 During last two decades a large attention has been paid to develop new high-temperature structural materials that could overcome properties, reliability and performance in service applications of existing ones.  Due to long-range ordered crystal structure and specific properties, the intermetallic alloys were assumed to fill an existing gap between structural ceramics and classical metallic alloys.

Nickel, titanium and iron based intermetallic alloys represent a group of advanced materials with low density, high melting temperature, ordered structure and resistance to high-temperature oxidation developed for high-temperature applications .

Multiphase nickel based intermetallic alloys

o Since some properties (mainly brittleness at room temperature and low creep resistance at high temperatures) of single phase intermetallic compounds Ni 3 Al and NiAl are not sufficient for many structural applications, recent research was focused on multiphase alloys and intermetallic matrix composites. o Several new original multi-component alloys with a complex type of microstructure were developed and prepared by casting technology .

Ternary system Ni-Al-Cr was doped by Fe, Ti, Ta, Mo, Zr and B additions in order to improve room temperature ductility and achieve superior creep strength at intermediate temperatures .

With γ

(

Ni based solid solution

)

primary solidification phase; With β

(

NiAl

)

primary solidification phase; Near eutectic Ni-Al-Cr-Fe alloy.

Main research activities within new nickel based intermetallic alloys •Fundamentals of solidification - growth at planar, cellular and dendritic solid-liquid interfaces •Microstructure characterization of Ni-Al-Cr based alloys •Heat treatments of Ni-Al-Cr based alloys Room and high-temperature mechanical properties of Ni Al-Cr based alloys

• (a) (b) (a)

Dendritic structure of multiphase Ni–21.9Al–8.1Cr–4.2Ta– 0.9Mo–0.3Zr (at.%) intermetallic alloy,

(b)

SEM micrograph showing coexisting regions in after directional solidification at V = 2.78 × 10 −5 ms −1 , D – dendrite, I – interdendritic region, P – Cr-based particles.

Properties

o Resistance to high temperature oxidation, nitridation and carburization ; o Fatigue resistance superior to that of nickel based superalloys ; o High yield strength in a large temperature range o Good tensile and compressive yield strength at 650 – 1100 °C ; o Inferior mechanical properties comparing to those of recent single crystalline nickel based superalloys.

Industrial applications

 Transfer rolls  Heat treating trays  Centrifugally cast tubes  Rails for walking beam furnaces  Die blocks  Nuts and bolts  Corrosion resistance tool bits  Single crystal turbine blades

Nickel-based superalloy TMS82 during the early stages of primary creep showing andislocation ribbon passing through both precipitates and matrix.

Mechanical Properties and Microstructure

Over the last 50 years turbine entry temperatures (TET` s ) have risen from 800ºC to 1600ºC. Materials developments in all turbine components, are critical to achieving this, but engine designers are looking for a TET of 1800ºC to increase engine efficiency and reduce environmental impact. We focus on understanding the fundamental mechanisms determining the mechanical properties of turbine materials and use this to produce tools and strategies for materials development and life prediction.

Alloy development of fourth-generation single-crystal alloys

 Nickel-base single-crystal superalloys can be strengthened by the addition of tungsten and rhenium, but doing so while maintaining reasonable density, stability and environmental resistance requires careful optimization of the composition and microstructure.

Creep strength comparison of binary NiAl, alloyed NiAl single crystals, and a first-generation single-crystal nickel-base superalloy made at 1026 o C

(1880 o F)

and a strain rate of 1x10-6 sec-1.

Microstructure of a creep-resistant NiAl 3Ti-0.5Hf single crystal alloy.

“Nimonics”

 Key component of the microstructure is precipitates of

(Ni,Fe) 3 Al:

γ

`.

 A modern superalloy might be 60 - 85% γ

` -

nickel is effectively a “glue” holding the γ

` t

ogether.

The yield stress of γ`increases with increasing temperature (up to about 700ºC)

Microstructure must be

stable

: Any finely divided precipitate distribution will tend to coarsen – driving force is lowering of interfacial energy.

γ` is nearly exactly lattice-matched to the Ni matrix.

Interfacial energy is nearly zero.

Alloy Additions

• •

Ti:

goes into γ` - Ni3(Al, Ti) solid soln strengthening of γ`

Cr:

goes into Ni matrix, • • solid soln strengthening, corrosion resistance

Co:

goes into both Ni and γ` oxidation and corrosion resistance lowers solubility of Al in Ni, so enhances γ` formation, improves g` high T stability

C:

combines with Cr, gives precipitates in Ni

• • • • • • •

Mo, W:

solid soln strengthening of Ni

Ta:

solid soln strengthening of γ`

B:

improves grain boundary and carbide / matrix adhesion, so suppresses cavity formation in creep

Hf:

impurities?)

Y:

<0.5%, improves high T ductility (scavenges improves oxidation resistance

Re:

the latest “magic dust”: 3% extends operating temperature considerably.

Typical Ni-based Superalloys



Nimonic 115

:

Nimonic 80A

Ni, 14.5% Cr, 13.3% Co, 3.8% Ti, 5.0% Al, 3.3% Mo, 0.15% C, 0.05% Zr, 0.016% B - an early wrought alloy 

MAR M200

: Ni, 9% Cr, 10% Co, 1.5% Ti, 5.5% Al, 0.15% C, 0.05% Zr, 0.015% B, 10% W, 2.5% Ta, 1.5% Hf - “standard” cast alloy

 SRR99: Ni, 8.5% Cr, 5% Co, 2.2% Ti, 5.5% Al, 9.5% W, 2.8% Ta.

- Rolls Royce single crystal alloy  CMSX-4: Ni, 6.5% Cr, 9% Co, 1% Ti, 5.6% Al, 0.6% Mo, 6% W, 6.5% Ta, 3% Re, 0.1% Hf - advanced single crystal alloy

Yield strength, UTS, fracture strain, etc, rather less important than creep behaviour and fatigue behaviour.

Nickel-based superalloys represent the current state-of-the-art for many high-temperature, nonnuclear, power-generation applications. However, these superalloys have not been tested in creep at the combination of high temperatures and very long service times anticipated in space nuclear power generation. Designers need to know the creep resistance of potential impeller materials at realistic temperatures, stresses, and environments.

 MAR-M 247LC is a representative of the cast superalloys currently used in impellers and rotors where the hub and blades are cast as a single unit, and was selected for the present evaluations at the NASA Glenn Research Center. Most creep tests were performed in air using conventional, uniaxial-lever arm constant-load creep frames with resistance-heating furnaces and shoulder-mounted extensometers.  However, two tests were run in a specialized creep-testing machine, where the specimens were sealed within environmental chambers containing inert helium gas of 99.999-percent purity held slightly above atmospheric pressure.  All creep tests were performed according to the ASTM E139 standard.

o The cast MAR-M 247LC had irregular, very coarse grains with widths near 700 μm and lengths near 800 to 12,000 μm. The grains were often longer in the direction of primary dendrite growth (

see the photomicrographs

).

o The microstructure was predominated by about 65 to 70 vol% of Ni3Al-type ordered intermetallic γ′ precipitates in a face-centered cubic γ matrix, with minor MC and M23C6 carbides.

o The sizes of the γ′ precipitates varied from about 0.4 μm at dendrite cores to 3.0 μm between dendrites, because of dendritic growth within grains.

Creep tests in air were designed to determine allowable creep stresses for 700 o , 820 o , and 920 o C that would give 1-percent creep in 10 years of service, a typical goal for this application. This service goal represented a target strain rate of 0.1 percent/year. Creep strain rate to 0.2-percent creep is shown versus stress in the following graph. Stresses of about 475, 150, and 70 MPa were estimated to achieve the target strain rate at 700 o , 820 o , and 920 o C, respectively.

Additional creep tests and analyses are necessary, but a preliminary creep analysis using current test results indicates quite good potential for an impeller fabricated of MAR-M 247LC for maximum temperatures to 920 o C .

Creep stress versus strain rate for MAR-M 247LC, showing estimated stresses necessary to achieve a maximum strain rate of 0.1 percent per year.

Tests to estimate the effects of air versus inert environments on creep resistance were also initiated. The results of single tests in air at 1-atm pressure and in helium at slightly above 1 atm at 820 o and 920 o C are compared in the following graphs. Creep progressed as fast or even faster in helium than in air at 820 o and 920 o C.

The creep tests in air reasonably approximate response in helium to low creep strain levels near 0.1 percent, but not at high strains. More tests are needed for confirmation, but this suggests that there may be no improvement in creep resistance due to the inert environment .

Comparison of creep response in air versus helium. Top: 820 o C. Bottom: 920 o C.

The new nickel-base alloys represent a major departure from previous alloy design practices used in industry for single-crystal superalloys. Advances in past superalloy development for turbine blade applications have been accomplished with continued increases in the refractory metal content, which significantly increase alloy density. High alloy densities have limited the use of the advanced superalloys to specialized applications.

Measured densities of new low-density superalloys compared with previously developed superalloys. The most creep resistant, low density alloys are shown here for comparison

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