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Influence of sintering process temperature on the formation and final microstructure of FeAl based intermetallics obtained from the elementary Fe and Al powders

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The results of the optimization process of liquid phase sintering of elemental Fe and Al powders in order to fabricate FeAl intermetallic sinters were studied in this paper. The oxidation of Al particles in the sintering process is responsible for the formation of the Al2O3 films in the sintered structure that are mainly located on the interparticle boundaries, which eventually lead to the de[...]

Optimization of Fe and Al elemental powders sintering parameters by DTA analysis

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Sintered iron aluminides are very promising novel structural materials mainly because of their attractive physical and mechanical properties, but also because of the low price of raw materials used for their manufacture [1]. Powder metallurgy (PM) eliminates many of the limitations during the fabrication process [2]. For example, PM enables forming of products with complex shape, choosing almost arbitrary chemical compositions, deals with problem of grain overgrowth, gives better chemical and structure homogeneity as well it is an energy-saving process [3÷5]. However, in case of the Fe-Al phases, the use of this method is limited by the necessity of controlling of the SHS (self-propagating high-temperature synthesis) reaction [3]. SHS occurring during sintering of Fe and Al elemental powders mixture is very rapid and strongly exothermic, which unfortunately may cause unwanted porosity of the sinters [6]. Our previous studies [7÷10] show the great complexity of phenomena taking place during presintering process of Fe and Al powders. These effects correlated with diffusion and phase transformations were also studied by other researchers [1, 3, 4]. It was concluded that the mechanism of exothermic phase transformations is strongly correlated with many factors, such as chemical composition of the sample, elemental powder particles size, atmosphere, and the heating rate. It is important that only the formation of the exothermic Fe2Al5 phase was reported prior to the formation of finally-wanted FeAl phase [1]. Our last investigations show however that not only Fe2Al5 but also other high aluminum phases - FeAl3 and FeAl2 could be found in the samples structure sintered at temperature lower than the SHS reaction temperature. In order to limit unfavorable influence of the SHS on sinter structure, the optimization of the powders particles size, the compact pressure and the heating rate was done in this work. EXPERIMENTAL Investigation[...]

Real kinetics of Fe-Al type intermetallic phases sintering analysis based on extended JMA model

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Production of certain advanced ceramics, composites, and intermetallic compounds in most cases is connected with complex and high energy consumption techniques. Increasing tendency to limit costs of fabrication requires development of efficient and energy-saving technologies. In case of Fe-Al intermetallic materials fabrication Powder Metallurgy (PM) methods associated by Self-propagating High-temperature Synthesis (SHS) become more popular [1]. Nevertheless, SHS assisted sintering is connected with high intensity, high synthesis temperature and limited possibilities of reaction control which lead to high porosity of sinters [2]. Such issues require deep study of intermetallic phases nucleation and transformation mechanisms during SHS reaction. Kinetic parameter of those transformations, for example the Avrami exponent n, could be calculated from DSC experiments according to the JMA model adapted to non-isothermal conditions [3÷12]. Unfortunately the JMA equation which allows to determine nucleation and growth mechanism on the basis of Avrami exponent n value has some limitations [13]. In theory Avrami plot should be a straight line of which slope is the Avrami exponent n. The experimentally observed kinetics show some deviations from this theory. They are connected with continuously changing reaction conditions forcing to calculate the slope of straight linear fragment of such plot in range from about 25 to about 75% of degree of phase transformation α. Thereby calculated value of n is affected by some error and does not give any insight in phase formation mechanism at the beginning and the final stage of the process. Some authors suggest that in order to eliminate this inaccuracy the local Avrami exponent should be calculated [14]. The aim of this study was to compute the local Avrami exponent n(T) from slope of lines parallel to tangent to Avrami plot lines in following points from entire range of particular phase form[...]

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