Evolution of microstructure and tensile strength of rapidly solidified Al-4.7 pct Zn-2.5 pct Mg-0.25 pct Zr- X wt pct Mn
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I.
INTRODUCTION
THE enhancement of mechanical properties by Mn additions has been extensively studied, and it is generally accepted that the formation of insoluble manganese dispersoids is responsible for the increased strength.[1,2] Since these dispersoids (mainly Al6Mn) have a high melting point and little solubility in aluminum, it would be advantageous to employ a rapid solidification/powder metallurgy (RS/PM) process to (1) extend the solubility of Mn to produce a higher volume fraction of dispersoids and to (2) apply subsequent heat treatment to control the size and distribution of dispersoids in the Al matrix.[3,4,5] To achieve a fine and uniform distribution of Mn dispersoids, it is important to understand the details of microstructural evolution in these alloys, including the formation and growth behavior of Mn dispersoids from the supersaturated solid solution. In addition, rapidly solidified powder contains metastable phases, which transform to the equilibrium phases during subsequent powder metallurgy processing, which involves moderate heating at 300 7C to 400 7C and hot deformation.[6,7] The improvement of tensile properties in RS/PM processing over conventional casting is significant, and the amount of the Mn addition can be extended to more than 3.6 wt pct, depending on the cooling rate in the RS/PM approach, while it is limited to only 1.2 wt pct in the casting JOONYEON CHANG, Research Scientist, and INGE MOON, Principal Research Scientist, are with the Division of Metals, Korea Institute of Science and Technology, Seoul 130-650, Korea. CHONGSOOL CHOI, Professor, is with the Department of Metallurgical Engineering, Yonsei University, Seoul 120-749, Korea. Manuscript submitted September 22, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
of Mn-containing Al alloys.[8,9] Most of the previous investigations on RS/PM-processed Al alloys with manganese have focused on the effect of Mn content on the tensile properties,[10,11,12] while detailed studies on the formation and growth of Mn dispersoids are generally lacking. Considering the existence of metastable phases in rapidly solidified Al-Mn alloys, the study of the formation of Mn dispersoids from the supersaturated solid solution or from metastable phases can provide valuable information, which can subsequently be used to exploit nucleation kinetics of fine Mn dispersoids. Furthermore, optimization of subsequent powder processing conditions can be made by selecting appropriate heating cycles or thermomechanical processes to enhance homogeneous nucleation and suppress growth of the Mn dispersoids in the Al alloys. To achieve this goal, it would be necessary to monitor the microstructural evolution and identify the phase transformation of the Mn dispersoids in Al-Mn alloys. It is the purpose of present study to (1) identify the formation of the equilibrium Mn dispersoid during consolidation and hot extrusion of rapidly solidified powder, and (2) correlate this behavior to the variation of tensile properties with processing temperature in Al-Zn-Mg-Z
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