Abstract
The Al-Mg phase diagram has been reinvestigated in the vicinity of the stability range of the Samson phase, β-Mg2Al3 (cF1168). For the composition Mg38.5Al61.5, this cubic phase, space group Fd3̄m (no 227), a = 28.242(1) Å, V = 22526(2) Å3, undergoes at 214°C a first-order phase transition to rhombohedral β′-Mg2Al3, (hR293), a = 19.968(1) Å, c = 48.9114(8) Å, V = 16889(2) Å3, (i.e. 22519 Å3 for the equivalent cubic unit cell) space group R3m (no 160), a subgroup of index four of Fd3̄m. The structure of the β-phase has been redetermined at ambient temperature as well as in situ at 400°C. It essentially agrees with Samson's model, even in most of the many partially occupied and split positions. The structure of β′-Mg 2Al3 is closely related to that of the β-phase. Its atomic sites can be derived from those of the β-phase by group-theoretical considerations. The main difference between the two structures is that all atomic sites are fully occupied in case of the β′-phase. The reciprocal space, Bragg as well as diffuse scattering, has been explored as function of temperature and the β- to β′-phase transition was studied in detail. The microstructures of both phases have been analyzed by electron microscopy and X-ray topography showing them highly defective. Finally, the thermal expansion coefficients and elastic parameters have been determined. Their values are somewhere in between those of Al and Mg.
Original language | English |
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Pages (from-to) | 259-288 |
Number of pages | 30 |
Journal | Zeitschrift fur Kristallographie |
Volume | 222 |
Issue number | 6 |
DOIs | |
State | Published - 2007 |
Externally published | Yes |
Funding
took place in the European Network of Excellence on Complex Metallic Alloys, financially supported by the EC under contract n· NMP3-CT-2005-500140. Part of this work has benefited from the use of NPDF at the Lujan Center at Los Alamos Neutron Science Center, funded by DOE Office of Basic Energy Sciences and Los Alamos National Laboratory funded by Department of Energy under contract W-7405-ENG-36. The upgrade of NPDF has been funded by NSF through grant DMR 00-76488. We acknowledge the European Synchrotron Radiation Facility (ESRF) for provision of synchrotron radiation facilities and we would like to thank Michela Brunelli for assistance in using beamline ID31 and supporting experiment HS-2968. We also want to thank the staff of the Swiss/Norwegian Beamline (SNBL) for its support. Valuable contribution of Prof. Phi-lippe A. Buffat (EPFL) to phase identification by electron diffraction and JEMS is kindly acknowledged. One of us (DF) thanks the National Science Foundation for financial support through a postdoctoral research fellowship (grant DMR-0502582).
Funders | Funder number |
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DOE Office of Basic Energy Sciences and Los Alamos National Laboratory | |
National Science Foundation | DMR 00-76488, DMR-0502582 |
U.S. Department of Energy | W-7405-ENG-36 |
European Commission | NMP3-CT-2005-500140 |
Keywords
- CMA
- Elastic parameters
- Electron microscopy
- Magnesium aluminides
- Microstructure
- Powder diffraction structure analysis
- Samson phase
- X-ray diffraction