The ordered structure NiAl has been investigated as a high-temperature gas-turbine material; however, brittle fracture is a problem with this alloy. Theoretical calculations have determined that the enthalpy of the ordered NiAl structure is 0.15 eV per atom less than for a disordered structure. The NiAl ordered structure is a cube with Ni at 0,0,0 and Al at 1/2,1/2,1/2.
(a) What is the Bravais lattice type for NiAl?
(b) What is the GFE change if this alloy disorders at 1900 K?
(c) Based upon your result for (b), predict if NiAl is ordered or disordered at 1900 K.
(d) From the Ni-Al phase diagram in Figure 5.15, to what temperature does the NiAl phase remain ordered?
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