Browsing by Author "Schwarz, Benjamin"

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    Gas nitriding of iron-based alloys
    (2014) Schwarz, Benjamin; Mittemeijer, Eric Jan (Prof. Dr. Ir.)
    Different binary and ternary iron-based alloys were gas nitrided to investigate the progression of nitride precipitation, their kinetics and morphology. Nitriding of an iron-based binary Fe-W alloy lead to a successive precipitation of different tungsten nitrides. Tungsten nitride precipitated not only in the bulk but also on the surface. Nitride particles at the surface show mostly an equiaxed morphology and obey a crystal structure which can be derived from the hexagonal delta-WN. Nitrides particles in the bulk are nano-sized, finely dispersed and have a platelet-like morphology. It is unclear whether these precipitates consist of binary alpha´´-Fe16N2 or ternary Fe-W-N. Prolonged nitriding lead to an assumed discontinuous precipitation of the finely dispersed nitrides ending up with regions consisting of alternating ferrite and tungsten-nitride lamellas. Pure iron and a series of binary iron-based Fe-Me alloys (with Me = Al, Si, Cr, Co, Ni and Ge) were nitrided to investigate the formation of pores under ferrite and austenite stabilizing nitriding conditions. Pores developed in pure iron and in all alloys due to the decomposition of a nitrogen-rich Fe-(Me)-N phase into nitrogen depleted Fe-(Me)-N phase and molecular N2 gas. Upon this decomposition such high pressures can occur that the pore surrounding ferrite matrix can yield and thermodynamic assessments indicate that a continued decomposition reaction is possible beyond the state where yielding of the ferrite matrix is initiated. Alloying elements which precipitate as nitrides retard the formation of pores as a consequence of the competition of alloying-element nitride precipitation and pore development. Alloying elements which reduce the solubility of nitrogen enhance pore formation. A single crystalline pure iron specimen, nitrided under ferrite stabilizing conditions, showed no pore formation at which the necessity of grain boundaries for the formation of pores was demonstrated. Iron-based ternary Fe-V-Si and binary Fe-Si alloy specimens were nitrided until nitrogen saturation in the specimens was achieved. In a first stage all vanadium precipitated as crystalline VN and subsequently all silicon precipitated as amorphous Si3N4. Moreover, the precipitation rate of Si3N4 in the nitrided ternary Fe-V-Si alloy was much lower than in the binary Fe-Si alloy nitrided under identical conditions. The lower Si3N4-precipitation rate is attributed to the presence of first precipitated VN and thus to coherency strains caused by the (semi-) coherent VN precipitates. This interpretation is supported by additional experiments where the first precipitated VN platelets were coarsened by annealing, before subsequent nitriding led to, now much faster, Si3N4 precipitation Based on the results gained upon nitriding ternary Fe-V-Si and binary Fe-Si alloy specimens the work was extended and specimens of ternary Fe-Me-Si alloys (with Me = Ti and Cr) were nitrided as well until nitrogen saturation in the specimens was attained. In contrast with recent observations in other Fe-Me1-Me2 alloys, no “mixed” (Me1, Me2) nitrides develop in Fe-Me-Si alloys upon nitriding. Instead, in a first step all Me precipitates as MeN and subsequently all Si precipitates as Si3N4. The MeN precipitates as crystalline, finely dispersed, nanosized platelets, obeying a Baker-Nutting orientation relationship (OR) with respect to the ferrite matrix. The Si3N4 precipitates as cubically, amorphous particles; the incoherent (part of the) MeN/alpha-Fe interface acts as heterogeneous nucleation site for Si3N4. The Si3N4-precipitation rate was found to be strongly dependent on the degree of coherency of the first precipitating MeN. The different, even opposite, kinetic effects observed for the various Fe-Me-Si alloys could be ascribed to the different time dependences of the coherent to incoherent transitions of the MeN particles in the different Fe-Me-Si alloys.