The Nucleation Potency of In Situ-Formed Oxides in Liquid Iron

Metallurgical and Materials Transactions A - The nucleation potency of iron oxides was verified experimentally through nucleation undercooling of liquid iron using aerodynamic levitation technology...     

Influence of Undercooling and Nd4Ba2Cu2O10 Content on the Growth Rate of Large-Grain Bulk Nd-Ba-Cu-O Superconductor

The dependence of growth rate on isothermal undercooling and composition has been investigated for Nd-Ba-Cu-O single grains containing various amounts of nonsuperconducting Nd₄Ba₂Cu₂O₁₀ (Nd-422) phase inclusions and fabricated under a 1% O₂ in N₂ atmosphere using a top seeded melt growth technique. The growth rate along the crystallographic c -direction is observed to exceed that along the a / b direction at all undercooling temperatures and exhibits a maximum for a Nd-422 content of ∼10 mol%. The samples have been examined by optical microscopy and the results interpreted within an established planar solidification model.     

Microstructural evolution in infiltration‐growth processed MgB2 bulk superconductors

The study reports phase and microstructural evolution in MgB₂ bulk superconductors fabricated by an infiltration and growth (IG) process. Three distinct stages, (1) intermediate boride formation, (2) bulk liquid Mg infiltration, and (3) MgB₂ layer formation, were identified in IG process after detailed examination of series of samples prepared with varied heating conditions. The intermediate phase Mg₂B₂₅, isomorphous to β‐boron, was detected prior to MgB₂ phase formation in stage (1). Due to volume expansion involved in stage 1, cracks formed in the β‐boron particles and propagated radially inwards during stage 3. The growing MgB₂ particles sintered simultaneously with formation of grain boundaries during the process, as evidenced by the measured hardness and critical current density in these samples. From our observations, we estimate the total time needed for complete transformation to MgB₂.     

A Homogeneous Billet Layer Casting Fabrication Method

A novel additive casting approach, termed as layer casting (LC), was proposed to fabricate ingots with homogeneous composition and grain structure distribution. Ingots of Al-4.5 wt pct Cu were fabricated using conventional and novel methods to verify the feasibility of this novel approach. The results show that the novel processing not only alleviates macrosegregation but also reduces the shrinkage cavity and improves the tensile properties of the as-cast condition.     

Infiltration-Growth Processing of NdBa2Cu3O7-delta Superconductor

NdBa₂Cu₃O_7-delta (Nd-123) superconductor has been processed to obtain a highly textured microstructure with micrometer-sized spherical Nd₄Ba₂Cu₂O₁₀ (Nd-422) inclusions through a process that involves the infiltration of liquid phases into a shaped preform followed by its slow cooling through the peritectic formation temperature of Nd-123. The process was carried out in high-purity argon (maximum of 4 ppm of oxygen) to reduce the formation of low- T _c solid solutions. A major advantage of the process is that it minimizes the possibility of Nd-422 coarsening in the liquid. The result is a very uniform distribution of nearly spherical Nd-422 inclusions with 80% of the particles having a diameter less than 1 µm, in the highly textured Nd-123 matrix. Since the present process eliminates shrinkage and large hollows within the interior of the samples, it can enable the fabrication of 3-dimensional complex-shaped objects with good dimensional tolerance and high J _c for applications. The critical current densities in the Nd-123/Nd-422 composites have been measured and the values obtained are discussed in the context of the microstructure of the material.     

The Effect of Post-grinding Heat Treatment of Alumina and Ag-Cu-Ti Braze Preform Thickness on the Microstructure and Mechanical Properties of Alumina-to-Alumina-Brazed Joints

Alumina-to-alumina-brazed joints were formed using 96.0 and 99.7 wt.% Al₂O₃ and TICUSIL^® (68.8Ag-26.7Cu-4.5Ti wt.%) preforms of different thicknesses. Brazing was conducted in a vacuum of 1 × 10^?5 mbar at 850 °C for 10 minutes. Joint strengths were evaluated using four-point bend testing and were compared to flexural strengths of standard test bars. Post-grinding heat treatment, performed at 1550 °C for 1 hour, did not affect the average surface roughness or grain size of either grades of alumina but affected their average flexural strengths with a small increase for 96.0 wt.% Al₂O₃ and a small decrease for 99.7 wt.% Al₂O₃. As the TICUSIL^® preform thickness was increased from 50 to 100 µm, the average strengths of both 96.0 and 99.7 wt.% Al₂O₃ brazed joints improved. Joints made using 100-µm-thick TICUSIL^® preforms predominantly consisted of Cu-Ti phases which formed due to excess Ti in the interlayers and non-uniform Ag-rich outflow. Brazed joints of 96.0 wt.% Al₂O₃ made using 100-µm-thick TICUSIL^® preforms achieved an average joint strength of 238 MPa with consistent failure in the ceramic.     

A practical route for the fabrication of large single-crystal (RE)-Ba-Cu-O superconductors

Single-crystal superconductors of the general formula (LRE)-Ba-Cu-O (light rare earth, LRE = Nd, Sm, Eu and Gd) have considerable potential for engineering applications because of their ability to trap magnetic fields significantly higher than those achievable with permanent magnets. But the lack of a process by which these materials can be fabricated reliably and economically in the form of large single grains has severely hindered their development. We report a practical processing method for the fabrication in air of single-crystal (RE)BCO. The technique is economical and offers considerable freedom in terms of the processing parameters and reproducibility in growth of oriented single grains. The process is based primarily on the development of a new type of generic seed crystal that can effectively promote the epitaxial nucleation of any (RE)BCO system, and secondly on suppressing the formation of RE-Ba solid solution in a controlled manner within large grains processed in air.     

A Comparison of 0°–0° and 45°–45° bridge‐Seeded,YBCO single grains

A variety of multiseeding techniques have been investigated over the past 20 yr in an attempt to enlarge bulk (RE)BCO superconducting samples fabricated by the top‐seeded melt growth (TSMG) process for practical applications. Unfortunately, these studies have failed to establish whether technically useful values of trapped field can be achieved in multiseeded bulk samples. In this work specially designed, 0°–0° and 45°–45° bridge seeds of different lengths have been employed to produce improved alignment of the seeds during the TSMG process. The ability of these bridge‐seeded samples to trap magnetic field, which is the key superconducting property for practical applications of bulk (RE)BCO, is compared for the samples seeded using 0°–0° and 45°–45° bridge seeds of different lengths. The grain boundaries produced by these bridge seeds are analyzed in detail, and the similarities and differences between the two bridge‐seeding processes are discussed.