Brittle Laves phases

There are several dozen metallic AB2 compounds called Laves phases that are superconducting they have either cubic or hexagonal crystal structures. Some have critical temperatures above 10 K and high upper critical magnetic fields Bc2- For example, Zri/2Hfi/2V2 has rc = 10.1K, B 2 = 24 T, and a compound with a different Zr/Hf ratio has similar and Bc2 values with the critical current density Jc 4 X 10 A/cm. These materials also have the advantage of not being as hard and brittle as some other intermetallics and alloys with comparable transition temperatures. 

Up to now the fracture toughness at room temperature is 2-4 MNm (see data in Sauthoff, 1990 a, b) for these alloys, and the brittle-to-ductile transition temperature is between about 500 °C and 700 °C depending on composition and preparation method. Analysis of the microstructures of the deformed alloys has shown that in the brittle-to-ductile transition range cracks in the Laves phase are stopped at the Laves phase-NiAl interface by producing a plastic zone in the NiAl phase ahead of the crack (Machon, 1992 Wunderlich etal., 1992). 

Since the transition metal elements in these ternary Laves phases can substitute for each other freely, and since the distribution of Laves phase can be controlled by thermomechanical treatments there are possibilities for optimizing such NiAl alloys with Laves phases with respect to creep resistance and the brittle-to-ductile transition temperature (BDTT) by controlling the composition and phase distribution, and this is being studied presently... 

Figure 33. Brittle-to-ductile transition temperature BDTT as a function of strain rate in bending (referred to the specimen edge) for various two-phase NiAl-(Ta,Nb)NiAl alloys with 23.5 vol.% Laves phase Tal0Ni45A145 (x), Ta9NblNi45A145 (-I-), Ta7.5Nb2.5Ni45A145 (o), TalNb9Ni45A145 ( ), and Nbl0Ni45A145 ( ) (Sauthoff, 1991a Zeumer etal., 1991 Zeumer and Sauthoff, 1992).

Ti(Al,Cr)2, and the y phase (Brady et al, 1997). The presence of the Laves phase stabilised the protective AI2O3 scale. However, the Laves phase is brittle, so its influence on the mechanical properties should be carefully examined. If this phase degrades the mechanical properties, it can only be used as a coating. 

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