H. M. Tawancy

On the Precipitation of Intermetallic Compounds in Selected Solid-Solution-Strengthened Ni-Base Alloys and Their Effects on Mechanical Properties

We have investigated the susceptibility of selected solid-solution-strengthened Ni-base alloys of commercial grades to precipitating intermetallic compounds during aging at elevated temperatures and the corresponding effects on mechanical properties. Depending upon the exact chemical composition and aging temperature as well as the precipitate morphology, some alloys are found to be susceptible to precipitating detrimental intermetallics, particularly Ni3Mo, Ni4Mo, mu, sigma, δ Ni3Nb and Laves phase. However, in some cases, it is found that certain intermetallics can produce a good combination of strength and ductility such as the Ni2(Mo, Cr) with Pt2Mo-type superlattice as well as the γ″ phase of Ni3Nb with DO22-type superlattice. Also, it is demonstrated that in some cases, small addition of an alloying element such as Fe to a given alloy can decelerate the kinetics of forming detrimental intermetallic compounds; however, a similar addition to another alloy can produce an opposite effect.

Influence of Microstructure on the Mechanical Properties of Ground State Structures in Substitutional Ordered Alloys

Experiments confirm the co-existence of ground state crystal structures in ordered alloys predicted from first-principles calculations using the disorder-to-order transformation in Ni4Mo alloy as a model system. However, it is demonstrated that knowledge of atomic arrangements in these structures may not be sufficient to predict properties due to the influence of microstructure. Spontaneous recrystallization in the Ni4Mo alloy is found to embrittle an otherwise intermetallic compound with ground state structure combining high strength and high ductility. It is concluded that ground state structures determined from first-principles calculation may not necessarily play a governing role in the case of Ni4Mo alloy due to the microstructural effect.

Enhancing the Corrosion Resistance of Near-Stoichiometric Ni4Mo Alloy by Doping with Yttrium

It is shown that doping with yttrium can significantly improve the corrosion resistance of near-stoichiometric Ni4Mo alloy, which is otherwise susceptible to severe intergranular attack in the ordered condition. A one-to-one correspondence is found to exist between corrosion resistance and morphology of ordered Ni4Mo. The susceptibility to intergranular corrosion is correlated with a discontinuous grain boundary reaction resulting from self-generated recrystallization to accommodate the strain associated with continued growth of the ordered crystals. Doping with Y is found to maintain an ultrafine ordered microstructure suppressing the occurrence of self-generated recrystallization and, therefore, improving the corrosion resistance.

Degradation of Alloy 800H Used in Ethylene Furnace Tube Application by the Combined Effects of Carburization and Thermal Fatigue

An ethylene furnace tube made of the wrought alloy 800H has developed through the thickness crack. The cracking has been correlated with massive precipitation of embrittling carbide phases due to carburization attack and the thermally induced stresses during decoking cycles. Changes in microstructure and state of stress are reflected on the crack propagation mode, which is observed to change from fatigue near the outer surface to intergranular near the inner surface. It is concluded that the tube has been subjected to an environment of low oxygen potential and high carbon activity precluding the formation of protective oxide by the Cr2O3-forming alloy 800H. Process modification to reduce the decoking frequency and/or replacing the tube material with one capable of developing more protective oxide such as the Al2O3-forming alloy 214 may be considered to combat the problem.

On the Role of Microstructural Characterization in Damage Analysis of Turbine Blade Superalloys

We emphasize the importance of detailed microstructural characterization in damage analysis of turbine blade superalloys. Examples representing a variety of damage modes sustained during engine operation in the utility industry are presented. A one-to-one correspondence between microstructure and a given type of damage is developed using various electron-optical techniques to serve as a guide in interpreting the microstructures of damaged blades. Damage modes considered in the study include loss of mechanical strength due to coarsening of the strengthening γ′-phase in the γ matrix, creep and fatigue damage, detrimental grain boundary reactions involving the Cr-rich M23C6 carbide and γ′-phase, as well as precipitation of detrimental intermetallic compounds. Degradation of environmental resistance related to loss of mechanical strength is also elucidated.

Microstructural Analysis of Corroded Carbon Steel Used in Process Equipment Handling Caustic Soda

Heat exchanger tubes and pipes made of carbon steel to handle aqueous solutions of spent caustic and crude caustic soda, respectively, at a petrochemical plant have prematurely sustained corrosion damage. Microstructural analysis of representative samples has been conducted using scanning electron microscopy combined with energy- and wavelength-dispersive spectroscopy and X-ray diffraction. It is shown that the tubes were corroded due to the presence of sulfate ions in the spent caustic solution. The corrosion damage sustained by the pipes has been related to the presence of carry over chloride ions in the caustic solution. It is concluded that for better performance, both media require the use of Ni or some Ni alloys rather than carbon steels.