Cavitation erosion of NiAl
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I.
INTRODUCTION
RECENT reviews of metallic, polymeric, ceramic, and composite materials[1,2,3] have shown that the intermetallic compounds having a B2 (cP2 in Pearson symbol) structure have a high resistance to cavitation erosion. Although the B2 phase of the well-known NiTi shape-memory alloy has excellent cavitation erosion resistance,[3] this phase remains in equilibrium only above 630 7C. Therefore, special processing is needed to retain the B2 phase of NiTi at room temperature in order to take advantage of its cavitation resistance.[1] The martensitic form of NiTi, which is the structure in equilibrium at room temperature, has a somewhat lower resistance to cavitation erosion.[3] However, the intermetallic compound NiAl exists as a stable B2 phase at lower temperatures over a range of stoichiometry between 45 and 59 pct at. pct Ni. The phase diagram of the Al-Ni binary system is shown in Figure 1. A schematic diagram of the ordered NiAl lattice is shown in Figure 2. The ordered B2 structure remains stable at high temperatures over a wide composition range between 44 and 69 at. pct Ni (Figure 1). For nickel-rich compositions, the excess Ni atoms distribute themselves randomly on the aluminum sublattice[5] of the ordered B2 structure. Upon cooling, some of these Ni rich alloys (having the B2 structure at high temperatures) undergo phase transformations at lower temperatures. The transformation products vary depending upon the time and the temperature involved in bringing about the phase transformation. The diagram shown in Figure 1 represents the phases and boundaries reasonably accurately for the equilibrium conditions. Refinements to these boundaries have been made by Khadkikar et al.[6] The available information suggests that NiAl (B2 structure) remains the equilibrium phase at room temperature for alloys containing up to 59 at. pct Ni. The equilibrium phases at room temperature involving higher Ni contents are as follows: a mixture of NiAl 1 Ni5Al3 beA. AKHTAR, Director of Materials Engineering, Powertech Labs Inc., Surrey, BC, Canada V3W 7R7, is Adjunct Professor, Metals and Materials Engineering Department, University of British Columbia, BC, Canada. R. SALVI, formerly Graduate Student, Metals and Materials Engineering Department, University of British Columbia, is with Soligen Technologies Inc., Northridge, CA. V.K. SIKKA, Leader, Materials Processing Group, is with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN. Manuscript submitted January 21, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS A
tween 59 and 63 at. pct Ni, Ni5Al3 between 63 and 67 at. pct Ni, and Ni2Al in a narrow range around 66.7 at. pct Ni. Martensitic transformation is known to occur in nickelrich NiAl alloys.[6–17] Thermally induced martensite, having a tetragonal L1o ordered structure, occurs in alloys containing nickel in excess of 63 at. pct.[7,8] At nickel contents lower than 63 at. pct, the high-temperature B2 transforms into the ordered 14M martensitic structure[9,10] (formerly referred to as 7R
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