A Combined Hot Dip Aluminizing/Laser Alloying Treatment to Produce Iron-Rich Aluminides on Alloy Steel
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INTRODUCTION
HIGH temperature oxidation/sulfidation is a major mechanism of material loss in different industries.[1–3] So, there is a great demand for new materials, as bulk or overlays, having the capability to protect against such severe conditions. Iron-rich FeAl and Fe3Al intermetallics are considered as new structural materials because of their excellent oxidation and sulfidation resistance, low cost, and the lack of costly strategic materials like Cr and Ni.[4] However, these compounds have low ductility at ambient temperature and lose their strength at temperatures above 873 K (600 C), which could restrict the use of bulk iron aluminides.[5,6] In order to take advantage of the exceptional benefits of these intermetallics, a number of surface engineering techniques were developed to produce iron aluminide coatings on suitable substrates.[7–26] The research has been pursuing two trends: the first one is the direct deposition of FeAl and Fe3Al coatings through processes like magnetron spark deposition,[7] cathodic arc vapor deposition,[8] aluminum powder feeding through plasma transferred arc,[9] solid state bonding,[10] electro spark deposition,[11] high velocity oxygen fuel,[12–16] chemical vapor deposition,[17] pack cementation,[18–20] and double glow plasma surface alloying.[21] The second approach is based on the generation of a primary surface layer and then employing a subsequent treatment in order to reach the desired compositions.[22–26] MOHAMMAD EMAMI, Ph.D. Student, and HAMID REZA SHAHVERDI, Associate Professor, are with the Department of Materials Engineering, Tarbiat Modares University, 14115-143 Tehran, Iran. Contact e-mail: [email protected] SHIGENARI HAYASHI, Associate Professor, is with the Faculty of Engineering, Hokkaido University, Sapporo, Japan. MOHAMMAD JAVAD TORKAMANY, Ph.D. Student, is with the Iranian National Center for Laser Science and Technology (INLC), 14665-576 Tehran, Iran. Manuscript submitted October 21, 2012. Article published online February 23, 2013 3176—VOLUME 44A, JULY 2013
Among these technologies, hot dip aluminizing of steels is the easiest and the least expensive technique for the production of iron aluminide coatings from a technical point of view. This process leads to the formation of very brittle Al-rich Fe2Al5 and FeAl3 compounds above which an Al layer with a thickness of several micrometers is formed.[27,28] Research has shown that Al-rich iron aluminides (e.g., Fe2Al5) are more brittle than Fe-rich ones (e.g., FeAl). This has been attributed to the crystal structures of these intermetallics. In the case of FeAl, the higher ductility is presumed to be a result of cubic structure, whereas the complex orthorhombic structure of Fe2Al5 with a limited number of slip systems is postulated to be the cause of its brittleness.[29] Moreover, the cross-sectional observations of aluminized steels have shown that these compounds are of a tongue-like interface with the substrate which leads to an uneven thickness of the coating. These may constrain industrial applications
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