Jyh-Wei Lee

Corrosion resistance and microstructural evaluation of the chromized coating process in a dual phase FeMnAlCr alloy

Abstract Chromizing is a suitable method for coating development for high temperature surface protection, corrosion and wear resistance. A homogenized dual phase Fe–24%Mn–8.3%Al–5%Cr–0.38%Si–0.34%Mo–0.45%C cast alloy was chromized with a pack cementation process at 1000 °C for 1–16 h. A chromized surface layer and an adjacent processing-related ferrite layer ranging from 80 to 100 μm in depth were formed. The thickness of the chromized layer X vs. the coating time t obeys the parabolic law: X=K t , where K is evaluated to be 5.89 μm/ h . The primary phase on the chromized surface is (Cr, Fe)2N1−x and the rest is Cr23C6, while the internal part of chromized layer is Cr7C3 phase. An appreciable increase of the surface hardness ranging from 2015 to 2458 HK is achieved due to the formation of chromium carbide layer. A significant improvement of the corrosion resistance in 3.5 wt.% NaCl aqueous solution is obtained. Chromized specimens exhibit only one-tenth weight loss compared to uncoated specimens when immersed in 90% Na2SO4 and 10% NaCl molten salt at 877 °C for 48 h.

Mechanical property evaluation of cathodic arc plasma-deposited CrN thin films on Fe–Mn–Al–C alloys

Abstract Chromium nitride is a promising hard coating applied in forming, drawing and plastic molding industries due to its good thermal stability and excellent resistance to wear and corrosion. An austenitic Fe–30Mn–5.8Al–1C alloy was coated with a CrN film by a cathodic arc plasma deposition process. The mechanical and adhesion properties were evaluated by microhardness, scratch, nanoindentation, Daimler–Benz Rockwell-C adhesion and pin-on-disk wear tests. The microhardness of CrN film reaches 2310±100 HK under a 10-gf load. The elastic modulus of the CrN film is 290 GPa, measured with a nanoindenter under a 3000-μN load. The adhesion of CrN film deposited on Fe–Mn–Al–C substrate is observed to be better than HF1. The wear resistance of the Fe–Mn–Al–C substrate is also significantly improved by the presence of the CrN film.

The determination of tensile properties from hardness measurements for Al-Zn-Mg alloys

AbstractThe correlations between tensile properties (yield strength, ultimate tensile strength and uniform strain) and indentation hardness are studied for two types of Al-Zn-Mg alloys. The reasons why Tabors equations do not well fit the experimental data when the strain-hardening coefficient is larger than 0.3 are discussed. New equations for the determination of tensile properties from hardness measurements are theoretically derived and found to be in excellent agreement with the experimental data for Al-Zn-Mg alloys. The equations areTu=(Hv/c2)[4.6(m−2)]m−2 and σy=(Hv/C2)1-(3−m>n) +25 (m−2), whereTu andσy are ultimate tensile strength and yield strength,Hv is Vickers hardness number,m is Meyers hardness coefficient,E is Youngs modulus,c2 is a constant about 2.9 in magnitude. In these equationsTu,σy,Hv andE are all expressed in kg mm−2.

High-temperature MgO–C–Al refractories–metal reactions in high-aluminum-content alloy steels

MgO-C-Al refractories were used in this study to evaluate high-temperature refractory-metal reactions with molten Fe-Mn-Al alloys. Dynamic reaction tests at 1570 °C and static reaction tests at 1500 °C and 1600 °C, respectively, were used. MgAl 2 O 4 and Al 4 C 3 phases were observed in the refractory bulk, and a large amount of protective MgAl 2 O 4 phase formed due to the decomposition of Al 4 C 3 phases on the refractory-metal reaction interface of the MgO-C-Al brick to retard the high-temperature attack of Fe-Mn-Al alloy melts. The oxygen partial pressure was substantially reduced by the oxidation of graphite in the MgO-C-Al brick during high-temperature test. This resulted in the nitridation of aluminum in molten Fe-Mn-Al alloy. White aluminum nitride (AlN) with the shape of whisker-like powder was formed and adhered to the surface of the MgO-C-Al brick. Aluminum was depleted from the Fe-Mn-Al alloy by nitridation or the oxidation reaction by CO gas. The alloy would also be carburized due to the absorption of CO gas or the reaction between aluminum and CO gas, which was produced by the oxidation of graphite in the MgO-C-Al refractory after static reaction test. It is argued that the MgO-C-Al refractory is not suitable to be used in the melting of Fe-Mn-Al alloys with high aluminum contents.

Growth, structure and electrical characteristics of epitaxial nickel silicide from chemically electroless Ni deposition on Si

The epitaxial NiSi2 has been successfully grown on (100) and (111) silicon single crystals by chemical electroless deposition and isothermal annealing for the first time. Transmission electron microscopy (TEM) was applied to study the structure and orientation relationship of the films and substrates. The nickel silicide formed on both (100) and (111) Si substrates was identified to be NiSi2 and was found to have epitaxial relationship with the substrates by bright field imaging and selected area diffraction pattern analysiS. Square, hexagonal and irregular dislocation networks were observed. The orientation relationships of silicide phase with respect to (100)Si substrate were identified to be (220)Si ∥ (220)NiSi2, (400)Si ∥ (400)NiSi2 and (001) Si ∥ (001)NiSi2. The orientation relationships of silicide phase with respect to (111)Si substrate were identified to be (220)Si ∥ (02¯2)NiSi2, (3¯11) Si ∥ (31¯1) NiSi2 and [1¯1¯4]Si ∥ [1¯1¯2]NiSi2. The average spacing of interfacial dislocations was about 90 nm for epitaxial silicide formed on (100)Si at 800° C, which is narrower than that on (111)Si. These spacings decreased with increasing annealing temperature.Sheet resistances were measured to be lower than that of poly-NiSi2 produced from electron gun evaporation with the same annealing condition. A linear relationship between resistivity and thickness was found and discussed.

NaCl-induced accelerated oxidation of 304 stainless steel and Fe-Mn-Al alloy at 900°C

The high-temperature corrosion behavior of cold-rolled and annealed 304 stainless steel (304SS) and Fe-29Mn-8Al-2.5Si-2Cr-0.74C alloy coated with 0.002 g cm−2 NaCl initially were studied at 900°C in air. The corrosion kinetics of the two alloys follow the parabolic rate law. The initial NaCl coating accelerates oxidation of these alloys by oxychlorination and chlorination/oxidation cyclic reactions, and catalytic actions of chloride or chlorine are thought to be the principal causes. A bulky, layered scale as well as some intergranular attack is noted on the annealed 304SS, and intergranular attack distributes over the alloy substrate of the cold-rolled 304SS during a 144 h exposure. With the formation of a compact Al2O3 scale to decrease further chlorine attack, the corrosion resistance of Fe-Mn-Al alloy is superior to that of 304SS in this study.