[Part 1]
If you are only going by the metal temperature handling capability alone, then pls note the following:
1) The DS casted blades provide ~14deg C advantage over equiaxed polycrystalline casted blades (e.g. MAR M247, with approx metal temp capability of 950-1050 deg C).
e.g. Kaveri/Kabini uses DS casted blades (and vanes) using Supercast 247A (a variant of CM247LC which is itself derived from equiaxed MAR M247)
2) The 1st Gen SC blades provide another ~20deg C advantage, over these DS casted blades.
e.g. RR2060/PW1480/CMSX3/ReneN4 - 1060 deg C
3) The 2nd Gen SC blades provide another ~30deg C advantage over these 1st Gen SC casted blades.
e.g. PWA1484/CMSX4/ReneN5 - 1120 deg C
4) After that, the 3-5th Gen SC blades are produced with an aim of adding further ~30deg C advantage, as follows:
CMSX10 - 1135 deg C
ReneN6 - 1110 deg C
TMS80/MC-NG/DMS4 - 1140 deg C
TMS196 - 1150 deg C
Pls further note DMS4 is the DMRL developed suddha-desi SC alloys for turbine blade application - and it's almost shoulder to shoulder to best available (i.e. published).
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Note - GE uses Rene-6 in F414.
DMRL developed a DS cast version of DMS4 called DMD4 - it was specifically developed for complex turbine aerofoil parts that are difficult to cast in SC form - and also as an cost-effective alternative. Further research continued with DMD4 by adding Ru and it was actaully proved to significantly improve the rupture life etc etc.
So it's obvious simply graduating from Equiaxed -> DS -> SC casting, it's not possible to reach the 1400-1650 deg C TeT capabilities of many modern turbofans.
Also the question obviously arises, how is the 1455deg C TeT of Kaveri achieved given the metal temperature capability of 1050 deg C of DS casted CM247LC based blades?
Answer, of course, and is well known in BRF as well is, from following two aspects:
1) Implementing various Blade Cooling techniques (e.g. Film/Convection cooling) - with this, a decent DS casted blade would provide for 200-250 deg C advantage (eg DS GTD 111) over metal-temperature-handling capabilities.
However a SC casted blade, again with a decently designed blade cooling techniques, would allow this advantage to go upto ~400 deg C.
2) Thermal Barrier Coating (TBC) application - Generally, the 7-8 wt% Yttria Stabilized Zirconia (8YSZ) TBC provides 150 deg C advantage (not to be confused with "thick layer"-TBC-application-capable-surfaces like the combustor walls etc, which can allow as much as 250-300 deg C advantages).
So Kaveri's DS blades gets to 1455 deg C via 1050 deg C (DS Cast) + 250 deg C thru blade cooling + 150 deg C via 8YSZ based TBC.
[Part 2]
Thus one assertion can be - that higher TeT is desired of, DS to SC graduation is inevitable in any modern turbofan development program.
But that is not because of incremental metal temperature capability addition etc (at best 20-60 deg C advantage).
The actual advantage of moving from DS to SC is not simply via the raw blade metal temp enhancement - it's more to do with thin section property optimisation that is inherent to SC processing. This thin section property optimisation (for e.g. in the thinnest section of a blade, the wall thickness (metal portion) is as small as 0.5 mm) allows for further intricate cooling passage designing etc which in turn allows drastic increase in overall TeT etc (200-250deg C of DS casted to 400+ deg C levels of SC casted blades).
This of course is in addition to the fantastic improvement of creep-resistance, tensile strength etc properties that entails from Equiaxed -> DS -> SC graduations (details in the Kaveri Sticky thread).
Also another aspect is advances in TBC ... recently we have seen
reports of Indian Rare Earths Limited developing bi-layer TBC of Lanthanum Zirconate (LZ) over Yttrium Stabilized Zirconia (YSZ).
A bilayer top-coat consisting of Lanthanum Zirconate (LZ) over 8YSZ applied over "traditional" bond coat of say NiCrAlY enhances the temperature capability of the coating by >100deg.
So basically this advanced TBC if successfully applied to Kaveri's DS casted HPT turbine blades will easily bring the TeT to mid-1500deg C regime.
And if (and when) DMRL graduates to SC casted blades (and vanes), of the already developed DMS4 material, the absolute cutting edge TeT regimes of 1600+ deg C, becomes well within reach.
[Part 3]
A word about 8YSZ and LZ TBCs - note, Wiki has good level of general details wrt the TBC concept:
1) The "traditional" 8YSZ TBC (that has been ther for 4 decades now) allows for maximum surface temperature capability of about 1200 °C - beyond that degradation of the coating (in form of reduced strain tolerance and a decrease in thermal fatigue life of the coating) takes place due to changes in microstructure.
2) Lanthanum Zirconate (LZ), has much higher thermal and phase stability - close to 2000deg C.
It also has lower thermal conductivity and sintering tendency compared to YSZ.
(Thermal Conductivity - 2 W/m/K of YSZ vs 1.56 W/m/K of LZ)
3) LZ is also less oxygen transparent than YSZ, providing better bond coat oxidation resistance and minimises the growth of TGO (Thermaly Grown Oxide layer) - Wiki has good details about TGO and it's impact on TBC.
4) LZ has lower coefficient of thermal expansion compared to YSZ - so it can not be applied directly on the NiCrAlY bond coat. Therefore LZ is applied as a top coat material over YSZ forming a bilayer TBC. Furthermore LZ has good chemical compatibility with YSZ, making them a very good candidate for bilayer top coat applications.
5) Nano-structured TBCs often exhibit excellent performance compared with conventional TBCs such as adhesive strength, thermal shock resistance, thermal insulation, corrosion resistance and so on.
Furthermore nano-structured bi-layer is also expected to reflect certain amount of radiations (std phyzziks says wavelength of the reflected light is directly proportional to the particle diameter) thus providing a more effective TBC. So for reflecting heat in the near IR spectrum, TBC micro-particles needs to be of the order of 1-3 μm.
6) In India Nano-structured high purity grade YSZ and LZ are prepared from beach sand containing monazite and zircon following wet chemical route i.e. co-precipitation method.
7) DRDO has tested air-plasma sprayed TBC comprising of NiCrAlY bond coat (of 50 μm thickness), YSZ top coat (thickness 100 μm) and LZ top-most coat (thickness 50 μm) on to cast Ni-base super alloy substrates. The total maximum thickness was kept well below 250 μm.
8 ) DRDO has already assembled and validated the bi-layer YSZ-LZ coated flaps in an aero-engine for test cases involving rapid thermal transients, supersonic flow of combustion products, vibratory loads of about 4 ‘g’, sustained 1,000 h equivalent of engine operation and more than 30,000 nozzle actuations.
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All of these in Parts 1-3, still doesn't clarify following two questions:
1) why is DMRL not able to create SC casted blades for Kaveri?
2) and if it was not, able to what are these SC casted turbine blade images from various AI and other published literature that's doing round for many years now?