High-temperature Strength Of Alloys Research Articles

Effect of W/Mo ratio on the microstructure of Ni-Mo-W-Cr-Nb based superalloys after thermal exposure

The structural materials used in molten salt reactors (MSRs) require enough high-temperature strength, good chemical compatibility with fuels and irradiation damage resistance. The Nb addition and W alloying are two effective optimization paths, which have been proven to respectively improve the embrittlement problems and high-temperature strength of alloys. It is worth exploring whether one full-featured alloy can be developed for the high-temperature MSRs by combining the benefits of Nb addition and W alloying. In this work, four Ni-Mo-W-Cr-Nb based superalloys with different W/Mo ratios were designed by substituting W for Mo in the standard Nb-modified alloy. The microstructural evolution and mechanical properties of these alloys were evaluated systematically after thermal exposure at 750 °C. As the W/Mo ratio increased, more undissolved primary MC carbides remained, while fewer secondary carbides formed in the matrix and at the grain boundaries after thermal exposure. Micro-hardness and tensile tests showed that the substitution of Mo by W can provide an obvious strengthening effect for alloys in the solid-solution treated state. After thermal exposure, all alloys exhibited precipitation-induced strengthening, but the strengthening degree decreased with the W/Mo ratio due to the fewer secondary MC carbides. Based on experimental observations and thermodynamic calculations, it was found that W atoms cannot easily occupy the lattice sites of MC carbides as Mo atoms do, and the absence of secondary carbides in the high-W alloys is due to the undissolved primary MC carbides and the low Mo concentration.

Influence of Surfacing Technologies on Structure Formation of High-Temperature Nickel Alloys

The paper presents the results of studying the formation of structure and properties of metals when using additive technologies, such as arc and laser surfacing of highly alloyed nickel alloys. The main problem of nickel-chromium alloys during treatment with highly concentrated energy sources (welding, surfacing, soldering, plasma and laser treatment) consists in low strength and heat resistance after heating to high temperatures and rapid cooling, which is typical for these methods. The maximum high-temperature strength of such alloys is achieved after quenching and subsequent aging resulting in the formation of finely-dispersed γ′- phase precipitates, which inhibit plastic deformation. However, the achieved level of high-temperature strength of nickel alloys is often reduced significantly as a result of thermal effects and unfavorable structural changes during subsequent treatment (welding, surfacing). The paper presents the results of studying the structure, phase composition and properties of EP648 alloy during argon-arc surfacing. A comparison with the EP648 alloy structures obtained by laser surfacing is provided. It was established that argon-arc welding of EP648 alloy in combination with ultrasonic treatment creates an additional effect of increasing phase dispersion, which leads to an increase in hightemperature strength of the alloy.

Unique x-ray scattering approach provides insight into high-temperature strength of alloys

Unique x-ray scattering approach provides insight into high-temperature strength of alloys

Effect of Alloying Elements on Nb-Rich Portion of Nb-Si-X Ternary Systems and In Situ Crack Observation of Nb-Si-Based Alloys

To find a new route for microstructure control and to find additive elements beneficial for improving high-temperature strength, a systematic investigation is performed on hypoeutectic Nb-15 at. pct Si-X ternary alloys containing a transition element, Fe, Co, Ni, Cu, Ru, Rh, Pd, Re, Os, Ir, Pt, or Au. Information on phase equilibrium is classified in terms of phase stability of silicide phases, α Nb5Si3, Nb4SiX, and Nb3Si, and the relationship between microstructure and mechanical properties both at room temperature and high temperature is investigated. All the additive elements are found to stabilize either α Nb5Si3 or Nb4SiX but destabilize Nb3Si. A microstructure of Nbss/α Nb5Si3 alloy composed of spheroidized α Nb5Si3 phase embedded in the Nbss matrix is effective for toughening, regardless of the initial as-cast microstructure. Also the plastic deformation of Nbss dendrites may effectively suppress the propagation of longer cracks. High-temperature strength of alloys is governed by the deformation of Nbss phase and increases with higher melting point additives.

Multi-scale modelling of Suzuki segregation inγ′ precipitates in Ni and Co-base superalloys

The high temperature strength of alloys with (γ + γ ′) microstructure is primarily due to the resistance of the ordered precipitate to cutting by matrix dislocations. Such shearing requires higher stresses since it involves the creation of a planar fault. Planar fault energy is known to be dependent on composition. This implies that the composition on the fault may be different from that in the bulk for energetic reasons. Such segregation (or desegregation) of specific alloying elements to the fault may result in Suzuki strengthening which has not been explored extensively in these systems. In this work, segregation (or desegregation) of alloying elements to planar faults was studied computationally in Ni3 (Al,Ti) and Co3 (W,Al) type γ ′ precipitates. The composition dependence of APB energy and heat of mixing were evaluated from first principle electronic structure calculations. A phase field model incorporating the first principles results, was used to simulate the motion of an extended superdislocation under stress concurrently with composition evolution. Results reveal that in both systems, significant (de)segregation occurs on equilibration. On application of stress, solutes were dragged along with the APB in some cases. Additionally, it was also noted the velocity of the superdislocation under an applied stress is strongly dependent on atomic mobility (i.e. diffusivity).

Recrystallization and grain growth in the nickel-based superalloy allvac 718Plus

Abstract The nickel-based superalloy Allvac 718Plus was developed in order to combine formability and cost advantages of Alloy 718 with high temperature strength of Alloy 720. Typical applications for this new material are turbine disks that are produced by closed die forging, e. g. by screw pressing. The final mechanical properties are strongly related to the microstructure, which forms during hot forming. It is therefore of great interest to model both the dynamic and static grain structure development during processing. In this work, dynamic and static recrystallization as well as normal grain growth models are described for this material. The calculated grain structure evolution in Allvac 718Plus is compared to that of Alloy 718 in the typical process range of disk forging. Additionally, both the experimental program and structure analysis methodology are specified.

Microstructure and high temperature strength of age hardenable AA2219 aluminium alloy modified by Sc, Mg and Zr additions

In the present work, it was envisaged to develop thermally stable Al–Cu alloy by modifying its chemistry. AA2219 alloys with different levels of scandium (Sc), magnesium (Mg) and zirconium (Zr) were prepared by melting with inert gas (argon) tungsten arc welding. Sc was added in three levels (0·2, 0·4, 0·8%), Mg one level (0·45%) and Zr one level (0·2%). Optical, scanning electron microscopy (SEM), X-ray diffractometry (XRD) and transmission electron microscopy (TEM) were carried out to study the microstructural details. Long time aging studies and hot tensile tests were used to study the high temperature strength of the alloys. High temperature tensile test was performed on the specimens at 250, 350 and 400°C. Room temperature tensile test was carried out for comparative purpose. Electron probe microscopy analysis (EPMA) was carried out to find out the copper segregation in the alloys. Microstructural studies showed that raising Sc content from 0·2 to 0·8% resulted in significant grain refinement. Grain refinement was much more effective with the addition of 0·2%Zr to AA2219 alloy containing 0·8%Sc and 0·45%Mg. Addition of Sc, Mg and Zr was found to increase in eutectic temperature and decrease in freezing range. Hot tensile results were found to be in agreement with results of hardness data. Among all the compositions, AA2219+0·8%Sc+0·45%Mg+0·2%Zr alloy was found to be the best in terms of its high temperature strength.

Improvement in the high-temperature strength of alloys based on cobalt by internal nitriding

Internal nitriding, as for internal oxidation, may be considered as a particular case of material dispersion strengthening. Internal nitriding is accompanied by preparation of a heterophase structure of a diffusion layer: The matrix is a solid solution with fine particles of alloying element nitride particles distributed within it. Internal nitriding may be special importance as a version of dispersion hardening for high-temperature alloys. Possibilities are considered in this article for internal nitriding of cobalt alloys.

Influence of sulphidation and carburisation on creep

The mechanical properties of structural materials can be strongly affected by environmental conditions. In particular, during long term exposure at elevated temperatures, i.e. under typical creep conditions, technical atmospheres must be expected, in general, to cause corrosive attack. Thus, to obtain reliable design data it is the interaction of corrosion and mechanical behaviour which must be considered as the lifetime determining factor. The present work is an overview of the influence of sulphidation and carburisation on the creep behaviour of high temperature alloys with emphasis on heat resistant steels. Sulphur is known to be a particularly aggressive species and severe attack on material due to sulphidation is encountered in gas turbines (nickel base alloys) as well as in coal gasification or fluidised bed combustion plants (heat resistant steels). Sulphidation is found to reduce the creep strength of high temperature alloys and the creep ductility in particular can be drastically reduced as a result of accelerated creep crack propagation. Carburisation, which is observed, for example, in ethylene cracker tubes, seems to be less dangerous than sulphidation. Creep experiments on heat resistant steels have shown that carburisation can degrade, but can also improve the high temperature creep properties, depending on the alloy. However, at low temperatures, complete embrittlement of the material always occurs when severe carburisation has taken place. For multicomponent gas mixtures containing, for example, sulphidising, carburising, and oxidising potentials, the corrosion–creep interaction can be rather complex. A high carbon level may eventually overcompensate any deleterious effect caused by sulphidation.MST/1147

Influence of sulphidation and carburisation on creep

The mechanical properties of structural materials can be strongly affected by environmental conditions. In particular, during long term exposure at elevated temperatures, i.e. under typical creep conditions, technical atmospheres must be expected, in general, to cause corrosive attack. Thus, to obtain reliable design data it is the interaction of corrosion and mechanical behaviour which must be considered as the lifetime determining factor. The present work is an overview of the influence of sulphidation and carburisation on the creep behaviour of high temperature alloys with emphasis on heat resistant steels. Sulphur is known to be a particularly aggressive species and severe attack on material due to sulphidation is encountered in gas turbines (nickel base alloys) as well as in coal gasification or fluidised bed combustion plants (heat resistant steels). Sulphidation is found to reduce the creep strength of high temperature alloys and the creep ductility in particular can be drastically reduced as a result of accelerated creep crack propagation. Carburisation, which is observed, for example, in ethylene cracker tubes, seems to be less dangerous than sulphidation. Creep experiments on heat resistant steels have shown that carburisation can degrade, but can also improve the high temperature creep properties, depending on the alloy. However, at low temperatures, complete embrittlement of the material always occurs when severe carburisation has taken place. For multicomponent gas mixtures containing, for example, sulphidising, carburising, and oxidising potentials, the corrosion–creep interaction can be rather complex. A high carbon level may eventually overcompensate any deleterious effect caused by sulphidation.MST/1147

Features of the phase composition of nickel alloys with high tungsten and molybdenum contents

1. In the high-temperature hardened condition with a total tungsten and molybdenum content in the alloys of not more than 8 at. % α2, σ, and μ type excess phases are not formed and only complex carbides are present. In the alloys without molybdenum but containing 6.5 at. % W hardening α2-phase is precipitated. 2. The high-temperature strength of alloys containing ≥6.5 at. % W is higher than of alloys containing about 8 at. % W+Mo, which is related not only to hardening by these elements of the γ-solid solution but also to formation of excess α2-phase. 3. In the alloys containing more than 5–6% each of W and Mo (or ΣW+Mo≥5 at. %) type M6C binary chromium carbide is present, while with ΣW+Mo≤5 at. % type M23C6 cubic carbide is precipitated, which has a positive influence on the high-temperature strength of the investigated alloys.

Regularities in the change in scatter characterisics for the stressrupture strength of high-temperature alloys

Regularities in the change in scatter characterisics for the stressrupture strength of high-temperature alloys

一方向凝固したNi-Al-Ti-(Ta)系γ′⁄γ共晶合金の引張性質

Directionally solidified γ/γ′ eutectic Ni-Al-Ti alloys with and without Ta were prepared and the mechanical properties were examined. Alloys were melted in Al2O3 crucibles 6 mm in inner diameter using a vacuum Mo furnace, and the crucibles were lowered at various speeds. The oriented structure was obtained by the lowering speed of 2.78 μm/s (10 mm/h). The growth direction of γ and γ′ was 〈100〉, and it inclined by 10°-15° from the axes of specimens. The tensile strength showed maximum values at about 973 K (700°C). Compositions of alloys with best mechanical properties are 80.5Ni-11Al-8.5Ti (alloy L) and 81.5Ni-9Al-5.5Ti-4Ta (alloy T) in at%. The high temperature strength of alloy “T” was superior to that of alloy “20” having the composition of 72Ni-15Al-4.5Ta-5Co-1.75V-0.75Re-1W in at% examined by Jakson.

銅合金及びチタニウムの高温度における静的・動的強さについて

In recently, as it should be considered the high temperature strength of some copper alloys and commercially pure titanium for practical purpose, the author has been begining the study of high temperature tests, such as tensile, creep and fatigue tests of those materials. Following results were obtained.(1) The short time tensile strength and creep strength of Albrac and cupro nickel alloys tend to show the different behaviour in the high temperature.(2) High temperature fatigue strength of cupro nickel alloy shows the maximum value in the temperature range from 200 to 300°C.(3) The relations between the speed of cyclic stressing and the fatigue strength of the aluminium alloys and commercially pure titanium tend to show quite contrary each other.(4) In the case of tough and harder materials, such as stainless steel, commercially pure titanium and cupro nickel alloys, they generate heat by large repeated stress cycles and decrease their fatigue strength.(5) When the high temperature strength of the alloys are studied, it is necessary to consider the effects of speed of cyclic stressing and internal heating phenomenon above mentioned.

Temperature dependence of the hardness of secondary phases common in turbine bucket alloys

Representative examples of the most common types of secondary phases in turbine bucket alloys have been synthesized in bulk and their hardness measured from room temperature to 800°C. It is concluded that high hot hardness of the dispersed phase is an important factor in the high temperature strength of alloys but that other (as yet not explicitly identified) factors are also significant.