Intermetallic Precipitation in Low-Density Steel
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ENSITY steels (LDS) are the alloys of iron and aluminum, the latter being a light element that helps to reduce the density of the material.[1–4] Aluminum, in solid solution with iron, replaces some of the heavy iron atoms in the three-dimensional lattice, thereby reducing the weight of the overall crystal. An estimated 10 pct drop in the density of iron is reported with about 8 wt pct aluminum in solid solution.[5] Such a decrease in density can be beneficial for the automobile industry, where the weight reduction of vehicles is the major driving force for the sustainable growth of modern civilization. Besides light weight, strength and ductility are the crucial properties of automotive materials. LDS with a fully ferritic microstructure rely on the solution hardening of aluminum and the grain size for their strength. With about 7 wt pct aluminum, the strengthductility product is reported to be approximately 15GPa pct, which compares well with that of dual-phase type steel that is used popularly in the manufacture of modern automobiles.[5] The property improves further to about 50GPa-pct with the addition of a large concentration of manganese that renders the material austenitic but makes the processing of the steel complicated due to the rich chemistry.[6] Improving the mechanical properties of the lean-chemistry ferritic LDS or imparting some unique characteristics to make it more attractive, therefore, may be a significant contribution. Aluminum reacts with various elements to form intermetallic compounds. In a recently published work, an addition of nickel successfully triggered the formation of nanosized iron aluminide particles in LDS that S. CHATTERJEE is with Research & Development, Tata Steel Limited, Jamshedpur 831001, India. Contact e-mail: sourabhchatterjee@ tatasteel.com A. CHATTERJEE and D. CHAKRABARTI are with the Metallurgical and Materials Engineering Department, Indian Institute of Technology, Kharagpur 721302, India. Manuscript submitted June 5, 2017. METALLURGICAL AND MATERIALS TRANSACTIONS A
led to a remarkable improvement of the mechanical properties.[7] Precipitation of intermetallic compounds, in fact, forms the basis of designing novel aluminum alloys that are often thought about to replace steels in many applications including automotive. For example, Duralumin, i.e., Al-Cu (3 to 5 wt pct) alloy, discovered initially by Alfred Wilm, is known to gain strength or hardness with aging at low temperature owing to precipitation of fine intermetallic (CuAl2) particles.[8,9] Interestingly, copper has almost nil solubility in iron. It precipitates in a fine metallic state when added to steels and increases hardness and strength without a significant loss of toughness.[10–12] The primary goal of the present article is to investigate the nature of the precipitates that form in LDS containing copper and the consequent age-hardening behavior of the material that has not been studied earlier for this class of steels. Thermo-Calc, the thermodynamic software with TCFE8 database,[13] was used in this work to und
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