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Research paves way to crack-resistant metals

25 June 2015

Research led by Arizona State University's Professor Karl Sieradzki is uncovering new knowledge about the causes of stress-corrosion cracking in alloys.

Corrosion of a silver-gold alloy spontaneously resulting in the formation of nanoscale porous structures that undergo high-speed cracking under the action of a tensile stress (image: Karl Sieradzki/Arizona State University)

Using advanced tools for ultra-high-speed photography and digital image correlation, the team has been able to observe events triggering the origination of stress-corrosion fracture in a model silver-gold alloy and to track the speed at which cracking occurs. They measured cracks moving at speeds of 200m/s corresponding to about half of the shear wave sound velocity in the material.

This is a remarkable result, claims Sieradzki, given that typically only brittle materials such as glass will fracture in this manner and that gold alloys are among the most malleable metals.

In the presence of corrosive environments, silver is selectively dissolved from the alloy causing porosity to form (see image). If this occurs while the alloy is stressed, then the material fails as if it were made of glass.

These results provide a deeper understanding of the stress-corrosion behaviour of metals such as aluminium alloys, brass and stainless steel that threatens the mechanical integrity of important engineered components and structures.

Sieradzki says the team's discoveries could provide a guide for designing alloys with different microstructures so that the materials are resistant to this type of cracking.

The research team's findings are described online in the journal, Nature Materials.

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