Strength grain boundary precipitate

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. 

The carbide precipitation that leads to sensitised grain boundary regions can be minimised by reducing the carbon to 0.03% or less but this increases the cost and reduces the strength. The alternative is to add stabilising elements such as titanium or niobium, which are stronger carbide formers than chromium. There are numerous texts that describe the metallurgy of... 

Al-Mg (5000 Series) and Al-Mg-Si (6000 Series) In the binary alloy system strength is obtained mainly by strain hardening. Stress corrosion is thought to be associated with a continuous grain boundary film of Mg,Alg which is anodic to the matrix . Air cooling prevents the immediate formation of such precipitates, but they form slowly at ambient temperatures. Thus only low Mg alloys are non-susceptible (Al-3% Mg). Widespread precipitation arising from plastic deformation with carefully controlled heat-treatment conditions can lower susceptibility. Al-5Mg alloys of relatively low susceptibility are subjected to such treatments. Mn and Cr... 

Fig. 20.SS Transmission electronmicrograph showing intergranular and imragranular precipitation and the precipitate-free zone adjacent to the grain boundary in a high-strength precipitation-hardening Al-Zn-Mg alloy (x24 000, courtesy G. Lorimer)...

At half of the solidus temperature on the absolute temperature scale (i.e., 1850°C for tungsten), solid-solution alloys lose much of their strength, and dispersion-strengthened or precipitation-hardened alloys are significantly stronger and creep resistant. This is caused by the interaction between the dispersoids and dislocations, as well as subgrains and grain boundaries. 

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