How do microvoids in a metal sample affect its strength?

[member 392791]

Hello,

I have two images taken of a metal sample that were snapped by a charpy impact test. The image with the black dots (microvoids) was the sample at room temperature, and the flat one was the sample that had been dipped in liquid nitrogen just before impact.

My question is, what role do the microvoids in the room temperature sample correspond to a higher strength of the material, since it was able to absorb more energy to snap compared to the other sample.

I know it must be something to do with the brittleness of the material, but I'm not too keen on what the microvoids are doing on the microscale that causes absorption of more energy.

Thank you

 

[SteamKing]

No images provided.

 

[member 392791]

The ductile image is the one at room temperature, and the brittle is the one that was in liquid nitrogen

 

[Alkim]

Cooling down makes the material so fragile that it breaks through the crystals, while at room temperature you separate the grains through the boundaries between them.

 

[Hyo X]

Brittleness or ductility can really be interpreted by atom movement during fracture. Ductile materials have significant atom movement during facture, while brittle materials do not.
Moving atoms takes energy. at room temperature, there is still enough energy for atoms to diffuse - overcome the energy barrier associated with adjacent bonding atoms. in brittle fracture, there is not enough energy (kT) for the atoms to diffuse, so fractures propogate regardless of grain boundaries, and fractures can propogate through grains (the strongest part).
microvoids are structures that form during the fracture process that represent atomic movement - diffusion - and thus energy absorbed during fracturecheck out http://en.wikipedia.org/wiki/Microvoid_coalescence