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Effects of Microalloying on the Microstructures and Mechanical Properties of Directionally Solidified Ni-33(at.%)Al-31Cr-3Mo Eutectic Alloys Investigated

Optical (a-h) and scanning electron transverse microstructures (i) of directionally solidified Ni-33Al-(31-x)Cr-3Mo-xX alloys
Optical (a-h) and scanning electron transverse microstructures (i) of directionally solidified Ni-33Al-(31-x)Cr-3Mo-xX alloys. (a) 0.25(at.%)Cu. (b) 0.25 Nb. (c) 0.25 Re. (d) 1.0 Re. (e) 0.5 Mn. (f) 1.0 Si. (g) 1.0 Ti. (h) 1.0 Ta. (i) 1.0 Hf. In a to h, the light phase is Cr and Mo, and the dark phase is NiAl. In (i), the white phase is Hf-rich.
Long description

Despite nickel aluminide (NiAl) alloys' attractive combination of oxidation and thermophysical properties, their development as replacements for superalloy airfoils in gas turbine engines has been largely limited by difficulties in developing alloys with an optimum combination of elevated-temperature creep resistance and room-temperature fracture toughness. Although single-crystal and polycrystalline NiAl alloys with superior specific creep strengths, comparable to or better than advanced superalloys, were developed by a combination of alloying and innovative processing techniques in the mid-1980's to mid-1990's, these materials had poor room-temperature fracture toughness, restricting their induction into service.

Alternatively, research has focused on developing directionally solidified NiAl-based in situ eutectic composites composed of NiAl and (Cr,Mo) phases in order to obtain a desirable combination of properties (refs. 1 to 4). Recently, it was demonstrated that the room-temperature fracture toughness KIC of the directionally solidified Ni-33(at.%)Al-31Cr-3Mo two-phase eutectic alloy is about 17 MPa (ref. 5). This is a considerable improvement over that of NiAl, for which KIC ~ 6 MPa. However, the elevated-temperature strength of this directionally solidified eutectic alloy is still less than that of advanced nickel-based superalloys.

Graphs of true compressive strain versus true compressive stress
Comparison of the true compressive stress - true strain rate behavior of directionally solidified Ni-33Al-31Cr-3Mo with those for eutectic alloys grown at 12.7 mm/h between 1200 and 1400 K. The open symbols represent data from constant velocity testing, whereas the solid symbols indicate constant load creep results. Left: Ni-33Al-30.5Cr-3Mo-0.5Hf. Right: Ni-33Al-30.75Cr-3Mo-0.25Ti.
Long description

A systematic investigation was undertaken at the NASA Glenn Research Center to examine the effects of small additions of 11 alloying elements (Co, Cu, Fe, Hf, Mn, Nb, Re, Si, Ta, Ti, and Zr) in amounts varying from 0.25 to 1.0 at.% on the elevated-temperature strength and room-temperature fracture toughness of directionally solidified Ni-33Al-31 Cr-3Mo eutectic alloy. The alloys were grown at 12.7 mm/hr, where the unalloyed eutectic base alloy exhibited a planar eutectic microstructure (ref. 4). The different microstructures that formed because of these fifth-element additions are included in the table. The additions of these elements even in small amounts resulted in the formation of cellular microstructures, and in some cases, dendrites and third phases were observed (see the preceding photomicrographs). Most of these elemental additions did not improve either the elevated-temperature strength or the room-temperature fracture toughness over that of the base alloy. However, small improvements in the compression strength were observed between 1200 and 1400 K when 0.5 at.% Hf and 0.25 at.% Ti were added to the base alloy (see the graphs). The results of this study suggest that the microalloying of Ni-33Al-31Cr-3Mo will not significantly improve either its elevated-temperature strength or its room-temperature fracture toughness. Thus, any improvements in these properties must be acquired by changing the processing conditions.

DESCRIPTION OF THE TRANSVERSE MICROSTRUCTURE IN ALIGNED REGIONS OF THE DS Ni-33Al-(31-x)Cr-3Mo-xX ALLOYS
Intended
fifth
element,
at.%		 Lamellar
eutectic
grains		 Cells Cell
diameter,
mm		 Cell
pattern		 NiAl
dendrites		 Intercellular
regions		 Globular
NiAl in
interdendritic
regions		 Distribution of fifth element
NiAl (Cr, Mo) Third
phase
0.25Co Yes --- --- ------ Yes --- --- Yes --- ---
0.25Cu --- Yes 350 Radial No Triple points --- --- --- ---
0.5Cu --- Yes 400 Radial No Yes --- --- --- ---
1.0Cu --- Yes 250 Radial No Yes Yes Yes --- ---
0.25Fe Yes --- --- ------ No --- --- --- --- ---
0.5Fe Yes --- --- ------ Yes --- --- --- --- ---
1.0Fe Yes --- --- ------ Yes --- --- Yes Yes ---
0.25Hf --- Yes 200 Radial Yes Yes Yes --- --- ---
0.5Hf --- Yes 250 Radial No Yes Yes --- --- ---
1.0Hf --- Yes 200 Radial No Yes Yes Yes --- Yes
0.25Mn Yes --- --- ------ No --- --- --- --- ---
0.5Mn Yes --- --- ------ No --- --- --- --- ---
1.0Mn --- Yes 400 Radial No Yes --- Not distinct --- ---
0.25Nb --- Yes 200 Straight Yes Yes Some --- --- ---
0.5Nb --- Yes 100 Straight Yes Yes Yes --- --- ---
1.0Nb --- A few 100 Not distinct Yes Yes Yes --- Yes Yes
0.25Re --- Yes 200 Nautilus No Not distinct --- --- --- ---
0.5Re --- Yes 150 Shell No Yes Yes --- --- ---
1.0Re --- Yes 100 Shell No Yes Yes --- Yes ---
0.25Si Yes --- --- Straight Yes --- --- --- --- ---
0.5Si --- Yes --- Straight Yes Yes --- --- --- ---
1.0Si --- Yes --- Straight Yes Yes --- --- Yes ---
0.25Ta --- Yes 300 Straight Yes Yes Yes --- --- ---
0.5Ta Yes 200 Straight Yes Yes Yes --- --- ---
1.0Ta --- Yes 150 Straight Yes Yes Yes --- Yes Yes
0.25Ti --- Yes 300 Radial No Yes --- --- --- ---
0.5Ti --- Yes 300 Radial No Yes --- --- --- ---
1.0Ti --- Yes 300 Radial No Yes Yes Yes --- ---
0.25Zr --- Yes 100 Shell No Yes Yes --- --- ---
0.5Zr --- Yes 100 Shell No Yes Yes --- Yes Yes

References

  1. Walter, J.L.; and Cline, H.E.: The Effect of Solidification Rate on Structure and High-Temperature Strength of the Eutectic NiAl-Cr. Metall. Trans . , vol. 1, no. 5, 1970, pp. 1221-1229.
  2. Yang, J.-M.: The Mechanical Behavior of In-Situ NiAl-Refractory Metal Composites. JOM, vol. 49, no. 8, 1997, pp. 40-43.
  3. Raj, S.V.; Locci I.E.; and Whitten-berger, J.D.: Creep Behavior of Advanced Materials for the 21st Century. Proceedings of Creep Behavior Advanced Materials for the 21st Century Symposium, R.S. Mishra, A.K. Mukherjee, and K. Linga Murty, eds., TMS, Warrendale, PA, 1999, pp. 295-310.
  4. Whittenberger, J.D., et al.: Effect of Growth Rate on Elevated Temperature Plastic Flow and Room Temperature Fracture Toughness of Directionally Solidified NiAl-31Cr-3Mo. Intermetallics, vol. 7, no. 10, 1999, pp. 1159-1168.
  5. Raj, S.V., et al.: Effect of Directionally-Solidified Microstructures on the Room Temperature Fracture Toughness Properties of Ni-33(at.%) Al-33Cr-1Mo and Ni-33(at.%) Al-31Cr-3Mo Eutectic Alloys Grown at Different Solidification Rates. Metall. Mater. Trans., vol. 33A, in press, 2002.

Glenn contact: Dr. Sai V. Raj, 216-433-8195, Sai.V.Raj@grc.nasa.gov
Authors: Dr. J. Daniel Whittenberger, Dr. Sai V. Raj, Dr. Ivan E. Locci, and Dr. Jonathan A. Salem
Headquarters program office: OAT
Programs/Projects: HITEMP, HOTPC

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Last updated: June 2002

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