Identification of dispersoid phases created in aluminum during mechanical alloying
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I. INTRODUCTION
IT is well known that precipitation hardened aluminum alloys lose some of their strength at high temperatures because of either overaging or dissolution of the hardening precipitates? Even at intermediate temperatures overaging can occur, particularly if the alloy is plastically deformed during aging.2 In an effort to overcome this limitation in the use of aluminum at elevated temperatures, a number of dispersion strengthened aluminum alloys have been developed. The first load bearing dispersion strengthened aluminum alloys were sintered aluminum powders (SAP). Bloch3 has reviewed the development of these materials in detail and has described the preparative techniques used in their manufacture. Generally, the fabrication of SAP includes ball milling of fine aluminum powders and compacting of the milled powders by powder metallurgical methods. During ball milling the oxide films on the surfaces of the powder particles are broken and imbedded into the aluminum matrix. This is achieved by the continuous fracturing and welding of the powder particles. A process control agent is added to prevent excessive welding and to establish a dynamic balance between fracturing and welding. Later work has shown that the quality of the SAP product can be improved by optimizing the ball milling conditions and by vacuum degassing of the powders prior to consolidation.4~6The ball milling conditions control the distribution of the dispersoids, and vacuum degassing prevents the formation of cracks on heating, caused by the release of hydrogen. In spite of this extensive research work, SAP has never been used in large quantities. The main reasons for this are the high costs of production and the low ductility exhibited by SAP at elevated temperatures. More recently, dispersion hardened aluminum alloys R. F. SINGER, formerly a Visiting Scholar in the Department of Materials Science and Engineering at Stanford is now a member of the Technical Staff at Brown Bovari Company in Baden, Switzerland. W. C. OLIVER and W. D. NIX are Graduate Research Assistant and Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305. Manuscript submitted January 30, 1980.
with very promising properties have been produced by mechanical alloying.7-1~This new process was developed by Benjamin 1~for use in the preparation of oxide dispersion strengthened superalloys. Instead of using a conventional ball mill, mechanical alloying requires the use of a high energy ball mill which results in a finer and more homogeneous dispersoid distribution. Some studies of mechanically alloyed aluminum have been conducted7 ~0, but there is still some uncertainty about the hardening phases. In particular, the relative importance of aluminum oxide dispersoids and aluminum carbide dispersoids (formed from the residual of the carbon containing control agents) is unknown. Some indication was found that the dispersoids present in the as-milled condition are amorphous, but conclusive evidence is still missing. The primary purp
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