Brittle cleavage of L1 2 trialuminides
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(Received 5 January 1990; accepted 10 April 1990)
Three trialuminide alloys, binary Al-25Sc, ternary Al-25Zr-6Fe, and quaternary Al-23Ti-6Fe-5V, all having the cubic Ll 2 structure, were investigated. All three alloys fracture in a brittle manner (fracture toughness, 2-3 MPa m1/2), predominantly by transgranular cleavage. Of nineteen cleavage facets examined in binary Al3Sc, seventeen were of the {110} type and only two were of the {100} type, consistent with our earlier work which showed that the cleavage plane occurring most frequently in quaternary Al-23Ti-6Fe-5V is also {110}. The room-temperature hardnesses and yield strengths (100-200 DPH and 100-270 MPa, respectively) of all three alloys are quite low (comparable to ductile Ll 2 alloys like Ni3Al), indicating that there is significant dislocation activity in these materials. Consistent with this, transmission electron microscopy identified several APB-coupled dislocations with b = a/2(110) gliding on the {111} planes in both binary Al-25Sc and quaternary Al-23Ti-6Fe-5V. The separations between the superpartials in Al-25Sc and Al-23Ti-6Fe-5V were measured to be 3.7 and 4 nm, respectively, giving APB energies of 313 and 274 mJ/mz, respectively. Auger analyses failed to detect any impurities on the cleavage facets themselves, or on second phase particles (or other potential cleavage crack nucleation sites). It is therefore concluded that brittle fracture in these alloys is not impurity-induced. Based on all the results obtained to date we conclude that the unusual brittleness of Ll 2 trialuminides is related to their intrinsically low cleavage strength. Possible reasons for their low cleavage strength are discussed.
I. INTRODUCTION
Recently, there has been considerable interest in trialuminides of the type A13X, where X is a transitionmetal element from groups IV-V of the periodic table (Ti, Zr, Nb, V, etc.),1 19 because of their potential for use as high-temperature structural materials. Trialuminides have several attractive properties for elevated temperature applications. Their high aluminum contents generally result in the formation of protective oxide layers, providing them with good oxidation resistance. Their densities are usually quite low, especially if one of the constituent atoms is a light element like titanium. In addition, many trialuminides have relatively high melting points, so that they are likely to have high specific strengths and moduli. However, they are as a class very brittle and, unless ways can be found to overcome their brittleness, they are unlikely to find extensive engineering application. In general, those trialuminides that have interesting physical properties (e.g., Al3Ti, Al3Zr, and Al3Nb) also have noncubic crystal structures.20 Therefore, it is not surprising that one of the approaches being tried to ductilize trialuminides is macroalloying to induce transformation to a cubic crystal structure, with the hope that the increased number of potential slip systems in the cubic structure will enable the alloys to deform
plastic
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