R.A. Buchanan

Dynamic evolution of nanoscale shear bands in a bulk-metallic glass

Dynamic shear-band-evolution processes in a bulk-metallic glass (BMG), an emerging class of materials, were captured by a state-of-the-art, high-speed, infrared camera. Many shear bands initiated, propagated, and arrested before the final fracture in tension, each with decreasing temperature, and shear-strain profiles. A free-volume-exhaustion mechanism was proposed to explain the phenomena. The results contribute to understanding and improving the limited ductility of BMGs, which otherwise have superior mechanical properties.

Localized Corrosion Behavior of a Zirconium-Based Bulk Metallic Glass Relative to Its Crystalline State

Abstract To date, few detailed corrosion studies of the new bulk metallic glasses (BMGs) have been presented. In the present work, the aqueous electrochemical corrosion properties of BMG-11, 52.5Zr–17.9Cu–14.6Ni–5.0Ti–10.0Al (atomic percent), were investigated. Cyclic-anodic-polarization tests were conducted on amorphous and crystalline specimens in a 0.6 M NaCl solution (simulated seawater) and on amorphous specimens in a 0.05 M Na 2 SO 4 solution (simulated moisture condensation, as related to ongoing fatigue experiments in humid air), all at room temperature. In the NaCl solution, both amorphous and crystalline materials were found to exhibit passive behavior with low corrosion rates (15 μm/year or less). However, susceptibilities to pitting corrosion were observed. The amorphous material was found to be more resistant to the onset of pitting corrosion under natural corrosion conditions. In the 0.05 M Na 2 SO 4 solution, the amorphous BMG-11 was found to exhibit passive behavior with a very low corrosion rate (0.4 μm/year), and to be immune to pitting corrosion. Furthermore, when the protective passive film was removed by scratching with a diamond stylus, it was found to quickly reform. This result suggested that a corrosion influence on the fatigue properties of BMG-11 in humid air would be minimal.

Fatigue behavior of Zr52.5Al10Ti5Cu17.9Ni14.6 bulk metallic glass

Abstract In the present study, fatigue tests were conducted on a zirconium-based bulk metallic glass (BMG), BMG-11 (Zr–10Al–5Ti–17.9Cu–14.6Ni, atomic percent), in air and vacuum to elucidate the possible environmental effects. In air, the fatigue endurance limit and the fatigue ratio were found to be 907 MPa and 0.53, respectively. These values are better than many conventional high-strength crystalline alloys. Unexpectedly, the fatigue lifetimes in vacuum were found to be lower than in air. Additional testing indicated that dissociation of residual water vapor to atomic hydrogen in the vacuum via a hot-tungsten-filament ionization gauge, and subsequent hydrogen embrittlement of the BMG-11, could have been a factor causing the lower fatigue lifetimes observed in vacuum.

The fatigue behavior of a zirconium-based bulk metallic glass in vacuum and air

Abstract Early fatigue studies on bulk metallic glasses (BMGs) have indicated fatigue strengths lower than anticipated. It was suspected that environmental effects degraded the fatigue life. In the present study, a zirconium-based BMG, BMG-11 (Zr–10Al–5Ti–17.9Cu–14.6Ni, at.%), was tested in vacuum and air. This comparison was made to define any detrimental environmental effects due to water vapor on the fatigue lifetime of BMG-11. The limited results obtained so far suggest that water vapor in air does not affect the lifetime. Moreover, the fatigue endurance limit for BMG-11 in air was found to be quite high, 907 MPa, and comparable to, or better than, those of many conventional high-strength alloys.

Assessment of thermal nitridation to protect metal bipolar plates in polymer electrolyte membrane fuel cells

Thermally (gas) nitrided Nb-30Ti-20W wt % exhibited excellent corrosion resistance and maintained good electrical conductivity in 80°C H 2 SO 4 environments designed to simulate proton exchange membrane (PEM) fuel cell bipolar plate conditions. This result supports proof of principle for thermally nitrided alloys as candidate bipolar plate materials, although such Nb-base alloys are too costly for most PEM fuel cell applications. Investigation of the corrosion resistance of a series of relatively inexpensive, thermally nitrided model Ni-X base alloys designed to form Cr 2 N, CrN, NbN, TiN, or VN surface layers have identified nitrided Ni-Cr base alloys as showing corrosion resistance nearly equal to that of the nitrided Nb-30Ti-20W.

A 200nm thick glass-forming metallic film for fatigue-property enhancements

In this letter, we report the fatigue-property enhancement by a thin layer of glass-forming film. The fatigue life of a 316L stainless steel is considerably improved by at least 30 times, depending on the maximum applied stress when it is coated with a 200nm thick Zr47Cu31Al13Ni9 film. The application of the sputtered film yields an increase of the fatigue limit by 30%. The smooth surface, good adhesion, and compressive residual stress are found to play beneficial roles in achieving superior fatigue properties, revealing the glass-forming film as a potential material to enhance fatigue properties.

Ion implantation for corrosion inhibition of aluminum alloys in saline media

Abstract The effects of ion implantation treatments on corrosion of 2014 and 1100 aluminum in saline media were investigated. Implanted constituents were N, Si, Ti, and Cr. Techniques included salt spray testing, electrochemical studies, Rutherford backscattering spectrometry, and profilometry. The principal conclusion was that ion implantation of Cr is of potential practical benefit for corrosion inhibition of 2014 Al in salt environments.

FATIGUE-INDUCED PHASE FORMATION AND ITS DEFORMATION BEHAVIOR IN A COBALT-BASED SUPERALLOY

The low-cycle fatigue behavior of a cobalt-based superalloy was studied in situ using neutron-diffraction experiments. The alloy exhibited stress-induced formation of a hexagonal-close-packed (hcp) phase within its parent face-centered-cubic (fcc) phase at ambient temperature under strain-controlled fatigue conditions with a total strain range, {Delta}{var_epsilon} = 2.5%. The (101) hcp peak was first observed during the 12th fatigue cycle under the given conditions following a period during which no hcp phase was detected. Subsequently, the intensity of the hcp peaks increased as fatigue progressed. Furthermore, within a single fatigue cycle, the intensity of the (101) hcp peak decreased during the compression half-cycle and increased again when the specimen was subjected to a subsequent tensile strain. The result suggests that the fcc to hcp transformation is partially reversible within one fatigue cycle.

Nitride Metallic Bipolar Plates for Proton Exchange Membrane Fuel Cells

Thermal nitridation of Ni- and Fe-base Cr-bearing alloys is under investigation as a means to enable use of metallic bipolar plates in proton exchange membrane fuel cells. Proof of principle for this approach was recently established by a successful single-cell fuel cell test using 50 cm2 active area anode and cathode plates made from a model nitrided Ni-50Cr alloy. Protection of the metal was accomplished by the formation of an electrically-conductive and corrosion-resistant CrN/Cr2 N surface layer. This paper presents an overview of efforts to form similar Cr-nitride surfaces on Ni-Cr and Fe-Cr base alloys in commercially available composition ranges.Copyright

Charge injection properties of iridium oxide films produced on Ti-6Al-4V alloy substrates by ion-beam mixing techniques

Abstract The charge injection capabilities of iridium oxide films, as produced on Ti-6Al-4V alloy substrates by ion-beam mixing techniques, were investigated. Iridium oxide is a valence change oxide, and therefore has high values of charge injection density upon voltage cycling in electrolytes. Because of this property, iridium oxide films are useful as working elements in neural prostheses. Iridium films of three thicknesses, produced by sputter deposition followed by ion beam mixing, were tested in cyclic voltammetry out of 1000 cycles or more. Two surface preparations, mechanical polishing and an acid passivation treatment, were also used as controls. Surface analysis was primarily by Rutherford backscattering spectrometry. Both the ion-beam mixing and the acid pretreatment increased the lifetimes of films, in comparison with the mechanically polished standards. Reductions in charge injection capability, when they occurred, were attributed to loss of iridium from the films, and there was a close correlation between the charge injection density and the iridium inventory.