(Mo,Co)x(Ni,Cr)y. The solution was to control
the chemistry of alloys using a formula to identify the susceptibility to topologically close packed phases using a complex formula known as Phacomp
Alloy Development 1940 - 2010
Low Grain Boundary Ductility This was an underlying issue with some alloys but became apparent with directionally solidified material. The problem was solved by the Martin Marietta DID (ductility improvement discovery) alloys containing 1-2% hafnium and a control of zirconium and other grain boundary elements.
1940 1950 1960 Figure 1
Contamination From Low Boiling Point Elements The chemistry of aerospace superalloys is closely controlled to confirm specification. An added control is the analysis of trace and tramp elements which could harm the alloy. Principal amongst these are the elements, bismuth, tellurium and selenium. On at least one occasion bismuth has been found in cast parts and on one unfortunate case it was present at a concentration of 4ppm in the surface of aerofoils. This was traced to the alumino-silicate refractory used to contain the shell moulds, lesser contamination was found resulting from this refractory used as a shell stucco. Mechanical testing at these levels of contamination showed a reduction of 80% in creep life.
Recrystallisation From Heat Treatment Recrystallisation (RX) occurs when material is heated above a temperature at which there is sufficient thermal energy to nucleate a second grain from an area with high residual strain. This is a serious problem with single crystals which cannot tolerate second grains but can also be an issue with conventional cast parts. Since internal strain can be present around cored surfaces it is not unknown for recrystallisation to occur with a potential loss in integrity should a large grain result.
Generation
1st 1970s 1st 1980s 3rd 1990s 4th 2000s Figure 2
® Alloy
SRR99 CMSX4 Rene N6 TMS138A
In-situ Composites These were an attempt to directionally solidify eutectic alloys which would produce a structure
1970 1980
1990
2000
similar to a
fibre composite. The resulting material had excellent longitudinal strength but poor transverse properties. As the solidification time was several times longer than a ‘normal’ DS casting the process remained a curiosity.
Intermetallic Alloys The intermetallic alloys based on nickel aluminide and titanium aluminide offer much potential as both have a lower density than either nickel or titanium alloys and with potentially superior high temperature properties. Titanium aluminide has service experience and further development potential and remains a strong contender for future service. Nickel aluminide remains a potential contender and remains under development. Whereas single crystal development may now
be mature, the stalled development of eutectic and intermetallic alloys indicate that there are still opportunities for further research, and with the possibility of new markets for the investment casting industry the door is far from shut to satisfy the curiosity of material scientists.
Cr Co Mo Re Ru W Al
8.0 6.5 4.0 3.2
5.0 9.0
0.6
12.0 1.0 5.8
2.8
3.0 6.0 5.7
10.0 5.5 6.0 6.0
3.60 5.9 Ti
5.6 5.8 5.6
2.2 1.0
Ta
3.0 6.5 7.0 5.6
Hf Ni BAL
0.1 BAL 0.2 BAL 0.1 BAL
October 2021 ❘ 13
Vitallium, Stellite Nimonics Vaccum Cast Superalloys D.S. alloys
1st Generation Single Crystals Titanium
Insitu Composites
2nd Generation Single Crystals 3rd Generation Single Crystals Ti & Ni Intermetallics
4th Generation Single Crystals
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