Metastabile Extensions of Intermediate Phases in Some Aluminum - Rare-Earth Metal Systems
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METASTABLE EXTENSIONS OF INTERMEDIATE PHASES IN SOME ALUMINUM - RARE-EARTH METAL SYSTEMS*
M.X. Quan, P. Haldar, J. Werth and B.C. Giessen, Materials Science Division, Barnett Institute and Department of Chemistry, Northeastern University, Boston, MA
ABSTRACT Using the arc-furnace hammer-and-anvil method, metastable alloys in the Al-rich portions of four Al-RE systems (10-30 at. pct. RE; Re = Gd, Ho, Er and Y) were prepared and examined by XRD. The principal results are: all equilibrium alloy phases [Al 4 RE (T phase), A1 3 RE (CN12 close-packed phases) and Al 2 RE (Laves phase)] have substantial metastable solid solubilities for excess Al; e.g., in the Al-Y system, L phase is extended from 66.7 up to 86 at. pct. Al, suppressing any formation of the A1 3 RE phase in that system. For early RE elements up to and including Gd, T phase is favored; for alloy chemically intermediate RE elements such as Y, L phase is dominant and for late RE elements (after Ho), the CP phases are extended most. DSC work indicates the formation of additional metastable phases occurring during equilibration.
INTRODUCT ION Recently, the alloy chemistry of metastable phases rich in Al has attracted research interest because of the finding of quasicrystalline alloy phases in rapidly quenched Al-rich Al-Mn alloys and other metastable alloys of Al with transition metals [1]. In the course of ongoing research on other Al-based alloy systems that might contain such phases, a survey of metastable, Al-rich Al-RE alloys produced by RSP was undertaken (where RE is a rare earth metal or Y) . An additional motivation for this work arises from the current interest in Al alloys strengthened by metastable solid solutions or precipitation products from such solutions [2]. The present paper gives preliminary results on the metastable alloy chemistry of the systems Al-Gd, Al-Y, Al-Ho and Al-Er in the regions with 10-30% RE (all percentage data are in atomic percent), reporting the structures of arc-furnace hammer-and-anvil quenched samples examined in the as-quenched state.
Alloy Chemistry of Al-rich Al-RE Alloys: The Al-rich portions of all Al-RE systems are similar in the major features [3] which are briefly reviewed in the following. Starting from 100% Al, one finds the following alloy phases and phase diagram characteristics: 1.
The solid solubility of RE elements in Al is very low (0.01% for Al(Ce) solid solution), as is readily explained by the substantial size difference [4].
Communication No.
281 from the Barnett Institute
Mat. Res. Soc. Symp. Proc. Vol. 58. , 1986 Materials Research Society
300
2.
There are phases of stoichiometry A14RE or A111RE3 comparatively complex structure [3; their occurrence light RE metals up to RE = Th.
3.
All systems contain phases A13 RE with 12-coordinated, close-packed (CN12-CP) structures of the Ni 3 Sn, BaPb 3 , HoA1 3 and AuCu 3 types. These A13 RE structures can be described as consisting of hexagonal A13 RE layers stacked such that minority atoms never are first nearest neighbors; the stacking sequence
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