Theoretical and practical implications of creep curve shape analyses for 8090
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I. INTRODUCTION
IN an earlier study, tensile creep and creep fracture properties were documented for two commercial aluminum alloys, 2124 and 2419. With these precipitation-hardened materials, over broad stress ranges at 373 to 463 K, dislocation processes control strain accumulation, with intergranular damage development causing failure. However, the behavior patterns observed for 2124 appeared to be increasingly affected by precipitate coarsening as the test duration increased at 427 K and above, although the direct evidence for particle growth was not clear cut. To evaluate these conclusions, results are now presented for another aluminum alloy, 8090. Alloy 8090 has a specific gravity of 2.53 Mg m3, as opposed to 2.8 Mg m3 for conventional aluminum alloys. Particularly for airframe applications, a technical interest then centers on alloys in the aluminum-lithium series, seeking to take advantage of their low density rather than attempting to exploit the possibly superior mechanical properties attainable with alternative high-strength products. Hence, a wide range of aluminum-lithium alloys have been evolved,[2] with 8090 based on an Al-Li-Cu-Mg formulation chosen to achieve a substantial density reduction. Yet, for these alloys, little creep property data seem to be openly available. In the present program, the tensile creep and creep fracture characteristics of 8090 are therefore reported for stress/temperature ranges matching the test conditions covered for 2124 and 2419.[1] Moreover, to clarify the effects of precipitate coarsening in relation to the processes governing strain accumulation and eventual failure, supplementary experimental evidence is derived from microstructural studies, yield stress determinations, and analyses of creep curve shape. [1]
II. EXPERIMENTAL PROCEDURES The composition (wt pct) of the 8090 sample was 2.35Li, 1.23Cu, 0.67Mg, 0.01Mn, 0.02Si, 0.03Fe, 0.01Cr, 0.02Ti,
H. BURT, Senior Research Officer, and B. WILSHIRE, Professor and EngD/IGDS Director, are with the Materials Research Centre, School of Engineering, University of Wales, Swansea, SA2 8PP, United Kingdom. Contact e-mail: [email protected] Manuscript submitted August 20, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
0.02Zn, 0.11Zr, balance Al. This material was supplied as 37.5-mm-thick plate in the T8771 condition, i.e., solution treatment at 819 K, cold stretching by 6 pct, and aging at 443 K for 32 hours. These processing operations resulted in an elongated grain structure, with grain lengths up to 500 m and widths of 20 m. Cylindrical testpieces, with 4-mm diameters and 9.5-mm threaded ends, were machined with 25.4-mm gage lengths parallel to the elongated grain axes. To allow direct comparisons to be made with the data obtained for 2124 and 2419, creep tests were again carried out using the constant truestress machines and procedures described previously.[1] However, as noted for 2124 and 2419, although high-precision equipment was employed,[3] the creep property values determined for 8090 showed si
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