August 3, 2008

Current Topics in Refractory Metals Research

By Ciulik, J

It has been an exciting year in refractory metals. We had a very interesting symposium at the TMS 2008 Annual Meeting in New Orleans. Our "Refractory Metals 2009" symposium covered three areas: processing of refractory metals, characterization of refractory metals, and properties of refractory metals. Twenty-three presentations in three sessions made the symposium this year the largest and best attended we have had since I began attending refractory metals symposia (in 2002). Several of the articles in this issue are based on presentations made at the New Orleans meeting, so by reading them you can get a flavor of the excellent presentations we had at our symposium. If you missed the meeting, abstracts are available on the TMS web site, and I'm certain that the authors would like to hear from you if you're interested in their work. This issue of JOM presents six articles on refractory metals. In their article. Michael C. Gao, Omer N. Dogan, Paul King, Anthony D. Rollett, and Michael Widom discuss a first-principles method to predict ductility in refractory metal alloys. Using density functional theory calculations and the observation that room- temperature ductility tends to be higher in metals and alloys with large Poisson ratios, predictions about ductile chromium-based alloys have been made.

Maria D. Moricca and S.K. Varma present follow-up article to a JOM article published in last year's refractory metals issue [JOM, 59 (6) (2007), pp. 46-49]. Their results this year describe the effects on high-temperature oxidation (700[degrees]C to 1,400[degrees]C) of adding carbon to Nb-W-Cr alloys. From their test results, improved oxidation resistance occurs when carbon is added to alloy Nb-20W-5Cr (a single-phase alloy), but oxidation resistance worsens when carbon is added to alloy Nb-20W-10Cr (an alloy containing NbCr^sub 2^ intermetallic compound particles).

A. Zamiri, H. Jiang, T.R. Bieler, and F. Pourboghrat have a follow-up article to a JOM article published in last year's refractory metals issue [JOM, 59 (6) (2007), pp. 50-55]. Their results this year describe the use of an evolutionary yield function to predict the plastic deformation behavior of high-purity niobium during fabrication of particle accelerator cavities. The evolutionary yield function is needed because the niobium sheet metal used is anisotropic (i.e., it has different mechanical properties in different directions, so it isn't easily deformed into symmetrical shapes). Worse still, the anisotropy coefficients traditionally used for deformation processing calculations aren't constant during deformation and the microstructure of the sheet metal can vary through the thickness. Consequently, evolutionary yield functions are needed for accurate modeling of the cavity forming process to account for the fact that the anisotropy coefficients are not constant during plastic deformation.

Jianhui Xu and Tongguang Zhai have two articles on the Mo-Re alloy in this issue: one on the small-scale resistance spot welding of 50Mo-50Re alloy thin sheet and one on the positive strain rate dependence of 5OMo-SORe thin sheet. In their first article, they describe the effect of strain rate on the ductility of recrystallized 50Mo-50R thin sheet and report that room-temperature ductility increases as the strain rate increases from 10^sup -6^ s^sup -1^ to 1 s^sup -1^. This increase is due to a "damage toughening" mechanism related to crack initiation locations (at low strain rates, cracks tend to initiate at grain boundary triple points, resulting in lower ductility). The second paper considers spot welding of thin sheet metal, which can be rather difficult. This article discusses the effects of seven important welding parameters (hold time, electrode shape, ramp time, weld current, electrode force and weld time) on the weld quality of 50Mo-50Re thin sheet.

From the extractive metallurgy side, Larry F. McHugh, Robert Balliett, and Jean A. Mozolic describe a novel method of roasting molybdenum sulfides and oxides. This "sulfide ore looping oxidation process" has been demonstrated on a lab scale to use less energy than traditional roasting processes and produce fewer by-products, making it a potentially economical alternative to current sulfide mineral processing methods.

Our next TMS refractory metals meeting will be part of a larger symposium co-sponsored by the TMS High Temperature Alloys Committee and the TMS Refractory Metals Committee at the TMS 2009 Annual Meeting, to be held February 15 through 19 in San Francisco, California. The "Next Generation Superalloys and Beyond" symposium will include a session on refractory-based alloys. Abstracts are currently being accepted for the symposium, so please consider presenting your work at the San Francisco meeting. I look forward to seeing you there.

J. Ciulik is with the Mechanical Engineering Department of the University of Texas at Austin in Austin, Texas, and is the advisor to JOM from the Refractory Metals Committee of the Structural Materials Division of TMS.

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