Resistance of a Mo–Si–B-Based Coating to Environmental Salt-Based Hot Corrosion

Oxidation of Metals - During operation of gas turbine engines, different salts develop that lead to a hot-corrosion attack. While the hot corrosion associated with Na2SO4 and Na3VO4 has received...     

Investigation of amorphous Ni0.60Nb0.40 diffusion barriers

Interactions of Ni_0.60Nb_0.40 amorphous alloys with polycrystalline overlayers of gold and copper and single-crystal substrates of silicon. GaAs and GaP were observed with Auger depth profiling. The Ni-Nb layer was deposited by r.f. sputtering and was approximately 5000 Å thick. The overlayers were evaporated to a thickness of 1000 Å. The amorphous metal reacted with the gold overlayers and the GaAs and GaP substrates at temperatures well below the nominal crystallization temperature of 650 °C. The Cu/Ni-Nb/Si system, in contrast, was stable at 600 °C for at least 1 h. Samples were also measured that had been contaminated with approximately 5–10 at.% O. Complete separation of the niobium and nickel into distinct layers was seen. For the samples on silicon substrates this separation was accompanied by the formation of a nickel silicide layer.     

Alloying and microstructure stability in the high-temperature Mo-Si-B system

Multi-phase alloys in the Mo-Si-B system are identified as high-temperature structural materials due to their high melting points (above 2000 °C) and excellent oxidation resistance attributed to the self-healing characteristics of borosilica layer up to 1400 °C. In the current study, the effect of alloying additions to achieve a reduced weight density has been examined in terms of changes in the microstructure and phase stability. The critical factor underlying the microstructural changes is related to the influence of the alloying additions on the stability of the high melting temperature ternary-based Mo₅SiB₂ (T₂) borosilicide phase.     

Oxidation-resistant coatings for ultra-high-temperature refractory Mo-based alloys

The synthesis of robust coatings that provide protection against environmental attack at ultra-high temperatures is a difficult challenge. In order to achieve this goal for Mo-based alloys the fundamental concepts of reactive diffusion pathway analysis and kinetic biasing are used to design a multilayer coating with a phase sequencing that provides for structural and thermodynamic compatibility and an underlying diffusion barrier to maintain coating integrity. The coating structure evolution during high-temperature exposure facilitates a prolonged lifetime as well as a self-healing capability. Both borosilicide and aluminide coatings that can be synthesized by a pack cementation process are demonstrated to yield superior environmental resistance on Mo-based systems at temperatures up to l,700°C and can be adapted to apply to other refractory metal systems.     

The undercooling of aluminum

An important parameter affecting microstructure development during solidification is the amount of undercooling prior to nucleation. The undercooling potential of aluminum has been assessed by thermal analysis measurements on powder dispersions of the liquid metal. A number of variables have been identified which influence the undercooling of powder Al samples including powder coating, powder size, melt cooling rate, and melt superheat. Surface analysis by Auger electron spectroscopy indicates that changing the medium in which the powders are produced is an effective method of altering the coating chemistry. Factorial design analysis has been employed to quantify the potential of processing variables to increase the undercooling level obtainable in aluminum. The factorial analysis indicates that control of the powder coating through changing the medium in which the powders are produced is most effective in decreasing the nucleation temperature. Additionally, the finest powders produced in the medium which induces the least catalytic coating, when cooled at high rates,T = 500 °C/s, from low superheats,T _s = 710 °C, are found to achieve the deepest undercooling, ΔT = 175 °C. These studies provide the basis for further increases in undercooling and for future investigations into the solidification reactions which produce both stable and metastable structures in aluminum alloys. Formerly Research Assistant in the Department of Metallurgical and Mineral Engineering, University of Wisconsin-Madison     

Interface reaction between Ni and amorphous SiC

A Ni/amorphous SiC (a-SiC) multilayered sample was prepared by ion-beam sputtering and was used as a model system to study the stability of metal contacts with a-SiC against interface reactions. The diffusion of Ni into the a-SiC layer as well as Si and C into the Ni layer takes place concurrently during the annealing process. An intermediate NiSi phase was identified in the Ni solution layer because of diffusion of Si and C resulting from the decomposition of a-SiC. A phase selection diagram has been developed that accounts for nucleation of the NiSi phase from the Ni solution layer.     

Solidification of undercooled Sn-Sb peritectic alloys: Part II. Heterogeneous nucleation

The undercooling behavior of fine droplet samples of Sn-rich, Sn-Sb alloys was investigated using differential thermal analysis (DTA). Undercooling levels measured during cooling from the liquid state follow the trend of the intermetallic phase liquidus, suggesting that solidification of all droplet samples (even those which solidify to yield a supersaturatedβ-tin product) was probably initiated with formation of primary intermetallic phase. Heterogeneous nucleation thermal cycling treatments were then used to measure the relative catalytic potency of primary intermetallic phases in this system for nucleation ofβ-tin during cooling. Crystallization reactions below the equilibrium peritectic temperature of 250 °C, at 187 °C and 230 °C, have been interpreted as corresponding to nucleation ofβ on Sn₃Sb₂ and SnSb substrates, respectively. The behavior observed in the Sn-Sb system can be generalized to guide the interpretation of heterogeneous catalysis and the analysis of solidification pathways in other peritectic alloy systems. Formerly Graduate Student, Department of Materials Science and Engineering, University of Wisconsin-Madison     

A high-resolution transmission electron microscopy study of interfaces between the γ, B2, and α2 phases in a Ti-Al-Mo alloy

Conventional and high-resolution transmission electron microscopy (HRTEM) were used to examine the interfacial structures in a Ti-50Al-5Mo (at. pct) alloy which was processed to produce combinations of γ, B2, and α₂ phases in a single sample. A small amount of a fourth phase labeled ζ was also found in the microstructure. It may be the phase Ti₂AlN but confirmation requires analysis of the N content in the phase.In this alloy, the orientation relationship between the γ and B2 phases is {111}_γ ∥}110}_B2 and 〈101]_γ ∥ 〈111〉_B2 with a coherent habit-plane interface parallel to (474)_γ. The orientation relationship between the B2 and α₂ (and also the ζ@#@) phases is {110}_B2 ∥(0001)_α2/ζ and 〈111〉_B2 ∥〈11-20〉_α2/ζ with a coherent interface parallel to the close-packed planes and along other orientations. The orientation relationship between the α₂ (and also the ζ@#@) and γ phases is (0001)_α2/ζ ∥{lll}_γ and (11•20)_α2/ζ ∥<10•1]_γ. The α₂ phase has a coherent interface parallel to the close-packed planes, while the ζ phase appears to adopt the (474)_γ interface plane of the γ phase, similar to the B2 phase. In some cases, the interface configuration between the γ and B2 phases appears to be altered by the presence of α₂ phase, resulting in a semicoherent interface. The phase labeled ζ in this study has the same orientation relationship with the γ and B2 phases as α₂ but consists of an ABABAC... stacking of close-packed basal planes. The (474)_γ habit plane interface between the γ and B2 phases is analyzed by several different theories of interfacial structure, and microstructural evolution in this system is also discussed. Formerly a Postdoctoral Fellow with the University of Wisconsin-Madison, is now with the This article is based on a presentation made during TMS/ASM Materials Week in the symposium entitled “Atomistic Mechanisms of Nucleation and Growth in Solids,” organized in honor of H.I. Aaronson’s 70th Anniversary and given October 3–5, 1994, in Rosemont, Illinois.