The crystallography of cleavage fracture in Al 3 Sc
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The intermetallic compound Al 3 Sc is a trialuminide with the L l 2 structure, which deforms easily in compression at room temperature, with yield stresses around 100 MPa. As shown by single crystal compression experiments, slip occurs on {111} planes. In tension Al 3 Sc fractures transgranularly in a brittle manner. The predominant cleavage plane is {Oil}. In a scanning electron microscope (SEM) numerous cleavage steps, which are aligned in three major crystallographic directions, are observed. The fracture surfaces almost always show microscopic waviness along the traces of intersecting slip planes. Regions that are flat within the experimental resolution of the SEM are only occasionally observed. Some of the cleavage steps consist of {111} or {001} planes, but others are not distinctly crystallographic. Plastic deformation involving dislocation motion or twinning may have occurred at these steps. Reactions among different types of steps are also observed. One type of cleavage pattern found is strikingly similar to the typical appearance of fracture surfaces of fee or fee related materials after stress corrosion cracking. However, this particular pattern is only rarely observed in Al 3 Sc and can therefore not be taken as evidence for stress corrosion cracking in this material. Our observations indicate that an interpretation of cleavage fracture in Al 3 Sc in terms of surface energies alone is unlikely to be successful. Similarly, any criterion that categorizes its fracture behavior into either fully brittle or fully ductile is faced with difficulties. A full understanding of the fracture morphology of Al 3 Sc will therefore require detailed atomistic simulations.
I. INTRODUCTION Intermetallic compounds based on trialuminides (e.g., Al3Ti, Al3Zr, Al 3 Nb, and Al 3 Sc) are of interest owing to their high melting points in the 1600 to 1850 K range, their low densities (as low as 3 Mg/m 3 ), and their potential oxidation resistance.1"6 One disadvantage of Al3Ti and Al3Zr is their tetragonal structures (D022 and D0 23 , respectively). However, by substitution of approximately 7 at. % of the aluminum by elements like V, Cr, Mn, Fe, Co, Ni, Cu, and Zn, cubic ordered L l 2 structures can be produced.24"8 In contrast to L l 2 compounds like Cu3Au or Ni3Al, trialuminides are brittle in tension.4 Whereas brittleness in some other L l 2 systems (e.g., Pt 3 Al 9 ) is plausible in view of their high yield stresses, the brittleness of trialuminides is more difficult to understand. Al 3 Sc, in particular, has a lower yield stress than Ni 3 Al, yet it is brittle in spite of its cubic structure.5 It tends to cleave on {011} planes, and not on {111} planes, which generally exhibit the lowest surface energy in fee materials. Owing to the brittleness of trialuminides, it has only recently been possible to demonstrate a small amount of ductility in bending experiments. Zhang et al.1 were the first to demonstrate some ductility in hot-isostatically pressed Al-8Cr-25Ti (at. %). Schneibel et al.10 verified 868
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