Quench Sensitivity of AA 7136 Alloy: Contribution of Grain Structure and Dispersoids
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INTRODUCTION
AL-ZN-MG-CU alloys have been used extensively as structural materials in aerospace industry.[1–3] These alloys are age hardenable and have to be subjected to solution heat treatment, quenching, and aging so as to obtain high strength.[4,5] Quenching is a critical step for these alloys. Their mechanical properties and corrosion resistance tend to decrease with the decrease of quenching rate,[6,7] and this phenomenon is called quench sensitivity.[8–10] Thus, fast quenching is desirable after solution heat treatment. However, fast quenching can often result in warpage of thin products,[11] and high residual stress in products with a large section,[12,13] and therefore it is required to decrease quenching rate in this case. Moreover, the quenching rate usually decreases from the surface to the mid-plane
JIANGUO TANG, SHENGDAN LIU, and LINGYING YE are with the School of Materials Science and Engineering, Central South University, Changsha 410083, P.R. China and with the Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education, Changsha 410083, P.R. China and also with the Nonferrous Metal Oriented Advanced Structural Materials and Manufacturing Cooperative Innovation Center, Changsha 410083, P.R. China. Contact e-mail: [email protected] ZHENSHEN YANG, QING WANG, JINGCHAO CHEN, and WENRU CHAI are with the School of Materials Science and Engineering, Central South University and Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of Education. Manuscript submitted December 18, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
of products with a large section. For instance, Lim et al.[14] showed that in an AA7175 alloy forging with a size of about 200 mm in diameter and 300 mm in length, the quenching rate decreases from 22 C/s in the surface layer to 4 C/s in the center layer during cold water quenching. This difference in quenching rate usually leads to heterogeneity and drop in properties of these thick products.[14,15] Therefore, it is an effective way to improve comprehensive properties of thick products and avoid warpage of thin products by decreasing quench sensitivity. Many investigations have shown that quenchinginduced precipitates form during slow quenching after solution heat treatment, which reduces solutes in the solid solution, and consequently fewer strengthening precipitates can be obtained after subsequent aging, resulting in lower strength.[16–18] Therefore, quench sensitivity can be decreased by inhibiting the formation of quenching-induced precipitates.[8,19] It is known that dispersoids, grain boundaries (GBs), and subgrain boundaries (SGBs) are the main nucleation sites for quenching-induced precipitates,[20–22] and thus a higher amount of these nucleation sites tends to result in higher quench sensitivity. In order to fully understand quench sensitivity of Al-Zn-Mg-Cu alloys, it is essential to know the quantitative relationship between quench sensitivity and these microstructure features; however, it is not easy because of
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