Designing Crack-Resistant Metals
A recent study into the interactions of metal alloys at the nanometer and atomic scales is likely to aid advances in preventing the failure of systems critical to public and industrial infrastructure.
Research led by Arizona State University materials science
and engineering professor Karl Sieradzki is uncovering new knowledge about the
causes of stress-corrosion cracking in alloys used in pipelines for
transporting water, natural gas and fossil fuels – as well as for components
used in nuclear-power-generating stations and the framework of aircraft.
Sieradzki is on the faculty of the School for Engineering of
Matter, Transport and Energy, one of ASU’s Ira A. Fulton Schools of
Engineering.
His research team’s findings are detailed in an advance
online publication June 22 of the paper “Potential-dependent dynamic fracture
of nanoporous gold” on the website of the journal Nature Materials.
The image shows 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. It helps demonstrate a discovery by an Arizona State
University
Using advanced tools for ultra-high-speed photography and
digital image correlation, the team has been able to closely observe the 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 200 meters per
second corresponding to about half of the shear wave sound velocity in the
material.
This is a remarkable result, Sieradzki said, 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 absence of a corrosive environment, these gold alloys
fail in the same manner as children’s modeling clay, Sieradzki explained: Roll
modeling clay into a cylindrical shape and you can stretch it by about 100
percent before it slowly tears apart. In the presence of corrosive
environments, silver is selectively dissolved from the alloy causing porosity
to form (see photo). If this occurs while the alloy is stressed, the material
fails as if it were made of glass.
These results provide a deeper understanding of the
stress-corrosion behavior of such metals as aluminum alloys, brass and
stainless steel that threatens the mechanical integrity of important engineered
components and structures.
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,” Sieradzki said.
The research has been funded by the Department of Energy’s
Basic Energy Science program.
His co-authors on the Nature Materials paper are former or
current ASU materials science and engineering graduate students: Shaofeng Sun
earned her doctoral degree in 2013; Xiying Chen is a third-year doctoral
student; and Nilesh Badwe earned a doctoral degree earlier this year.
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