Identification of dispersoids in Al-Mg alloys containing Mn
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I.
INTRODUCTION
I N a previous publication J it has been shown that submicron dispersoids in an alloy of AI-4.85 pct Mg-0.74 pct Mn0.22 pct Cr were important to the hot ductility of the alloy. The chemical composition had been studied with energy dispersive X-ray spectroscopy (EDS). The major components were either A1 or Mg or both. The minor components were mostly Mn, together with small amounts of Fe and Cr. Whether A1 or Mg, or both, were major components could not be determined from EDS spectra because the Mg ka-peak could not be resolved from the A1 k~-peak. A preliminary identification using transmission electron microscope (TEM) suggested that the dispersoids had an orthorhombic structure based on MnA16 with a superlattice whose exact nature needed to be clarified. In the present study, evidence suggesting that the dispersoids belong to the phase of MnAI42 has been found. In addition, dispersoids of E phase and CrA17 have also been found. It has been known 3 that substructures are formed during hot working through the processes of dynamic recovery and recrystallization. Fracture during hot working can be decisively influenced by dynamic recrystallization. 4 9 Both dynamic processes are strongly affected by the presence of dispersoids. 4'9,~~ Consequently, the addition of manganese to aluminum alloys ll increases the recrystallization temperature, slows down recovery, and hinders grain growth. In AA 5056, grain growth has been shown to be substantially affected by the contents of both Mn and CrJ z All these experiences may be related to the interaction between the dispersoids and dislocations, grain boundaries and subboundaries. Therefore, the identification of the dispersoids is of fundamental importance to the science of the hot working of AA 5000 series of alloys. In the following, the experimental procedure is described and the results of experimeots and analysis are discussed in detail before conclusions are drawn.
II.
EXPERIMENTAL PROCEDURE
Four different alloys were prepared in a graphite crucible, l Emission spectroscopy gave the chemical compositions shown in Table I. Alloy A is a ternary alloy similar to commercial AA 5083 and AA 5456 alloys. The impurities
SHENG-LONG LEE, Lecturer. and SHINN-TYAN WU, Professor, are with the Department of Materials Science and Engineering, National TsingHua University, Hsinchu, Taiwan, Republic of China. Manuscript submitted June 26, 1986. METALLURGICALTRANSACTIONS A
of Fe, Si, and Ti are deliberately excluded to avoid the complications from interdendritic precipitates.1 Chromium is also excluded because EDS results ~ show it to be a minor alloying element. Alloy B was prepared for investigating the role played by Mg. Alloy C has a composition similar to that of AA 5083 except for the absence of the impurities of Fe, Si, and Ti. It is for studying the dispersoids other than those of MnAI4 in a commercial-like alloy. Alloy D contains impurities like those of commercial alloys. The first three alloys were given a soaking treatment at 500 ~ for twenty-four hours
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