Metallurgical control of fatigue crack propagation in superalloys

Low-cycle fatigue life of turbine engine disk alloys is determined by the initiation and propagation of fatigue cracks. Performance improvements can be achieved through the combination of clean melting technology, to reduce the defect size, and a new generation of high-strength superalloys with fatigue cracking resistance. Metallurgical control of fatigue crack propagation in high-strength superalloys becomes feasible only through a clear understanding of the fatigue cracking mechanism, as well as the micro-structure/property relationships. Many metallurgical parameters have been identified to control the fatigue cracking resistance at high temperatures. One of the most effective methods, applicable to all high γ′ content superalloys, is to modify the grain boundary structure by means of a controlled cooling from a supersolvus solutioning. The precipitation reaction occurring on the grain boundaries during cooling generates a serrated structure that exhibits a good stress oxidation resistance for fatigue cracking.     

Joining ductile refractory metal inserts in x-ray tube targets

Niobium alloy C103 (Nb-10Hf-1Ti) is being investigated for use as an anode shaft material for x-ray tubes. This shaft is used to support the x-ray target and to inhibit heat flow from the target to the rest of the anode. C103 is an attractive shaft material because it has a higher thermal resistance than the conventionally used material—titanium-zirconium-molybdenum (TZM). The primary challenge to using C103 is the joining of the shaft to the powder-metallurgy TZM target. This article discusses a variety of approaches toward accomplishing this, including direct diffusion bonding, the use of a diffusion-enhancing interlayer, and the incorporation of a ductile metal insert into the x-ray target.     

Intrinsic stress in DC sputtered niobium

The intrinsic mechanical stress of DC-magnetron-sputtered Nb films is characterized as a function of sputtering parameters and target erosion. The zero-stress point shifts to lower cathode voltages as the target erodes. The zero-stress point is always characterized by the same cathode-current-Ar-pressure relationship     

Comparison of initial fixation strength between biodegradable and metallic interference screws and a press-fit fixation technique in a porcine model

We describe nuchal cord changes in four children with os odontoideum who had cervical spine magnetic resonance imaging (MRI) before surgery. In these four children, nuchal cord changes consistent with previous trauma were seen. The presence of the nuchal cord changes supports the concept of trauma as an etiologic factor in these four cases. This study supports trauma as an associated factor in patients with os odontoideum.     

Intra and intermolecular relaxations 2,3-dihydroxypropyl methacrylate and 2-hydroxyethyl methacrylate hydrogels

The dynamic mechanical transitions present in the 2-hydroxyethyl methacrylate (HEMA) and 2,3-dihydroxypropyl methacrylate (DHPMA) homopolymers and two random copolymers, with one-to-one and three-to-one HEMA-to-DHPMA molar ratios, were examined by analyzing the dry xerogels and hydrogels at various states of hydration. The temperatures of the primary α transition and the secondary β and γ transitions were determined in the tension mode; the storage modulus and loss modulus as a function of temperature and frequency were recorded. The results were compared to the results obtained from dielectric analysis at low hydration using tan δ. The frequency dependence of the dispersions was calculated for the dry and hydrated states, using mechanical and dielectric data. The information obtained was used to elucidate the interaction between the polymer and the sorbed water. Analysis of the low temperature secondary γ transition and secondary transitions resulting from polymer-water interactions was emphasized. During the initial hydration, the temperature maxima of the dielectric and mechanical secondary γ transitions, and the transitions that appear in the presence of absorbed water increased as the DHPMA content increased. The apparent activation energy associated with the secondary transitions increased as well. All of this is the result of stronger intermolecular interactions due to the increased density of hydrophilic moieties in DHPMA.