Enhancement of Strength and Ductility of Al-Ag Alloys Processed by High-Pressure Torsion and Aging
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ODUCTION
IN 1938, Gunier and Preston reported the so-called GP zones in Al-rich, fcc Al-Cu plate-like precipitates and later Al-Ag spherical precipitates.[3] Precipitation in Al-Ag alloys was studied by Guinier, Geisler, Ko¨ster, and their various collaborators.[4] Hardy and Heal[5] suggested that the most probable sequence of precipitation is spherical G.P. zones (possibly ordered) fi c¢ plates (Ag2Al, formed on stacking faults or faulted during growth) fi c (formed by discontinuous precipitation, a solute-rich hexagonal close-packed (hcp) phase inside face-centered cubic (fcc) solid solution). The strength of the alloys is improved along with such a well-dispersed fine precipitation sequence during aging. Grain refinement also increases the strength of metallic materials through the Hall–Petch relation.[6,7] [1]
[2]
r ¼ r0 þ kd1=2
½1
where r0 is the friction stress required to move a dislocation and k is a material-dependent intensity factor related to hardening by grain boundaries. According to this equation, hard materials can be produced by refining the grain size. However, it is not easy to achieve strengthening due to grain refinement and precipitation hardening simultaneously because grain refinement by a conventional deformation-recrystallization process (so-called thermomechanical treatment) is difficult while maintaining a supersaturated state for the subsequent precipitation through aging. It is well established that the severe plastic deformation (SPD) process achieves grain refinement regardless
SEUNGWON LEE, Academic Researcher, and ZENJI HORITA, Professor, are with the Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Fukuoka 819-0395, Japan, also with the WPI, International Institute for Carbon-Neutral Energy Research (I2CNER). Contact e-mails: [email protected], [email protected] Manuscript submitted January 15, 2012. Article published online April 30, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
of sample states including supersaturation. There are a few reports aimed at the combined effects of grain refinement and fine precipitation in age-hardenable alloys. Horita et al.[8] showed that age hardening is feasible in an Al-11 wt pct Ag alloy after grain refinement using equal-channel angular pressing (ECAP). It was also shown that not only tensile strength but also ductility is improved by such a dual effect of grain refinement and precipitate dispersion. In this research, one of the SPD processes, high-pressure torsion (HPT),[9,10] is adopted for grain refinement because a finer grain size is attained by intense shear strain using HPT. The objective of this research is thus to investigate hardening behavior due to the dual effect of grain refinement and fine dispersion of precipitates in Al-Ag alloys.
II.
MATERIALS AND EXPERIMENTAL PROCEDURES
Ingots of Al-X wt pct Ag (X = 5, 11, 20) alloys having dimensions of ~100 mm length and ~15 mm diameter were fabricated by arc melting using high purity Al (99.99 pct) and Ag (99.99 pct) chips in argon atmo
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