Evaluation of Solute Clusters Associated with Bake-Hardening Response in Isothermal Aged Al-Mg-Si Alloys Using a Three-D
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I.
INTRODUCTION
Al-Mg-Si alloys are widely used as automobile body panels because of their desirable combination of good formability and high strengthening potential. Despite this high strengthening potential, processing factors such as the short duration of artificial aging impedes the strengthening process. The so-called paintbake cycling process, a thermal cycling process[1] designed to provide both paint curing and precipitation hardening for auto panels, involves a relatively shortduration artificial aging process. The alloys are usually stored for a period at room temperature (RT) for logistical reasons, referred to as natural aging (NA), before it is given an artificial aging heat treatment. A typical paint-bake cycle of 30 minutes at 448 K (175 C) is much too short to reach peak strength, which requires 10 to 20 hours at 448 K (175 C) for naturally pre-aged Al-Mg-Si alloys.[2] The precipitation sequence in Al-Mg-Si alloys is generally accepted as the following: YASUHIRO ARUGA, Senior Researcher, and MASAYA KOZUKA, Researcher, are with the Materials Research Laboratory, Kobe Steel, Ltd., 1-5-5 Takatsukadai, Nishi-ku, Kobe 651-2271, Japan. Contact e-mail: [email protected] YASUO TAKAKI, Manager, is with the Aluminum Sheets and Coils Research Department, Aluminum & Copper Business, Kobe Steel, Ltd., 15 Kinugaoka, Moka, Tochigi 321-4367, Japan. TATSUO SATO, Professor, is with the Precision and Intelligence Laboratory, Tokyo Institute of Technology, Nagatsuta 4259-R2-18, Midori-ku, Yokohama 226-8503, Japan. Manuscript submitted December 21, 2013. Article published online September 18, 2014 5906—VOLUME 45A, DECEMBER 2014
aðSSSSÞ ! solute clusters ! GP zones ! b00 ! b0 ! b; ½1 where SSSS is the super saturated solid solution.[3] Since the SSSS is very far from equilibrium, even NA at RT causes cluster formation. This is evidenced by a significant increase in hardness observed during NA.[4] Clusters strongly influence the precipitation of the b¢¢ phase, widely recognized as the strengthening phase formed during artificial aging.[5] Hence, atom probe tomography (APT) has been used actively to characterize the clusters formed during aging after solution treatment.[6–12] It has been described that clusters formed near RT do not enhance the formation of the b’’ phase during artificial aging. This causes the so-called negative effect of NA, which results in reduced strength after paint-bake treatment. It is known that a pre-aging (PA) heat treatment in the temperature interval 343 K to 393 K (70 C to 120 C) after quenching leads to a higher number density of the b¢¢ precipitates after subsequent artificial aging, resulting in an improved bake-hardening (BH) response.[13,14] It was believed that clusters formed during PA can either transform to b¢¢ or serve as nuclei for precipitation of b¢¢.[15] The difference between these two cluster types (i.e., clusters formed near RT and during PA) has been investigated actively, but some points of confusion still exist, especially in terms of temporal changes in size and the
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