Daniel J. Sordelet

Effects of Platinum on the Interdiffusion and Oxidation Behavior of Ni-Al-Based Alloys

Thermal barrier coating (TBC) systems, needed for higher thrust with increased efficiency in gas turbines, typically consist of an alumina-scale forming metallic bond coat and a ceramic topcoat. The durability and reliability of TBC systems are critically linked to the oxidation behavior of the bond coat. Ideally, the bond coat should oxidize to form a slow-growing, non-porous and adherent thermally grown oxide (TGO) scale layer of α-Al2O3. The ability to promote such ideal TGO formation depends critically on the composition and microstructure of the bond coat, together with the presence of minor elements (metal and non-metal) that with time diffuse into the coating from the substrate during service. An experimental program was undertaken to attain a more detailed fundamental understanding of the phase equilibria in the Ni-Al-Pt system and the influences of alloy composition on the formation, growth and spallation behavior of the resulting TGO scales formed during isothermal and thermal cycling tests at 1150°C. Additional studies were conducted to determine the influence of platinum on interdiffusion behavior in the Ni-Al system, and how this influence would impact coating/substrate interdiffusion. It will be shown that platinum has a profound effect on the oxidation and interdiffusion behaviors, to the extent that novel advanced coating systems can be developed. Introduction The demand for improved performance in high-temperature mechanical systems has led to increasingly severe operating environments, particularly for the components in advanced gas-turbine engines. Future improvements in gas-turbine performance will require even higher operating efficiencies, longer operating lifetimes, reduced emissions and, therefore, higher turbine operating temperatures. Advanced cooling schemes coupled with thermal barrier coatings (TBCs) can enable the current families of nickel-base superalloys to meet the materials needs for the engines of tomorrow. Thermal barrier coating systems currently provide average metal temperature reductions of about 80°C, while potential benefits are estimated to be greater than 170°C. However, lack of reliability, more than any other design factor, limits the general use of TBC systems for gas turbines. Commercial advanced TBC systems are typically two-layered, consisting of a ceramic topcoat and an underlying metallic bond coat. The properties of the ceramic topcoat are such that it has a low thermal conductivity, high oxygen permeability, and a relatively high coefficient of thermal expansion. The topcoat is also made “strain tolerant” by depositing a structure that contains numerous pores and/or pathways. The consequently high oxygen permeability of the topcoat imposes the constraint that the metallic bond coat must be resistant to oxidation attack. The bond coat should therefore be sufficiently rich in aluminum to form a protective, thermally grown oxide (TGO) scale of α-Al2O3. In addition to imparting oxidation resistance, the TGO serves to bond the ceramic topcoat to the substrate/bond coat system. Notwithstanding, it is generally found that spallation and/or cracking of the growing TGO scale is the ultimate failure mechanism of Materials Science Forum Online: 2004-08-15 ISSN: 1662-9752, Vols. 461-464, pp 213-222 doi:10.4028/www.scientific.net/MSF.461-464.213

Preparation of spherical, monosized Y2O3 precursor particles

Abstract Spherical, monosized yttria precursor particles were obtained by homogeneous precipitation in aqueous solutions by reaction with the thermal decomposition products of urea. Increasing [Y 3+ above 0.05 M resulted in a deviation from spherical morphology and caused agglomeration of particles. Over the concentration range studied, excess urea did not affect particle morphology, but increased the yield. Increasing aging time appeared to increase particle size as well as to improve yield, as long as the urea was not depleted. The approximate chemical composition of the precipitate was YOHCO 3 . The YOHCO 3 particles formed were amorphous to X-rays, and underwent a two-stage thermal decomposition, first forming Y 2 O 2 CO 3 near 180°C, and then cubic Y 2 O 3 above 610°C. Generation of CO 3 2− appeared to be crucial to the formation of the solid phase. Heating the aqueous yttrium solution with trichloroacetic acid (CCl 3 COOH) instead of urea as the precipitating agent produced a solid phase, while heating the same solution with formamide (HCONH 2 ) substituted for urea formed no precipitate.

Development of suitable interatomic potentials for simulation of liquid and amorphous Cu―Zr alloys

We present a new semi-empirical potential suitable for molecular dynamics simulations of liquid and amorphous Cu–Zr alloys. To provide input data for developing the potential, new experimental measurements of the structure factors for amorphous Cu64.5Zr35.5 alloy were performed. In this work, we propose a new method to include diffraction data in the potential development procedure, which also includes fitting to first-principles and liquid density and enthalpy of mixing data. To refine the new potential, we used first-principles and liquid enthalpy of mixing data published earlier combined with the densities of liquid Cu64.5Zr35.5 measured over a range of temperatures. We show that the potential predicts a liquid-to-glass transition temperature that agrees reasonably well with experimental data. Finally, we compare the new potential with two previously developed semi-empirical potentials for Cu–Zr alloys and examine their comparative and contrasting descriptions of structure and properties for Cu64.5Zr35.5 liquids and glasses.

Using atomistic computer simulations to analyze x-ray diffraction data from metallic glasses

We propose a method of using atomistic computer simulations to obtain partial pair correlation functions from wide angle diffraction experiments with metallic liquids and their glasses. In this method, a model is first created using a semiempirical interatomic potential and then an additional atomic force is added to improve the agreement with experimental diffraction data. To illustrate this approach, the structure of an amorphous Cu64.5Zr35.5 alloy is highlighted, where we present the results for the semiempirical many-body potential and fitting to x-ray diffraction data. While only x-ray diffraction data were used in the present work, the method can be easily adapted to the case when there are also data from neutron diffraction or even in combination. Moreover, this method can be employed in the case of multicomponent systems when the data of several diffraction experiments can be combined.

Plasticity in Ni59Zr20Ti16Si2Sn3 metallic glass matrix composites containing brass fibers synthesized by warm extrusion of powders

Deformation behavior of centimeter-scale Ni-based metallic glass matrix composites reinforced by brass fibers, synthesized by warm extrusion of gas atomized powders, has been investigated under the uniaxial compression condition at room temperature. Throughout the extrusion process, all blended spherical powders are elongated along the extrusion direction. The brass fibers are well distributed in the metallic glass matrix for the metallic glass matrix composites containing the brass up to 0.4 in volume fraction and no pores are visible. With increasing the brass content, elastic modulus and strength decrease due to the softness of the brass, but enhanced macroscopic plasticity is observed due to the formation of multiple shear bands, initiated from the interface between brass fiber and metallic glass matrix, as well as their confinement between the brass fibers. These behaviors are not observed in the sample synthesized by warm extrusion of only metallic glass powders.

Synthesis of Cu47Ti34Zr11Ni8 Bulk Metallic Glass By Warm Extrusion of Gas Atomized Powders

Cu 4 7 Ti 3 4 Zr 1 1 Ni 8 amorphous gas atomized powders were consolidated by warm extrusion. After consolidation near 723 K using an extrusion ratio of 5, the material retains between 88% and 98% of the amorphous structure found in the gas atomized powder. The onsets of the glass transition and crystallization temperatures of this extruded material are observed respectively at slightly higher and lower temperatures than those of the starting powders. These temperature shifts are attributed to a composition change in the remaining amorphous phase during partial devitrification throughout the extrusion process. Powders extruded at the same temperature, but using higher extrusion ratios of 9 and 13, exhibit substantial devitrification during the consolidation process yet still deform homogeneously.

Abrasive wear behavior of Al–Cu–Fe quasicrystalline composite coatings

Abstract Quasicrystals are a relatively new class of materials which exhibit unusual atomic structure and useful physical and chemical properties. The inherent brittleness of quasicrystals has limited their potential use to primarily surface coating applications. This study examined the effect on abrasive wear behavior of Al–Cu–Fe quasicrystalline coatings through the addition (e.g. 0–100 v/o) of a relatively ductile Fe–Al phase. Coatings were deposited by plasma arc spraying techniques. The incorporation of discrete Fe–Al particles into the quasicrystalline coating matrix improves the abrasive wear resistance. Moreover, it is observed that low-level additions of the Fe–Al phase (e.g. 1 v/o) produce the most abrasive wear-resistant coating. The wear behavior of the quasicrystalline and composite coatings is discussed in terms of wear mode and coating hardness.

Experimental and ab initio molecular dynamics simulation studies of liquid Al[subscript 60]Cu[subscript 40] alloy

X-ray diffraction and ab initio molecular dynamics simulation studies of molten Al{sub 60}Cu{sub 40} have been carried out between 973 and 1323 K. The structures obtained from our simulated atomic models are fully consistent with the experimental results. The local structures of the models analyzed using Honeycutt-Andersen and Voronoi tessellation methods clearly demonstrate that as the temperatures of the liquid is lowered it becomes more ordered. While no one cluster-type dominates the local structure of this liquid, the most prevalent polyhedra in the liquid structure can be described as distorted icosahedra. No obvious correlations between the clusters observed in the liquid and known stable crystalline phases in this system were observed.

Synthesis of Ni-based bulk amorphous alloys by warm extrusion of amorphous powders

Abstract A high strength Ni-based bulk amorphous alloy is synthesized by warm extrusion of gas atomized amorphous powder. The Ni59Zr20Ti16Si2Sn3 amorphous powder obtained by a high pressure Ar gas atomization method has a wide super-cooled liquid region of 63 K. Warm extrusion of the amorphous powders in the super-cooled liquid state successfully yielded a fully consolidated bulk amorphous Ni59Zr20Ti16Si2Sn3 alloy. The processing conditions for extrusion are obtained from the time–temperature-transformation curve for the onset of crystallization of the amorphous powder. Lower extrusion ratio of 5 is preferred for the retention of the single amorphous phase during extrusion at 848 K. The extruded bulk amorphous Ni59Zr20Ti16Si2Sn3 alloy exhibits a high strength level (∼2.0 GPa) similar to that of an as-cast bulk amorphous Ni59Zr20Ti16Si2Sn3 alloy (∼2.2 GPa). The mechanical behavior of the extruded alloy under the compressive condition shows no anisotropy in the longitudinal and transverse directions to the extrusion direction.

Synthesis of Ni-based bulk metallic glass matrix composites containing ductile brass phase by warm extrusion of gas atomized powders

To prevent catastrophic failure by propagating highly localized shear bands and to overcome the limited dimension of metallic glass, centimeter-scale Ni 5 9 Zr 2 0 Ti 1 6 Si 2 Sn 3 bulk metallic glass matrix composites were fabricated by warm extrusion of a mixture of gas-atomized fully amorphous powders and ductile brass powders. After consolidation, the composite retained the fully amorphous matrix found in the gas-atomized powder combined with the brass second phase. The glass-transition and crystallization temperatures of the extruded material were the same as those of the starting powders. The confined ductile brass phase enabled the bulk metallic glass matrix composites to deform plastically under uniaxial compression at room temperature. The combination of strength and ductility in the inherently brittle Ni-based monolithic materials could be obtained by introducing a ductile phase in the bulk metallic glass matrix. However, control of the volume fraction and distribution of the ductile brass phase was important for the proper combination of the strength and plasticity.