Discontinuous precipitate

The structures and phase transformations observed in steels have been dealt with in some detail not only because of the great practical importance of steels, but also because reactions similar to those occurring in steels are also observed in many other alloy systems. In particular, diifusionless transformations (austenite -> martensite), continuous precipitation (austenite -> pearlite) and discontinuous precipitation (austenite -> bainite and tempering of martensite) are fairly common in other alloy systems. 

Composition Profile Measurement. Results of Zieba et al. (1997) will be given as an example of the measurement of solute distribution in an alloy undergoing a phase transformation. They studied discontinuous precipitation in cobalt-tungsten alloys, in which a Co-32 wt% W alloy was aged in the temperature range 875 K to 1025 K, and high spatial resolution X-ray microanalysis of thin foils by STEM was used to measure the solute distribution near the reaction front. 

In order to explore the microchemical changes accompanying the growth process of the discontinuous precipitates high spatial resolution thin foil microprobe analyses were made by Zieba et al. (1997) across the moving cell boundary and across each set of several eCo and Co3W lamellae. 

Discoloration, of acrylic fibers, 11 194 Discontinuous furnaces, 12 598 Discontinuous precipitation reaction,... 

Benzodiazepines have a low abuse potential when they are properly prescribed and their use is supervised (American Psychiatric Association 1990). However, physical dependence often occurs when benzodiazepines are taken at higher-than-usual doses or for prolonged periods. If benzodiazepines are discontinued precipitously, withdrawal effects (including hyperpyrexia, seizures, psychosis, and even death) may occur. Signs and symptoms of withdrawal may include tachycardia, increased blood pressure, muscle cramps, anxiety, insomnia, panic attacks, impairment of memory and concentration, perceptual disturbances, and delirium. In addition, withdrawal-related derealization, hallucinations, and other psychotic symptoms have been reported. These withdrawal symptoms may begin as early as the day after discontinuation of the benzodiazepine, and they may continue for weeks to months. Evidence indicates that withdrawal reactions associated with shorter-half-life benzodiazepines peak more rapidly and more intensely. 

Fig. 5.6 (a) Byzantine paten (ca. 600 AD, The MenU Collection, Houston) (b) intergranular corrosion and (c) EDX analysis showed primarily silver at location 1 and discontinuous precipitation of copper at locations 2-4 SEM metaUographs and analysis courtesy off Joosten ICN (after [255])... 

A discontinuous precipitate often was observed at citrus oil/ aqueous phase interfaces kept at 50 C for prolonged periods. The precipitate particles congregate on the water side of the citrus oil/aqueous phase interface and disperse into the aqueous phase upon agitation. If the aqueous phase is distilled water, or a supernatant phase, the precipitate particles cause the aqueous phase to become noticeably cloudy. Precipitate particles or interfacial films were not detected at citrus oil/complex coacervate phase interfaces. However, such interfaces normally were not kept for prolonged periods, because their IFT values rapidly decayed to a value too low to measure. 

Fig. 7.50 Schematic representation of microstructures susceptibleto intergranular corrosion, (a) Continuous precipitation of B-rich AB2. (b) Discontinuous precipitation of E-rich DE3...

Tin additions to lead-calcium alloys change dramatically the method of precipitation and age-hardening from discontinuous precipitation of PbsCa to a mixed discontinuous and continuous precipitation of PbsCa and (PbSn)3Ca and, finally, to a continuous precipitation of SnsCa. Such precipitation reactions have been described for alloys that contain low tin contents [45,48,68-70]. The reactions are not influenced or modifled by impurities in the lead alloys [71,72]. A ternary phase diagram has been proposed [41], which sets the areas of stability of PbsCa, SusCa, and mixed (PbSn)3Ca precipitates, and this is shown in Fig. 2.5. The phase diagram has been conflrmed [73]. 

At low tin contents, the calcium initially precipitates rapidly as PbsCa in the same manner as in binary lead-calcium alloys. These alloys have a fine grain structure and quickly reach high hardness as discontinuous precipitation predominates. At higher tin contents, the mode of precipitation changes to a mixed discontinuous precipitation of PbsCa followed by a continuous precipitation reaction of Pb cSn Ca. These reactions lead to overageing of the precipitates and a decrease in mechanical properties. 

Silver has a significant effect in delaying the discontinuous precipitation reaction as well as the overageing of calcium precipitates [90]. Silver also increases the corrosion resistance of lead-calcium tin alloys — particularly under conditions that simulate end-of-discharge (high pH) conditions — and improves the cycle-life and capacity of VRLA batteries. Silver decreases the thickness of the PbO layer but produces a harder, more compact, corrosion layer than tin. Silver does not, however, increase the conductivity of the corrosion layer. 

Figure 4.35 illustrates the different temperature—time zones of age-hardening of the Pb—0.11 wt% Ca—0.57 wt% Sn alloy. Zone A covers the discontinuous transformations zone B is the zone of continuous precipitation of (Pbi xSnx)3Ca and zone C is the zone of discontinuous precipitation of lamellar (Pbi xSnx)3Ca phase [75]. 

Senl] Sennour, M., Jouneau, P.H., Esnouf, C., TEM and EBSD Investigation of Continuous and Discontinuous Precipitation of CrN in Nitrided Pure Cr-Fe Alloys , J. Mater. Set., 39(14), 4521-4531 (2004) (Experimental, Interface Phenomena, Morphology, Phase Relations, 20)... 

Discuss whether the grain boundary migration during discontinuous precipitation or dissolution is basically different from DIGM. 

Baik, Y.-J. and Yoon, D. N., The discontinuous precipitation of a liquid phase in Mo-Ni induced by diffusional coherency strain, Acta Metall. Mater., 38, 1525-34, 1990. 

In the previous chapter, we considered the problem of selection of the path for the reactive diffusion when the choice is made among a finite number of phase formation modes. Let us now treat the problems allowing for an infinite set of solutions, all of which are compatible with the matter balance equations. These are mainly the problems of morphology choice when different modes are possible, and the task is to find the optimal one to be realized in practice. In the first two sections of the chapter, we consider the issue of low-temperature phase diffusion transformations, namely discontinuous precipitation and DIGM. At that, the evolution equations based on matter conservation laws allow an infinite number of solutions corresponding to different thicknesses of the phase formed and different velocities of phase transformation front movement. 

First, we treat a model of discontinuous precipitation of binary polycrystalline supercooled alloys at low temperatures as a result of DIGM. In the proposed approach, we independently determine the main parameters interlamellar distance, maximum velocity of the phase transformation front, and residual supersaturation at the front. This is achieved by using a set of equations for... 

Model of Self-Consistent Calculation of Discontinuous Precipitation Parameters in the Pb-Sn System... 

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