Nucleation time-temperature-transformation

A molten metal alloy would normally be expected to crystallize into one or several phases. To form an amorphous, ie, glassy metal alloy from the Hquid state means that the crystallization step must be avoided during solidification. This can be understood by considering a time—temperature—transformation (TTT) diagram (Eig. 2). Nucleating phases require an iacubation time to assemble atoms through a statistical process iato the correct crystal stmcture... 

This section will begin by looking at how thermodynamic and kinetic modelling has been combined to understand time-temperature-transformation diagrams in steels. The woric, for the most part, is semi-empirical in nature, which is forced upon the topic area by difficulties associated with the diffusional transformations, particularly where nucleation aspects have to be considered. The approaches have considered how best to predict the time/temperature conditions for austenite to... 

A discontinuous transformation generally occurs by the concurrent nucleation and growth of the new phase (i.e., by the nucleation of new particles and the growth of previously nucleated ones). In this chapter we present an analysis of the resulting overall rate of transformation. Time-temperature-transformation diagrams, which display the degree of overall transformation as a function of time and temperature, are introduced and interpreted in terms of a nucleation and growth model. 

Experimental time-temperature-transformation (TXT) diagram for Ti-Mo. Xhe start and finish times of the isothermal precipitation reaction vary with temperature as a result of the temperature dependence of the nucleation and growth processes. Precipitation is complete, at any temperature, when the equilibrium fraction of a is established in accordance with the lever rule. Xhe solid horizontal line represents the athermal (or nonthermally activated) martensitic transformation that occurs when the p phase is quenched. 

Discuss the kinetic and thermodynamic factors governing liquid-solid and solid-solid phase transformations. Explain and predict nucleation, growth, and time-temperature-transformation (1 IT) processes in solid-state systems both qualitatively (through diagrams) and quantitatively (through equations). 

The dependence of the nucleation on time and temperature can be presented in a lucid form by the time-temperature-transformation (TTT) diagram. The amount transformed corresponds to the crystal density All time-temperature conditions leading to the same crystal density are connected by lines. Figure 2.11 shows the TTT diagram derived from the data of Fig. 2.9. The density line p = 1 is the outermost boundary of the nucleation area. It indicates the conditions for the creation of the first effective nucleus. The characteristic shape of the density curves results from the interplay of the two different mechanisms mentioned above, which are responsible for the limitations of the nucleation area at high and low temperatures, respectively. This shape delineates a temperature range of minimum times for the formation of the first nucleus. 

Fig. 2.11. Time-temperature-transformation (TTT) diagram of nucleation showing several crystal density lines (p)...

In order to monitor the mechanical properties in relation to the microstructure, the knowledge of the precipitation state at the end of a thermo-mechanical treatment is of prime importance. In this purpose, Arcelor develops models that allow for the prediction of the influence of the process parameters on the state of precipitation. The model Multipreci, developed at IRSID is one of them. It (Hedicts the precipitation kinetics of mono- and di-atomic particles in ferrite and austenite as a function of the time-temperature history. It is based on the classical theories for diffusive phase transformation and treats simultaneously the nucleation, growth and ripening phenomena. The state of precipitation that is predicted includes the particle size distribution, their number and volume fraction. From these values, the effect of the precipitates on the mechanical properties can be calculated. 

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