TTT diagrams

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... 

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.

In order to get the iron to transform displaeively we proceed as follows. We start with f.c.c. iron at 914°C which we then cool to room temperature at a rate of about 10 °C s . As Fig. 8.6 shows, we will miss the nose of the 1% curve, and we would expect to end up with f.c.c. iron at room temperature. F.c.c. iron at room temperature would be undercooled by nearly 900°C, and there would be a huge driving force for the f.c.c. b.c.c. transformation. Even so, the TTT diagram tells us that we might expect f.c.c. iron to survive for years at room temperature before the diffusive transformation could get under way. 

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.

Fig. 8.11. The TTT diagram for a 0.8% carbon (eutectoid) steel. We will miss the nose of the 1% curve if w quench the steel at = 200°C s. Note that if the steel is quenched into cold water not all the i/will transform to martensite. The steel will contain some "retained" / which can only be turned into martensite if the steel is cooled below the Mf temperature of -50°C.

Sketch the time-temperature-transformation (TTT) diagram for a plain carbon steel... 

Explain briefly the shape of the lines drawn on the TTT diagram. 

Fig. 10.2. Semi-schematic TTT diagram for the precipitation of MgjAlg from the Al-5.5 wt% Mg solid solution.

Fig. 10.5. TTT diagram for the precipitation of CuAh from the Al + 4 wt% Cu solid solution. Note that the equilibrium solubility of Cu in Al at room temperature is only 0.1 wt% (see Fig. 10.3). The quenched solution is therefore carrying 4/0.1 = 40 times as much Cu as it wants to.

Fig. 10.10. Detailed TTT diagram for the Al-4 wt% Cu alloy. We get peak strength by ageing to give 8". The lower the ageing temperature, the longer the ageing time. Note that GP zones do not form above 1 80°C if we age above this temperature we will foil to get the peak value of yield strength.

Fig. 11.8. TTT diagrams for (a) eutectoid, (b) hypoeutectoid and ( ) hypereutectoid steels, (b) and ( ) show (dashed lines) the C-curves for the formation of primary a and FejC respectively. Note that, os the carbon content increases, both A s and Mf decrease.

The final note is that pearlite and bainite only form from undercooled y. They never form from martensite. The TTT diagram eannot therefore be used to tell us anything about the rate of tempering in martensite. 

Figure 11 Example of time-temperature transformation (TTT) diagram. (Increasing time in vertical direction). Reprinted from Roller and Gillham [4], published by the Federation of Societies for Coatings Technology, with permission of publisher.

Time-temperature-transformation (TTT) diagram, 70 422-423 diagram(s), 72 567, 23 278 Timet, titanium contract with Boeing, 24 846... 

Figure 5.31 TTT diagram for a pyroxene of composition similar to C2 in figure 5.30. Curves a, b, c, and d cooling rates of a lava flow at increasing depths from surface (g) to 1 m inside the body ((i), based on calculations of Provost and Bottinga (1974). From Bu-seck et al. (1982). Reprinted with permission of The Mineralogical Society of America.

Figare 10.47 TTT diagrams for a formation for U720 and U720LI after Keefe... 

Figure 10.48 TTT diagrams for U720 and U720LI based on calculated

Figure It.14. Schematic TTT diagram for a low-alloy steels showing feirite and bainite formation with overall C curve behaviour assumed by Kiikaldy et al.

The prediction of transformation diagrams after Bhadeshia (1982). Later work by Bhadeshia (1982) noted that the approach of Kirkaldy et al. (1978) could not predict the appearance of the bay in the experimentally observed TTT diagrams of many steels, and he proposed that the onset of transformation was governed by nucleation. He considered that the time period before the onset of a detectable amount of isodiermal transformation, r, could be reasonably defined as the incubation period, r necessary to establish a steady-state nucleation rate. The following expression for r, was then utilised... 

Q, p and Ci were then determined by empirically fitting curves, derived using Eq. (11.7), to experimentally observed TTT curves. Unfortunately, although good correlation coefficients were obtained during the fitting process, the results proved insufficiently accurate to predict further TTT diagrams. However, it was observed... 

Figure 11.18. Comparison between predicted and experimental TTT diagrams for steel types (a) 5140 and (b) 8630 from Kirkaldy and Venugopolan (1983).

One of the earliest attempts to explicitly combine thermodynamics and kinetics in rapid solidification was by Saunders et al. (1985). They examined the equations derived by Davies (1976) and Uhlmann (1972) for predicting TTT diagrams. These were based on Johnson-Mehl-Avrami kinetics for predicting glass formation during rapid solidification where the ruling equation could be given as... 

The WLF equation holds over the temperature range from Tg to about + 100 K. The constants in Eq. (5.76) are related to the free volume. This is a procedure analogous to the one we used to generate time-temperature-transformation (TTT) diagrams for metallic phase transformations in Section 3.1.2.2. 

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