Candidate Alloys Research Articles

Materials for ultra-supercritical coal-fired power plant boilers

The efficiency of conventional fossil power plants is a strong function of the steam temperature and pressure. Research to increase both has been pursued worldwide, since the energy crisis in the 1970s. The need to reduce CO 2 emission has recently provided an additional incentive to increase efficiency. The main enabling technology in achieving the above goals has been the development of stronger high-temperature materials. Extensive R&D programs have resulted in numerous high strength alloys for heavy section piping, and tubing needed to build boilers. The study reported here is aimed at identifying, evaluating and qualifying the materials needed for the construction of the critical components of coal-fired boilers capable of operating with 760 °C (1400 °F)/35 MPa (5000 psi) steam. The economic viability of such a plant has been explored. Candidate alloys applicable to various ranges of temperature have been identified. Stress rupture tests have been completed on the base metal and on welds to a number of alloys. Steamside oxidation tests in an autoclave at 650 (1200 °F) and 800 °C (1475 °F) have been completed. Fireside corrosion tests have been conducted under conditions simulating those of waterwalls and superheater/reheater tubes. Weldability and fabricability of the alloys have been investigated. The capability of various overlay coatings and diffusion coatings have been examined. This paper provides a status report on the progress achieved to date on this project.

Development of a Fastener Structural Element Test for Certifying Navy Fastener Materials

Abstract New materials intended for use in critical U.S. Navy applications are evaluated by a Fracture Toughness Review Process (FTRP) for acceptability. In addition to developing data on the mechanical properties, fatigue life, and fracture toughness, the FTRP requires that a representative structural element test be performed to verify that the material can withstand plastic deformation at high loading rates in the presence of a detectable flaw. A Fastener Structural Element Test (FSET) has been developed to certify fastener materials such that the results of fracture mechanics testing on candidate alloys can be verified. The testing required the development of a suitable specimen that best represented a fastener, flaw seeding, and pre-cracking techniques, load and displacement measurement techniques suitable for high loading rates, and material performance acceptance criteria. The complete results of the testing on titanium alloy Ti-5AI-1Sn-1Zr-1V-0.8Mo (a.k.a. Ti-5111) and a Co-Ni-Cr-Mo-Al-Fe alloy will be presented along with the test methodology, data acquisition, and analysis details. The challenges encountered and areas that require further refinement will also be discussed.

Development of Inert Anode Materials for Electrowinning in Calcium Chloride Melts

The electroreduction of solid titanium dioxide in a calcium chloride melt is an attractive method for the production of titanium metal. Several groups have proposed versions of this route, but no commercial process yet exists. In laboratory studies, consumable carbon anodes are used, and these can give rise to a number of cell design and operational issues. The replacement of the carbon anode with an inert anode offers numerous benefits that include: elimination of carbon dioxide emissions; more efficient cell operation and reduced electrode costs. We have evaluated a wide range of candidate inert anode materials, including metals, alloys and conductive ceramics. The studies examined the chemical stability of these materials in a calcium chloride-based melt, and also performance in laboratory-scale electrolysis tests. All metals and alloys tested were either dissolved in the melt and/or formed a thick, highly resistive crust. A conductive ceramic, chromium titanate, was the best-performed candidate and gave an acceptable cell voltage during a 24 h electrolysis test. This work was undertaken as part of the CSIRO Light Metals Flagship.

Irradiated microstructure of alloy 800H

Austenitic alloy 800H has the same basic composition as INCOLOY alloy 800 (Fe–20Cr–32Ni) but with significantly higher creep-rupture strength due to a required 60 μm minimum grain size. It is one of the high temperature candidate alloys being considered for Generation IV nuclear reactor systems. The radiation resistance of 800H has not been previously studied. This work provides information on the microstructural changes in 800H after irradiation using 5.0 MeV Ni ions at 500 °C to 5 and 50 dpa. Following irradiation, changes in microstructure and phase stability were studied using transmission electron microscopy (TEM). At a dose of 50 dpa, no voids were found and the density and size of the faulted loops were measured to be 2.3 × 10 16 cm −3 and 8.4 nm, respectively. There are fine precipitates distributed in 800H with an average size approximately 6 nm and a density greater than 9.1 × 10 15 cm −3. The high Ni content and the presence of precipitates are believed to be responsible for the resistance to void formation at dose up to 50 dpa.

Creep Behavior of Double Tempered 8%Cr-2%WVTa Martensitic Steel

Creep testing was carried out at around 650°C for a martensitic 8Cr-2WVTa steel (F82H), which is a candidate alloy for the first wall of the fusion reactors of the Tokamak type. Rupture strength of the double tempered steel (F82HD) is lightly higher than that of simply tempered steel (F82HS). On the other hand, creep rate of F82HD is obviously smaller than that of F82HS in acceleration creep, though creep strain of F82HD in transition creep, where creep rate decreases with increasing strain, is larger than that of F82HS. Hardness of the crept F82HD decreases with increasing creep strain, which corresponded with the transmission electron microscopy (TEM) observation. On the contrary, X-ray diffraction and electron back-scattered diffraction pattern measurements show that fine sub-grains are created during transition creep. The creep curves were analyzed using an exponential type creep equation and the apparent activation energy, the activation volume and the pre-exponential factor were calculated as a function of creep strain. Then, these parameters were converted into two parameters, i.e. equivalent obstacle spacing (EOS) and mobile dislocation density parameter (MDDP). While EOS decreases with increasing creep strain, MDDP increases with increasing strain during transition creep. The decrease in EOS and the increase in either EOS or MDDP are rate-controlling factors in transition and acceleration creep, respectively. On the other hand, in case of F82HS, EOS increases and MDDP decreases during transition creep. In this case, the decrease in MDDP controls the creep rate during transition creep of F82HS. It is concluded that both EOS and MDDP are representative parameters of the change in substructure during creep.

一方向凝固によるSn-Cu二元共晶系合金のカップルドゾーンの解明

The Sn-Cu binary eutectic alloy is one of candidate alloys for lead-free solder. In order to investigate the evolution of solidified structure of Sn-Cu binary eutectic alloy, the unidirectional solidification technique has been applied using a Bridgman type furnace. Primary β-Sn exhibits dendrite morphology, on the other hand, a primary Cu6Sn5 indicates a faceted crystal, the cross section of which is typically H-shaped and eutectic structure is rod-like. Coupled growth zone of this alloy system is found to be extended to Cu-rich side and to be rather symmetric. This may be due to the difference in undercooling for growth of Cu6Sn5, β-Sn and eutectic phases. Jackson parameter of Cu6Sn5 is deduced to be high comparing with β-Sn.

Candidate Materials for High-Strength Fastener Applications in Both the Aerospace and Automotive Industries

There are many commercially available titanium alloys that have exhibited the capability of achieving high strength. Many of these alloys have not been seriously considered for fastener applications due to their cost or availability as coil or bar product. However, because new designs, increased material requirements, and larger aircraft are being built, the need to reduce weight and improve performance continues to be a major issue. The possibility of reducing weight by replacing currently used steel or Ni-based fasteners in various sizes is a great incentive. Over the past few years, many of these titanium alloys have been processed to bar and coil products to evaluate their capabilities as potential fastener materials. This article will review and summarize the mechanical properties, tensile, shear, notch tensile, and available fatigue, as well as the microstructure of these candidate alloys.

Modeling of Ni–Cr–Mo based alloys: Part I—phase stability

The CALPHAD approach is applied to the study of the stability of several candidate Ni–Cr–Mo based alloys for use in waste disposal canisters for the Yucca Mountain Project (YMP). The stability of occurring phases in the as-fabricated materials is investigated since temporal evolution of precipitates at service temperatures can significantly alter their corrosion resistance and mechanical properties. To account for the stability of the Ni 2Cr phase and the hypothetical Ni 2Mo and Ni 2W phases, available thermodynamic data and ab initio electronic structure-based results were employed. A comparative study of property diagrams for the nominal composition of several YMP candidate alloys is presented. These results form the basis for experimental and theoretical studies of kinetics of phase transformation and aging to be presented in Part II.

U.S. Program on Materials Technology for Ultra-Supercritical Coal Power Plants

The efficiency of conventional fossil power plants is a strong function of the steam temperature and pressure. Research to increase both has been pursued worldwide, since the energy crisis in the 1970s. The need to reduce CO2 emissions has recently provided an additional incentive to increase efficiency. More recently, interest has been evinced in advanced combustion technologies utilizing oxygen instead of air for combustion. The main enabling technology in achieving the above goals is the development of stronger high temperature materials. Extensive research-and-development programs have resulted in numerous high-strength alloys for heavy section piping and for tubing needed to build boilers. The study reported on here is aimed at identifying, evaluating, and qualifying the materials needed for the construction of the critical components of coal-fired boilers that are capable of operating with steam at temperatures of 760 °C (1400 °F) and pressures of 35 MPa (5000 psi). The economic viability of such a plant has been explored. Candidate alloys applicable to various ranges of temperatures have been identified. Stress rupture tests have been completed on the base metal and on welds to a number of alloys. Steamside oxidation tests in an autoclave at 650 °C (1200 °F) and 800 °C (1475 °F) have been completed. Fireside corrosion tests have been conducted under conditions simulating those of waterwalls and superheater/reheater tubes. The weldability and fabricability of the alloys have been investigated. The capabilities of various overlay coatings and diffusion coatings have been examined. This article provides a status report on the progress achieved to date on this project.

Influence of halide ions and alloy microstructure on the passive and localized corrosion behavior of alloy 22

Alloy 22 (N06022) is the current candidate alloy used to fabricate the external wall of the high-level nuclear waste containers for the Yucca Mountain repository. It was of interest to study and compare the general and localized corrosion susceptibility of Alloy 22 in fluoride and chloride solutions at 90 °C. Standard electrochemical tests such as cyclic potentiodynamic polarization, amperometry, and electrochemical impedance spectroscopy were used. Studied variables included the solution pH and the alloy microstructure (thermal aging). Results show that Alloy 22 is highly resistant to general corrosion in all the solutions tested. Thermal aging is not detrimental and even seems to be slightly beneficial for general corrosion at the higher solution pHs. Pitting corrosion was never observed. Crevice corrosion was found only for high chloride-containing solutions after anodic polarization. The presence of fluoride ions together with chloride ions seems to increase the susceptibility of Alloy 22 to crevice corrosion compared to pure chloride solutions.

Candidate plasma-facing materials for extreme ultraviolet lithography source components

Material selection and lifetime issues for extreme ultraviolet (EUV) lithography are of critical importance to the success of this technology for commercial applications. This work reviews current trends in production and use of plasma-facing electrodes, insulators, and wall materials for EUV-type sources. Ideal candidate materials should be able to: withstand high thermal shock from the short pulsed plasma; withstand high thermal loads without structural failure; reduce debris generation during discharge; and be machined accurately. We reviewed the literature on current and proposed fusion plasma-facing materials as well as current experience with plasma gun and other simulation devices. Both fusion and EUV source materials involve issues of surface erosion by particle sputtering and heat-induced evaporation/melting. These materials are either bare structural materials or surface coatings. EUV materials can be divided into four categories: wall, electrode, optical, and insulator materials. For electric discharge sources, all four types are required, whereas laser-produced plasma EUV sources do not require electrode and insulator materials. Several types of candidate alloy and other materials and methods of manufacture are recommended for each component of EUV lithography light sources.

Decomposition of martensite α″ during aging treatments and resulting mechanical properties of Ti−Ta alloys

Abstract Effects of Ta content and aging temperature on the decomposition of martensite α″ in the Ti−Ta alloys with Ta contents from 30 mass% to 50 mass% and the resulting mechanical properties, including the dynamic Young’s modulus, were investigated for biomedical applications. The microstructures were examined using a scanning electron microscopy (SEM), a transmission electron microscopy (TEM) and an X-ray diffraction (XRD) analysis. The decomposition of martensite α″ phase in the Ti−Ta alloys depends on both Ta content and aging temperature. Various microstructures such as α, (β + ω) and (α + β) phases precipitate in the Ti−Ta alloys after the aging treatments are performed between 723 K and 873 K for 259.2 ks. The dynamic Young’s modulus and mechanical properties of the Ti−Ta alloys are found to be sensitive to the microstructures caused by Ta content and aging temperature. The Ti–30% Ta alloy with α phase values much lower modulus and higher strength-to-modulus ratio than the currently used implant biomaterials, and thus has a great potential to become a new candidate alloy for biomedical applications.

Selecting metal alloy electric contact materials for MEMS switches**The views expressed in this paper are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense, or the US Government.

This paper presents a method for selecting metal alloys as the electric contact materials for microelectromechanical systems (MEMS) metal contact switches. This procedure consists of reviewing macro-switch lessons learned, utilizing equilibrium binary alloy phase diagrams, obtaining thin film material properties and, based on a suitable model, predicting contact resistance performance. After determining a candidate alloy material, MEMS switches were designed, fabricated and tested to validate the alloy selection methodology. Minimum average contact resistance values of 1.17 and 1.87 Ω were measured for micro-switches with gold (Au) and gold–platinum (Au–(6.3%)Pt) alloy electric contacts, respectively. In addition, ‘hot-switched’ life cycle test results of 1.02 × 108 and 2.70 × 108 cycles were collected for micro-switches with Au and Au–(6.3%)Pt contacts, respectively. These results indicate increased wear with a small increase in contact resistance for MEMS switches with metal alloy electric contacts.

Assessing lead‐free intellectual property

This paper presents the analysis of information collected from numerous patent searches on lead‐free alloys. The significance of claim structure and content is discussed in view of the growing number of lead‐free patents. Patent analysis software was developed to effectively compare over 350 lead‐free alloy patents. A case study was conducted to assess Sn‐Ag‐Cu and special purpose lead‐free candidate alloy intellectual property. The results show that there are a number of patents and patent applications that may affect the use of “popular” Sn‐Ag‐Cu formulations.

Improving the fracture toughness of constituent phases and Nb-based in-situ composites by a computational alloy design approach

A computational alloy design approach has been used to identify a ductile matrix for Nb-based in-situ composites containing Ti, Hf, Cr, Si, and Ge additions. Candidate alloys in the form of cast buttons were fabricated by arc melting. Coupon specimens were prepared and heated treated to vary the microstructure. Backscattered electron (BSE) microscopy, quantitative metallography, energy-dispersive spectroscopy (EDS), and X-ray diffraction (XRD) were utilized to characterize the morphology, volume fraction, composition, and crystallography of individual phases in the microstructure. The fracture toughness of the composites was characterized by three-point bending and compact-tension techniques, while the fracture toughness of individual phases in the in-situ composites was determined by an indentation technique. The composition, crystallography, and volume fraction of individual phases were correlated with the fracture-toughness results to assess (1) the role of constituent properties in the overall fracture resistance of the composites and (2) the effectiveness of the computational design approach. The results indicated that the effects of alloy addition and plastic constraint on fracture toughness were reasonably predicted, but the conditions for relaxing plastic constraint to attain higher fracture toughness were not achieved.

Sliding wear performance in molten Zn–Al bath of cobalt-based overlayers produced by plasma-transferred arc weld-surfacing

The purpose of this investigation was to improve the wear resistance of part materials (bush and sleeve in sink roll) by applying plasma transferred arc (PTA) surfacing technique in the continuous hot-dip galvanizing facilities. Candidate alloys from the preliminary investigation were cobalt-based Stellite 6 and Tribaloy 800 with WC, WC–Co, TiC, NbC, and Cr 3C 2. The resultant deposits were subsequently characterized using X-ray diffraction, scanning electron microscopy, and the microhardness tests. Microstructural development during the solidification of each deposit is discussed. Pin-on-disk sliding wear tests were conducted in a molten Zn–0.18%Al (wt.%) bath at 470 °C to evaluate the wear performance of the PTA weld-surfaced cobalt-based alloys. Based on microstructural observation and the microhardness measurement, sliding wear performance of the PTA weld-surfaced cobalt-based alloys in molten Zn–Al bath is reported and compared with pin and disk combination.

Phase Equilibria in the Ni-Si-B System

A thermodynamic study has been carried out on the Ni-Si-B ternary system, which is an important system in view of the development of Ni-base filler metals. A regular solution approximation based on the sublattice model was adopted to describe the Gibbs energy for the individual phases in the binary and ternary systems. Thermodynamic parameters for each phase have been evaluated using the available experimental information on phase boundaries and other related thermodynamic properties. Thermal analysis experiments have also been conducted on several ternary alloys to re-examine the available ternary experimental data on phase boundaries. The set of evaluated parameters in this study enables reproducible calculations of the liquidus and solidus temperatures and vertical section diagrams satisfactorily. Ni-base brazing filler metals are widely used in the fields of aircraft, various engines, and nuclear engineering due to their high strength at high temperatures, corrosion resistance and oxidation resistance. At present, the study on the development of Ni-base filler metals is done by trial and error. The melting point has to be measured for many alloy specimens in spite of only a few candidate alloys out of them are used for brazing processes. Therefore, the prediction of liquidus and solidus temperatures and phase equilibria is significantly useful in the design of candidate alloy compo- sition. The CALPHAD (CALculation of PHAse Diagrams) approach 1) of phase equilibria calculation using thermody- namic descriptions from databases provides a powerful tool for obtaining such information. In the present study, a thermodynamic analysis of the phase equilibria in the Ni-Si-B system, which is an important system of Ni-base brazing filler metals, has been carried out. And also thermal analysis experiments have been conducted on several ternary alloys in order to re-examine the available experimental data on phase boundaries. 2. Experimental Procedures The phase boundaries of the ternary system were deter- mined by differential scanning calorimetry (DSC). The starting materials are powders of Ni(99.9%), Si(99.9%), and B(99%). The alloys were prepared by arc melting of cold-pressed pellets in an atmosphere of argon. A titanium button was melted to getter the residual oxygen in the chamber, prior to melting the actual charges. The arc-melted alloys were re-melted in vacuum by induction heating in order to ensure homogeneity. The as-cast alloys were encapsulated in quartz tubes under vacuum and then annealed at 850 � C for 18 days before water quenching. No chemical analysis for the alloys was conducted, since the weight losses in preparing alloys were generally less than 1%. The prepared alloy compositions are shown in Table 1. Thermal analysis (DSC) was carried out using a Rigaku ThermoPlus DSC8270 (Rigaku Corp., Tokyo, Japan). A cylindrical specimen was heated and cooled in Al2O3 crucible at a rate of 5 � C/min under a purified argon-flow atmosphere with � -Al2O3 as the standard materials. However, in order to avoid experimental error caused by supercooling, the peak temperature values during heating were adopted in a thermodynamic analysis. The peak temperatures in heating processes are shown in Table 1. In addition, in order to confirm the eutectic temperature of the Si-B system, two alloys of the Si-B binary system were Table 1 Experimental results for phase boundaries of the Si-B binary and the Ni-Si-B ternary alloys determined by DSC. Alloy composition (mol%) Peak temperature ( � C)

Phase diagrams for lead-free solder alloys

The need for new, improved solder alloys and a better understanding of reactions during the soldering process grows steadily as the need for smaller and more reliable electronic products increases. Information obtained from phase equilibria data and thermodynamic calculations has proven to be an important tool in the design and understanding of new lead-free solder alloys. A wide range of candidate alloys can be rapidly evaluated for proper freezing ranges, susceptibility to contamination effects, and reactions with substrate materials before the expensive process of preparing and testing candidate alloys is initiated.

Stability and Aging of Candidate Alloys for the Yucca Mountain Project: CALPHAD Results

ABSTRACTThe CALPHAD approach is applied to the study of stability and aging of several Ni-Cr-Mo alloys similar to those considered for use in waste disposal containers for the Yucca Mountain Project (YMP). The stability of occurring phases in the as-fabricated materials must be investigated since their precipitation with time at temperature can reduce their corrosion resistance and mechanical properties. We first show that excellent agreement exists between assessed and calculated phase diagrams for the three major binary alloys, Ni-Cr, Ni-Mo, and Cr-Mo, and for the ternary Ni-Cr-Mo alloy. Then we present a comparative study of phase fractions versus temperature for the nominal composition of several multi-component Ni-Cr-Mo alloys. Finally we discuss the aging properties of the Ni2Cr-ordered phase in Ni-Cr and in Ni-Cr-Mo alloys, and of the complex P phase in Alloy 22.

A Study of the Corrosion Behaviour of Magnetron-Sputter Deposited Coatings in a Reducing-Sulphidising Atmosphere at 600°C

Heat exchangers in coal gasification plants are exposed to highly reducing and sulphidising process gases. This paper presents high temperature corrosion test results for two candidate heat exchanger materials, P91 and Alloy 800H, coated using an unbalanced magnetron-sputter deposition technique. The coatings of plain and modified type 310SS stainless steel on P91 provided a reasonable improvement of corrosion resistance compared to uncoated P91. The Ni-Cr-Al coatings with up to 5 wt-% Al enhanced the corrosion resistance of the already very stable Alloy 800H base material considerably, by the formation of an Al and O rich intermediate corrosion layer.