Key Engineering Materials Vol. 751

Abstract: Five multi-component alloy (MCA) formulations of CoFeMnNiZn (MCA01), Al_0.5CoFeMnNiZn (MCA02), Al_1.0CoFeMnNiZn (MCA03), Co₅Fe₅Mn₃₀Ni₂₀Zn₄₀ (MCA04) and Al_8.4Co_4.6Fe_4.6Mn₂₇Ni_18.4Zn₃₇ (MCA05) were prepared by mechanical alloying and melting process (MAM). Five-component alloys of MCA01-MCA05 were designed using empirical formulae for high entropy alloys. Phase formation and microstructure were evaluated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that MCA01 was partially melted by MAM process. However, MCA02-MCA05 could be melted and cast by MAM process. The microstructures of as-cast MCA02 and MCA03 showed dendritic solidification. Nevertheless, the as-cast MCA04 showed microstructure similar to that of Ni-based superalloy, i.e., the as-cast MCA04 consisted of γ matrix and γ′ phase. Moreover, egg type core shell structure was found in the interdendritic regions of the MCA05 alloy. In addition, the Al-added MCA02 and MCA03 alloys showed crystal structures of FCC1, FCC2 and BCC. MCA04 alloy demonstrated crystal structure of FCC whereas MCA05 alloy had crystal structures of FCC and Primitive Cubic.

Abstract: This research investigated mechanical degradation of powder injection moulded SiC_p-reinforced aluminium composite subjected to moderate temperature exposures. Aluminium composite of 20 vol.% SiC_p reinforcement was produced by powder injection moulding and sintering at 680°C, followed by 500°C solution treatment plus 150°C for 6 hours artificial aging, and subsequent exposures at 100, 200 and 300°C for 10 and 100 hours. It was found that short-term exposure for 10 hours provided increasing hardness with increasing exposure temperature, while long-term exposure for 100 hours led to an opposite result. The maximum micro Vickers hardness was obtained at 182.2 H_v for Al-SiC_p composite exposed at 300°C for 10 hours. Tensile strength was however found deleterious with increasing both exposure temperature and time. The maximum tensile strength was achieved at 191.2 MPa for Al-SiC_p composite exposed at 100°C for 100 hours. The formations of AlN, Mg₂Si and Al₂Cu were observed in both age-hardened and as-exposed conditions. Furthermore, the highest temperature exposure at 300°C and extended exposure time at 100 hours resulted in the lowest hardness and tensile properties due possibly to the loss of coherency of precipitates. SiC_p clusters were the main cause of the tensile failure.

Abstract: Hardfacing weld is a technique which mainly improves and extends the useful life of engineering components. The purpose of this research is to improve welding procedure for one layer and three layers hardfacing of 3.5% Chromium cast steel and to study wear behavior of hardfacing layers. Flux Cored Wire Arc Welding (FCAW) process has been used as a welding process of this research by choosing austenitic stainless steel and martensitic hardfacing wire to weld the buffer and hardfacing layer respectively. Preheating was also used in this study. Abrasive wear test of hardfacing deposit were conducted in accordance with procedure “A” standard of ASTM G65. In addition, microstructures and macrostructure of worn surface deposits were analyzed by using optical microscope. These results showed that there is no crack and defect in the Heat Affected Zone (HAZ) and other regions. The hardness of preheating sample in HAZ regions was lower than the ones without preheating. Therefore, preheating samples should be done before welding. The abrasive wear resistance of three layers hardfacing deposit was better than one layer hardfacing deposit because one layer hardfacing deposit was more diluted from buffer layer than three layers hardfacing deposit. Moreover, weight loss of one hardfacing layer was also higher than three layers.

Abstract: In the present investigation, WC–10Co–4Cr coating was deposited by high velocity oxy-fuel (HVOF) process on CA6NM hydro turbine steel to improve its erosion resistance. The coating was characterized in term of crossectional microstructure, phase, microhardness and fracture toughness using a field emission scanning electron microscope (FESEM), X-ray diffractometer and microhardness tester respectively. Solid particle erosion resistance of the substrate and coating were evaluated by air jet erosion tester at two different impingement angles (30° and 90°). Coating microstructure has shown a homogeneous and well-bonded laminar morphology. The microhardness of the coating was observed more than three times higher than CA6NM substrate. This resulted in significant improvement in erosion resistance of coated CA6NM steel at both impingement angles.

Abstract: Nanocrystalline CrAlN thin films were deposited on silicon substrates by reactive DC magnetron co-sputtering technique. The effect of deposition time on crystal structure, chemical composition, thickness, microstructure and hardness of the thin films were characterized by XRD, EDS, AFM and FE-SEM and Nanoindentation, respectively. The as-deposited films were formed as a (Cr,Al)N solid solution with (111), (200) and (220) plane. The lattice constants were in range of 3.9916 - 4.0455 Å. The as-deposited films exhibited a nanostructure with a crystallite size in range of 15-35 nm. The thickness and roughness increased from 197 nm to 998 nm and 1.6 nm to 8.1 nm, respectively, with increasing the deposition time. The chemical composition of the films varied with the deposition time. The cross section analysis by FE-SEM showed columnar structure and dense morphology. The film hardness decreased from 39 GPa to 25 GPa with increasing the deposition time and crystallite size.

Abstract: Solid particle erosion on surface of material effects on life time for them such as the turbine brade, boiler tube and chemical processing equipments. Thermal spray is one of various technique which used for the repairing or building up new surface. The objective of this experiment is to study the influence of thermal spray Ni-Si-B coating parameters on the solid particle erosion resistance at room temperature. The coating parameters were the distances between coating nozzle and the substrate of 10, 15, and 20 mm and travel speeds of 1.0 and 1.5 mm s-1 respectively, leading to the difference of heat inputs. The hardness of coating surfaces was measured by Vickers hardness. The microstructure of coating surfaces was examined by using an optical microscope (OM) and scanning electron microscope (SEM). Moreover, solid particle erosion resistance of coating surfaces was investigated on the erosion test rig at room temperature. Erosion rates of specimens were assessed by the calculation of weight loss which measured using an electronic balance (0.1 mg resolution). The relationship between erosion resistance and hardness was studied coincide with the microstructure examination. According to the erosion resistance evaluation, it was found that an erosion rate of coating depended on the coating condition. Finally, wear scar morphology on the top of an eroded surface and cross-section area was observed by SEM. However, the coating parameters played an important role in the erosion resistance. The microstructure of coating concerned with coating parameters was further discussed with the erosion behavior in this research.

Abstract: For better mechanical properties and lifetime of sintered products, it is suggested that an improvement can be made by a combination of mechanical-and chemical-surface treatments. In this study, the effect of deep-rolling on surface properties and microstructure is investigated. It is found that both compactness and hardness is improved by deep-rolling process where high force is applied. The outer surface hardness can be doubled when the deep-rolling is applied prior to carburizing. Nonetheless, a reduction in the thickness of the martensite-transformed layer due to an increment of applied force is observed.

Abstract: A challenge in producing a sacrificial anode through conventional casting method is that the alloying elements in casting segregate during solidification, which further causes non-uniform anode corrosion reducing anode performance. In this paper, we investigated the performance of Al-5Zn-0.02In anode produced by conventional casting compared with by semi-solid casting technique. The performance of produced anodes were measured in terms of anode potential, current capacity, consumption rate and anode efficiency in 3.5% NaCl solution for 14 days. We found that the microstructure of the conventional cast anode had dendrites and coarse grains and the corrosion caused pitting corrosion. In contrast, the semi-solid cast anode had fine grains without any dendrites. The corrosion attacked mainly the grain boundaries and less on the matrix. Surprisingly, the conventional cast anode has about 10% higher efficiency than that of semi-solid cast anode.

Abstract: Pitting corrosion caused by wet-dry cycles under corrosive media droplet is one of the key concerns for passive film of metallic materials, particularly stainless steels and aluminum alloys, exposed to atmosphere during service. In this context, the formation of corrosion can lead to high investment cost dealing with corrosion mitigation strategy, e.g. materials selection, electrochemical corrosion control, etc. Based on materials selection perspectives, it is very necessary to have proper understanding of localized corrosion behaviors of metallic materials under solution droplet. Therefore, the present study aims to develop a methodology for pitting corrosion monitoring that can be suitably used for extending better understanding on corrosion phenomena occurring under wet-dry cycles of droplet. A special liquid handling apparatus controlled by Arduino software was constructed and used for generating NaCl solution droplet at given dimension on the surface of stainless steel specimens based on Pendant drop principle. This was recognized as wet cycle. During dry cycle, such NaCl solution droplet was naturally dried off in various conditions of relative humidity. Pitting initiation was observed through a high-resolution CCD camera. Droplet morphology and evaporation time were evaluated at the temperature of 27°C and relative humidity of 10% and 60%. The research results revealed that pitting corrosion started at 1^st cycle without rust formation. Afterwards, the rust formation was clearly noticed when testing cycles of 15 were exceeded.

Abstract: In this study, CrN_x thin films were prepared on 304 stainless steel substrate by DC reactive magnetron sputtering technique. Different N₂ gas partial pressure from 10 to 30% was employed in the sputtering process while sputtering power, sputtering pressure and total film thickness were kept constant. In order to improve structural, morphological, and mechanical properties, vacuum annealing process was adopted on the CrN_x thin films at the temperature of 400 oC. The standard characterization techniques such as X-ray diffraction, scanning electron microscope, atomic force microscope and hardness (load force of HV0.1) were used to reveal the properties of as-deposited and annealing films. The as-deposited films show two-phase crystal structure of Cr₂N and CrN depending on N₂ gas partial pressure. After the annealing process, the films effectively enhance the crystal structure and found the phases change from Cr₂N to CrN for the film deposited at low N₂ partial pressure. The surface roughness of the films was between 5 - 20 nm, and as expected the annealing films shows smoother surface than the as-deposited films. Hardness of the CrN_x films is in the range of 7 to 10 GPa. The mechanism of improvement in structural and mechanical properties of annealing films is introduced based on strain relaxation.