Nanophase Fe alloys consolidated to full density from mechanically milled powders
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L.F. Allard and K. Breder High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831
E. Maa) Mechanical Engineering Department, Louisiana State University, Baton Rouge, Louisiana 70803 (Received 7 September 1999; accepted 13 January 2000)
Nanophase elemental Fe powders prepared by mechanical milling were sinter forged to full density with an average grain size in the nanophase range (below 100 nm). If Cu additions are introduced during milling to form supersaturated solid solutions (Fe85Cu15 and Fe60Cu40), grain sizes can be easily controlled to below 50 nm after consolidation. For Fe–Cu, it was observed that atomic level alloying between the two elements during milling was very helpful for obtaining a homogeneous microstructure and nanocrystalline grain/domain sizes in the consolidated product. The advantages of using sinter forging (upset die forging), as well as the role of the Cu addition, in the retention of nanocrystalline grain sizes are discussed. The consolidated Fe alloys exhibit very high strength under compression, further demonstrating that low populations of flaws and nanophase grain structures were attained in the consolidated products.
I. INTRODUCTION
Nanocrystalline materials are of considerable scientific and technological interest at present.1 Such materials can be produced through a number of chemical and physical processing routes and often in the form of powders. For example, mechanical milling is known to be an effective processing method, 2 because this roomtemperature (RT) process requires simple equipment and yields relatively large quantities of nanophase powders. Such loose powders, however, have limited applications. For example, it is obviously necessary to sinter the powders into compacts for mechanical property testing and structural applications. In fact, full density is often required if one is to measure reliably, and eventually exploit in applications, the intrinsic mechanical response of nanophase materials. It has been well recognized in recent years that the presence of significant porosity as well as the lack of sufficiently large samples often leads to inconsistent observations of mechanical properties of nanophase materials. Hot consolidation of the powders into fully dense bulk samples is obviously a challenging a)
Address all correspondence to this author. Present address: Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, MD 21218.
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J. Mater. Res., Vol. 15, No. 4, Apr 2000 Downloaded: 14 Mar 2015
step in processing. Since significant heat is supplied, considerable grain growth occurs so that the consolidated product often loses the desired nanostructure. This is a particularly serious problem for single-phase elementalmetals. For example, Fe is easy to mill to reach nanometer-scale grain sizes, but it has been difficult to maintain the nanophase grains upon consolidation to fulldensity bulk samples. A number of studies have resorted to adding grain growth im
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