Dear Veerendra,

The dislocation movement in metals having FCC crystalline structure is less dependant than in those having BCC crystalline structure. In metals having FCC crystalline structure, the dislocation movement at low temperatures is high enough to maintain a ductile behaviour. Therefore, FCC crystalline structure metals can be used for cryogenic applications as they will not have a brittle failure.

The FCC materials have ductile to brittle transiton temperature ( DBTT)  more lower than the other materials such as BCC materials.Therefore FCC materials don't be brittle in temperature conditions that we work it.  

The cryogenic treatment induces compressive stresses in the material. Due to this material becomes brittle. The materials with face centred cubic (FCC) or Hexagonal lattice have more slip planes than BCC structure. The added compressive stresses causes plastic deformation at cryogenic temperature and the material becomes brittle with time. Due to the more slip planes, the FCC structural material exhibit more deformation for the reliving stresses as compared to BCC structure. i.e. the BCC structural material will easily become brittle as compared to FCC structural material. Hence to avoid brittle failure; FCC structural material is used for the Cryogenic Dewar.