Formation of Nanocrystalline Matrix Composite during Spray Forming of Al 83 La 5 Y 5 Ni 5 Co 2
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RECENTLY, the synthesis of Al-RE-TM-based amorphous or nanocrystalline bulk materials have attracted unprecedented interest due to their high specific strength along with improved physical properties.[1–6] In general, the bulk amorphous or nanocrystalline material is synthesized by consolidation of pulverized melt-spun amorphous ribbons or gas-atomized amorphous powders in the size range of 10 to 20 lm.[7,8] The amorphization of Al-based alloys involves high cooling rate due to their low glass-forming ability compared to the other systems, e.g., Cu-, Zr-, and Fe-based alloys. Therefore, the melt-spinning process and the high-pressure gas atomization are the most preferred routes for producing amorphous Al alloys. However, the large number of processing steps and the small size of the product materials involved in these routes do not make these routes economically viable for V.C. SRIVASTAVA, Scientist, National Metallurgical Laboratory, Jamshedpur-831007, India, is AvH Fellow, Institut fu¨r Werkstofftechnik, Universita¨t Bremen. V. UHLENWINKEL and A. SCHULZ, Scientists, and H.-W. ZOCH, Director, are with the Institut fu¨r Werkstofftechnik, Universita¨t Bremen, D-28359 Bremen, Germany. Contact e-mail: [email protected] K.B. SURREDDI, Doctoral Student, is with the Institute for Complex Materials, IFW Dresden. J. ECKERT, Director, Institute for Complex Materials, IFW Dresden, D-01171 Dresden, Germany, is Professor, Institute of Materials Science, Technical University Dresden, D-01062 Dresden, Germany. Manuscript submitted September 1, 2008. Article published online January 6, 2009 450—VOLUME 40A, FEBRUARY 2009
commercialization. In addition, it is difficult to achieve high yield in atomization processes due to the fact that the standard deviation of powder size is generally very large, and the fraction of large size particles crystallize because of their slow cooling rate. Furthermore, a small operating temperature window DTx, which is the difference between crystallization (Tx) and glass transition (Tg) temperatures, during hot consolidation limits proper control of the process and mostly leads to the crystallization of amorphous phases.[9] Therefore, it has become extremely important to look for some other alternative routes that can give rise to bulk amorphous/ nanocrystalline material in a single step. In one of their works in 1991, Oguchi et al.[10] demonstrated the ‘‘supercooled liquid-quenching’’ method to produce 7-mm-thick Al84Ni10Mm6 (at. pct) amorphous sheet. This was carried out by highpressure gas atomization of liquid metal into a spray and its subsequent deposition on a rotating drum to achieve high cooling rate similar to melt spinning. The major aspect of the microstructural evolution in this technique is the rapid heat extraction from the highly undercooled droplets during deposition. In the similar line of spray atomization and deposition, Afonso et al.[11,12] attempted to develop Al-Y-Ni-Co alloys by spray forming at a very high gas to melt flow rate ratio of 10 m3/kg and found a considerable
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