Effect of directionally solidified microstructures on the room-temperature fracture-toughness properties of Ni-33(at. pc
- PDF / 5,435,926 Bytes
- 16 Pages / 612 x 792 pts (letter) Page_size
- 104 Downloads / 178 Views
DUCTION
DESPITE an attractive combination of oxidation and thermo-physical properties,[2] the development of NiAl alloys as replacements for superalloys in turbine airfoil applications has been largely limited by the difficulty in designing alloys with an optimum combination of high elevated-temperature creep properties and room-temperature fracture toughness. Although single crystal and polycrystalline alloys with superior specific creep strengths comparable or better than advanced superalloys were developed by a combination of alloying and innovative processing techniques in the mid 1980s to the mid 1990s,[3,4] these materials possessed poor room-temperature fracture toughness, which has restricted their induction into service. More recently, research has focused on developing directionally solidified (DS) NiAl in situ eutectic composites in order to obtain this desirable combination of properties.[5–17] In particular, the NiAl/(Cr,Mo) eutectic system consisting of the NiAl and the (Cr,Mo) phases has been extensively studied following the earlier research by Walter et al.[18–20] The latter studies revealed that the addition of Mo, varying between 0.7 to 6 (at. pct),* transforms [1]
*Unless otherwise mentioned, all compositions are reported in atomic percent in this article.
the rodlike eutectic microstructure to lamellar plates. Conventionally, it has been assumed that these in situ composites require the two phases to be grown parallel to
S.V. RAJ, Materials Research Engineer, and J.A. SALEM, Materials Research Engineer, are with the NASA Glenn Research Center at Lewis Field, Cleveland, OH 44135. I.E. LOCCI, Principal Researcher, is with Case Western Reserve University at NASA Glenn Research Center, Cleveland, OH 44135. R.J. PAWLIK, Research Associate, is with the Ohio Aerospace Institute, Cleveland, OH 44135. Manuscript submitted April 6, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
each other and to the growth direction, as in a planar eutectic microstructure, in order to maximize the mechanical strength in the longitudinal direction. However, planar eutectic microstructures develop in the NiAl/(Cr,Mo) system only when the solidification rate, VI , is typically less than 25 mm h⫺1 for Mo ⬍ 6 at. pct.[15,17,19] These rates are generally much too slow for economic commercial production of any alloy even if it is demonstrated to possess all the requisite properties required for turbine airfoil applications. However, solidification at faster rates leads to the development of cellular eutectic microstructures, where the (Cr,Mo) and the NiAl plates emanate radially from the cell interior to the cell boundaries, at least in the range 20 ⱕ VI ⱕ 508 mm h⫺1.[15–17] Cellular microstructures are also likely to form in the unalloyed NiAl/(Cr,Mo) material, even with small amounts of alloying elements varying between 0.25 and 1 pct, when VI is sufficiently slow to produce a planar eutectic microstructure.[21] Clearly, it would be technologically relevant to study the mechanical properties of these cellular microstruct
Data Loading...
