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A
coherent boundary is one for which atomic positions on either side of the
boundary are correlated. This is illustrated for a coherent precipitate
(yellow) in the top diagram. The lower diagram shows a similar precipitate,
but with an incoherent boundary. Atoms in the matrix and the lattice are
no longer in register across the interface.
If
the atomic spacing in the two regions is different, a coherent boundary
will create strain fields in the lattice at the interface. The stresses
due to this condition will hamper the movement of dislocations in the material
and increase its yield stress. This occurs in precipitation hardened alloy
such as the Cu-Al binary alloy where Guinier-Preston zones and q'' precipitates
have incoherent boundaries with the (Al)-matrix. |
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