Time-temperature-transformation curve

Fig. 20.48 Isothermal time temperature transformation curves for (a) a eutectoid steel and...

Fie. 7.60 Comparison of the time-temperature-transformation curves of Hastelloy alloys C and C-276. The latter contains less carbon and silicon. Redrawn from Ref 95... 

Fig. 15 Graphical illustration of method for determination of r in quench factor analysis. Cn is the critical time to achieve a fraction transformed at a certain temperature indicated by the time-temperature-transformation curve. (Ref [43], reprinted with permission from ASM International.)...

Figure 2.2 A time-temperature-transformation curve for a glass forming melt...

T-T-T Curve. Time-temperature-transformation curves describing mineralogical changes. 

Time-temperature-transformation curve forTi-51 Ni, which shows precipitation reactions as a function of temperature and time. Source M. Nishida, C.M. Wayman, and T. Honma, Melall. Trans. A Vol 17,1986, p 15(B... 

Competition between Sintering and Crystallization compares the kinetics of the processes and shows how time-temperature transformation curves can be used to establish optimal sintering schedules. 

Figure 12.14 Time-temperature transformation curves for Hastelloy Alloys C and C-276. Intermetallics and carbide phases precipitate in the regions to the right of the curves. (Reproduced with permission from Daubert Cromwell.)...

We are now in position to construct the time-temperature-transformation curve for glass formation. A 1 ppm crystallization is just detectable, so we set the transformation threshold to 10 For kt [Pg.288]

Fig. 2. Time—temperature—transformation (TTT) diagram where A represents the cooling curve necessary to bypass crystallization. The C-shaped curve separates the amorphous soHd region from the crystalline soHd region. Terms are defined ia text.

Fig. 8.5. The diffusive f.c.c. —> b.c.c. transformation in iron the time-temperature-transformation (TTT) diagram, or "C-curve". The 1% and 99% curves represent, for oil practical purposes, the stort and end of the transformation. Semi-schematic only.

Time-temperature-transformation (T-T-T) diagrams are used to present the structure of steels after isothermal transformation at different temperatures for varying times. The T-T-T diagram for a commercial eutectoid steel is shown in Fig. 20.48a. Also shown on the curves are the points at which the microstructures illustrated in Figs. 20.46 and 20.47 are observed, and the thermal treatments producing these structures. When a steel partially transformed to, say, pearlite, is quenched from point a in Fig. 20.48a to below nif, the untransformed austenite transforms to martensite. 

At this point, the fundamental question, posed at the outset of this section, namely. How fast must a melt be cooled to avoid the formation of a detectable volume fraction of the crystallized phase can be addressed somewhat more quantitatively. The first step entails the construction of a time-temperature-transformation (TTT) curve for a given system. Such a curve defines the time required, at any temperature, for a given volume fraction to crystallize. Here the procedure, not unlike that used to solve Worked Example 9.2, is generalized. 

An analysis of crystallisation rates is conveniently performed in terms of the so-called time-temperature-transformation (TTT) curves, which relate the time taken to crystallise a given fraction of the undercooled liquid or the supersaturated solution to the temperature. Experimentally, the crystallisation rates are measured by quenching the liquid phase to some predetermined temperature T and measuring the time taken for the solid to crystallise at that temperature, either by monitoring the latent heat of crystallisation or by microscopic observation. The volume fraction 4>(T) that crystallises out in time 1 is given by one form of the Avrami equation ... 

Problem 6.7. Figure 6.42 gives a set of rate of transformation [F(t)] curves for the crystallization of a glass. Using this information, calculate and plot (using a computer program of your choice) a ITT (time-temperature-transformation) diagram for this crystallization that includes curves for 10,50, and 90% transformed with temperature on the vertical axis versus log (time) on the horizontal axis. 

This critical cooling rate ( q ) has been estimated by use of isothermal time-temperature transformation ( IT T) diagrams (Uhhnann 1972) or continuous cooling transformation (CT) curves Onorato and Uhlmann 1976). 

From equations (10) to (23), the time-temperature-transformation (ITT) curve can be obtained. The critical cooling rate Rc necessary for amorphous phase formation with a melt quenching method can be evaluated from TTT curve calculated and ap>proximated as follows... 

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