An interdiffusion study of a NiAl alloy using single-phase diffusion couples
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I. INTRODUCTION
DUE to their high strength and high melting temperatures, ordered intermetallic compounds have been explored as structural materials for applications at high temperatures. One such class of intermetallic compounds being considered is the transition-metal aluminides, such as NiAl and FeAl. Both compounds are resistant to high-temperature oxidation and hot corrosion. For example, NiAl has been used for decades as a protective coating for nickel-based superalloys. More recently, a high-pressure turbine vane made of NiAl was successfully engine-tested.[1] In spite of the increasing interest in, and attractiveness of, the transition-metal aluminides, there is little understanding of the structure-property relationships, nor is there a knowledge base concerning the processing of these materials. Yoo et al.[2] have pointed out that there is an urgent need for fundamental research in the area of the diffusion behavior of ordered intermetallic compounds such as these aluminides. The transition-metal aluminide NiAl exists over a wide range of homogeneities and exhibits an ordered bcc (or B2) structure, consisting of Ni atoms on one of the two sublattices occupying the [0,0,0] position, referred to as the a sublattice, and Al atoms on the other sublattice [1/2, 1/2, 1/2] position, referred to as the b sublattice. In contrast to ionic compounds, which exist only at the stoichiometric composition, NiAl exists over a large range of homogeneities, as a result of forming constitutional vacancies on the a sublattice, with an excess of Al over stoichiometry, and of forming antisite Ni atoms on the b sublattice, with an excess of Ni.[3] Accordingly, its phase boundary extends from 28 at. pct Al at 1381 8C to 58 at. pct Al at 1133 8C.[4] The dominant intrinsic defects in NiAl are the triple defects, consisting of two vacancies created on the a sublattices for each antisite Ni atom on the b sublattice. Extensive data exist in the literature, SUNGTAE KIM, formerly Graduate Student, is Research Associate, Department of Materials Science and Engineering, University of WisconsinMadison. Y. AUSTIN CHANG, Wisconsin Distinguished Professor, is with the Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706. Manuscript submitted August 24, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
as given in the review by Chang and Neumann,[3] to show that NiAl is highly ordered even at 1000 8C. The morerecent results of Xiao and Baker[5] and Kogachi et al.[6] are consistent with the literature data. The lattice parameter of NiAl increases linearly with Al concentration until the stoichiometric composition is reached, and then decreases abruptly with an additional increase of the Al composition. According to Henig and Lukas,[7] the enthalpy of formation of NiAl has a minimum at the stoichiometric composition, as shown in Figure 1. This is not surprising, since the number of strong Ni-Al bonds is the highest at the stoichiometric composition. This minimum in the enthalpy of formation suggest
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