Grain-boundary precipitate

In the case of the aluminium-magnesium system, most commercial alloys are usually supersaturated, so that elevated service temperatures and inexpert heat treatment are inadvisable, since any resultant grain boundary precipitation may induce susceptibility to intercrystalline attack. The extent of this susceptibility may be approximately deduced from the continuity of... 

The Telated alloy system, Al-Zn-Mg-Cn is also well documented " Overageing is reported to be beneficial since modification of the grain boundary precipitate aspect ratio occurs. ... 

Another type of nickel alloy with which problems of intergranular corrosion may be encountered is that based on Ni-Cr-Mo containing about 15% Cr and 15% Mo. In this type of alloy the nature of the grain boundary precipitation responsible for the phenomenon is more complex than in Ni-Cr-Fe alloys, and the precipitates that may form during unfavourable heat treatment are not confined to carbides but include at least one inter-metallic phase in addition. The phenomenon has been extensively studied in recent years . The grain boundary precipitates responsible are molybdenum-rich M C carbide and non-stoichiometric intermetallic ix... 

As with alloys of other metals, nickel alloys may suffer stress-corrosion cracking in certain corrosive environments, although the number of alloy environment combinations in which nickel alloys have been reported to undergo cracking is relatively small. In addition, intergranular attack due to grain boundary precipitates may be intensified by tensile stress in the metal in certain environments and develop into cracking. Table 4.28 lists the major circumstances in which stress corrosion or stress-assisted corrosion of nickel and its alloys have been recorded in service and also shows the preventive and remedial measures that have been adopted, usually with success, in each case. 

Fig. 16. Transmission electron micrograph of grain-boundary precipitate in translucent Al203 with EDS, energy dispersive spectroscopy, identification of Mg, Al, and O and convergent beam electron diffraction pattern of the [111] zone axis showing presence of MgAl204.

If the solute-depleted solid solution adjacent to the grain-boundary precipitate is anodic, and the cathodic reaction is supported by the pre-... 

Although the scales were too thin to be observed usefully in cross section, such examinations revealed extensive grain-boundary precipitation in the substrate, often penetrating through much of the specimen thickness, for FA 57 and FA 58 (Figures 4(a) and (b)) no such precipitation was observed for FA 49 and FA 56. EDX analysis... 

Condition of container material (grain-boundary precipitation, second phase, stressed or annealed)... 

Fig. 14 Grain boundary precipitate growth showing solute transport path during precipitate growth according to the collector-plate mechanism. Solute B is transported to the a-a grain boundary and then along the boundary to form the P precipitate. Diffusion-controlled precipitate growth results in solute depletion from the a phase along the homophase boundary due to fast boundary transport [44].

Fig. 22 (a) Schematic of heterogeneous grain boundary precipitates formed along a... 

T. Ramgopal, P.I. Gouma, G.S. Frankel, Role of grain-boundary precipitates and solute-depleted zone on the intergranular corrosion of aluminum alloy 7150, Corrosion 58 (2002) 687—697. 

Yield strength of STDA Udimet 718 (which contained essentially no carbide films nor heavy grain boundary precipitates) was quite similar to that of the various Inconel 718 materials. Udimet 718 demonstrated superior ultimate strength at cryogenic temperatures compared with all Inconel 718 materials except that which was not solution treated after cold working. Although some of the various pro-cessing/heat treatment schedules applied to Inconel 718 resulted in improved ductility, the maximum ductility demonstrated by Inconel 718 still fell far short of that displayed by Udimet 718. 

Of these causes, depletion of alloying elements has the greatest practical significance. This occurs when grain boundary precipitates are formed that contain one or more alloying elements in a markedly higher concentration than the matrix. 

In addition to carbides, grain boundary precipitates in this material also consist of an intermetal-lic chromium- and molybdenum-rich intermetallic phase, which leads to depletion of chromium and molybdenum in the grain boundary zones. The critical temperature range for precipitation is very wide (500-1150°C). More recent types of this family of alloys, such as NiMol6Crl6Ti (material no. 2.4610) and NiCr21Mol4W (material no. 2.4602), exhibit a considerably reduced susceptibility due to their lower carbon and silicon contents (both <0.1%) thus, there is no further risk of intergranular corrosion in practical applications (Shell et al. 1964). 

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