Transformation, isothermal temperature

In an amorphous material, the aUoy, when heated to a constant isothermal temperature and maintained there, shows a dsc trace as in Figure 10 (74). This trace is not a characteristic of microcrystalline growth, but rather can be well described by an isothermal nucleation and growth process based on the Johnson-Mehl-Avrami (JMA) transformation theory (75). The transformed volume fraction at time /can be written as... 

On cooling to an isothermal temperature below the martensite start (Mg) jwint of790 5 °C (1454 9 °F), first some a phase is formed above Mg and then the remaining, predominate portion of the p phase is transformed into a supersaturated hexagonal martensite (a"). Below Mg and above the martensite finish (Mf) temperature of 740 5 °C (1364 9 °F), there remains a residual P phase, which is probably transformed isothermally to a phase. The resulting structure for isothermal reaction is a + a", where the a" phase below 750 °C (1380 °F) decomposes discontinuously into a two-phase a + p structure and a metastable P phase enriched with p-stabilizing elements. 

The hydration shell is formed with the increasing of the water content of the sample and the NA transforms from the unordered to A- and then to B form, in the case of DNA and DNA-like polynucleotides and salt concentrations similar to in vivo conditions. The reverse process, dehydration of NA, results in the reverse conformational transitions but they take place at the values of relative humidity (r.h.) less than the forward direction [12]. Thus, there is a conformational hysteresis over the hydration-dehydration loop. The adsorption isotherms of the NAs, i.e. the plots of the number of the adsorbed water molecules versus the r.h. of the sample at constant temperature, also demonstrate the hysteresis phenomena [13]. The hysteresis is i( producible and its value does not decrease for at least a week. 

Stmctures that form as a function of temperature and time on cooling for a steel of a given composition are usually represented graphically by continuous-cooling and isothermal-transformation diagrams. Another constituent that sometimes forms at temperatures below that for peadite is bainite, which consists of ferrite and Fe C, but in a less well-defined arrangement than peadite. There is not sufficient temperature and time for carbon atoms to diffuse long distances, and a rather poody defined acicular or feathery stmcture results. 

Fig. 17. Isothermal transformation (IT) diagram for a plain carbon eutectoid steel (1). Ae is A temperature at equiUbnum BHN, BrineU hardness number ...

In austempering the article is quenched to the desired temperature in the lower bainite region, usually in molten salt, and kept at this temperature until transformation is complete (Fig. 22). Usually, the piece is held twice as long as the period indicated by the isothermal transformation diagram. The article may then be quenched or air-cooled to room temperature after transformation is complete, and may be tempered to lower hardness if desired. 

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