Mechanisms of ductility improvement in L1 2 compounds
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I. INTRODUCTION Material properties of the intermetallic compounds are regarded as lying in an intermediate position and having widely distributed spectra between metals and ceramics. A number of intermetallic compounds have significant characteristics regarding the mechanical properties of stiffness and strength. For example, some intermetallic compounds show an increase of yield strength with increasing temperature, accompanied with good structural and chemical stabilities. Also, the restricted atomic mobility produced from the lattice constraint of the ordered structure brings slower rates of all metallurgical processes like recrystallization, creep, and corrosion. These properties give significant advantages for use at high temperatures. In addition, many intermetallic compounds combined with aluminum, titanium, and silicon, i.e., aluminide, titanide, and silicide have low densities. This character is very useful in rotating and aerospace applications. However, it has been known for a long time that the intermetallic compounds are usually very brittle, particularly at low temperatures. Therefore, they are difficult to fabricate into useful shapes and have not seen practical uses. The brittleness of the intermetallic compounds is caused by strong tendencies to cleave in grain interiors or at grain boundaries. The former is attributed to the usual large unit cell in these systems, i.e., low symmetry compared with ordinary metals and alloys, which produces a higher Peierls stress, resulting in a lack of operative slip systems. The latter, which appears in many Ll 2 -type intermetallic compounds, is associated with an inherently lower cohesive strength at grain boundaries in intermetallic compounds. Among a number of crystal structures reported for the intermetallic compounds,1 two are particularly simple: the fcc-derivative Ll 2 structure with composition 426
J. Mater. Res. 3 (3), May/Jun 1988
A3B, in which A atoms occupy the face center sites and B atoms occupy the corner sites, and the bcc-derivative B2 structure with composition AB. Examples of fee are Ni3Al and Cu3Au. In Ll 2 -type intermetallic compounds, the atomistic structure and derivative properties of zero-dimensional defects (such as vacancy, interstitial atom, and nonstoichiometric atom), and of one-dimensional defects (such as dislocation) have been vigorously studied for the last few decades2^1 since these defects have been recognized to be uniquely responsible for their mechanical behavior. Their simple crystal structures suggest they may be ductile. Although some of these intermetallic compounds, for example, Cu3Au are ductile in the form of polycrystal as well as single crystals, many polycrystals are very brittle at ambient temperatures. Most of LI 2-type intermetallic compounds, however, are highly ductile in the single-crystal form. With regard to the brittleness of the poly crystalline Ll 2 -type intermetallic compounds, two mechanisms have been demonstrated: that due to the segregation of impurities to grain boundaries,5 which has been tra
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