Critical cooling rates

It is proposed to make the shaft from a NiCrMo low-alloy steel. The critical cooling rates of NiCrMo steels are given quite well by the empirical equation... 

Using the fluxing technique, Lau and Kui [33] determined that the critical cooling rate for forming a 7-mm diameter bulk amorphous Pd4QNi4()P2o cylinder was 0.75 K/sec. From this value, they estimated that the steady-state nucleation frequency was on the order of lO" m s. On the other hand, Drehman and Greer [34] estimated that the steady state nucleation frequency at 590 K is 10 m" s, which is also the maximum... 

Sutton [1.15] studied the question of how quickly solutions with certain CPAs (GL, dimethylsulfoxide (DMSO) and others] have to be cooled in order to avoid crystallization. At 100 °C/min concentration of 42.1 % DMSO and 48.5 % for GL are necessary to achieve the glass phase. With a 32.5 % solution of (2R.3R)-(-)butan-2,3-dio, the same effect can be accomplished at = 50 °C/min. In Fig. 1.18 Sutton (Fig. 11 from [1.114]) showed, that polyethylene glycol with a molecular weight of 400 (PEG 400) reduced the critical cooling rate down to approx. 25 °C/min. The addition of PEG 8000 [1.115] improved the protection of lactate dehydrogenase (LDH) by maltodextrins, if maltodextrins with low dextrose equivalents are used. 

Fig. 1.18. Critical cooling rates for butane-2,3-diol and dextran as a function of butane-2,3-diol concentration.

Sutton, R. L. Critical cooling rates to avoid ice cristallisation in solutions of cryoprotective agents. Journ. Chem. Soc. Faraday Trans. Vol. 87, p. 101-106, 1991... 

Sutton, R. B. Critical cooling rates for aqueous cryoprotectants in the presence of sugars and polysaccharides. Cryobiology 29, p. 585-598, 1992 Copyright 1992, Academic Press. Inc. 

Critical care analytes, measuring, 24 55 Critical cooling rate, of steel, 23 283 Critical current density, 23 821-823, 825 optimization of, 23 829 in PIT conductors, 23 833-834 temperature and field dependence of, 23 847-848... 

The ternary alloys of a lanthanide with Al and a late transition metal have very high glass-forming ability and low critical cooling rate bulk samples may be cast. 

Figorc 11J8. Calculated critical cooling rates for suppression of various phases in (a) Ag-Si and (b) Au-Si from Kambli et al. (1985). Horizontal bars indicate experimental glass-forming range by laser quenching. 

Figure 11.40. (a) Calculated critical cooling rates for glass formation in Ti-Zr-Be (Saunders and Miodownik 1988) and (b) observed glass-forming range (Saunders... 

The above results may be applied to infer the critical cooling rate for the concentration of the core to be affected by diffusion. It is necessary to define precisely what is meant when we say "the center is affected by diffusion." If we use center concentration of Ci + 0.01 (Cmid - Ci) as the criterion for center concentration to be affected by diffusion, then it would occur at z = 2(fDdt) /a = 0.3947. For an asymptotic cooling history, this means that 2 Dox) la = 0.3947, or Doxla = 0.0389. Combining with Equation 5-137 that J = RTq/ xE), we obtain the critical q ... 

Watson and Cherniak (1997) used numerical solutions to investigate the critical cooling rate for the center concentration of zircon core to be affected. Equation 5-142 is similar to their result and may be viewed as the analytical proof of it. The general equation (Equation 5-141) may be applied to diffusion of other species in zircon, as well as other minerals. 

More recently magnesium-base, iron-base, and zirconium-titanium-base alloys have been developed that do not require such rapid cooling. In 1992, W. L. Johnson and co-workers developed the first commercial alloy available in bulk form Vitreloy 1, which contains 41.2 a/o Zr, 13.8 a/o Ti, 12.5 a/o Cu, 10 a/o Ni, and 22.5 a/o Be. The critical cooling rate for this alloy is about 1 K/s so glassy parts can be made with dimensions of several centimeters. Its properties are given in Table 15.3. 

Fig. 1.18. Critical cooling rates for butane-2,3-diol and dextran as a function of butane-2,3-diol concentration. 1, Butane-2,3-diol 2, butane-2,3-diol and dextran 20 3, butane-2,3-diol and PEG 400 (Figure 11 from [1.114])...

Sutton, R.L. Critical cooling rates to avoid ice crystallisation in solutions of... 

Sutton, R.B. Critical cooling rates for aqueous cryoprotectants in the presence... 

Critical cooling rate, Rc minimum quenching rate of a melt that results in a glassy product... 

It is also interesting to notice that the crystallization kinetics are also strongly affected by the presence of impurities, especially oxygen at a level below one percent molar. This contamination can alter the critical cooling rate by two order of magnitude. For instance, the glassy alloy V105, which has... 

---