M. A. Imam

Fatigue thresholds of Ni-Ti alloy near the shape memory transition temperature

The fatigue thresholds of nickel-titanium alloy with martensite start temperature of 80°C were measured in air from room temperature to 150°C and load ratios of 0.1 to 0.9. The ΔK versus K max fundamental threshold curves are characteristic of ductile metals at all temperatures tested and are consistent with the existence of two threshold parameters, ΔK th * and K max,th * , for cyclic and maximum stress intensity, respectively. ΔK th * increases linearly with temperature, while K max,th * , exhibits a transition between 80 and 100°C, decreasing by half as a function of temperature. Compliance curves suggest the existence of crack closure in the stable martensite regime below 100°C, but little or no crack closure in the stress-induced martensite regime above 100°C up to 150°C. The results suggest that both crack closure and the existence of an intrinsic fundamental threshold curve with both ΔK and K max thresholds as a function of load ratio are necessary to fully account for R dependence of ΔK th .

Overview of nanophase metals and alloys for gas sensors, getters, and hydrogen storage

Abstract Nanophase metallic materials produced by inert gas condensation and ball-milling, including zirconium and magnesium alloys, have been the subject of several exploratory studies on potential gas-reactive applications of nanophase metals. Such applications include gas sensing, vacuum gettering, and hydrogen storage. A brief overview is given of the background, rationale, and some results from the literature of gas-reactive applications of nanophase metals and alloys.

Characteristics of plasma processed SiC nanocrystallites and nanorods

Nanoparticles and nanorods of SiC were synthesized using arc-plasma processing of coarse particles. X-ray diffraction and Raman spectroscopic studies showed the presence of β-SiC and carbon nanotubes in the starting coarse particles and SiC nanorods in the ultrafine particles produced by plasma processing. Scanning electron microscopy and transmission electron microscopy confirmed the presence of carbon nanotubes in the starting material and nanorods of SiC in the plasma-processed samples.

Microwave surface resistance of bulk Tl‐Ba‐Ca‐Cu‐O superconductors

The first measurements of the microwave surface resistance at 18 GHz of bulk Tl‐Ba‐Ca‐Cu‐O superconductors produced by the hot isostatic pressing (HIP) process are reported. The superconducting samples, prepared by solid‐state reaction with subsequent sintering and consolidation to obtain ideal density, were measured by replacing the end wall of a TE011 circular mode gold‐plated copper cavity with the sample and determining the cavity Q for the temperature range 4–300 K. Results indicated that HIP samples which underwent subsequent annealing exhibit, below the critical temperature, a surface resistance approaching an order of magnitude less than copper.

Transformation of a quasicrystalline state to a crystalline state by ion-beam bombardment

Abstract Heating that occurred during argon-ion bombardment of a thin film of a quasicrystalline phase of an Al─Li─Cu alloy resulted in distortion and subsequent phase transformation. The distortion causes staggering of the diffraction spots in the fivefold symmetry as well as contraction and expansion of the systematic rows parallel to the axis of the symmetry of the distortion which is one of six fivefold axes. The results support the quasicrystalline model of the icosahedral phase which transforms possibly to a cubic phase through an intermediary metastable configuration corresponding to the distorted phase.

On dispersions of voids as sources of strength

Abstract Materials of reduced density and increased strength can be achieved by the creation of dispersed voids. A procedure for the manufacture of such materials is described and their characteristic strength estimated.

Effects of alloying on the superconducting properties of ErBa/sub 2/Cu/sub 3/O/sub 7/

In high-T/sub c/ superconductors, the low critical current densities in polycrystalline materials have been attributed to a combination of critical current anisotropy and poor superconducting coupling across grain boundaries. Theoretical calculations indicate that although the flux pinning should vary roughly inversely as grain size, the polycrystalline critical current behavior could possibly be understood in terms of stresses due to the grain boundary. Experiments have been conducted to increase the coupling between adjacent grains by modifying grain boundary chemistry. These include adding either a conducting layer or a superconducting layer at the interfaces. The effect of additions such as Ag, B, Bi, Ga, and In to produce a conducting layer and the alloying of RBa/sub 2/Cu/sub 3/O/sub 7/ with another superconductor to produce a superconducting layer were analyzed by measuring T/sub c/ and J/sub c/ and observing changes to the microstructure. Early results indicate some J/sub c/ enhancement with silver addition. However, the addition of a different superconductor appears more promising. >

Microstructural Changes Produced by Ion-Beam Thinning in Al-Li-Cu Alloys

Thin specimens for transmission electron microscopy examination may be prepared by several techniques, such as electropolishing and ion-beam thinning. Ion-beam thinning, which employs the use of an argon ion-beam at high voltage (e.g. 6kV) to bombard the specimen, is a commonly used technique for preparing and/or cleaning thin foil specimens. The experimental results show that even short-time thinning of Al-Li-Cu alloys with argon ion-beams introduce dramatic microstructural changes. Interesting observations concerning the effect of the ion-beam on quasi-crystals in Al-Li-Cu alloys during the thinning process are presented. The results and causes of the ion-beam thinning in Al-Li-Cu alloys are discussed.

Preliminary studies for the development of superconducting composite wires

The results reported are part of a larger effort, the basic aim of which is to develop high-T/sub c/ superconducting composite wires, films, and thin-gauge panels with large current carrying capacity and engineered thermal and mechanical properties. The results relate to the following aspects of this ongoing research effort: the sputtering parameters used to deposit Bi-Sr-Ca-Cu-O films on MgO and other substrates; a dipping technique to produce Bi-Sr-Ca-Cu-O superconducting layers and wires; the postannealing heat treatments; the characterization of the microstructure and crystallographic features of the deposited films; and the superconducting properties of the films, layers, and wires. >

Preparation of dense bulk high T/sub c/ superconducting materials using hot isostatic pressing

Sintered superconducting materials were encapsulated in evacuated stainless-steel or pyrex glass containers and processed in a hot isostatic pressing (HIP) unit at high temperatures and pressures. Optimum HIP conditions to obtain bulk samples of nearly theoretical density were determined. Results indicate that in addition to density, HIP also improved T/sub c/. For the one-two-three system, the grain size is significantly reduced during HIP. This is attributed to fracturing of coarse particles in the sintered compact due to high strain rates experienced during the pressurization. HIP materials can be machined to any shape and size, and the process being used for making specimens for J/sub c/ measurements and superconducting devices. >