microstructural observations confirmed the increase of Topologically Close Packed (TCP) phases in TMS-138Y. This was due to the formation of a yttrium-containing compound, Al0.06Ni0.73Co0.04Y0.18 (at.), which made incoherent interfaces that allowed TCP phases to precipitate by heterogeneous nucleation.

Strengthening at High Temperatures in an Iron-Aluminium Alloy by the Precipitation of Stable and Coherent Intermetallic Particles

AbstractDespite decades of intensive research iron aluminides remain characterised by relatively poor ductility at room temperature and low strength at high temperatures, especially under slow strain rate or creep conditions. A variety of strengthening particles has been tested for improving high temperature strength, but each has serious limitations: typical carbide precipitates are unable to resist dissolution or coarsening at high temperatures; as-solidified iron aluminides with sufficient amounts of transition elements such as Nb or Mo show heavy solidification segregation and are embrittled by a network of Laves phase; mechanical milling with stable oxides appears an excessively expensive processing route. A new iron-aluminium alloy has been developed with Zr and Cr additions that forms fine coherent precipitates even after extended annealing at temperatures as high as 900ºC. These precipitates have a complex Fe3Zr structure and form in a cube-on-cube orientation relationship in the bcc matrix. The low solubility and diffusivity of the solute, as well as the low energy, near-coherent interface ensures excellent stability of these intermetallic precipitates. Interesting strengthening is possible for this material under the relevant high temperature creep conditions.