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Mechanics of crack deflection at a twist grain boundary

  Heterogeneous brittle solids such as ceramics, lamellar intermetallics, and  olycrystalline hexagonal-close-packed (hcp) metals such as Zr, Zn and Cd are   echnologically important and broadly used. Zirconium, for example, has a low  bsorption cross section for neutrons, and is therefore used in nuclear energy pplications. Titanium aluminide (TiAl) is a candidate material for many
structural applications, and TiAl alloys have the potential to replace nickel-based uperalloys in some sections of jet engines. The most common mode of failure in ll these materials is cleavage cracking along weak planes or interfaces in the
solid. The low fracture toughness associated with cleavage cracking is a major
concern in practical applications, and there is currently great interest in finding
ways to optimize their microstructure so as to make them resistant to fracture.

In this project, the fracture toughening mechanism by twisted grain boundaries
(GBs) are investigated. Fig. A shows the diagram of two grains adjoined by a
twisted GB. As a crack front propagates to the GB, it may (a) deflect along the
GB, and (b) penetrate into the second grain by initiate multiple cleavage cracks
with certain space between each one. This work reveals the interaction of GB
fracture (GBF) and cleavage fracture (CF) at several twisted angles and different
toughness ratio in the GB (àŽ“gb) and in cleavage planes (àŽ“0).

Our investigation predicts (i) a critical toughness ratio between the GB and the
cleavage planes for the crack to propagate into the adjacent grain; (ii) an array of
cracks in the GB and the twisted grain; (iii) the macroscopic mode I toughness of
the solid as a function of crack length; and (iv) the influence of GB toughness
and twist misorientation on the effective toughness of the solid (àŽ“eff/àŽ“ef). Fig. B
gives then normalized effective toughness versus effective crack length (ae/w) for
a crack crossing a 14 degree twist GB, for several values of normalized GB
strength. If the GB strength is infinite, the effective toughness is unbounded.