Microstructure/Phase Evolution in Mechanical Alloying/Milling of Stainless Steel and Aluminum Powder Blends
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INTRODUCTION
MECHANICAL alloying (MA) is a convenient and inexpensive solid-state processing route for synthesis of a wide range of novel materials such as nanocrystalline and amorphous alloys, nanostructured composites, and bulk metallic glasses.[1–5] Among other devices, MA is routinely carried out in planetary ball mills. The process involves high strain rate plastic deformation of a given powder charge within a confined volume that induces concomitant deformation, cold welding, fragmentation, interdiffusion, and dynamic recrystallization. In the recent past, the present group of authors have synthesized several Al-based amorphous and nanocrystalline alloys by MA.[6–9] The present report aims to demonstrate microstructural evolution in the course of the synthesis of Al + stainless steel powder composites by MA of either elemental powder blend with appropriate average composition or a mixture of aluminum and prealloyed stainless steel powder. This study is of interest for, apart from developing Al-based high specific strength composite, two separate practical applications: mechanical joining of aluminum with stainless steel (say, by friction welding or contact rolling) and prediction/simulation of microstructural damages/changes in stainless steel or aluminium-based coatings subjected to high strain rate wear or erosion. While joining metallic couples with restricted or no P.P. CHATTOPADHYAY, Professor, and A. SAMANTA, Scholar, are with the Department of Metallurgy and Materials Engineering, Bengal Engineering and Science University, Shibpur, Howrah 711 103, West Bengal, India. W. LOJKOWSKI, Professor, is with Unipress, Warsaw, Poland. H.-J. FECHT, Professor, is with the Division of Materials, University of Ulm, 89081, Ulm, Germany. I. MANNA, Professor, is with the Metallurgical and Materials Engineering Department, IIT, Kharagpur 721 302, India. Manuscript submitted May 1, 2006. Article published online August 9, 2007.
2298—VOLUME 38A, SEPTEMBER 2007
solubility and prone to formation of brittle intermetallic phases often necessitate joining of dissimilar metals/materials by mechanical force/pressure as an alternative to fusion welding, railroad or tribological coatings undergo degradation due to progressive friction and wear during prolong use. Microstructural and phase evolution in both the processes or degradations can be studied and simulated by subjecting the concerned material to high-energy ball milling.[10–12]
II.
EXPERIMENTAL
Elemental powder (99.5 wt pct purity), equivalent to AISI 316 stainless steel (316SS) composition, were subjected to MA by high-energy planetary ball milling in a Fritsch P6 planetary ball mill in wet (toluene) medium with a ball-to-powder ratio of 10:1 using WC vial and balls (10-mm diameter). Similar MA operation was carried out with powder mixtures comprising prealloyed 316SS (commercially available) and elemental aluminum powders in three different proportions (316SS + 25, 65, and 85 wt pct Al). For comparison, mechanical milling of prealloyed 316SS powder and elemental powder b
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