On the influence of Si-Ge additions on the aging response of Al-Cu
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IN the authors’ previous article, it was demonstrated that by adding Cu to the ternary Al-Si-Ge, a dramatic hardening result could be achieved.[1] Alloys of the composition Al5.7 wt pct Cu-0.5 wt pct Si-1.3 wt pct Ge (which is Al-2.5 at. pct Cu-0.5 at. pct Si-0.5 at. pct Ge) and Al-5.4 wt pct Cu-1 wt pct Si-2.63 wt pct Ge (which is Al-2 at. pct Cu-1 at. pct Si-1 at. pct Ge) were aged at 190 °C. The hardening response of the Al-Cu-Si-Ge alloys was compared to two commercial Al-Cu–based alloys (2219 and 2014) and was shown to be superior in terms of achieving a substantially higher peak hardness at a shorter aging time while displaying equal if not better microstructural stability after prolonged aging. It was shown that at peak hardness, the microstructure consisted of a dense distribution of u⬘ precipitates heterogeneously nucleated on Si-Ge precipitates. However, a detailed examination of the microstructure during the entire aging regime was not done. The present study seeks to further explain the unique hardening response of the quarternary Al-Cu-Si-Ge alloy by examining its microstructure in the postquench, room-temperature-aged condition, during initial stages of precipitation, at peak hardness, and in the overaged condition. To further understand the role of Si-Ge additions, binary Al-5.5 wt. pct Cu (which is Al-2.4 at. pct Cu) is examined for comparison. II. EXPERIMENTAL PROCEDURE Bulk alloys of the composition Al-5.7 wt pct Cu-0.5 wt pct Si-1.3 wt pct Ge (Al-2.5 at. pct Cu-0.5 at. pct Si-0.5 at. pct Ge) and Al-5.5 wt pct Cu (Al-2.4 at. pct Cu) were made
DAVID MITLIN, Postdoctoral Fellow, formerly with the Materials Science Department, University of California-Berkeley and the Lawrence Berkeley National Laboratory, is with Los Alamos National Laboratory, Los Alamos, NM 87545. Contact e-mail: [email protected] V. RADMILOVIC, Staff Member, and U. DAHMEN, Head of National Center for Electron Microscopy, are with the Lawrence Berkeley National Laboratory, Berkeley, CA 94720. J.W. MORRIS, Jr. is Professor, Materials Science Department, University of California-Berkeley, Berkeley, CA 94720, and Staff Member, Lawrence Berkeley National Laboratory. Manuscript submitted November 14, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
by arc melting 99.999 Si, 99.9999 Ge, 99.999 Cu, and 99.99 Al (all in wt pct). The cast samples were cold swaged to achieve 10 to 15 pct plastic deformation in order to break up the dendritic as-cast microstructure and promote homogenization during the high-temperature anneal. They were then encapsulated in a sealed quartz glass tube that was backfilled with argon and annealed for 24 hours at 500 °C. From that temperature, they were quenched in an ice-water bath. The final shape of the bulk alloy was roughly cylindrical, approximately 20 mm in length and 10 mm in diameter. The cylindrical ingots were sliced into discs 0.5 cm in thickness cut normal to the cylinder axis. The samples were aged at 190 °C. The temperature was monitored with an external thermocouple and varied by no more th
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