Ni(Fe)S Sample

Alloy name: Ni(Fe)S
Diagram No.: 874
Type of diagram: TTT
Chemical composition in atomic %: NiFeS - 50.52 at% S - 0.6 at% Fe
Alloy group: Nickel-based alloys
Note: Time Temperature Transformation (TTT) diagram for Ni(Fe)S sample of different composition for Fv=90%: the transformation time range, corresponding to Fv=90%, is less then 1h for T>240C for the studied compositions.
Using the information on transformation time range and related volume fraction at different composition, a TTT curve can be built: see for instance, the TTT curve in Figure which corresponds to a 90% transformed volume fraction, for different overstoichiometry and iron content. It is worth noting that it is not necessary here to investigate temperatures higher than 280C since we know from the phase diagram and the microstructural study that the transformation is not complete except for near stoichiometric compositions.
Indeed the set of TTT curves is very appropriate tool to synthesise all the information on the temperature and composition effect. For near stoichiometric sample (50.4 at% S), Fv~90% is reached after about 17 min at 200C and 3 min at 260C. For the overstoichiometric sample 51.14 at% S after 115 min at 200C and about 40 min at 260C. The acceleration with temperature increase is clearly illustrated by the reduction of the time range: from 125 min at 200C to 50 min at 260C. The same remark holds for the composition effect: for an increase of 1 at% S the transformation time range increases from about 20 min to about 110 min. According to the TTT curves in Figure, the transformation time range is less than 60 min for temperature between 240C and 280C and for all studied compositions. Of course, at temperature high than 280C, as the transformation becomes partial, the time of transformation is not reported.
The rather flat shape of the TTT curve at low temperature indicates a strong influence of the temperature. On the contrary, the curved shape at higher temperature corresponds to a smaller effect of the temperature on the transformation time range. Such behavior can be related to the shape of the TTT curve which correspond to half of a C shape (the high temperature domain being not investigated because of non complete transformation). In solid state phase transformation, the C shape of TTT curve is known to result from competition of two mechanisms which have inverse evolution according to temperature: a driving force for nucleation and growth which increases with decreasing temperature, in competition with the mobility of species which decreases with an Arrhenius law when temperature is decreasing. Such mechanisms should be involved for the Alpha->Beta transformation.
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