Structure and deformation behavior of T 1 precipitate plates in an Al- 2Li- 1 Cu alloy
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I.
INTRODUCTION
IN recent years, considerable effort has gone into the development of A1-Li-Cu alloys due to their potential for use as high-strength, light-weight aerospace alloys, t1'2'3] These alloys derive their strength mainly from precipitation hardening, and in commercial alloys containing Li and Cu, three precipitate phases usually form during aging to peak strength: (1) a coherent spherical-shaped L12 ordered 8' phase (AlaLi), (2) a semicoherent plate-shaped T~ phase (A12CuLi), and (3) a semicoherent plate-shaped 0' phase (A12Cu), often coated with 8' depending on the Li/Cu ratio. (4-71 All of these phases play important roles in strengthening [8'9'~~ and, in addition, the T~ precipitates have been found to affect the yield-strength anisotropy of AI-LiCu alloys, till The crystal structures of the 8' and 0' phases are well established because of the extensive research that has been performed in understanding precipitation processes in binary AI-Li and A1-Cu alloys. [~2]However, it has only been recently that significant effort has gone into determining the structure of the T 1 phase, mainly in response to the development of commercial A1-Li-Cu alloys. [4-1~ In one of the most thorough studies of T~ precipitates performed so far, Huang and Ardell [131proposed a crystal structure for the T~ phase which was derived by comparing calculated X-ray intensities for various models with Debye-Scherrer X-ray intensity measurements reported for a cast Al2CuLi alloy by Hardy and Silcock. t~4] The T~ structure of Huang and Ardell t~3]has the space-group symmetry P6/mmm and consists of an A B C B . . . stacking of planes, where the A and C planes are close-packed with ordered arrangements of Li and A1 in 2 : 1 and 1 : 2 ratios, respectively, and the B planes have a disordered hexagonal arrangement of Cu and A1 that is not close-packed. Their model is consisJ.M. HOWE, ALCOA Assistant Professor, and J. LEE, Research Assistant, are with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213 A.K. VASUDEVAN, formerly a Staff Scientist, ALCOA Laboratories, is with Applied Research & Technology Directorate, Office of Naval Research, Code-1216, Arlington, VA 22217-5000. Manuscript submitted January 15, 1988.
METALLURGICAL TRANSACTIONS A
tent with rotation X-ray photographs obtained by Hardy and Silcock t14]which showed that no glide planes or screw axes were present in the T~ structure and that it belonged to one of the space groups P622, P6mm, P6m2, or P6/mmm. The TI structure of Huang and Ardell [13] was able to produce electron diffraction patterns in six low-index matrix orientations which were in agreement with those observed experimentally.[~sl Within the last year, two groups have investigated the structure of T~ precipitate plates using high-resolution transmission electron microscopy (HRTEM). Radmilovic and Thomas [~6]have shown a simulated HRTEM image of the T 1 structure in a (10T0)r~ [I(110)~ orientation that matched experimental images in this zone axis
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