Second phase precipitates

Breakdown of passivation and pitting. The local breakdown of passivity of metals, such as stainless steels, nickel, or aluminum, occurs preferentially at sites of local heterogeneities, such as inclusions, second-phase precipitates, or even dislocations. The size, shape, distribution, as well as the chemical or electrochemical dissolution behavior (active or inactive) of these heterogeneities in a given environment, determine to a large extent whether pit initiation is followed either by repassivation (metastable pitting) or stable pit growth.27... 

Localized corrosion of passivating metals initiates at local heterogeneities, such as inclusions and second-phase precipitates as well as grain boundaries, dislocations, flaws, or sites of mechanical damage. In the case of stainless steel surfaces, pit initiation occurs at sites of MnS inclusions. Exclusion of inclusions and precipitates, nonequilibrium... 

The performance of a cast nickel-based alloy is generally based on the microstructural quality, such as the amount of interdendritic segregation, secondary carbides, and intermetallic phases. With the same overall chemical composition, the corrosion rate of the same alloy can vary by several orders of magnitude, depending on its particular microstructure. The most important metallurgical factors that need to be considered are second-phase precipitation by thermal instability and the presence of cold work. The latter is especially important in cases where SCC may be expected. (Rebak)5... 

Studies to determine the effects of doping Nb Ge films grown by CVD have included the synthesis of NbiGci. Mx pseudobinary phases, as well as incorporation of C- and N-dopants into Nb3Ge films. The introduction of second phase precipitates has been shown to enhance values of NbjGe films at the expense of suppressed 7(. values. These results are now discussed in the following paragraphs. 

Phase segregation — A second phase precipitates or segregates at the fiber-matrix interface to promote debonding. 

The Cu-Al-Ni alloys are advantageous because of their higher stability at higher temperatures compared with the Cu-Zn-Al alloys. However, second-phase precipitation cannot be suppressed and embrittles the Cu-Al-Ni alloys and precludes cold working, i.e. such alloys can only be hot finished (Van Humbeek and Delaey, 1989 Hodgson, 1990). Thermomechanical treatments and microalloying additions - in particular Mn, Ti, and Zr - are used for... 

The TTT behavior governing precipitation of the second phase precipitate in the supersaturated solid solution is an approximate indicator of the time-temperature-corrosion diagram. Such a diagram is illustrated by Fig. 13 for an Al-Cu-Mg alloy [42], Plastic deformation prior to precipitation, low aging temperatures, and nucieation promoters that promote nucieation but retard growth... 

A more sophisticated and appropriate description of the depletion of a beneficial alloying element when precipitates are formed on grain boundaries is described by the coUector-plate mechanism and other more advanced treatments. This mechanism describes heterogeneous precipitation of a second-phase precipitate... 

Severe IGC is not observed in high purity Al, or Alclad products that do not form second-phase precipitates at grain boundaries [74]. It is also not observed in alloys that form coarse constituents possessing similar electrochemical properties as the matrix. However, small levels of impurities can induce mild IGC. The mechanisms responsible for IGC is high-purity alloys are complex. They range from... 

Microstructure after casting of Rodent consists of grains/dendrites sizes up to a few hundred micrometers. Inside the grains are imiformly distributed second phase precipitations that originated during alloy solidification as a result of a superfusion on growing dendrites boimdaries (Fig. la). 

Information on grain boundary structure, the structure of second-phase precipitates, and crystal defects can be obtained using high-resolution electron microscopy... 

In this type of composite materials the main load-bearing constituent is the matrix. Small hard particles are distributed evenly in the matrix to block dislocations. The mechanism of the strengthening is described by several authors, among others several years ago by Kelly (1964) and later by Ashby and Jones (2005a). The matrices are made of metals and polymers. The particles are of different origin silica powder or fine sand are used for polymer matrices and, for metallic matrices, second phase precipitates and particles made of oxides, nitrides, carbides and borides. Precipitates are small particles which crystallise from impurities dissolved in metal alloys. The crystallization during the cooling process results in very small and hard particles, closely distributed in the metal matrix. When ceramic particles... 

For irradiations at low temperamres, < 400°C, the density of point defects in FM steels formed from irradiation increases significantly mainly because of the reduced mobihty of point defects. In addition, second-phase precipitation can occur depending... 

Han et al. [135] demonstrated that in polycrystalline (Zn,Mn)0 synthesized by solid state reaction, the (Zn,Mn)Mn204 precipitates are responsible for the observed ferromagnetic transition. In Figure 5.20, the X-ray absorption spectroscopy at Mn I2 3 edges indicates the existence of Mn charge states in the sample that has the (Mn, Zn)Mn204 precipitates. The bulk (Zn,Mn)0 without the second phase precipitates showed paramagnetic behavior. 

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