Noam Eliaz

Hot corrosion in gas turbine components

Abstract The macroscopic and microscopic characteristics as well as the proposed mechanisms of Type I (high-temperature) and Type II (low-temperature) hot corrosion are reviewed. Two case histories of gas turbine blade failures are presented. Different practical approaches to minimize hot corrosion are described.

Characteristics of hydrogen embrittlement, stress corrosion cracking and tempered martensite embrittlement in high-strength steels

Abstract Characteristics of tempered martensite embrittlement (TME), hydrogen embrittlement (HE), and stress corrosion cracking (SCC) in high-strength steels are reviewed. Often, it is important to determine unambiguously by which of these mechanisms failure occurred, in order to suggest the right actions to prevent failure recurrence. To this aim, samples made of high-strength AISI 4340 alloy steel were embrittled by controlled processes that might take place, for example, during the fabrication and service of aircraft landing gears. The samples were then fractured and characterized using light and scanning electron microscopy, microhardness tests, and X-ray diffraction. Fractography was found to be the most useful tool in determining which of these mechanisms is responsible for a failure, under similar conditions, of structures made of AISI 4340 alloy steel.

Positive effects of hydrogen in metals

Hydrogen is often present in metals as a result of production, fabrication and processing operations or service conditions. Thus, it can be regarded as an alloying element. Although, high hydrogen levels in metals can have a devastating effect on the mechanical properties, many positive effects can also be derived from its high solubility. The objective of this paper is to review some positive effects of hydrogen in metals. An emphasis will be made on enhancements in the processing properties due to hydrogen (thermohydrogen processing (THP)), though other uses of hydrogen, such as an energy storage device and in the electronics industry will also be presented.

Fracture of cementless femoral stems at the mid-stem junction in modular revision hip arthroplasty systems.

BACKGROUND Mechanical failure of femoral stems at the modular junction of revision hip arthroplasty systems has been reported only infrequently. In the current study, the cause of six stem fractures, which occurred in vivo, was analyzed with use of clinical data and failure analysis. METHODS Six patients with a fracture at the mid-stem junction of a modular revision hip implant were identified in our database of patients who had undergone revision arthroplasty. The characteristics of the patients with a fractured stem were compared with those of 165 patients from the same prospective database who had a modular stem implanted, had at least two years of follow-up, and had not had a fracture of the stem. Failure analysis of three implants (six fracture surfaces) was carried out, with use of microscopic, chemical, and microhardness characterization techniques. RESULTS Patients with a fractured stem had significantly higher body mass indices than patients without a stem fracture. Radiographs demonstrated that these femoral implants lacked adequate osseous support of the junction area of the stem. All stems failed approximately 1 to 2 mm proximal to the body-stem junction, thus indicating the presence of a bending moment. The chemical composition and microhardness matched those of Ti-6Al-4V. Evidence of wear and fatigue were found on the fracture surface. A wear strip was also observed along the circumference of the stem near the junction. CONCLUSIONS We concluded that the stem failure was initiated by a fretting fatigue mechanism and was propagated by a pure bending fatigue mechanism. Risk factors for fractures of the modular junction include excessive body weight and inadequate proximal osseous support because of trochanteric osteotomy, reduced preoperative bone stock, osteolysis, loosening, and/or implant undersizing. Surgeons should consider the use of implants with strengthened junctions when using modular stems in such patients.

Electrochemical and electrophoretic deposition of hydroxyapatite for orthopaedic applications

Abstract The basic calcium phosphate mineral, hydroxyapatite (HAP) (Ca10(PO4)6(OH)2), is the prototype of one of the major constituents of bone and teeth. Thin layers of HAP were coated on the surface of type 316L stainless steel by electrophoretic deposition (EPD) from a 2·5% suspension in isopropyl alcohol, and this was followed by vacuum sintering at 800°C for 1 h. The development of HAP coatings was affected by the applied potential and time. In addition, HAP coatings were synthesised on pure Ti by electrochemical deposition and fully characterised to validate their use in orthopaedic implants. Electrodeposition was carried out from a bath containing low concentrations of Ca(NO3)2 and NH4H2PO4 at pH 6·0 by cathodic polarisation. Both types of HAP coating were fully characterised, including with respect to corrosion resistance. The advantages of these techniques include: (i) control over the composition and structure of the coating; and (ii) the ability to coat irregular surfaces easily.

Induced Codeposition of Alloys of Tungsten, Molybdenum and Rhenium with Transition Metals

In this section, the process of electrodeposition is reviewed briefly, and its place in the general context of electrode reactions and charge transfer across the metal/solution interface is set (Section 1.1). In Section 1.2, special emphasis is given to deposition of alloys, and particularly to anomalous deposition of alloys (Sections 1.2.3 and 1.2.4). Next, the phenomenon of induced codeposition is defined, and possible mechanisms are discussed briefly (Section 1.2.5). Several electroless (Section 1.2.6) and electrodeposition processes, in which induced codeposition plays a role, are mentioned. A more extensive discussion of electrodeposition of W-, Moand Re-based alloys is included in Section 2. Typical

The effect of surface treatment on the surface texture and contact angle of electrochemically deposited hydroxyapatite coating and on its interaction with bone-forming cells.

This work demonstrates the effects of both surface preparation and surface post-treatment by exposure to electron beam on the surface texture, contact angle and the interaction with bone-forming cells of electrochemically deposited hydroxyapatite (HAp) coating. Both the surface texture and the contact angle of the ground titanium substrate changed as a result of either heat treatment following soaking in NaOH solution or soaking in H(2)O(2) solution. Consequently, the shape of the current transients during potentiostatic deposition of HAp changed, and the resulting coatings exhibited different surface textures and contact angles. The developed interfacial area ratio Sdr and the core fluid retention index Sci were found more reliable than the mean roughness R(a) and the root-mean-square roughness Z(rms) in correlating the adhesion of the coating to the metal substrate and the cellular response with surface texture. The NaOH pretreatment provided the highest surface area and induced the highest cell attachment, even though the H(2)O(2) treatment provided the highest hydrophilicity to the metal substrate. Electrodeposition at pH 6 was found preferable compared to electrodeposition at pH 4.2. The ability to modify the cellular response by exposure to unique electron-beam surface treatment was demonstrated. The very high hydrophilicity of the as-deposited HAp coating enhanced its bioactivity.

Enhanced osseointegration of grit-blasted, NaOH-treated and electrochemically hydroxyapatite-coated Ti-6Al-4V implants in rabbits

Osseointegration, in terms of the bone apposition ratio (BAR) and the new bone area (NBA), was measured by backscattered electron imaging. The results were compared for four implant types: grit-blasted and NaOH-treated Ti-6Al-4V (Uncoated-NaOH), electrodeposited with hydroxyapatite without alkali treatment (ED-HAp), electrodeposited with hydroxyapatite after alkali treatment (NaOH-ED-HAp), and plasma sprayed with hydroxyapatite (PS-HAp). No heat treatment was done after soaking in NaOH. The implants were press fitted into the intramedullary canal of mature New Zealand white rabbits and analyzed, both at the diaphyseal and at the metaphyseal zones, either 1week or 12weeks after surgery. NaOH-ED-HAp already exhibited a higher BAR value than the ED-HAp at 1week, and was as good as the commercial PS-HAp at 12weeks. The NBA value for NaOH-ED-HAp at 12weeks was the highest. The higher content of octacalcium phosphate in NaOH-ED-HAp, as evident from the X-ray photoelectron spectroscopy analysis of the oxygen shake-up peaks, and the associated increase in the solubility of this coating in vivo are considered responsible for the enhanced osseointegration. Taking into account also the reduced occurrence of delamination and the inherent advantages of the electrodeposition process, electrodeposition of HAp following soaking in NaOH may become an attractive alternative for the traditional plasma-sprayed process for coating of orthopedic and dental implants.

An increase of the spall strength in aluminum, copper, and Metglas at strain rates larger than 107 s−1

Measurements of the dynamic spall strength in aluminum, copper, and Metglas shocked by a high-power laser to hundreds of kilobars pressure are reported. The strain rates in these experiments are of the order of 107 s−1, which cannot be reached in impact experiments. The free-surface velocity behavior associated with spallation is characterized by oscillations caused by the reverberations of the spall layer. An optically recording velocity interferometer system was developed to measure the free-surface velocity time history. This diagnostic method has the advantages of being a noninterfering system and produces a highly accurate continuous measurement in time. The spall strength was calculated from the free-surface velocity as a function of the strain rate. The results show a rapid increase in the spall strength, suggesting that a critical phenomenon occurs at strain rates ∼107 s−1, expressed by the sudden approach to the theoretical value of the spall strength.

Hydrogen-assisted processing of materials

Abstract Under certain conditions, hydrogen can degrade the mechanical properties and fracture behavior of most structural alloys; however, it also has some positive effects in metals. Several current and potential applications of hydrogen for enhancing the production and processing of materials are reviewed. These include thermohydrogen processing (THP) and forming of refractory alloys, processing of rare earth-transition metal magnets by hydrogen decrepitation (HD) and hydrogenation–decomposition–desorption–recombination (HDDR), hydrogen-induced amorphization (HIA) and microstructural refinement, extraction of elements from ores and alloys, and the use of hydrogen as a reducing gas for welding and brazing. Hydrogen is found to enhance the formability, microstructure and properties of a large variety of materials, including steels, Ti-based alloys and metal matrix composites (MMCs), refractory metals and alloys, rare earth-transition metal alloys, metalloid-containing metallic glasses, etc.