K. Ted Hartwig

Microalloyed aluminum for cryogenic conductors

The addition of low levels of various elements can enhance the strength of high-purity aluminum with minimal effects on resistivity. The base aluminum is 99.9998% pure with a residual resistivity ratio (RRR = ρ273K/ρ42K) of 10,500 and the microalloy levels range from 2 atomic ppm up to 170 atomic ppm (5–90 weight ppm). Solute elements (B, Ca, Ce, Ga, or Y) are added to the base in binary form. The lowest RRR value obtained is 2,500. The effects of solute concentration, solute element type, and heat treatment on grain size, resistivity, and flow stress are presented. Optical micrographs also show the presence of particles that occur as a result of microsegregation during solidification.

Refinement and Densification of Aluminum Nickelides by Severe Plastic Deformation

Two nickel (Ni)-coated aluminum (Al) powders were consolidated to full or near-full density by severe plastic deformation using the equal-channel angular extrusion (ECAE) technique. Mixtures of 78Al-22Ni at.% (63Al-37Ni wt.%) or 39Al-61Ni at.% (23Al-77Ni wt.%) were placed in square-shaped copper (Cu) or Ni casings, sealed, preheated to a range of temperatures from ambient to 700 °C, and, once at a uniform temperature, dropped into the ECAE die and single-pass extruded. It was found that the preheating temperature affected the transformation of the initial Al-Ni composition into aluminum nickelide (Al-Ni) intermetallics. It was also found that the use of the nickel can significantly affect the densification of the powders. Scanning electron microscopy, energy dispersive x-ray spectroscopy, x-ray diffraction, and microhardness measurements were used to examine the nature of the resultant intermetallics. The onset and nature of the transformation from the precursors into the products were further studied by differential thermal analysis. These results and the role of ECAE on the transformation are discussed.

Use of High Intensity Milling and Equal Channel Angular Extrusion to Produce and Consolidate Nanostructured Titanium Silicide

The focus of this research is on (a) the use of mechanical alloying (MA) to produce nanostructured titanium silicide (Ti5 Si3 ) powder and (b) use of equal channel angular extrusion (ECAE) for powder consolidation to preserve the fine scale structure achieved via mechanical alloying. MA is a high-intensity ball milling process that can be used to produce nanocrystalline and amorphous powders. A 23 factorial design is applied to optimize the process and study the effect of MA parameters (milling time, milling speed, ball-to-powder ratio) on the grain size of powder. ECAE is a process that produces intense and uniform plastic deformation caused by simple shear of the material. A 4 × 2 factorial design was used to study the effect of ECAE parameters (number of extrusions, billet orientation, temperature) on the properties of the final product. The final product possesses a grain size of less than 20 nanometers.Copyright

Effects of grain size and purity on the low temperature cyclic strain degradation of stabilizer aluminum

Aluminum specimens of 99.99% to 99.9998% purity and with grain sizes ranging from 0.1 mm up to 2.2 mm have been mechanically and electrically tested at 4.2 K to determine strain hardening and resistivity degradation characteristics. RRR values of the fully annealed (large grained) materials range from 1,150 up to 10,400. Results show that for a cyclic strain range of ±0.001 at 4.2 K, the effects of grain size are significant when purities are greater than about 99.9995%. At purities below 99.999%, the grain size has a minimal effect on the rate of strain hardening and on resistivity degradation. Mechanisms for the observed behavior are associated with increased strain hardening rates and a reduction in the total number of point defects (vacancies) generated during the low temperature deformation.

Electrical and Mechanical Behavior of Micro-Alloyed Aluminum (RRR>2500) at 4.2 K

The electrical and mechanical properties of micro-alloyed high purity aluminum are examined as a function of cyclic strain (±0.1%) at 4.2 K. The base metal purity is 99.9998% (RRR ≈ 10500) and the levels of micro-alloying (4 to 90 wppm) are such that the nominal RRR values range from about 2500 to 10000. Five different alloying elements (B, Ca, Ce, Ga, and Y) are examined with up to four solute levels for each element (in binary alloys). In addition to the results of cyclic testing, a discussion of the effects of micro-alloying on resistivity degradation and strain hardening is given. The results show that the effects of micro-alloying at levels of about 20 wppm are beneficial. The results also show that the benefits of micro-alloying are closely related to behavior through a solute atomic weight effect; on an atomic ppm basis, the solute atomic weight is more important than the solute type. In addition, resistivity coefficients of the micro-alloying elements and flow stress values at 0.001 strain are given.

Eddy Current Determination of the Effects of Cyclic Strain and Temperature on the Magnetoresistivity of Pure Aluminum

An important characteristic of pure metals for use in low temperature conductor applications is the behavior of the material in the presence of a high magnetic field. Using the eddy current decay method, the magnetoresistivity of 99.99% and 99.999% aluminum is examined at temperatures of 4.2, 20, and 27.2 Kelvin and at fields of 0 to 7 Tesla. The effects of cyclic strain are also closely examined Comparisons of magnetoresistivity behavior of materials at different temperatures and with different defect concentrations, although with similar zero field resistivities, are made. The results show that magnetoresistivity is strongly dependent on the zero field resistivity. Furthermore, the effects of temperature are small, but noticeable, and the effects of strain and impurities on magnetoresistivity are apparently similar in nature. For materials with RR (ρrt/ρtemp) values of 400 or more, resistivity increases linearly at fields above 1 to 2 T. The onset of this linear behavior occurs at lower fields when the zero field resistivity of the material is lower, regardless of the test temperature, strain level, etc. In the range of variables covered, materials with a lower zero field resistivity always have a lower in-field resistivity for similar applied fields. The results also show that the measurement of transverse magnetoresistivity in situations where values of H*RR are greater than several thousand tesla may be unacceptable if the eddy current decay method is used; an anomalous voltagetime record is generated by the eddy current pickup coil.