Intermetallic precipitates

IGC of aluminum alloys is controlled by material selection and proper selection of thermal (tempering) treatments that can affect the amount, size, and distribution of second-phase intermetallic precipitates. Resistance to IGC is achieved by the use of heat treatments that cause general uniform precipitation throughout the grain structure. General guidelines for selecting suitable heat treatments for these alloys are available (45). 

The nickel base alloys are produced from a group of alloys which have chemical compositions generally over 50 % nickel and less than 10 % iron. They are mainly strengthened by intermetallic precipitation in an austenitic matrix. The cobalt base alloys have a high Co content (40 to 70 %X high Cr (over 20 %), high W (7 to 15 %) and they are strengthened by a combination of carbides and solid solution hardeners. 

Pitting of industrial metals and alloy almost always starts at non metallic inclusions, notably sulfides, or intermetallic precipitates. It is therefore not surprising that the presence of inclusions or precipitates on a metal surface lowers the value of the pitting potential. As an illustration, Figure 7.53 presents anodic polarization curves... 

Inclusions or intermetallic precipitates can facilitate pit initiation and growth in different ways. Figure 7.54 shows the effect of inclusions that are anodic, cathodic or inert with respect to the base metal. Inert inclusions (a) do not electrochemically interact with the base alloy, but they can still play a role in the initiation of pitting if... 

Nickel-based alloys, which form the bulk of alloys produced, are basically nickel-chrome alloys with a face-centered cubic solid-solution matrix containing carbides and the coherent intermetallic precipitate y-NijlAfTi). This latter precipitate provides most of the alloy strengthening and results in useful operating temperatures up to 90% of the start of melting. Further additions of aluminum, titanium, niobium, and tantalum are made to combine with nickel in the y phase, and additions of molybdenum, tungsten, and chromium strengthen the solid solution matrix. 

In an aluminum alloy sheet which requires high formability, the presence of strontium is reported to reduce the number of intermetallic precipitates. This improves the alloy s formability and reduces the incidence of edge cracking. 

Unlike Sn-Pb joints, which have a dual phase structure and block the path of corrosion due to the existence of phase boundaries, the SAC305 joint is basically pure Sn with coarse islands of A n and CueSns intermetallic precipitate (Fig. 5). A corrosion crack can propagate and lead to additional corrosion along the way, without interruption from the Sn phase structure. Although both materials show strong resistance to corrosion, the localized nature of the corroded area at critical locations causes significant degradation in Sn-Ag-Cu solder joints[40]. 

Microstructural constituents. A heterogeneous microstructure forms anodic and cathodic sites which promotes corrosion. The intermetallic precipitate serves as anodic and cathodic sites and they may be anodic or cathodic to the matrbc. For instance, a CuAl2 precipitate is cathodic to the aluminum matrbc and Mg2Si is anodic to the matrix. Microstructural variation plays a leading role in metal matrbc composites. For instance, A1 6013-SiC composite corrodes at the A1 matrbc/SiC interface because... 

Moreover, most international works, except the British one, tend to eliminate also for unacceptable postirradiation ductility, the high Ni alloys hardened via intermetallic precipitation. Therefore, investigations then focused on a selection of Fe bases with low Cr content and Ni content of up to 25%, hardened in solid solution by additions of Mo, Si, Ti -I- possibly Nb and V and also including variable additions of C, N, P, and B. The objective being to check the following points on these advanced austenitic bases compared with the reference 15-15Ti ... 

Inter- metallic phases Negative Precipitates with accompanying depletion of alloying elements (Cr, Mo). If steel manufacturers recommendations are followed, intermetallic precipitation should not occur during heat treatment or welding. 

For precipitation hardenable alloys, the size and distribution of intermetallic precipitates depends on the quenching and artihcial ageing conditions. The continuous precipitation of intermetalhcs at grain boundaries can thus be avoided. 

Metallurgists have proposed AlFeNi alloys containing roughly 1% iron and nickel, such as 8001 (Fe 0.5, Ni 0.9). The addition of iron and nickel forms uniformly distributed, very small intermetallic precipitates of AlaFe and Al9NiFe that reduce the sensitivity to intercrystalline corrosion in hot water [28, 29]. 

The commercial ferritic steel 446, containing -27% Cr, did not show any metal dusting corrosion in the 550-650°C temperature range. SFM images for the surface and cross-section are presented in Fig. 5.21. A continuous chromium-rich surface oxide is evident. No experiments were carried out at temperatures below 550°C. Chromium-rich intermetallic precipitates (sigma phase) can be seen in the alloy subsurface region (Fig. 5.21) - these brittle precipitates adversely affect the mechanical properties of the alloy. 

SEM cross-section of 446 steel after metal dusting corrosion at 650°C showing protective surface oxide and intermetallic precipitates... 

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