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Posted: March 26, 2008

Ageing of alloys: a mayonnaise that separates more quickly than expected

(Nanowerk News) Why do certain electronic components undergo spontaneous, irreversible breakdown? Why do certain mechanical parts, without any apparent wear, suffer failure? An initial, empirical answer to such questions has been provided by observations and measurements made by French researchers (1) (CEMES / CNRS), associated with foreign research teams (2). In fact, for the first time, they have succeeded in directly monitoring one of the processes that accelerates the ageing of alloys. Their results clearly show that the presence of certain defects in alloys causes their components to separate more rapidly. This discovery should enable the lifetime of electronic components to be predicted with more accuracy. Their work is published in the March 21st issue of the journal Science.
Electronic components and mechanical parts fail because, over time, the alloys they are made of undergo ageing. All metals and alloys have defects, known as dislocations, responsible for most of their mechanical properties. For the last fifty or so years, it has been suspected that these very defects are the cause of premature ageing of alloys. Thanks to observations made by a CEMES-CNRS team in Toulouse, the researchers have recently demonstrated that the presence of such defects actually accelerates the ageing process of alloy based materials.
They studied a material widely used in electronics for metal connections in microprocessors. Constituted of a film of aluminum and inclusions of silicon nanoparticles, this alloy is like a mayonnaise (fine droplets of oil emulsified in water). Certain defects in the aluminum crystalline structure create microscopic channels that interconnect the silicon nanoparticles. This configuration allows the silicon atoms to move rapidly from one particle to another; Marc Legros even goes so far as describing these defects as “atom slides”. Over time, the smaller particles dissolve and the atoms composing them swell the largest particles. Whereas before they were intimately mixed, the silicon and aluminum separate, just like the oil and water of a mayonnaise that de-emulsifies. This dynamic phenomenon then leads to the destruction of the alloy and the loss of its properties.
Using transmission electronic microscopy, CEMES-CNRS researchers directly monitored the very rapid disappearance of a small “drop of silicon” to the benefit of a larger drop, the first time this has been done. This phenomenon is known as “pipe-diffusion”. Although the silicon atoms can move about slowly in the aluminum, the researchers showed, by repeating the experiment at different temperatures, that the presence of a crystalline dislocation increases one thousand fold the rate of transfer of silicon atoms from one nanoparticle to another. Therefore, the “mayonnaise” separates more rapidly when defects are present.
This research adds a piece to the puzzle of understanding the ageing of alloys and has enabled the modeling of this very complex phenomenon to be improved. In particular, the researchers hope to be able to control the ageing of aluminum based interconnections in microprocessors and acquire a better understanding of the mechanical behavior of alloys used, for example, in airplane engines.

1) Marc Legros, CEMES-CNRS, Toulouse

2) Gerhard Dehm, Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Department Materials Physics, University of Leoben, Austria, Eduard Arzt, INM-Leibniz Institute for New Materials , Saarbrücken, Allemagne, T. John Balk, Department of Chemical and Materials Engineering, University of Kentucky, Lexington, United States.

Giant diffusivity along dislocation cores, Marc Legros, Gerhard Dehm, Eduard Arzt, T. John Balk, Science, 21 March 2008.
Source: CNRS
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