Kenneth Easterling

The Role of Alloy Composition on the Stability of Nitrides in Ti-Microalloyed Steels during Weld Thermal Cycles

The effect of weld thermal cycling on titanium nitride stability in a range of normalized Timicroalloyed steels containing various alloying additions of V, Al, and N has been investigated. Nitride dispersions and the chemical analysis of individual particles are studied using transmission electron microscopy and a quantitative 200 kV STEM-EDX-EELS microanalysis system. It is found that whereas the normalized material contains various nitrides of mixed compositions, only nitrides based on TiN survive high energy simulated weld cycles. Grain growth in weld cycled material is highly dependent upon the stability of nitrides during the weld cycle, and this stability depends on the original composition of the nitrides and hence of the steel itself. The presence of aluminum in the particles is particularly detrimental in this respect. The best grain growth control and highest toughness are found in steels based on optimum ratios of Ti/V/N and on low Al levels. The possible way in which the steel’s composition affects particle stability and hence grain growth during weld thermal cycles is discussed.

The influence of particle shape on zener drag

Practically all papers concerned with grain refinement based on pinning by particle dispersions have considered spherical particles. This paper avoids this restriction and develops Zener pinning eq ...

Improvement of critical current density in the Bi-Pb-Sr-Ca-Cu-O system through hot isostatic pressing

Abstract The effects of hot isostatic pressing (HIPing) on densification and the superconducting properties of the Bi-Pb-Sr-Ca-O system were investigated. A relative density up to 95% and a critical current density greater than 1100 A/cm 2 at 77 K were achieved through HIPing at 650°C for 2 h under 200 MPa argon. Under these conditions, the product gave a value of J c four times that without HIPing. T o was unaffected by HIPing for samples encapsulated with a combination of glass and silver tubes. However, T o was suppressed from 103 K to 86 K for samples encapsulated with stainless steel and silver tubes while J c increased from 260 A/ cm 2 to 1086 A/cm 2 during HIPing, indicating that the weak links were significantly improved. A new minor phase, having a composition of Bi/Pb/Sr/Ca/Cu=0.58/2.8/3.0/2.1/1.1, was observed in HIPed samples but its effect on J c is not clear.

Microstructure and flux pinning in superconducting Bi-Pb-Sr-Ca-Cu-O wires

Abstract The critical current density ( J c ) of Ag-clad Bi-Pb-Sr-Ca-Cu-O wire has been measured to be 1.2×10 4 A/cm 2 at 77 K in zero field. The high J c is attributed to a combination of elimination of the poisoning effect of Ag on superconductivity, grain alignment, and enhancement of flux pinning. J c - H dependence was significantly improved in the Ag-clad tape, which has a J c of 1.0×10 3 A /cm 2 at 77 K and 4000 Oe, while the J c of the sintered pellet drops two orders of magnitude at only 100 Oe. A pronounced anisotropy in J c under high magnetic field is attributed to the grain alignment. Planar defects, such as heavy stacking faults parallel to the a - b plane in the rolled tape, are considered to be effective pinning centres.

Stress-relief heat treatment of submerged-arcvvelded microalloyed steels

A wide range of microalloyed steels containing various combinations of Al, Ti, V, Nb, and submerged-arc welded using wires of either high or low Mo additions, were studied in both the as-welded and ...

On the rotation of precipitate particles

Abstract An energetically unfavourable situation can develop when coherent particles are bypassed by migrating grain boundaries. This is due to the precipitates being exposed to incoherent interfaces in the new matrix. In this work, the rotation of precipitate particles to low energy, coherent orientations in the new matrix is shown to be one of several possible responses to this situation. A physical and kinetic model for the rotation is put forward and the results of calculations of rotation rate presented. It is shown that particle rotation is controlled by interfacial diffusion and depends upon alloy composition, time, temperature and particle size and shape. The possibility of particle rotation occurring during particle/boundary contact is also discussed. This is shown to depend upon the nature of the boundary type. Generally, boundaries moving under large driving pressures with high velocities (e.g. phase transformation interfaces), are less likely to allow rotation during particle/boundary contact than less mobile boundaries such as those in grain growth and recrystallization. Experimental results from SAD and TEM of a Ti stabilized austenitic stainless steel containing a dispersion of coherent TiC precipitates is also presented and supports the particle rotation model.

Cu valence states in superconducting BiPbSrCaCuO system

Abstract The oxidation states of Bi, Pb, and Cu in the BiPbSrCaCuO (BPSCCO) system have been determined by a combination of volumetric measurement technique and iodometric titration. It was found that, in contrast to previous reports, the concentration of the Cu3+ ions decreased with increasing Pb content, and Cu3+ ions were absent in samples of Bi1.6Pb0.4Sr1.6Ca2Cu3O9.8, while a Tc at 108 K and a Jc of greater than 12,000 A/cm2 at 77 K were observed. The 110 K phase in BPSCCO was stabilized and showed a high tolerance to change in oxygen partial pressure during sintering. In Pb-doped materials, Bi appears to be trivalent while Pb was determined to be mixed-valence Pb 4+ Pb 2+ . It is suggested that superconductivity in BPSCCO may result from a dynamic transfer of holes from Bi PbO layers toward CuO2 planes.

Improved single crystal growth in the Bi-Sr-Ca-Cu-O system using a sealed cavity technique

Bi-Sr-Ca-Cu-O single crystals obtained thus far commonly have a very small thickness in the c-axis because of the weak bonding between two adjacent BiO layers and the high vapour pressure of Bi2O3 at high temperature. To overcome these difficulties a presintered SrCaCu4O6 was used to seal the Bi-Sr-Ca-Cu-O melt from which the single crystals grew during slow cooling. In the SrCaCu4O6 sealed cavity the Bi loss through vaporization was prevented and Sr and Ca were enriched in the superconducting crystals. The resulting single crystals have a dimension of up to 3.0×1.5×0.7 mm3, which is two orders of magnitude thicker than those grown from alkali chloride flux. The composition of the crystals Bi2.2Sr1.8Ca0.75Cu1.8Ox and they possess a zero resista nce at temperature above liquid nitrogen.

Formation of the 124-phase superconductor YBa2Cu4O8 by retaining oxygen in a reaction HIP sintering process

The ceramic superconductor YBa2Cu4O8 has been produced by high temperature sintering of a mixture of CuO and YBa2Cu3O7 in a glass capsule under high hydrostatic argon pressure. The resulting highly dense material is investigated by X-ray diffraction, optical and electron microscopy, resistance measurements and hardness measurements, and shown to be a homogeneous High transition temperature superconductor.

Electrical transport properties of dense bulk YBa2Cu4O8 produced by hot isostatic pressing

Abstract Highly dense sintered YBa 2 Cu 4 O 8 has been produced by hot isostatic pressing (HIP). The electrical resistivity ϱ of this material has been measured as a function of temperature T and pressure ϱ in the range 40–650 K and 0–0.7 GPa. Both the temperature dependence and the pressure dependence of ϱ are found to be well described by a model based on the standard Bloch-Gruneisen theory. It is pointed out that ϱ is liner in T only under isobaric conditions, while ϱ is strongly nonlinear in all high- T c superconductors under isochoric (constant volume) conditions. The critical current density of the material is 900 A/cm 2 at 4 K, while the resistivity is 630 μΩ cm at 294 K.