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It reads,"Hardening due to coherent precipitate".Firstly,what has hardening to do with coherence of the precipitate?Why would dislocations shear the precipitate?

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a coherent precipitate is one in which the precipitate exists as a separated phase embedded in the matrix of the material being hardened- as opposed to being dispersed as a solute within the atomic lattice.

For a dislocation to travel past a coherent precipitate particle which is in the movement plane of the dislocation it must propagate the same displacements through the precipitate particle as it did within the matrix lattice. If it cannot shear the particle as illustrated in the picture above, then the dislocation is pinned, its movement is halted, plastic deformation is inhibited and the material exhibits increased hardness.

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