Mechanism of order-induced embrittlement in Ni-Ni4Mo alloys

Intergranular embrittlement of Ni-Mo alloys by long-range ordering to Ni₄Mo was examined for an off-stoichiometric alloy and a stoichiometric alloy of known impurity contents. Both tensile properties and corrosion resistance in HCl were measured as functions of exposure time at 700°C. Various techniques employed for microstructural characterization and microchemical analysis included analytical electron microscopy, X-ray diffractometry and Auger electron spectroscopy. During exposure at 700°C, the yield-strength maxima in both alloys were reached before the ductility minima. Homogeneous matrix ordering was observed to cause a moderate loss of ductility and the fracture mode remained to be transgranular. However, a considerable loss of ductility, intergranular embrittlement and extensive intergranular corrosion attack were found to be associated with heterogeneous grain-boundary ordering, which occurred by a discontinuous mechanism. Discontinuous ordering resulted in molybdenum-depleted zones alongside grain boundaries as evinced from microchemical analysis and localized corrosion attack. It was concluded that the observed intergranular embrittlement was caused by highly localized deformation in the molybdenum-depleted zones.     

Order-Strengthening in a nickel-base superalloy (hastelloy alloy S)

HASTELLOY* alloy s is a commercial, solid solu* HASTELLOY is a registered trademark of Cabot Corporation tion-strengthened, nickel-base superalloy developed for applications where oxidation resistance, low thermal expansion and retained ductility after long-time exposure at elevated temperatures are prerequisites. Its typical heat treatment consists of annealing at 1340 K (1950 °F) followed by air cooling to produce an essentially single phase material. When specimens from annealed heats were aged at 810 K (1000 °F) for 1000 to 8000 h and then tensile tested at room temperature, it was found that relative to the annealed condition, the 0.2 pct yield strength had nearly doubled while about 70 pct of the tensile elongation was retained. It is the objective of this note to report on the formation of a long-range ordered phase that caused the observed strengthening.     

On the behaviour of minor active elements during oxidation of selected Ni-base high-temperature alloys

We have examined the distribution of active minor elements in the oxide scales developed by selected Ni-base alloys with commercial grades. Emphasis is placed upon Mn, La and Si in a chromia-forming alloy and Y in an alumina-forming alloy. Initially, La and Y have been segregated at free surfaces and then become constituents of the oxides in contact with the substrates. A continuous layer of MnCr₂O₄ is formed above La- and Si-modified inner chromia layer. Silicon has been homogenously distributed throughout the grain structure, however, some La is present as LaCr₂O₃ particles and most of the remainder has been segregated at grain boundaries. The results indicate that the collective effect of Mn, Si and La is to extend protection by chromia to temperatures in excess of 1000 °C. Yttrium in the alumina- forming alloy is found to predominantly segregate at grain boundaries of nanostructured oxide with improved mechanical strength.     

Comparative Isothermal Oxidation Resistance of Selected Single-Crystal Ni-Based Superalloys With and Without Pt-Modified Surface Layers

Oxidation of Metals - Isothermal oxidation tests at 1150 °C show that the commercial grade of the single-crystal Ni-based superalloy CMSX-4 is superior to that of the CMSX-10 version....     

Enhancing the photovoltaic effect in the infrared region by germanium quantum dots inserted in the intrinsic region of a silicon p-i-n diode with nanostructure

We show that a strong photovoltaic response in the infrared region of the solar spectrum (1.1–1.4 μm wavelength) is obtained by inserting a multilayer structure of germanium quantum dots and silicon spacer layers into the intrinsic region of a silicon p-i-n diode. The multilayer structure (active layer) is deposited on an n-type silicon wafer using the technique of ultra-high vacuum chemical vapor deposition. Reflection high-energy electron diffraction has been used to in situ monitor the transition from the two-dimensional to three-dimensional growth mode of germanium on silicon. The p-type layer of the diode is formed in situ by doping a layer of silicon with boron. Prototype solar cells have been fabricated in situ to measure the energy conversion efficiency. Photoluminescence spectroscopy has been used to probe the presence of any defect-related energy levels within the band gap, and the quality of the diode is determined from measurement of dark current. Scanning electron microscopy, atomic force microscopy, and transmission/scanning transmission electron microscopy have been used to characterize the structure of the active layer. It is demonstrated that by optimizing the structure of the active layer to minimize recombination of charge carriers in the quantum dots, a short-circuit current of 24 mA/cm² and an open-circuit voltage of 0.6 V could be achieved leading to an energy conversion efficiency of about 11.5% corresponding to an active layer with a thickness of 300 nm.     

On the Degradation Modes and Oxidation Behavior of Platinum Aluminide Bond Coats in Thermal Barrier Coating Used as Surface Protection System for a Turbine Blade Superalloy

Adhesion of thermally grown oxide (TGO) to the bond coat is known to limit the useful life of thermal barrier coatings used in gas turbine blade applications. This is determined by the structure and composition of the bond coat as well as its thermal stability and in turn, its ability to develop and maintain a protective oxide. In this study, the degradation modes of platinum aluminides of the β-(Ni,Pt)Al- and PtAl₂ + β-(Ni,Pt)Al-types used as bond coats in thermal barrier coatings deposited on Ni-base superalloy and utilizing zirconia-7 wt% yttria and as top coat have been examined. Thermal exposure tests have been carried out at 1150 °C with cycling to room temperature every 24 h. Various electron-optical techniques have been used to characterize the microstructures of the bond coats and TGO. Particular emphasis has been placed upon the susceptibility of the bond coat to degradation by interdiffusion, oxidation, rumpling and formation of internal cavities. It is shown that the oxidation behavior and thermal stability characteristics are functions of the exact distribution of Pt in the bond coats. The β-(Ni,Pt)Al-type bond coat is found to have higher thermal stability and oxidize at a slower rate in comparison with the PtAl₂ + β-(Ni,Pt)Al₂-type. However, both bond coats are observed to exhibit a similar behavior in that the Al-rich and Pt-modified β-phase is progressively transformed into the Al-depleted γ′- and γ-phases with continued thermal exposure but at a slower rate in the β-(Ni,Pt)Al bond coat. Under the test conditions used in the study, there has been no evidence for rumpling, however, internal cavities are observed near the surface of each bond coat during the later stages of thermal exposure showing that rumpling is not necessarily a prerequisite. Failure of the respective thermal barrier coating systems is found to occur by loss of adhesion between the TGO and bond coat whose composition has approached that of the superalloy substrate by interdiffusion.     

Failure of a furnace outlet pipe in a benzene plant by internal oxidation due to improper welding practice

A furnace outlet pipe made of INCOLOY^® alloy 800H to handle gaseous hydrocarbon in a benzene plant developed cracks in the weld heat-affected zone during operation at 595 °C. Microstructural characterization revealed that the cracks were of the ductile intergranular mode, which could be related to localized plastic deformation alongside the grain boundaries. The microstructure of the heat-affected zone was distinguished from the base metal by a coarser grain structure and intergranular oxidation in addition to higher hardness indicating the presence of residual stresses from the welding process. Intergranular oxidation was found to result in a mixture of Cr and Fe oxides enveloping a Ni-rich solid-solution adjacent to the grain boundary. Therefore, the observed ductile intergranular cracking could be related to localized plastic deformation in the relatively “soft” zone of Ni-rich solid-solution. Most evidence indicated that the failure occurred because of improper welding atmosphere leading to internal oxidation under relatively low oxygen potential, which is oxidizing to Cr and to a lesser extent Fe, and reducing to Ni.     

Long-range ordering behaviour and mechanical properties of Ni-Mo-based alloys

Thermal ageing for up to 1000 h at 600–800 °C was used to study the long-range ordering behaviour to Ni₄Mo and related ordered phases in selected Ni-Mo based alloys with varying Fe and Cr contents, and the corresponding effects on mechanical properties. Analytical electron microscopy and X-ray diffraction were used to characterize the microstructure. Mechanical properties were determined from microhardness and tensile tests. During the early stages of ordering, the crystallographically-related Pt₂Mo-type, DO₂₂ and D1_a superlattices coexisted in all alloys studied. However, depending upon the exact chemical composition, particularly the Mo, Cr and Fe contents, some of these superlattices became unstable as the ordering reaction progressed. Chromium was found to act as a stabilizer of either a Pt₂Mo-type superlattice or Ni₃Mo depending upon the Mo content; however Fe acted as a stabilizer only of Ni₃Mo. For both Cr and Fe, the tendency to stabilize Ni₃Mo was realized at relatively higher Mo contents. Ordering behaviour of commercial alloys containing minor concentrations of Cr and Fe was found to significantly vary from one heat to another depending upon the exact Mo content. Although ordering to Ni₄Mo and Ni₃Mo could lead to severe embrittlement, ordering to a Pt₂Mo-type superlattice was found to have beneficial effects on mechanical strength.