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Material Physics

The Premature Ageing of Alloys

pipe diffusion

© M. Legros

An exemple of the pipe diffusion process on aluminum, observed by transmission electron microscopy. The silicon precipitate P1 uses the dislocation d1 as a short circuit for diffusion toward P2.

Why do certain apparently serviceable machines suddenly and irreversibly break down? An international team1 around Marc Legros from CEMES2 has shed light on this everyday enigma by successfully monitoring, for the first time, a complex process behind the premature ageing of alloy-based electronic components and mechanical parts.
Alloys are often employed for their useful material properties, one being strength that derives from dislocations–or defects–in their crystalline lattice structures. For the last 50 years, it was suspected that these same dislocations also caused premature alloy ageing. This theory has now been proven3 by Legros and his colleagues.
Experiments were based on a material widely used for microprocessor interconnections: aluminum film containing silicon nanoparticles. The team followed and controlled the effects of dislocations using transmission electron microscopy.
While silicon atoms in aluminum are generally free to roam, the researchers demonstrated that dislocations can increase as much as a thousand times the rate at which atoms migrate from one nanoparticle to another. Dislocation lines develop into what Legros describes as “atom slides,” which speed up the transfer of atoms, a phenomenon known as pipe diffusion. The team also showed that pipe diffusion is exacerbated at high temperatures, making the heat generated by a microprocessor a major concern.
The transfer of atoms along dislocation lines gradually leads smaller particles to dissolve into larger ones. This tendency drives the once-interlocked silicon and aluminum to separate, just as an emulsion like salad dressing breaks down over time into oil and vinegar. The alloy will then lose its properties. If, for example, non-ductile silicon precipitates develop, the material’s conductivity and strength are at risk. These observations should lead to a more accurate modeling of premature alloy ageing, with implications for products ranging from microprocessor interconnections to airplane engine components. Results point to the prospect of reducing pipe diffusion effects by modifying thermic cycles imposed on electronic parts during manufacturing. While the unexpected failure of electronic machines and devices remains an unpleasant fact of life, the findings have cracked a common mystery and paved the way for future developments.

Fui Lee Luk

Notes :

1. Gerhard Dehm (Austria), Eduard Artz (Germany), and T. John Balk (USA).
2. Centre d'élaboration de matériaux et d'études structurales (CNRS).
3. M. Legros et al., “Observation of Giant Diffusivity Along Dislocation Cores,” Science, 2008. 319: 1646-9.

Contacts :

Marc Legros, CEMES, Toulouse.


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