Electronic Structure and Mechanical Properties of Intermetallics: Apb Energies in Ni-Al-Based Systems
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ELECTRONIC STRUCTURE AND MECHANICAL PROPERTIES OF INTERMETALLICS: APB ENERGIES IN Ni-Al-BASED SYSTEMS TAO HONG AND A.J. FREEMAN Department of Physics and Astronomy, Northwestern University, Evanston, 60208
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ABSTRACT The possible origin of the high degree of brittleness (i.e., low ductility) of the Ni-Al-based alloys in the B2 structures is investigated by means of all-electron self-consistent total energy LMTO calculations. Using a supercell approach, the energetics of the two simplest anti-phase boundaries (APB) for NiAl in the B2 structure - namely the ! on (110] and ; on (112] - are calculated for the first time assuming no 2 relaxation at the interface. We find APB energies of order of 800 ergs/cm for both cases. Since the calculated APB energies are very high, slip is hardly likely to occur - as suggested experimentally. By substituting Ni or Al with V, Cr or Mn at the APB interface plane, remarkably decreased APB energies are obtained. These first results on these simplified model systems may suggest a way to decrease the APB energy contribution to the ductility of NiAl-based alloys.
INTRODUCTION The intermetallic compound NiAl has been studied intensively because of its
potential aerospace applications at high temperatures: it
has a high
melting point, high ordering energy, good oxidation resistance and is relatively light [1-3). However, its intrinsic brittleness at room temperature has been an unsolved problem. Deformation experiments have shown that the nature of slip is (110) in [4] NiAl, which implies that only 3 independent slip systems exist. As the independent slip systems are orthogonal, there are no cross slips and therefore, von Mises criterion for ductility is not satisfied I5]. Since the Burgers vectors In the disordered bcc structure are - it is quite reasonable to believe that the 2 r I type slip was shown by an alloying ductility might be improved if i process. Although some mixed results have been obtained [6,7] with the addition of Cr or Mn, the chance of achieving positive results by adding other elements has not been excluded. As a first step, we have calculated anti-phase boundary (APB) energies for two most probable APB's, namely, ; on (110] and -'on (112] using the linear muffin-tin orbital [8) (LMTO) method, which is one of the most accurate all-electron first principle methods based on local density functional (LDF) theory. For simplicity, we considered three smallest APB cells with 4, 6, 8 and 10 layers of (110) and (112) planes, corresponding to unit cells with 8, 12, 16, and 20 atoms, respectively. In these first calculations, no relaxation was alloyed at the APB interface. Figure I shows such a structure for the 8 layer case. The cells for i[{110] calculations are orthorhombic, while the cells for the j{112] calculations are monoclinic. As discussed in more detail later, the quite different nearest neighbors and second nearest neighbors in the 2ciii>(i10], [(i12] APB's and in the original B2 cell makes the lattices behave differently. In Section II, the APB results for pu
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