G.E. Fuchs

Solution heat treatment response of a third generation single crystal Ni-base superalloy

Abstract The standard solution heat treatment of the third generation, single crystal Ni-base superalloy, CMSX-10, requires temperatures up to 1365°C and lasts a total of approximately 45 h. These high temperatures and long times result in a heat treatment that is costly. To determine if the heat treatment could be simplified and/or shortened to reduce the cost, a detailed study was completed on the standard heat treatment. The solution anneal heat treatment dissolves the eutectic γ/γ′ regions early in the heat treatment cycle at temperatures up to about 1340°C. However, the chemical segregation from the partitioning of elements during solidification, was not eliminated until much higher temperatures were reached. In particular, the segregation of W and Re to the dendrite core was not significantly reduced until temperatures in excess of 1360°C were reached in the heat treatment cycle. Reducing the heat treatment temperature and/or shortening the time of the heat treatment would be expected, therefore, to result in residual segregation of W and Re to the dendrite cores, a locally unstable microstructure, and, possibly, the formation of TCP phases.

Modeling of the partitioning and phase transformation temperatures of an as-cast third generation single crystal Ni-base superalloy

Abstract Cast single crystal Ni-base superalloys exhibit severe solidification segregation that must be subsequently removed by solution heat treatment. In order to understand how some of the elements in these alloys effect the solidification partitioning, the as-cast microstructure of the third generation single crystal Ni-base superalloy, CMSX-10, was examined. In addition, the solidification partitioning was calculated using Thermo-Calc™. The solidus, liquidus and γ ′-solvus were also calculated for the compositions of the base alloy, and determined compositions of the dendrite cores and the interdendritic regions. All of the calculated values were compared to the experimentally determined results of this study and areas for future work will be discussed.

Effect of W additions to Ti-48Al-2Nb-2Cr alloys

Abstract Alloying additions of 0, 0.5 and 1.0 at.% W were made to Ti-48Al-2Nb-2Cr alloys for increased strength and creep resistance. The alloys were processed by both ingot metallurgy (IM) and powder metallurgy (PM) routes. Duplex and near-lamellar microstructures were produced by heat treatment. The tensile properties of the materials were examined in the temperature range 25–1000 °C in air. The creep properties were determined at 850 °C at stresses ranging from 103 to 207 MPa. The interrelationship between composition, microstructure, processing and properties are discussed.

The microstructure and tensile properties of mitrogen containing vacuum atomized alloy 690

The mechanical properties and microstructure of a heat of nitrogen containing vacuum atomized A690 have been characterized. Although wrought A690 exhibits extensive grain growth during solution annealing heat treatments, only limited grain growth was observed in P/M690N{sub 2}. The presence of the nitrogen in the P/M690N{sub 2} resulted in the formation of a fine dispersion of Ti(C,N) which limited grain growth during elevated temperature exposures. The yield and ultimate tensile strength of the P/M690N{sub 2} was significantly greater than wrought A690 and elevated temperature exposures did not greatly affect the properties of the P/M690N{sub 2}. Although the P/M690N{sub 2} did exhibit appreciably higher strengths than wrought A690, the ductility was not adversely affected. In general, the resulting microstructure and, hence, mechanical properties of the P/M690N{sub 2} were very stable, uniform, and reproducible, even after long-term elevated temperature exposures of up to 24 hours at 1100{degree}C. 14 refs., 5 figs., 1 tab.

Calculating solidification and transformation in As-Cast CMSX-10

Advanced single-crystal nickel-based superalloy s contain significant levels of refractory metal alloying additions and exhibit severe solidification segregation that must be subsequently removed by solution heat treatment. The solidification partitioning and transformation temperatures of the third-generation single-crystal nickel-base superalloy, CMSX-10, was calculated using Thermo-Calc™. All of the calculated values were compared to the experimentally determined results of this study.

Microstructural evaluation of as-solidified and heat-treated γ-TiAl based powders

Powders with nominal compositions (in atomic %) Ti-48Al and Ti-48Al-2Nb-2Cr were prepared by the plasma rotating electrode process (PREP) and gas atomization (GA) techniques. As-solidified and heat treated (1000{degrees} C/3 hr) powder samples were examined by metallography, SEM, X-ray diffraction, and TEM. The microstructures of the powders were characterized as a function of atomization technique, alloy content, powder particle size (solidification rate), and thermal history. All of the as-solidified powders were comprised of disordered {alpha}, and ordered {alpha}{sub 2}-Ti{sub 3}Al and {gamma}-TiAl. For both alloys, a larger volume fraction of {alpha} and {alpha}{sub 2} was observed in the PREP powders relative to GA powders of comparable size. Additionally, for both alloys and both atomization techniques, the volume fraction of {alpha}{sub 2} was observed to increase with decreasing powder particle size. In general, the PREP powders appeared to solidify more rapidly than the GA powders. Possible reasons for the differences in microstructures and solidification rates observed for the two atomization techniques will be discussed. Upon heat treatment, the microstructure of the powders was similar. The heat treated powders were comprised predominently of equiaxed {gamma} grains with limited amounts of lamellar {alpha}{sub 2}/{gamma} grains also present. The GA powders did exhibit amorexa0» slightly higher volume fraction of {alpha}{sub 2} than the PREP powders, possibly due to slight differences in the Al content of the alloys or the higher oxygen content of the GA alloys.«xa0less

Investigation of Oxide Bifilms in Investment Cast Superalloy IN100: Part I. Mechanical Properties

Oxide bifilms are a proposed casting inclusion reported to have been observed in vacuum investment cast polycrystalline Ni-based superalloys. Ongoing research seeks to determine if current superalloy casting practices can result in the formation of oxide bifilms, and subsequently if it is possible to observe and characterize this phenomenon. The effects of casting atmosphere, turbulence, filtering, hot isostatic pressing (HIP), and heat treatment have been investigated to identify the critical parameters that have been reported to result in bifilm formation in Ni-based superalloys. Room temperature tensile and room temperature fatigue testing are used to identify the effects of each casting and processing parameter on casting defect formation and the resultant effects on mechanical properties. Characterization of mechanical test specimens seeks to identify the role of casting defects and microstructural features on the fracture mechanisms of the specimen conditions analyzed, and in particular, evidence of bifilm formation and the chemical composition(s) of oxide bifilms. Analyzed tensile and fatigue data did not indicate an influence of bifilms on the tensile or fatigue strength of vacuum processed IN100. Bifilms were not observed, via the characterization methods utilized, to be an active mechanism in tensile or fatigue fracture.

Investigation of Oxide Bifilms in Investment Cast Superalloy IN100: Part II. Characterization

Oxide bifilms are a proposed casting inclusion reported to have been observed in vacuum investment cast polycrystalline Ni-based superalloys. Ongoing research seeks to determine if current superalloy casting practices can result in the formation of oxide bifilms, and subsequently if it is possible to observe and characterize this phenomenon. The effect of casting atmosphere, turbulence, filtering, hot isostatic pressing, and heat treatment has been investigated to identify the critical parameters that have been reported to result in bifilm formation in Ni-based superalloy IN100. Scanning Auger microscopy (SAM), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HR-TEM) were utilized to characterize samples from each casting condition. In situ ultrahigh vacuum Auger fractography did not indicate the presence of bifilms on the fracture behavior of IN100 in any processing condition. SAM analysis identified a sulfur-enriched monolayer on the surface of dendritic casting porosity, and identified heterogeneous Ti oxycarbide inclusions in air cast IN100. SEM analysis also indicated the presence of Ti oxycarbide inclusions in air cast IN100, and determined that these inclusion structures consist of fine blocky external M(Ti, Mo)C carbide enveloping an internal core of alumina. HR-TEM analysis indicated that none of the oxycarbide inclusion interfaces exist as discontinuous unbound interfaces, and that the internal alumina core is an ultra-fine polycrystalline structure.

An Innovative Ceramic Corrosion Protection System for Zircaloy Cladding

Light Water reactor (LWR) fuel performance is currently limited by thermal, chemical and mechanical constraints associated with the design, fabrication, and operation of the fuel in incore operation. Corrosion of the zirconium based (Zircaloy-4) alloy cladding of the fuel is a primary limiting factor. Recent success at the University of Florida in developing thin ceramic films with great adhesive properties for metal substrates offers an innovative breakthrough for eliminating a major weakness of the Zircaloy clad. ?The University of Florida proposes to coat the existing Zircaloy clad tubes with a ceramic coating for corrosion protection. An added bonus of this approach would be the implementation of a boron-containing burnable poison outer layer will also be demonstrated as part of the ceramic coating development. In this proposed effort, emphasis will be on the ceramic coating with only demonstration of feasibility on the burnable outer coating approach. This proposed program i s expected to give a step change (approximately a doubling) in clad lifetime before failure due to corrosion. In the development of ceramic coatings for Zircaloy-4 clad, silicon carbide and zirconium carbide coatings will first be applied to Zircaloy-4 coupons and cladding samples by thermal assisted chemical vapor deposition, plasma assisted chemical vapor deposition or by laser ablation deposition. All of these processes are in use at the University of Florida and have shown great potential. The questions of adhesion and thermal expansion mismatch of the ceramic coating to the Zircaloy substrate will be addressed. Several solutions to these conditions will be examined, if needed. These solutions include the use of a zirconium oxide compliant layer, employment of a laser roughened surface and the use of a gradient composition interlayer. These solutions have already been shown to be effective for other high modulus coatings on metal substrates. Mechanical properties and adhesion of the coatings will be monitored as a function of the coating process parameters. The corrosion protection of the various coatings will be evaluated by accelerated corrosion testing. Engineering requirements for coating a full size Zircaloy clad tube will be determined. It is expected that the coating process will add approximately 10 dollars or 10% to the price of a tube. In the second approach, the University of Florida will demonstrate the feasibility to add a boron carbide outer layer to functions as a burnable poison.(B204)

Response by M.A. Kaplan and G.E. Fuchs

In his letter, Professor Campbell comments on Kaplan and Fuchs papers ‘‘Oxides Bifilms in Superalloy: IN100, Parts I and II and provides some very interesting thoughts on the impact of casting procedures on the formation of defects referred to as bifilms. Campbell also notes that these papers are the first reports of the use of powerful characterization techniques such as high resolution TEM and Auger microscopy. The vast majority of Campbell’s comments are re-interpretation of the photomicrographs and the data reported by Kaplan and Fuchs, which Campbell claims to be proof of the presence of bifilms throughout the test material examined. However, Campbell appears to ignore the extensive surface characterization results reported that clearly indicate that there are no oxide films or bifilms on the fractures surfaces examined. In a similar manner to Prof. Campbell, each of the comments will be addressed from each paper in order.