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Submitted on 1 Jan 1987
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HIGH RESOLUTION TEM STUDY OF PRECIPITATES IN A RAPIDLY SOLIDIFIED
Al-Li-Zr ALLOY
N. Kim, J. Howe, E. Boden
To cite this version:
N. Kim, J. Howe, E. Boden. HIGH RESOLUTION TEM STUDY OF PRECIPITATES IN A
RAPIDLY SOLIDIFIED Al-Li-Zr ALLOY. Journal de Physique Colloques, 1987, 48 (C3), pp.C3-
457-C3-463. �10.1051/jphyscol:1987352�. �jpa-00226583�
JOURNAL DE PHYSIQUE
Colloque C3, suppl6men-t au n 0 9 , Tome 48, septembre 1987
HIGH RESOLUTION TEM STUDY OF PRECIPITATES IN A RAPIDLY SOLIDIFIED Al-Li-Zr ALLOY
N . J . KIM , J.M. HOWE* and E . G . BODEN*"
Metals and Ceramics Laboratory, Allied-Signal Incorporation, P.O. Box 1021 R, Morristown, NJ 07960, U.S.A.
" ~ e p a r t m e n t of Metallurgical Engineering and Materials Science, Carnegie-Mellon University, Pittsburgh, PA 15213, U.S.A.
* * Applications Laboratory, Philips Electronic Instruments Incorporation , Mahwah, NJ 01430, U.S.A.
In this investigation, high resolution transmission electron microscopy and image simulations are used to characterize the atomic structure, coherency and chemistry of 6' and A1,Zr precipitates in a rapidly solidified A1-3.4Li-0.5Zr alloy. During aging, 6' precipitates around A1,Zr while maintaining coherency across the interphase boundary. Coalescence occurs when S' precipitates whose sublattices are in phase coarsen and impinge. On the other hand, 6' precipitates whose sublattices are antiphase appear not to coalesce and a layer of aluminum matrix exists between them. Deformation of the alloy results in a shearing of 6 ' precipitates which occurs in some cases, by multiple cross slip along intersecting
(111) planes. Finally, the results suggest that there may be some incorporation of lithium in the A1,Zr precipitates.
INTRODUCTION
During aging of aluminum-lithium alloys, there is a precipitation of metastable, coherent, L1, ordered 6 ' (A1,Li) in the microstructure. This 6' precipitate suffers from its propensity to dislocation shear, resulting in a loss in ductility and fracture toughness. It has been shown that the ductility and fracture toughness of aluminum-lithium alloys can be improved by the addition of zirconium 11-51, The improvements in mechanical properties come, in part from the unrecrystallized microstructure achieved by the addition of zirconium, but are mainly due-to the effect of zirconium on modifying the precipitation behavior.
Zirconium promotes the formation of a metastable, L12 A1,Zr precipitate which is isostructural with the 6' precipitate. During aging of an alloy which contains Al,Zr, 6' precipitates heterogeneously around the Al,Zr, forming so-called composite precipitates [I-81. This composite precipitate is characterized by a spherical inner core of AlaZr surrounded by an envelope of 6'. The existence of A1,Zr or composite precipitates in the microstructure has important effects on the mechanical behavior of aluminum-lithium alloys in that A1,Zr precipitates are quite resistant to dislocation shear, thereby remedying the slip localization in
aluminum-lithium alloys.
There have been a relatively large number of investigations on microstructural development in Al-Li-Zr alloys; however, there still is a lack of detailed
knowledge of the phase transformation behavior in Al-Li-Zr alloys, e.g., formation of antiphase boundaries (APBs) during coarsening of 6 ' precipitates, interaction of dislocations with 6' and A1,Zr precipitates and chemistry of the A1,Zr
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987352
C3-458 J O U R N A L DE PHYSIQUE
precipitate. This lack of understanding is mainly due to the inability of most analytical techniques to detect the structures on an atomic level. In the present investigation, high resolution electron microscopy (HREM) and image simulations have been used to perform detailed analyses of phase transformations and their consequences in Al-Li-Zr alloys.
EXPERIMENTAL PROCEDURES
An alloy of composition of A1-3.4 wt%Li-0.5 wt%Zr was rapidly quenched from the melt into continuous ribbons by the jet casting process. These ribbons were mechanically comminuted to - 4 0 mesh powder and then consolidated into bulk compacts by vacuum hot pressing followed by hot extrusion to a final rectangular shape of 63.5 mm X 10.2 mm. An extrusion was solution treated at 540°C for 2 hrs., quenched in cold water and aged at various temperatures. Thin foils were obtained from the as-heat treated extrusion as well as from the gauge section of the deformed tensile specimen. TEM foils were prepared by jet polishing with an electrolyte of 33% HNO, and 67% methanol or with one of 10% perchloric acid and
908ethanol. Conventional TEM and tilted illumination HREM observations were made using Philips CN-12 and EM400T. Axial illumination HREM study was conducted using
JEOL