, I

PROCESSING AND OPERATING EXPERIENCE OF Ni,ALBASED INTERMETALLIC ALLOY IC-221M

V. K. Sikka and M. L. Santella Metals and Ceramics

Oak Ridge National Laboratory P.O. Box 2008

Oak Ridge, Tennessee 37831

and

J. E. Orth United Defense LP

2101 West loth Street Anniston, Alabama 36201

ABSTRACT

The cast Ni,Al-based intermetallic alloy IC-221M is the most advanced in its commercial applications.

This paper presents progress made for this alloy in the areas of: (1) composition optimization; (2) melting process

development; (3) casting process; (4) mechanical properties; (5 ) welding process, weld repairs, and thermal aging

response; and (6) applications. This paper also reviews the operating experience with several of the components.

The projection for future growth in the applications of nickel aluminide is also discussed.

INTRODUCTION

Development of intermetallics for structural applications have been tallred about in the scientific literature

[l-141 for nearly two decades. From the thousands of possible intermetallic compounds [15] that exist based on

elements in the periodic table, the aluminides of nickel, iron, titanium, and molydisilicide have shown the most

potential for commercial applications. Among the aluminides, a cast Ni,Al-based alloy, developed at the Oak

Ridge National Laboratory ( O W ) and known as IC-221M, has reached the most significant stage of - -

commercialization. The purpose of this paper is to provide the processing details, properties, and operating

experience for this alloy.

*Research sponsored by the U.S. Department of Energy, Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Industrial Technologies, Advanced Industrial Materials Program, under contract DE-ACO5-96OR22464 with Lockheed Martin Energy Research Corp.

SP

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, make any warranty, express or implied, or assumes any legal liabili- ty or responsibiuty for the accuracy, completeness, or usefulness of any information, appa- ratus, product, or proces disclosed, or represents that its use wouid not infringe privately owned rights. Reference herein to any specific conunem'al product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessan'ly constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessar- ily state or reflect those of the United States Government or any agency thereof.

DISCLAIM=

Portions of this document may be illegible in electronic image product& h a g s are produced from the best available original dOr?umf!llL

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COMPOSITION

The IC-221M is a cast alloy with composition [15-161 based on controlled additions of boron, chromium,

zirconium, and molybdenum to the Ni,Al base. The nominal composition of IC-221M, given in

Table 1, can be produced in the laboratory environment with good control of each element. However, the alloy

production under commercial environment resulted in the following described issues which have been dealt with

for commercial applications:

1.

2.

3.

Melting procedures not known for such high alumiprwn content alloy. This issue stems from two aspects of

commercial melting practices. First, only small aluminum additions are typically used in commercial alloys.

Such aluminum additions are made to the melt near the end of melting for the purposes of deoxidation. Large

additions of a l d u m such as in IC-221M were feared to chill the melt and mostly oxidiz~ and go to slag.

The oxidation process was expected to cause p r chemistry control The second concern for melting was

based on the assumption that the only way to melt an alloy with such high aluminum content is to load the

entire aluminum content at the beginning of the melt. In this scenario, the large melting point difference

between duminum and nickel will cause overheating of the aluminum prior to the start of the nickel melting.

The overheated aluminum of very high fluidity will seep through any cracks in the furnace crucible and attack

the induction coil. Such an attack of the induction coil will bring molten aluminum in contact with water and

result in an explosion. Both of these concerns have been eliminated with the development [17-191 of a melting

process known as Exo-Meltm. The key features of this melt practice are described in some detail in a later

section.

Recovery of various elements. The oxidation of aluminum, zirconium, and boron during air melting were

considered of concern for their recovery. However, our work with melting of many experimental heats have

demonstrated that the use of the Exo-Melt” process results in essentially 100% recovery of all alloying

elements except zirconium, which requires a loading of 120% in the melt stock.

Pick-up of elements other than nominal from the commercial melt stock. The commercial grade melt stock

tends to introduce impurities such as carbon and sulfur. A systematic study of a large number of heats to

determine the effect of impurity elements on the weldability has allowed us to set acceptable limits of carbon,

sulfur, silicon, and boron. The pick-up of silicon and iron during the foundry practices is described in issue

2

No. 4 below. Since limited opportunity occurs to remove these impurities during air-induction melting,

carbon and sulfur contents can be kept to a minimum by proper selection of the melt stock. The contents of

carbon and sulfur may also be controlled by starting with a new furnace lining or using pure nickel wash heat

prior to melting the nickel-aluminide alloy.

4. Pick-up of certain elementsfrom the foundry environment. The elements silicon and iron are commonly

picked up when melting nickel aluminide in an iron-based alloy foundry. The silicon is picked up from the

fact that nickel aluminide tends to mct with the sand (SiO,), especially when extra metal is poured into sand

pig molds. If the pig is used in melting revert heats without removing the sand, the aluminum and zirconium

contents of nickel aluminide reduces SiO, to silicon, which is picked up by the alloy. The silicon pick-up

from this source can be reduced by two approaches: (1) to pour the extra metal in Zro, washcoated pig

molds (such a wash eliminates the reaction of molten IC-221M with sand); and (2) if sand is stuck to the pigs,

it must be removed by grit blasting prior to melting the revert.

Another source of silicon pick-up is by the attack of molten nickel aluminide on SiO, in zirconia

crucibles. Silicon pick-up from this source can be lllllllfIllzed by two steps: (1) use A&03 as a furnace

crucible or lining rather than zirconia, and (2) minimize the time that molten IC-221M stays in contact with the

crucible. The contact time can be reduced by proper scheduling of melting and casting of heats so that the

. . .

molten metal does not set in the furnace for long periods waiting for it to be poured.

The iron pick-up can occur in at least two different ways. First, being in an iron foundry, there is

always a chance of a small piece of iron or steel from other heats getting into the nickel-aluminide melt stock.

Pick-up &om this source can be minimized by careful controls in the foundry practice. The second source for

the pick-up of iron is the steel liner that is typically used for setting a new furnace lining. The best method to

reduce such a pick-up is to run a wash heat of nickel after setting the new lining and prior to melting IC-221M.

The pick-up of iron over 1 wt % has the tendency to precipitate the beta phase, which lowers the high-

temperature strength of IC-22 1M.

5 . Melting of foundry revert stock. The foundry revert stock consists of several items: (1) pigs cast from metal

left over after casting all planned molds, (2) runners and risers removed from the castings, and (3) any

defective castings that are beyond weld repairing. Any of these three revert stocks have a potential of adding I

3

impurities to the alloy. The potential of silicon from the pigs has already been described above. The runners

and risers can also add silicon if all of the sand stuck to the castings is not completely removed. The remelting

of the castings can also add iron impurities if some of the steel beads become trapped in the defective area.

Proper removal of any sand from the runners and risers and trapped steel beads can minimize the pick-up of

silicon or iron fkom the foundry revert stock.

6 . Oxidation of molten metal. The zirconium in the IC-221M is the first element to oxidize followed by

aluminum, if molten metal is exposed to air for long periods of time. Such oxidation can be rmrllIz11zed using . . .

an argon cover during &-induction melting. Similarly, the oxidation of molten metal flowing through the

sand molds can be minimized by flushing the molds with argon prior to casting.

MELTING PROCESS

The most significant progress towards commercialization of IC-221M has occurred through the

development of the Exo-Meltm process for melting of IC-221M using the currently available melt technology at

most foundries. The process is based on the fact that the formation of aluminides from their alloying elements is

exothermic [17-191 and uses a furnace-loading sequence to effectively use the heat of formation in the melting

process. The most significant benefits of the Exo-Meltm process for melting IC-221M include:

1. It permits the commercial melting of IC-221M and uses the currently available foundry equipment.

2. It produces reproducible alloy chemistries.

3. For virgin heats, it save 50% energy and 50% time as opposed to the conventional process, which industry

was not willing to try for reasons listed above.

4. The energy and time savings and extended crucible life result in 50% lower cost than the conventional process.

5 . One ORNL licensee has melted over 22,700 kg of the IC-221M alloy in 1996, and two licensees have melted

over 22,700 kg so far in 1997.

6. The use of the Exo-Meltm process has helped accelerate the commercialization process.

4

CASTING PROCESSES

The IC-221M produced by the Exo-Meltm process can be cast into complex shapes by the static sand

mold casting process. It can be cast into tubes and pipe by the centrifugal process. The components of nickel

aluminide can be produced by welding the centrifugal and static castings. The alloy can also be investment cast

into ceramic molds. Examples of sand casting of IC-22 1M are shown in Fig. 1. These castings vary sigruficantly

in section thickness from 6.35 mm for the heat-treating fmture to 203 mm for the forging die. The casting

complexity also increases from a simple shape such as pallet tip to very complex heat-treating fixtures. Not only

can the complex shapes be cast, but they can also be produced with good dimensional control as demonstrated in

Table 2.

PROPERTIES

The physical and mechanical properties and corrosion data on nickel aluminide alloys were compared with

commercial alloys in a recent paper [20]. The characteristic physical and mechanical properties [21 J of the

IC-221M alloy are listed for a range of temperahms in Table 3.

WELDING

The continuing effort at ORNL has demonstrated that the cast IC-221M can be successfully welded for

either the repair of castings or fabrication of components. The gas tungsten arc is the best welding process, and

IC-221LA and IC-221W are the two recommended filler wires. The composition for IC-221LA in weight percent

is 4.5 Al-16.0 Cr-1.2 Mo-1.5 Zr-0.003 B-76.8 Ni, and the composition for IC-221W in weight percent is 8.0 Al-

7.7 Cr-1.4 Mo-3.0 Zr-0.003 B-79.9 Ni. The welding process development has progressed to a point of being

able to successfully weld large-diameter centrifugalca& pipe to sand-cast trunnions. The welding success of such

large components has resulted from optimized weld design, a tight control of base metal chemistry, and the use of

stainless steel backing ring to minimize root pass cracking.

The weldment properties reported elsewhere [22] show that tensile properties are similar to base metal with

somewhat lower ductility and creep rupture strengths, 70% of the base metal.

5

NICKEL-ALUMINIDE APPLICATIONS

The applications of nickel aluminide continue to grow. They range from heat-treating fixtures for

carburizing and oxidizing environments, rolls for steel-hardening furnaces, dies for hot-forging applications, tube

hangers for high-temperature processing furnaces, and components for calcination furnaces. A new application is

developing in the area of radiant burner tubes for furnace heating.

FUTURE PROJECTIONS

The calendar year 1996 has been an excellent year for commercialization of nickel aluminides. Over

22,700 kg of the IC-22 1M alloy were melted and cast into a variety of components. The performance of 1996 has

extended into 1997. This is evidenced by the extension of a single foundry licensed in 1996 to three licensees at

the time of this writing. The current application development activity suggests that the production of IC-221M is

expected to double in 1997 and has the potential to grow to five times during 1998.

CONCLUSIONS

The cast nickel-duminide alloy IC-221M is well on its way to commercialization. This paper has

presented the compositions, melting, casting processes, properties, welding, applications, and future projections.

ACKNOWLEDGMENTS

The authors wish to thank C. Randy Howell for coordinating the testing and data analyses, Robert W.

Swindeman and Craig A. Blue for paper review, and Millie L. Atchley for preparing the manuscript.

REFERENCES

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Materials Research Society, Pittsburgh, 1985.

2. N. S. Stoloff, C. C. Koch, C. T. Liu, and 0. Izumi, eds., High Temperature Ordered Intermetallic Alloys

II,81, Materials Research Society, Pittsburgh, 1987.

3. C. T. Liu, A. I. Taub, N. S. Stoloff, and C. C. Koch,eds., High Temperature Ordered Intermetallic

Alloys 111, 133, Materials Research Society, Pitsburgh, 1989.

4. L. A. Johnson, D. P. Pope, and J. 0. Stiegler, eds., High Temperature Ordered Intermetallic Alloys

IV, 213, Materials Research Society, Pittsburgh, 199 1.

5. I Baker, R. Darolia, J. D. Whittenberger, and M. H. Yoo, eds., High Temperature Ordered Intermetallic

Alloys V, 288, Materials Research Society, Pittsburgh, 1993. L

6. J. A. Horton, I. Baker, S. Hanada, R. D. Noebe, and D. S. Schwartz, eds., High Temperature Ordered

Intermetallic Alloys VI, Materials Research Society, Pittsburgh, 1995.

7. S. H. Whang, C. T. Liu, D. P. Pope, and J. 0. Stiegler, eds., High Temperature Aluminides und

Intermetallics, The Mineral, Metals and Materials Society, Warrendale, 1990.

8. S. H. Whang, D.P. Pope, and C. T. Liu, eds., “High-Temperature Aluminides and Intermetallics,”

Proceedings of the Second TMS/ASM Symposium, Mater. Sci. Eng., A15WA153 (1992) 746.

9. Izumi, O., ed., Intermetallic Compounds - Structure and Mechanical Properties, Japan Institute of Metals,

Tokyo, Japan, 199 1.

10. C. T. Liu, R. W. Cahn, and G. Sauthoff, eds., Ordered Intennekrllics - Physical Metallurgy and

Mechanical Behavior, Kluwer Academic Publishers, Dordrecht, The Netherlands, 1992.

11 . Y. W. Kim and R. R. Boyer, eds., Micro-structurdProperties Relationships in Titanium Aluminides and

Alloys, The Mineral, Metals and Materials Society, Warrendale, 1991.

12. R. Darolia, J. J. Lewandowski, C. T. Liu, P. L. Martin, D. B. Miracle, and M. V. Nathal, eds., Structural

Intermetallics, The Mineral, Metals and Materials Society, Warrendale, 1993.

13. N. S. Stoloff and V. K. Sikka, eds., Physical Metallurgy and Processing of Intermetallic Compounds,

Chapman and Hall, New York, 1995.

14. G. Welsch and P. D. Desai, eds., Oxidation and Corrosion of Intemtallic Alloys, Purdue University,

West Lafayette, 1996.

15. K. Aoki and 0. IzUmi, Nippon Kinzoku Gakkaishi, 43 (1979) 1190.

16. R. L. Fleischer, Intermetallic Compounds - Structure and Mechanical Properties, Proceedings of JIMIS-6,

ed. 0. Izumi (Tokyo: Japan Institute of Metals, 1991), 157-63.

17. C. T. Liu, C. L. White, and J. A. Horton, Acta Metall., 33 (1985) 213.

18. V. K. Sikka, S. C. Deevi, and J. D. Vought, Adv. Mater. Process., 147 (1995) 29.

19. S. C. Deevi and V. K. Sikka, Intermetallics, 96 (1997) 17.

20. S. C. Deevi and V. K. Sikka, Intermetallics, 4 (1996) 357.

2 1. V. K. Sikka, “Commercialization of Nickel and Iron Aluminides,” Nickel and Iron Aluminides: Processing,

Properties, and Applications, eds., S . C . Deevi, P. J. Maziasz, V. K. S u a , and R. W. Cahn

(Materials Park ASM International, 1997), 361-76.

22. M. L. Santella, “An Overview of the Welding of Ni3AI and Fe3AI Alloys,” Nickel and Iron Aluminides:

Processing, Properties, andApplications, eds., S . C . Deevi, P. J. Maziasz, V. K. Sikka, and R. W. Cahn

(Materials Park: ASM International, 1997), 321-28.

FIGURE CAPTION

1. Examples of sand casting of IC-221M alloy.

f

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Table 1. Comparison of nominal chemical analysis of IC-22 1M with the range observed for heats made using virgin and revert stock in a pilot commercial melt run of 94 heats

carried out at Alloy Engineering & Casting Company

Virgin heats (wt %) Revert heats (wt %’.)” Element Nominal

(wt %) Range Average Range Average

Al

Cr

Mo

zr

B

C

Si

Fe

Ni

8.0

7.7

1.43

1.70

0.0080

81.1

7.5 - 8.2

7.63 - 8.11

1.38 - 1.50

1.73 - 2.02

0.004 - 0.008

0.012 - 0.032

0.021 - 0.055

0.03 - 0.15

b

7.86

7.8 1

1.45

1.93

0.0054

0.022

0.036

0.077

80.8 1

7.3 - 8.3

7.56 - 8.5

1.34 - 1.56

1.62 - 2.05

0.003 - 0.008

0.01 - 0.05

0.026 - 0.155

0.03 - 0.91

b

7.74

7.88

1.43

1.86

0.0054

0.024

0.06 1

0.194

80.8 1

“50% virgin and 50% revert. %dance.

... Table 2. Variability of casting dimensions for a pilot commercial run

of 63 sets of IC-221M castings

Component Average k standard deviation Average k standard deviation Oength) (width)

Tray 672.7 k 0.7 672.5 2 1.2

Lower fixture 657.7 k 1.8 657.4 f 1.9

Upper fixture 632.8 f 2.2 632.3 f 2.0

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Table 3. Physical and mechanical properties of Ni,Al-based cast alloy IC-221M

Temperature ("C)

Room 200 400 600 800 900 lo00 1100

200 190 174 160 148 139 126 114

12.77"

11.9

555

770

14

13.08 13.72b 14.33 15.17 15.78 16.57

13.9 16.7 20.3 25.2 27.5 30.2

570 590 610 680 600 400 200

800

14

--

40

-- -- -- -- -- -- -- Density (g/cm3) 7.86 Hardness (R,-) 30 Microhardness 260 270 280 290 280 230 120 -- @Ph) ModuIus (GPA) Mean Coeff. of the& expansion

Thermal conductivity (w/m k) 0.2% Tensile yield strength (Mpa) ultimate tensile strength W a ) Total tensile elongation (%) le h Rupture strength (MPa) Id h Rupture

-- -- -- -- - -- --

(lOd/'C)

strength W a ) lo4 h Rupture -- --

C h a r P Y impact 40 35 strength (MPa)

toughness (J) Fatigue 106 cycle -- 63@ life (MPa) Fatigue lo7 cycle -- -- -- 550 life (MPa)

-- -- -- --

850 850 820 675 500 200

17 18

-_

--

40

5 5 7

252 124 55

172 83 36

124 55 24

15 10

lo

28

18

11

--

"Room temperature to 100°C. Qmm temperature to specified temperature. ?>ah at 650°C for investment-cast test bars.