Split patterns in ordered precipitates

March 19, 2007

Do L1_2 ordered Ni_3Al (\gamma^{\prime}) precipitates in the Ni-rich (\gamma) matrix undergo elastic stress driven splitting, or, is it particle coalescence which gives rise to split-looking patterns?

Luo et al, in a paper which is to appear in Acta Materialia, advance an altogether new mechanism, namely, nucleation of ordered particles at dislocations and the subsequent growth.

Even though the fact that ordered precipitates preferentially nucleate at dislocations in Ni-base alloys due to lattice mismatch is well known (See the 1966 classic paper of Ardell and Nicholson–with an appendix by Eshelby, for example), I have never come across any mention of the same in the context of particle splitting–though, from the paper, I understand that such a mechanism was discussed in the PhD thesis of Prof. Wang back in 1995.

Paper: Nucleation of ordered particles at dislocations and formation of split patterns

Authors: W Luo, C Shen and Y Wang

Abstract:

We investigated the effect of nucleation of ordered precipitates at dislocations and the subsequent growth of particle morphology through computer simulations using a phase-field model. The model treats simultaneously precipitates, dislocations and precipitate–dislocation interactions within a single algorithm. In particular, the model takes into account the structural discontinuity associated with a dislocation that leads to the formation of antiphase domains if the dislocation is within an ordered particle. Three long-range order parameters are used to describe the antiphase domains associated with L12 ordering and an additional set of non-conserved order parameters is introduced to characterize dislocations. We show that heterogeneous nucleation and subsequent growth of ordered precipitates at dislocations yield various “split” patterns, whose formation has been attributed to different mechanisms in literature.

Very interesting indeed!

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