Effect of manganese addition on aluminizing of steels with cathodic arc based diffusion process

Intermetallics are ordered phases which form between two metallic compounds and have different crystal structure from their constituent metals. Intermetallics are in an attractive class of materials due to showing a good combination of physical and mechanical properties. In a comparison with any metals, their tremendous properties such as low density, high melting point, high strength and corrosion resistance make them the promising structural applications for high temperature. These comprise high temperature gas turbine hardware, corrosion resistant materials, heat treatment fixtures, magnetic materials and hydrogen storage materials. Thanks to their ordered structure, they have long range ordered superlattice and their dislocation mobilities are decreased at high temperature, therefore; their high temperature properties are extremely good. Intermetallics are produced by powder metallurgy, casting, arc melting, physical vapor deposition technique. Although intermetallics have so many good properties, their brittle nature at room temperature make them hard to fabricate as structural materials. Because of this reason, researches are underway to produce them as coatings by various methods. The most studied and common group of intermetallics are aluminides, Fe-Al, Ni-Al and Ti-Al. To overcome their brittleness, which arises from an extrinsic environmental effect, alloying, microstructural control and coating techniques are used. Coating is the most prominent technique among them due to allowing the substrate material choose. Iron aluminide coatings are deposited by pack cementation, chemical vapor deposition, slurry fusion, hot dip, thermal spray, physical vapor deposition. Most of these techniques are suitable for industrial scale applications and largely used to produce thin films. However, these techniques need secondary processes, for instance hot dip, substrates are dipping into molten metal and annealing at high temperature to synthesize intermetallic coating on the surface. After this annealing process, surface may not be annealed equally or diffusion process may not be completed. As a result of this, extra processes are becoming necessary. In some techniques, such as thermal spray, plasma spray and PVD, coating material sources are prepared as a desired coating composition. In thermal spray method, if the molten particle has higher melting temperature than substrate surface, which has lower melting temperature, could be damaged and as a consequence of this impact, substrate material could be melted locally. In plasma sprayed coating is not only composed of the deposited material, but also pores, cracks and oxides and generally a surface finishing process is necessary after coating process. Fe-Al binary system is a complex system which consists of many intermetallics both line compounds and solid solutions. These intermetallics' crystal structure could change with the temperature. Long range ordered or disordered phases could affect their mechanical properties and diffusion rates. In this kind of situation, even though the chemical composition is proper to form an intermetallic phase, this could not form due to the ordered-disordered transition. In this study, unalloyed and manganese alloyed steel surfaces are aluminized and effect of manganese on the diffusion of aluminum is investigated thermodynamically and kinetically. To investigate the mechanism at elevated temperatures, a classical diffusion couple method was not chosen. Generally, Fe-Al diffusion couples are formed by Fe and Fe-Al alloy or two Fe-Al alloys with a different compositions because of aluminum's low melting temperature. These couples are attached each other and held at high temperatures for hours. After these processes, newly formed phases and their diffusion mechanism are analyzed. Apart from these, diffusion researches could be performed by several techniques which are mentioned before as intermetallic coating techniques. Unlike other techniques that used layered growing or annealing processes, it is aimed to produce intermetallic rich surfaces for investigate the diffusion mechanism, thanks to cathodic arc physical vapor deposition method without any secondary processes. To investigate diffusion behavior of aluminum at different temperatures, AC is applied to bias voltage instead of DC. In negative values of bias, substrate is coated, while in positive values of bias, substrate surface is heated by electrons. Thanks to this new method, diffusion coatings were perfomed at 800, 900, 1000, 1100 and 1200°C without any extra diffusion processes. XRD, SEM, EDS and micro hardness measuring methods were performed to examine the phase structure and diffusion mechanism. There were two main parameters in experimental sets, temperature and alloying element. Consequently, in unalloyed sample group, samples which are formed below phase transformation temperature 912°C, have Fe2Al5 Al rich phase on their surfaces. At 912°C, iron has a phase transformation from body centered cubic structure to face centered cubic structure and with this transformation, solubility of aluminum in iron is increasing and it reaches to maxima at 1100°C. Above 912°C, instead of Al rich phases, Fe rich phases are observed on the surface of samples. Correspondingly, below 912°C diffusion rate of Al is slower than diffusion rate of Fe and above this temperature diffusion rate of Fe is slower than diffusion rate of Al. At 1000°C two phase structure is observed, on the surface, first a thin layer of FeAl forms and after this a layer of long range ordered Fe3Al appears. When the temperature is increasing another 100°C, with increase in the diffusion rate of Al, structure turns into fully Fe3Al. As it is mentioned before, at 1100°C, solubility of Al in the face centered cubic Fe is reaching maximum value. As a result of this, Kirkendall voids are observed on surface of the sample. At 1200°C, diffusion rate of Al is getting higher, and Fe-Al solid solution is formed by the diffusion of Al atoms into Fe lattice instead of an Fe-Al intermetallic. In Mn alloyed sample group, samples which are produced at 1000°C and below 1000°C have Fe2Al5 phase on their surfaces. In a comparison with the unalloyed samples formed at the same temperature, the diffusion rate of Al in Fe is getting slower and diffusion depth of Al is getting shorter with the effect of manganese. At 1100 and 1200°C, Fe rich phases are observed on the surface of samples. At 1100°C, in Mn alloyed sample group Kirkendall voids are observed too. However, solid solution phase is not ran across at 1200°C. Hardness measurement is performed on the intermetallic zones, interdiffusion zones and substrate of both two sample groups. Samples which have the same composition in the intermetallic zone, have similar hardness values in this zone. However, hardness values of manganese alloyed substrates is almost two times greater than unalloyed substrates' hardness values. In the light of these observations and analysis, by the effect of manganese, thickness of intermetallic layer is increased, manganese is not a part of intermetallic components which are formed between Fe and Al, and manganese had a strong hindering effect on diffusion rate of aluminum.