Isothermal annealing

Fig. 24. Transforniation diagram for isothermal annealing. The product is ferrite and peadite.

Isothermal annealing can be conveniendy adapted to continuous annealings usually in specially designed furnaces. This is commonly referred to as cycle annealing. 

Figure 3 shows a series of isothermal anneals at neighbouring temperatures small step annealing) for recrystallized samples with Fe content between 5 and 20at%. The following typical features are observed ... 

Figure 6. Relative change of residual resistivity and LRO-parameter of Ni76Al24+0.19at%B during isothermal annealing at indicated temperatures ...

Figure 43 illustrates the possible current transients during thermal treatment of Al-anodic Al203-Au structures at linearly increasing temperature (a) and during isothermal annealing (b). The first case is characterized by a TSC maximum at —400 K followed by a change in current direction and a second maximum (Fig. 43a). In the case of isothermal treatment, jTSC follows a t n dependence, where n is close to unity. These findings are usually interpreted in terms of a release from deep traps of those electrons that were initially captured there in the process of anodization. There are no clear ideas as to the physical nature of these traps. Parkhutik and Shershulskii249 have postulated that traps are associ-...

When the samples were etched mildly, the anomalous increase upon annealing was not observed. In an isothermal annealing experiment performed at 423 K for As—H complexes, the exponential decay given by Eq. (3) was verified for a 50 times reduction in concentration. In Fig. 11 the results of a series of 30 min isochronal anneals are shown for each of the donor-H complexes. The curves are given by Eq. (3) with an assumed attempt frequency of 1013 s-1 and binding energies of 1.32 eV for P—H and 1.43 eV for As—H and Sb—H. 

Isothermal annealing, 23 288—290 transformation diagram for, 23 289 Isothermal dehydrogenation, 23 337 Isothermal evaporation, general separation heuristics for, 22 319-320 Isothermal forging, of titanium, 24 859 Isothermal furnace liners, 13 239-240... 

Figure 5.30. Schematic drawing showing the construction of an isothermal transformation diagram from measurements of the progress of the transformation at various constant temperatures. This may be done, for instance, by metallographic examination of several specimens, quenched from the 7-field quickly enough to miss the nose of the C-curve and then isothermally annealed for various length of time. Notice that curves for the transformation of different samples may be shown on the same diagram and that more complex trends may be observed in real diagrams of specific alloys. In the example reported, Ms is the temperature at which the alloy will begin to show the martensitic transformation, Mf indicates the temperature below which no additional martensite forms.

Schlenz H., Kroll H., and Phillips M.W. (2001) Isothermal annealing and continuous cooling experiments on synthetic orthopyroxenes temperature and time evolution of the Fe, Mg distribution. Eur. J. Mineral. 13, 715-726. 

A diffusion measurement at the temperature To is made by annealing a diffusion couple comprised of two semi-infinite bars. However, there is a complication after the completion of the isothermal anneal, carried out at To for the time to, the specimen must be cooled to room temperature at a finite rate. During this cooling period, a small amount of additional nonisothermal diffusion occurs. If an expression can be found for the amount of time, At, required to produce this same additional increment of diffusion at the constant temperature To, the specimen could be analyzed very simply at the end of the experiment by assuming that it was annealed at To for the time to + At. Assume that D = D0 exp —E/(kT)] and that the temperature during the cooling period is... 

The rate of recovery is normally highest at the start of an isothermal annealing cycle because the driving force is largest at that time. As recovery continues, the driving force diminishes as the available strain energy is used up and the rate of recovery falls continuously toward zero. A plot of the rate of recovery as a function of time yields a curve that is somewhat similar in appearance to an exponential decay curve. The rate of recovery is also temperature dependent and may be expressed, in a number of cases, by a simple empirical equation of the form... 

Isothermal Annealing. This i, n process of heating to the correct temperature above the critical for proper austenizing, followed hy rapid cooling to a suitable temperature and holding sufficiently long for completion of the transformation. 

From the system of equations (2.46), it follows that the layers of the ApBq and ArBs compounds already present in an A-ApBq-ArBs B specimen should not necessarily simultaneously grow during its further isothermal annealing. If their initial thicknesses, x0 and y0, are such that, for example, the derivative 6x/ t)t=h is negative and the derivative (dy/ dt)l=l is positive, then the thickness of the ApBq layer will decrease, while the thickness of the ArBs layer will increase until the x/y ratio falls into the range defined by inequality (2.48). Subsequently, both layers will grow simultaneously. 

During the natural course of the process of formation of the ApBq and ArBs layers between elementary substances A and B when an A B specimen is given to itself at constant temperature and pressure, a correct ratio of their thicknesses is established automatically. However, if an A-ApBq-ArBs B specimen was prepared artificially, this ratio can hardly be expected to be correct. Therefore, during subsequent isothermal annealing of the specimen, one of the layers will shrink and can even disappear as occurred before its turn if, of course, by that time the other layer has not reached a minimal thickness required for the former to occur. Such a phenomenon was observed, for example, by G. Ottaviani and M. Costato74 with the PtSi layer in Pt-Pt2Si-PtSi-Si specimens and by K.N. Tu et alm with the CoSi layer in Co-Co2Si-CoSi-Si specimens. 

G. Ottaviani and M. Costato have investigated the process of compound formation at the platinum-silicon interface. Platinum films, 115, 205, 268 and 357 nm thick, were sputtered on Si(l 11) single crystals under hard vacuum. The interaction of the Pt and Si phases during isothermal annealing was followed using Rutherford backscattering spectroscopy of helium ions. 

Fig. 2.16. Schematic diagram to illustrate the diffusional stage of the growth process of the Al3Mg2 and Ali2Mgi7 intermetallic compound layers (a) and microstructure of the transition zone between aluminium and magnesium after isothermal annealing at 400°C for 172800 s (48 h) (b).208 Reprinted with permission of Trans Tech Publications Ltd. Photograph kindly provided by Prof. H Mehrer.

In fact, what follows from the equilibrium phase diagram of any binary system is (z) which compounds may, not should, form individual layers at the interface between substances A and B and (zz) the final state of the A-B reaction couple after prolonged isothermal annealing, which only depends on the amounts of initial substances taken. The phase diagram by no means dictates that those compound layers must necessarily occur simultaneously. Moreover, from the point of view of phase equilibria, the number of com-... 

It should be emphasised that according to the diffusional theory any chemical compound layer once formed cannot then disappear during isothermal annealing of the A-B reaction couple because its growth rate increases with decreasing thickness dx/dt ac ox, tending to infinity at... 

When without the plots of Fig. 3.9b, the microstructure of Fig. 3.9a might provide undisputable evidence for the simultaneous occurrence of all the platinum antimonides between platinum and antimony in the course of isothermal annealing a Pt-Sb reaction couple. Layer thickness-time plots clearly show, however, that this is far from being the case. The PtSi layer is seen to shrink. Therefore, though no crack is visible due to application of pressure, the Pt-Sb couple was evidently split at some (uncertain) moment of time into at least two independent couples in which the other platinum antimonide layers could readily occur. Generally, in perfect reaction couples any compound layer survived in the initial linear stage of interaction can hardly be expected to shrink on its own during the further course of the reaction. 

Both systems are suitable to check whether or not there is a directly proportional relationship between the width of the homogeneity range of a compound and the growth rate of its layer, predicted by the diffusional theory.5 It is clear that in view of the presence of the liquid zinc phase during preparation of Ni-Zn and Co-Zn reaction couples, all the inter-metallic phases had equal and favourable conditions to form their nuclei at the interface between nickel or cobalt and zinc, which could then readily grow during subsequent isothermal annealing. 

In the couples of the type shown in Fig. 3.12b the cohesion between dissimilar metals after prolonged isothermal annealing was so poor that the nickel or cobalt plate could be taken out without any effort by hand from the zinc matrix. Therefore, more or less acceptable kinetic data were only obtained with the couples of the type shown in Fig. 3.12c, in which the nickel or cobalt plate is seen to be surrounded by the zinc phase. However,... 

In general, there may be no full correspondence between the equilibrium phase diagram of a multiphase binary system and the microstructure of the A-B transition zone occurred after isothermal annealing of the A-B reaction couple. 

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