As materials scientists know well, one reliable
way to make strong metals even stronger is
to shrink their already-tiny crystalline grains.
It’s a time-tested technique that’s made
today’s cars, planes, and armor safer than
ever.
But at the nanoscale, grains are notoriously
fickle.
Their strong tendency to grow makes it nearly
impossible for researchers to chase higher
levels of strength.
But that could soon change.
A new computer model developed by researchers
from MIT shows how nano-sized grains might
be stabilized in metal alloys.
Their findings could provide the blueprint
for constructing harder and stronger metals.
Alloying one metal with another is one technique
that has helped researchers push grain sizes
to smaller and smaller scales—thanks to
a process known as segregation.
As the grains in a metal shrink, the addition
of a small amount of an alloying metal segregate,
or adhere, to the boundaries between different
grains.
Like police officers containing an unruly
crowd, these secondary atoms keep grains from
growing out of control.
The thermodynamics of this process are well
understood for relatively large grains.
There are libraries of maps that tell metal-makers
how to achieve the desired strength or hardness.
But no such maps are available for alloys
made up of nano-sized grains.
The problem is a poor grasp of how nanograins
in alloys behave under real conditions, which
naturally involve fluctuations in temperature:
How do policing atoms corral an increasingly
disorderly crowd?
Most models treat these enforcers individually.
That tends to make calculations easy to perform.
But it comes at the expense of producing results
that aren’t exactly true to life.
To tackle this problem, the MIT researchers
built a simulation model that treats the full
network of enforcers as a material all its
own.
The result is the kind of two-phase system
that materials researchers are used to working
with.
As such, the researchers could create phase
maps that predict the conditions that give
rise to stable nanograins of various sizes.
This new approach isn’t perfect.
For one, the resolution of the model could
be improved to capture smaller changes in
grain structure with rising temperature.
And it remains to be seen how the phase maps
translate into real materials in the lab.
But with further improvements, the new method
could be a useful tool for creating stronger
metal alloys.
