Development Of High-temperature Alloys Research Articles

Development of High Temperature Materials for Aero Engine Hot End Components: An Overview

The present paper overviews the development of materials and superalloys for aero engine hot end components to meet the increasing trend of turbine inlet temperature. Requirement of higher and higher thrust is steadily increasing the turbine inlet temperature and the development of nickel-chromium superalloys in early 1940s could not fulfil the material capability for long. Various nickel-base superalloys developed in 1950s and 1960s could increase the life of hot end components by retaining strength and resisting oxidation at extreme temperatures. In the 1960s and 1970s, with almost stagnation in high temperature alloy development, metallurgists changed focus from alloy chemistry to alloy processing which evolved the directional solidification and single crystal casting technologies. At present, almost all fighter class engines and high bypass commercial engines are using nickel and cobalt base superalloys for hot end components and single crystal superalloys particularly for turbine blades. This paper covers the developmental phases of superalloys and casting technologies for engine hot end components. This paper will be an invaluable asset for the researchers as well as for designers of future gas turbine engines.

An excellent synergy in yield strength and plasticity of NbTiZrTa0.25Cr0.4 refractory high entropy alloy through the regulation of cooling rates

The solidification process often determines the microstructure of alloys from the atomic scale to the macroscopic scale, which distinguishes the performance of one alloy from another. Among all modification measures, the cooling rate plays a vital role in the solidification process. Hereinto, to thoroughly investigate the cooling rate effects on phase components, microstructures and mechanical properties, a series of NbTiZrTa0.25Cr0.4 refractory high entropy alloys were prepared under a wide range of cooling rates. The results show that the microstructures with combined dendrites and interdendrites gradually coarsened as the cooling rate decreased, coupled with the transition from a single BCC phase to BCC + Laves dual-phase. During the microstructural evolution, the prominent strengthening mechanism (i.e., solution strengthening) of the alloy prepared under high cooling rates became weak, while the secondary strengthening mechanism (i.e., second phase strengthening associated with Laves phase) was gradually reinforced. The reinforced second phase strengthening compensated for the weakened solution strengthening; as a result, the yield strength first decreased and then increased as the cooling rate decreased without sacrificing the plasticity of the alloys. It was found that the alloy prepared under an appropriate cooling rate of 62.5 K/s exhibited the maximum yield strength of 1452 MPa and maintained a competitive plasticity of approximately 23.2%, which exceeded the NbTiZrTa0.25Crx system and other reported refractory high entropy alloys. This study provides insight into the development of high-temperature alloys and a promising avenue to enhance the performance of an alloy by regulating the cooling rates.

High-Temperature Tensile Properties of a Cobalt-Based Co-20Cr-15W-10Ni Superalloy with a Bimodal Grain Structure

Cobalt-based superalloys are common materials for the manufacturing of various components used in aerospace applications. Conventional cobalt-based superalloys with a unimodal grain structure generally exhibit low strength and ductility at high temperatures. A bimodal grain structure of a cobalt-based superalloy, Co–20Cr–15W–10Ni (CCWN), was designed to achieve both high strength and ductility at high temperatures. The deformation behavior and tensile properties of a CCWN alloy with unimodal fine-grain (FG), coarse-grain (CG), and bimodal (FG/CG) structures were investigated at 900 °C. The microstructures and substructures after high-temperature deformation were examined via electron backscatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) to determine the deformation mechanisms. The microstructural observation showed that the bimodal grain structure consisted of FG and CG domains. During high-temperature deformation at 900 °C, the FG structure was mainly deformed by dynamic recrystallization (DRX), maintaining a similar FG structure. The CG structure was mainly deformed by DRV, resulting in a small amount of DRX grains and a large amount of dynamic recovery (DRV) grains. However, the bimodal grain structures were mainly softened via DRX and transformed into a new bimodal structure, ultrafine grain (UFG) and FG. The FG domains tended to deform via dislocations, and the CG domains via twinning. The high-temperature tensile tests revealed that the bimodal-structured alloy exhibited both higher strength and ductility than those of the alloy samples with unimodal FG or CG structure. This is associated with the newly developed UFG/FG structures in the bimodal grain-structured samples during high-temperature deformation. This work may provide new insight into the development of high-temperature alloys with bimodal grain structures.

Entropy-driven melting point depression in fcc HEAs

High Entropy Alloys (HEAs) are an increasingly dominant alloy design paradigm. The premise of entropic stabilization of single-phase alloys has motivated much of the research on HEAs. Chemical complexity may indeed help stabilize single alloy phases relative to other lower-entropy competing solid phases. Paradoxically, this complexity may de-stabilize these alloys against the liquid phase, potentially limiting the application space of HEAs at elevated temperatures. In this work, we carry out a comprehensive investigation of the phase stability in the fcc CoCrFeMnNiV-Al HEA space using a state of the art CALPHAD database. By using modern visualization techniques and statistical analysis we examine the trade-off between chemical complexity and stability against the liquid state and identify a potentially difficult to overcome barrier for development of high temperature alloys, at least within the conventional fcc HEA space. Limited experimental data seem to be consistent with this analysis.

Low-Density, High-Temperature Co Base Superalloys

Co base superalloys strengthened by coherent L12 ordered γ′ precipitate in a disordered face-centered cubic γ matrix represent a new opportunity for high-temperature alloy development. The emergence of alloys with low density and high specific yield strength at elevated temperatures has further energized the research and development efforts in the last 5 years. Initially stabilized by the addition of small amounts of Nb and Ta, these new generations of alloys with multiple alloying additions to form basic quaternary and ternary alloys have steadily expanded the property envelopes to raise hope for a modern class of superalloys with higher-temperature capabilities. This article reviews the work of a vibrant set of researchers across the globe whose findings are constantly unlocking the potential of these alloys. These developments have achieved high-temperature strength (at 870°C) >0.6 GPa, γ′ solvus temperature exceeding 1,100°C, and densities between 7.8 and 8.6 g/cm3.

Mo-Silicide Alloys for High-Temperature Structural Applications

The challenges of a high-temperature environment (T > 1,400°C) impose severe material performance constraints in terms of melting point, oxidation resistance, and structural functionality. A number of ceramic materials, intermetallic compounds, and refractory metals with high melting temperatures are available as material choices. However, in a single-component single-phase form, these materials do not satisfy all the aforementioned requirements. One clear message from the evolutionary development of high-temperature alloys is the importance of developing multicomponent alloys with multiphase microstructures and the capability to control phase fractions and morphologies to satisfy a number of mechanical property requirements. Besides the essential structural requirements, elevated temperatures often also involve aggressive environments that require a material to display an inherent oxidation protection that can be further enhanced by coating. Among the leading candidates to advance beyond the capability of the current nickel (Ni)-base superalloys, the multiphase microstructures that can be developed in the molybdenum-silicon-boron (Mo-Si-B) system involving a high melting temperature (>2,100°C) ternary-based intermetallic Mo5SiB2 (T2) offer an attractive performance. Most of the attention has been on three-phase alloys comprised of Mo(ss), T2, and Mo3Si that offer high-temperature stability and robust microstructures, but new alloy designs are in development. In this review the recent advances in the development of Mo-silicide alloys are discussed in terms of alloy design, microstructure control, structural performance, environmental resistance, and component analysis.

Mysterious failure in load-free superalloys under repeated thermal shocks

The increase of heat engine efficiency is highly demanded in applications such as gas turbines and fossil-fired power plants, which has posed prime challenges and opportunities for the development of high temperature alloys under extreme creep-fatigue-oxidation conditions. Besides the widely investigated turbine blade alloys and their coatings, many other components could be load-free or nearly so, but fracture failure and preferential oxidation are nevertheless found when subjected to repeated thermal shocks. This work is based on a specific contrast of oxidation behavior in load-free Co-Cr-C-W superalloys under isothermal and cyclic thermal conditions. First, we establish a mechanistic model on the stress generation arising from concomitant oxidation-diffusion-creep processes, based on the Stokes-Herring-Suo formalism. Because chromium diffuses from the substrate to the oxide-alloy interface, the divergence of this uniaxial diffusion flux is nontrivial, so that the lateral creep deformation is invoked to satisfy the kinematic constraint. Second, the preference of the oxide scale or “fingers” is found to depend on the degree of the above oxidation stress. Under the conditions of long fingers (as compared to the chromium depletion zone) and low oxidation stresses (as compared to the oxide strength), these fingers will grow and penetrating into the substrate, and thus subsequently the material fracture resistance is degraded. The above two lines of thought provide a mechanistic interpretation of thermal fatigue failure without external load, and help the further design of alloys under extreme thermal, mechanical, and corrosive conditions.

Mysterious Failure in Load-Free Superalloys Under Repeated Thermal Shocks

Abstract The increase of heat engine efficiency is highly demanded in applications such as gas turbines and fossil-fired power plants, which has posed prime challenges and opportunities for the development of high temperature alloys under extreme creep-fatigue-oxidation conditions. Besides the widely investigated turbine blade alloys and their coatings, many other components could be load-free or nearly so, but fracture failure and preferential oxidation are nevertheless found when subjected to repeated thermal shocks. This work is based on a specific contrast of oxidation behavior in load-free Co-Cr-C-W superalloys under isothermal and cyclic thermal conditions. First, we establish a mechanistic model on the stress generation arising from concomitant oxidation-diffusion-creep processes, based on the Stokes-Herring-Suo formalism. Because chromium diffuses from the substrate to the oxide-alloy interface, the divergence of this uniaxial diffusion flux is nontrivial, so that the lateral creep deformation is invoked to satisfy the kinematic constraint. Second, the preference of the oxide scale or “fingers” is found to depend on the degree of the above oxidation stress. Under the conditions of long fingers (as compared to the chromium depletion zone) and low oxidation stresses (as compared to the oxide strength), these fingers will grow and penetrating into the substrate, and thus subsequently the material fracture resistance is degraded. The above two lines of thought provide a mechanistic interpretation of thermal fatigue failure without external load, and help the further design of alloys under extreme thermal, mechanical, and corrosive conditions.

High Temperature Alloy Development for Future Nuclear Systems in Korea

Abstract A sodium-cooled fast reactor (SFR) and a very high temperature reactor (VHTR) are being developed according to the national road map of Korea. Until now, the R&D activities on the structural materials for these Gen-IV reactor systems can be summarized to the development of new alloys and evaluation of candidate materials. Extensive effort is being focused on the development of ferritic-martensitic (FM) steels and Fe-base oxide dispersion strengthened (ODS) alloys for the SFR applications. Research activity on Ni-base ODS alloys for the VHTR applications has been also performed, aiming at developing the intermediate heat exchanger materials with superior mechanical properties at high temperatures above 900 °C. Extensive studies have been made on the development of ceramic and composite material for VHTR application and on the simulation of radiation damage. Recently, reduced activation ferritic martensitic (RAFM) steel development is planned to develop structural materials for Korean Test Blanket Module (TBM) as one of the International Thermonuclear Experimental Reactor (ITER) development program.

Isothermal oxidation behaviour of Nb–W–Cr Alloys

A study of the effect of Cr content on the microstructure and isothermal oxidation behaviour of four alloys from the Nb–Cr–W system has been performed. Selection of specific alloy compositions has been based on the ternary isothermal sections. Oxidation experiments were conducted in air at 900 and 1300 °C for 24 h under isothermal conditions. Weight gain per unit area as function of the temperature has been used to evaluate the oxidation resistance. The phases present in the alloys and the oxide scales were characterized by XRD, SEM, and EDS. Microstructure consists of Nb solid solution and NbCr 2, Laves phase. The oxidation kinetics follows a parabolic behaviour at 1300 °C; the addition of 30% Cr resulted in the significant reduction of the parabolic oxidation rate. At 900 °C, alloys with higher Cr content exhibit higher oxidation rates in comparison to alloys with lower Cr content. The oxidation products are a mixture of CrNbO 4 and Nb 2O 5 and the amount of each oxide present in the mixture is related to the intermetallic phase content and the oxidation temperature. The characterization results delineate the effect of the Cr content on the oxidation mechanisms of these alloys that represent a promising base for high-temperature alloy development.

High-temperature alloys research: Still going strong

In this issue of JOM, the High Temperature Alloys Committee and the Seven Springs International Symposium Committee have compiled seven papers to provide a snapshot of the dynamic world of high-temperature alloys research. The search for structural materials that can be used at higher temperatures, under increased stresses, in more aggressive environments, and for longer times is a never-ending battle. In particular, the development of the gas turbine engine has been paced by the development of high-temperature materials, and more specifi cally, superalloys. The goal to achieve higher effi ciencies, increased performance, and decreased emissions in gas turbines has and will continue to require improved materials. The papers in this issue are broken down into two categories: processing and properties of high-performance coatings and alloys and recent development of high-temperature alloys. Most of the papers are on nickel-based superalloys, the most commonly used materials in the hot sections of gas turbines. Also included are two examples of other high-temperature materials. Within the fi ve high-performance coatings and alloys papers, the development of new alloys and the effects of composition and processing on the properties of high-temperature alloys are discussed. M.P. Brady et al. describe in “The Development of Alumina-Forming Austenitic Stainless Steels for High-Temperature Structural Use” the development of a new family of Al 2 O 3 -forming austenite stainless steels High-Temperature Alloys Research: Still Going Strong

Differential barothermal analysis in the course of reactive powder barothermal processing of RuAl alloys

Thermostable high-temperature structural alloys based on the refractory (melting temperature Tm=2060°C) RuAl intermetallic (IM) with an ordered B2 crystal structure (CsCl type) are developed. This IM surpass other aluminides (NiAl, TiAl and Ni3Al) used as the base for the development of high-temperature alloys and matrices of high-temperature composites (CM) intended for hot parts of supersonic engines, which serve at the temperatures exceeding operation temperatures of modern nickel-base superalloys. The differential barothermal analysis was used to develop the basic technological process of barothermal reaction sintering to produce near-net shape billets from RuAl-based structural materials.

Strengthening of iron aluminide alloys for high-temperature applications

ABSTRACTAn overview is given on materials developments of ferritic and Fe3Al-based iron aluminium alloys with strengthening precipitate phases for high-temperature applications currently underway at the Max-Planck-Institut für Eisenforschung GmbH (MPIE). The development of high-temperature alloys for structural applications is to be focussed on optimisation of strength, creep and corrosion resistance at high temperatures and sufficient ductility at lower temperatures. This is discussed with respect to recent studies on Fe-Al-based alloys with strengthening precipitates, such as κ-phase Fe3AlCx, MC-carbides, Laves phase, and the B2-ordered intermetallic phase NiAl. The following alloy systems have been investigated: Fe-Al-X (X=C, Ti, Ta, Mo, Zr), Fe-Al-Ti-Nb, Fe-Al-Ni-Cr, and Fe-Al-M-C (M=Ti, V, Nb, Ta).The investigations have been focussed on the microstructure, constitution, mechanical properties, and high-temperature corrosion behaviour of Fe-Al-based alloys with Al contents ranging from 10 to 30 at. %.

The AMC – CAST Alliance for Advanced Magnesium Research and Development

Australian Magnesium Corporation (AMC) and the Cooperative Research Centre for Cast Metals Manufacturing (CAST) have strengthened their relationship by creating an alliance to further R & D on the processing and applications of magnesium alloys. This research is necessary in order to address a number of issues that are critical for continued growth of the market for magnesium alloys in the automotive industry. The paper describes recent progress that has been made in R & D on such important issues as: continuous direct chill casting; cover gas protection solidification; mechanical properties, high temperature alloy development and design. Many aspects of this research program are being conducted in close collaboration with manufacturers and end-users of magnesium alloys.

Magnesium technology 2002, part II: Wrought products, alloy processing, R&D strategies, corrosion, welding

Particularly interesting among the topics covered at the annual TMS Magnesium Symposium are sessions devoted to new materials and processes. While high-temperature alloy development (reviewed in Part I) has been a major emphasis in recent magnesium symposia, the surprise development during the 2002 TMS Annual Meeting in February was a new interest in magnesium wrought products. This Part II conference review article describes key conference papers on magnesium wrought products and processes as well as other processes (e.g., semisolid processing, welding), and themes reviewed will be corrosion, always of interest to magnesium applications, and general R&D strategies.

Magnesium technology 2002, part I: Primary production, environmental issues, high-temperature alloys

It is expected that the research in high-temperature alloy development will reach a critical mass in about five years. This will not only raise the materials science of magnesium to a higher level but will also increase the confidence of the industry in magnesium as a structural performance material. Currently, applications are being actively pursued in United States Council for Automotive Research power train programs and European Council for Automotive R&D engine block programs to develop the technology for elevated-temperature magnesium. More research is still needed in this interesting materials field.

Alloy design strategies for promoting protective oxide-scale formation

This article discusses general strategies for designing alloys to form protective oxide scales. Approaches based on classical alloyoxidation theories work reasonably well for single-phase alloys. However, high-temperature alloy development has been and will increasingly be based on multiphase microstructures in order to achieve many of the needed, but usually opposing properties, such as high-temperature strength and room-temperature toughness. No theoretical-based, well-defined strategies exist for the design of oxidation-resistant multiphase alloys. Still, key factors are beginning to emerge, which can provide guidance for promoting the formation of protective scales on multiphase alloys and for taking advantage of some unique mechanisms that are operative in multiphase alloys but not in single-phase alloys.

High-temperature performance evaluation of adirectionally solidified nickel-base superalloy

The application of a new approach, design for performance, for high-temperature alloy development, design analysis, and remaining life assessment, based on short-time high-precision testing, is described in this paper. The material tested was a directionally solidified nickel-base alloy, GTD111. It was found that the creep strength at 850 °C was indeed superior to that of a competitive alloy, IN738, but was not necessarily enhanced by the preferred alignment of grain boundaries and crystal orientation. In contrast, the fracture resistance at 800 °C was improved in the longitudinal direction compared with transverse and diagonal orientations in terms of susceptibility to gas phase embrittlement (GPE) by oxygen. Specimens cut transversely and diagonally to the growth direction were more sensitive to GPE than specimens taken from conventionally cast IN738. The new conceptual framework allows account to be taken of GPE and other embrittling phenomena, which may develop in service, leading to rational life management decisions for gas turbine users. Additionally, straightforward design analysis procedures can be developed from the test data, which for the first time allow separate measurements of creep strength and fracture resistance to be used for performance evaluation.

Advanced materials and coatings for energy conversion systems

Following an historical review of the development of high-temperature alloys for energy conversion systems including turbine engines, some of the current advances in single crystal materials. intermetallics, metal-matrix composites, and ceramic-matrix composites are discussed. Particular attention is directed at creep phenomena, fatigue properties, and oxidation resistance. Included within the discussions is the current status of carbon/carbon composites as potential high-temperature engineering materials and the development of coating systems for thermal barrier and oxidation protection. The specific influences of combustion gas compositions, i.e., oxidation potential, sulfur, halides, etc. are discussed. A current list of eligible advanced materials and coatings systems is presented and assessed. Finally, the critical failure mechanism and life-prediction parameters for some of the new classes of advanced structural materials are elaborated with the view to achieving affordability and extended life with a high degree of reliability. Examples are drawn from a variety of energy conversion systems.

High-temperature structural (β+γ) alloys based on NiAl with increased low-temperature plasticity

Development of a new generation of gas-turbine engines calls for development of high-temperature alloys with better operating parameters than those of the current nickel superalIoys. The most important factor is an increase in the working temperature of the alloy and decrease in its density while retaining the level of its mechanical properties. These problems can be solved by using intermetallic compounds, in particular, nickel monoaluminide, as the base for new high-temperature alloys. Nickel monoaluminide NiAI ([3-phase) has an ordered bcc crystal lattice of the type CsCI (B2), a high melting point (1640~ and a low density (5.86 g / cm 3 ). The low plasticity of NiAI at low temperatures and a relatively low high-tempera:ure strength above 800~ restrict its use as a structural material. It is possible to increase the high-temperature strength of alloys based on NiA1 by alloying and / or developing special structures in the alloy: controlled crystallized structures or controlled recrystallized structures that provide a decrease in the fraction of the lateral boundaries. One of the methods of increasing high-temperature plasticity is grinding of the grains. However, fine-grained alloys have a decreased creep resistance. Therefore it is of interest to increase the low-temperature plasticity or increase the resistance to crack propagation by developing heterophase structures, in particular, heterophase alloys based on NiAI in which plastic, ductile laminas of "soft" phases with a disordered fcc crystal structure (e.g., a )'-solid solution based on Ni, ct ()')-solid solutions based on Fe) coexist with disperse segregations of more "rigid" strengthening phases, e.g., bcc crystal solid solutions based on refractory metals of group VIA Cr, Mo, W and / or refractory implantation phases such as carbides and borides [ 1 ]. Among the promising bases for developing alloys with the ductile component in the form of a )'-solid solution are (13 + "/)-alloys of the system N i AI C o , in which there is a wide two-phase region parallel to the side N i Co [2, 3]. In the system Ni AI Co fusible excess phases do not evolve and low-temperature transformations do not occur. It is also known that the value of the diffusion coefficient at 900~ de-