Abstract
Tracer diffusivities provide the most fundamental information on diffusion in materials, and are the foundation of robust diffusion databases that enable the use of the Onsager phenomenological formalism with no major assumptions. Compared to traditional radiotracer techniques that utilize radioactive isotopes, the secondary ion mass spectrometry (SIMS)-based thin-film technique for tracer diffusion is based on the use of enriched stable isotopes that can be accurately profiled using SIMS. An overview of the thin-film method for tracer diffusion studies using stable isotopes is provided. Experimental procedures and techniques for the measurement of tracer diffusion coefficients are presented for pure magnesium, which presents some unique challenges due to the ease of oxidation. The development of a modified Shewmon-Rhines diffusion capsule for annealing Mg and an ultra-high vacuum system for sputter deposition of Mg isotopes are discussed. Optimized conditions for accurate SIMS depth profiling in polycrystalline Mg are provided. An automated procedure for correction of heat-up and cool-down times during tracer diffusion annealing is discussed. The non-linear fitting of a SIMS depth profile data using the thin-film Gaussian solution to obtain the tracer diffusivity along with the background tracer concentration and tracer film thickness is demonstrated. An Arrhenius fit of the Mg self-diffusion data obtained using the low-temperature SIMS measurements from this study and the high-temperature radiotracer measurements of Shewmon and Rhines (Trans. AIME 250:1021-1025, 1954) was found to be a good representation of both types of diffusion data over a broad range of temperatures between 250 and 627 °C (523 and 900 K).
| Original language | English |
|---|---|
| Pages (from-to) | 762-778 |
| Number of pages | 17 |
| Journal | Journal of Phase Equilibria and Diffusion |
| Volume | 35 |
| Issue number | 6 |
| DOIs | |
| State | Published - Nov 18 2014 |
Funding
The authors are grateful for the support provided by the U.S. Department of Energy (DOE), Assistant Secretary for Energy Efficiency and Renewable Energy (EERE), Office of Vehicle Technologies as part of the Automotive Lightweight Materials Program under contract DE-AC05-00OR22725 with UT-Battelle, LLC. The authors thank the ORNL isotope processing facility, including Scott Aaron and Lee Zevenbergen for providing the enriched 25Mg isotopic foils used in this work and for helpful discussions. The technical expertise provided by Edward Kenik (EBSD analysis) and Harry Meyer (XPS analysis) at the High Temperature Materials Laboratory (HTML) is recognized. The authors acknowledge Edward Dein at the Advanced Materials Processing and Analysis Center (AMPAC) clean room facility, University of Central Florida (UCF) for his assistance with the UHV PVD system and the isotopic deposition experiments. The assistance of Jay Tuggle and the students of one of the authors, J. Hunter, at Virginia Tech during the course of this project are appreciated. The authors thank Sarah Brennan and the graduate students of one of the authors, Y. Sohn, and Mikhail Klimov (SIMS specialist) at UCF for their assistance during various stages of this work. The authors appreciate the many fruitful discussions related to SIMS with Peter Todd, formerly at ORNL and now at Nebulytics, Inc. in Oak Ridge. The authors thank John Allison, Robert McCune and the staff associated with the Mg-ICME initiative for their support and encouragement. The support of Carol Schutte, William Joost and Joe Carpenter with the DOE Vehicle Technologies Program, and Phil Sklad and David Warren at ORNL are gratefully acknowledged.
| Funders | Funder number |
|---|---|
| Carol Schutte | |
| Office of Energy Efficiency and Renewable Energy | |
| U.S. Department of Energy | |
| William Joost and Joe Carpenter | |
| U.S. Department of Energy | |
| Office of Energy Efficiency and Renewable Energy | |
| Vehicle Technologies Office | DE-AC05-00OR22725 |
Keywords
- SIMS
- database
- diffusion
- isotope
- magnesium
- self diffusivity
- tracer diffusivity