Effects of Combined Additions of Mn and Zr on Dispersoid Formation and Recrystallization Behavior in Al-Zn-Mg Alloys
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INTRODUCTION
MICROALLOYING is an effective way to inhibit recrystallization and, therefore, improve properties of aluminum alloys.[1–6] The additions of transition elements or rare earth elements lead to the formation of various dispersoids, which can pin grain boundaries.[7–10] For instance, a small addition of Zr to AlZn-Mg alloys can lead to the formation of coherent L12 Al3Zr dispersoids, which can offer a strong Zener-drag effect on grain boundaries during solution heat treatment.[11,12] However, Zr tends to segregate to dendrite/grain centers during casting (since the partition SHENGDAN LIU, LINGYING YE, and JIANGUO TANG are with the School of Materials Science and Engineering, Central South University, Changsha 410083, China and with the Key Laboratory of Non-ferrous Metals Science and Engineering, Ministry of Education, Changsha 410083, China and also with the Nonferrous Metal Oriented Advanced Structural Materials and Manufacturing Cooperative Innovation Center, Changsha 410083, China. Contact email: [email protected] JINGCHAO CHEN, WENRU CHAI, QING WANG, and ZHENSHEN YANG are with the School of Materials Science and Engineering, Central South University and also with the Key Laboratory of Non-ferrous Metals Science and Engineering. Manuscript submitted December 13, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
coefficient K > 1),[13] and the concentration gradients cannot be removed during subsequent heat treatment.[14] As a result, there is a higher number density of Al3Zr dispersoids in the interior of the dendrite/grain than near the dendrite/grain boundaries, and a particle-free zone often exists adjacent to grain boundaries and leads to partial recrystallization in the deformed alloy after solution heat treatment. The addition of Mn to aluminum may lead to various Mn-containing dispersoids, such as Al3Mn, Al4Mn, and Al6Mn,[15,16] and in multicomponent alloys containing Zn, Mg, or Cu, dispersoids such as Al20Cu2Mn3, Al24Mn5Zn, and Al20(Zn,Cu)2Mn3 can form.[17–19] In 2XXX, 5XXX, and 6XXX series aluminum alloys, it has been reported that Mn-containing dispersoids play an important role in the control of recrystallization behavior and refining recrystallized grains.[20–24] Mn tends to segregate toward the dendrite/grain boundaries (partition coefficient < 1) during casting.[25] Therefore, the number density of Mn-containing dispersoids in dendrite/grain boundaries is higher than that in the interior of the dendrite/grain. Based on their opposite microsegregation patterns, the combined additions of Mn and Zr are expected to create a greater uniformity of dispersoid coverage and, consequently, increase recrystallization resistance.
In fact, the combined additions of Mn and Zr exhibit different effects on recrystallization behavior in different aluminum alloys. For instance, Walsh et al.[26] found the addition of Mn from 0.31 to 1.02 wt pct had little effect on recrystallization of 2134-type Al alloys and all the alloys were in the unrecrystallized condition. Cheong and Weiland.[27] reported that the combin
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