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Failure by simultaneous grain growth, strain localization, and interface debonding in metal films on polymer substrates

Nanshu Lu's picture

In a previous paper , we have demonstrated that a microcrystalline copper film well bonded to a polymer substrate can be stretched beyond 50% without cracking. The film eventually fails through the co-evolution of necking and debonding from the substrate. Here we report much lower strains to failure (around 10%) for polymer-supported nanocrystalline metal films, whose microstructure is revealed to be unstable under mechanical loading. We find that strain localization and deformation-associated grain growth facilitate each other, resulting in an unstable deformation process. Film/substrate delamination can be found wherever strain localization occurs. We therefore propose that three concomitant mechanisms are responsible for the failure of a plastically deformable but microstructurally unstable thin metal film: strain localization at large grains, deformation-induced grain growth and film debonding from the substrate. 

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Ying Li's picture

Have you done the detailed work on the grain size distribution according to your SEM or FBI imagines? I think the statistical data may be more helpful to understanding the deformation-induced grain growth.

Ying Li Department of Engineering Mechanics Tsinghua University Beijing, 100084, P. R. CHINA

Nanshu Lu's picture

Ying, thanks for your interest and comment.

We haven't found any efficient way to determine the statistical distribution of nano grains so that we are trying to show straightforward micrographs at this point.

TEM is not suitable because the film microstructure is very unstable and easy to change during sample preparation.

EBSD works well for annealed films with micron-sized grains but the indexing rate is very low for nano grains.

XRD does not show much broadening because our grain size is not small enough.

I will try AFM to see if it can give reasonable results.

Manually sketching grain boundaries will always work but I will keep it as the last choice. 

Do you have any experience or suggestions on this to share with us? Many thanks.

 

Nanshu 

Teng Li's picture

Nanshu,

Neat experiments! The paper reveals another critical mechanism governing the failure of metal films on polymer substrates.

The films reported in paper are about 1 micron thick. I'm curious about the effect of film thickness on the deformation-induced grain growth. For example, does similar phenomenon occur in a thinner film (say, a few hundred nm thick) on a polymer substrate.

Nanshu Lu's picture

Hi, Teng,

Thanks for pointing this out. Actually it is what I am doing now.

Basically the film failure mode exhibits a ductile-brittle transition as film thickness reduces. But we found that neither the thickness nor the grain size alone will be sufficient to understand the phenomenon. I am trying to take both factors into consideration to give better explanations now.

I'll send you some preliminary results through email.

 

Nanshu 

Ying Li's picture

Sorry for the late reply. EBSD is a good method to monitor the texture of the film. By the EBSD, the grain size of the Cu could be clearly investigated. Especially, the in-situ SEM loading with the EBSD could check the grain growth when the Cu film is under tension. Such a in-situ SEM with EBSD method has been successfully applied to investigate the equal channel angular extruded Mg-3Al-Zn magnesium alloy. However, as you say, the indexing rate is very low for nano grains.

Ying Li Department of Engineering Mechanics Tsinghua University Beijing, 100084, P. R. CHINA

Dear Nanshu,

I see that you have a very good experience with ABAQUS.  I hope you can help me with the following very simple problem.

I have a problem simulating earthquake effect on a very simple structure. The structure is a 2 story steel plan frame structure. with lumped mass at its corners.

I am applying the Northbridge earthquake on it but I am getting very strange results.

 I am applying the Material density beside the lumped mass at corners. I do not know if using lumped masses is correct like this or not.

 Both, eignvalues and frequencies and also responses are not correct.

 I attached  the input file and also the acceleration time history for the earthquake.

The input file extension was changed to txt so the server can accept it.

Here is the link

http://imechanica.org/node/5090

 Looking forward to hearing from you.

 Regards for all,

 Maher Elabd

 

 

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