An Atomistic-Based Hierarchical Multiscale Examination of Age Hardening in an Al-Cu Alloy
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INTRODUCTION
NEAR the beginning of the 20th century, Alfred Wilm[1] discovered that the hardness of quenched Al alloys evolves in time. While Wilm’s discovery laid the foundation for many modern engineering alloys, the phenomenon had already been inadvertently utilized by the Wright brothers during their historic flight of 1903.[2] In the 50 years following Wilm’s discovery, the evolving hardness in Al alloys was linked to an evolving microstructure, thanks to advancements in dislocation theory[3–5] and the observation of nanometer-sized solute clusters by X-ray scattering[6,7] and transmission electron microscopy (TEM).[8] In the last 50 years, utilization of this phenomenon, now known broadly as precipitation hardening, has consistently progressed,[9,10] with the most recent precipitation hardened Al alloys demonstrating strengths approaching 1 GPa while maintaining significant ductility.[11,12] In Al-Cu alloys, the evolution of the microstructure at common aging temperatures [383 K (110 C) to 423 K (150 C)] and Cu concentrations (~4 wt pct Cu) consists of[13] SSSa ! GP zones ! h00 (GP2 zones) ! h0 ! h (CuAl2) with SSSa representing a supersaturated solid solution and GP zones being nanometer-sized Cu diskshaped monolayers on {100} planes. h¢¢ precipitates are larger than GP zones and consist of two disk-shaped Cu
CHANDRA VEER SINGH, Assistant Professor, is with the Department of Materials Science and Engineering, University of Toronto, 184 College St., Toronto, ON M5S 3E4, Canada. DEREK H. WARNER, Assistant Professor, is with the School of Civil and Environmental Engineering, Cornell University, Ithaca, NY 14853. Contact e-mail: [email protected] Manuscript submitted April 16, 2012. Article published online February 14, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
monolayers on {100} planes separated by three planes of Al atoms. h¢ precipitates are Al2Cu platelets having a tetragonal structure. h precipitates constitute the equilibrium precipitate phase with Al2Cu stoichiometry and bodycentered tetragonal structure. GP zones and h¢¢ precipitates are fully coherent with the Al matrix, h¢ is semi-coherent, and h is incoherent. Although multilayer GP zones have been observed,[14] there is a general agreement that they are a single atomic layer thick.[15] The Cu content of GP zones, however, has not been settled with different experimental observations showing significant variation from 40 to 100 pct[14,16] with an average value of about 80 pct.[16] The presence of precipitates restricts the motion of dislocations and thus affects macroscopic strength. During the initial stages of age hardening, precipitate size and volume fraction increase, leading to an increase in strength. However, in the later stages, precipitates grow at the expense of others, leading to an increase in precipitate spacing and a decrease in strength. In binary Al-Cu alloys, optimum strength typically occurs near the transition from h¢¢ precipitates to h¢ precipitates.[17] A variety of dislocation–precipitate interaction mechanisms have been
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