Title page for ETD etd-272172849731471

Type of Document

Dissertation

Author

Mutasa, Batsirai M.

Author's Email Address

batsirai@vision.mse.vt.edu

URN

etd-272172849731471

Title

Defect Structures in Ordered Intermetallics; Grain Boundaries and Surfaces in FeAl, NiAl, CoAl and TiAl.

Degree

PhD

Department

Materials Science and Engineering

Advisory Committee

Advisor Name	Title
Aning, Alexander O.
Kampe, Stephen L.
Kriz, Ronald D.
Mishin, Yuri M.
Reynolds, William T. Jr.
Farkas, Diana	Committee Chair

Keywords

NiAl
CoAl
FeAl
grain boundaries
surfaces
point defects
intermetallics
ductility
atomistic simulations
TiAl

Date of Defense

1997-05-16

Availability

unrestricted

Abstract

Ordered intermetallics based on transition

metal aluminides have been proposed as

structural materials for advanced aerospace

applications. The development of these

materials, which have the advantages of low

density and high operating temperatures, have

been focused on the aluminides of titanium,

nickel and iron. Though these materials exhibit

attractive properties at elevated temperatures,

their utilization is limited due to their

propensity for low temperature fracture and

susceptibility to decreased ductility due to

environmental effects. A major embrittlement

mechanism at ambient temperatures in these

aluminides has been by the loss of cohesive

strength at the interfaces (intergranular

failure). This study focuses on this mechanism

of failure, by undertaking a systematic study

of the energies and structures of specific grain

boundaries in some of these compounds. The

relaxed atomistic grain boundary structures in

B2 aluminides, FeAl, NiAl and CoAl and

L1₀ g-TiAl were investigated

using molecular statics and embedded atom

potentials in order to explore general trends

for a series of B2 compounds as well as TiAl.

The potentials used correctly predict the

proper mechanism of compositional disorder

of these compounds. Using these potentials,

point defects, free surface energies and

various grain boundary structures of similar

energies in three B2 compounds, FeAl, NiAl

and CoAl were studied. These B2 alloys

exhibited increasing anti-phase boundary

energies respectively. The misorientations

chosen for detailed study correspond to the

S5(310) and S5(210)

boundaries. These boundaries were

investigated with consideration given to

possible variations in the local chemical

composition. The effects of both boundary

stoichiometry and bulk stoichiometry on grain

boundary energetics were also considered.

Defect energies were calculated for

boundaries contained in both stoichiometric

and off-stoichiometric bulk. The surface

energies for these aluminides were also

calculated so that trends concerning the

cohesive energy of the boundaries could be

studied. The implications of stoichiometry, the

multiplicity of the boundary structures and

possible transformations between them for

grain boundary brittleness are also discussed.

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