By embedding
ductile, large-grained columns (shown here as colors) in a harder,
ultrafine-grained
matrix (shown here as black), researchers were able to improve
titanium's
strength without impairing its ductility. Image credit: Yuntian Zhu.
(November 9, 2015) Researchers
at North Carolina State University and the Chinese Academy of Sciences have
developed a technique to make titanium stronger without sacrificing any of the
metal’s ductility – a combination that no one has achieved before. The
researchers believe the technique could also be used for other metals, and the
advance has potential applications for creating more energy-efficient vehicles.
“Historically, a material is either strong or ductile, but
almost never both at the same time,” says Yuntian Zhu, a professor of materials
science and engineering at North Carolina State University and co-corresponding
author of a paper describing the work. “We’ve managed to get the best of both
worlds. This will allow us to create strong materials for use in making lighter
vehicles, but that are sufficiently ductile to prevent the material from
suffering catastrophic failure under strain.”
The key idea here is grain size, or the size of the crystals
in the metal. Metals with a small grain size are stronger – meaning they can
withstand more force before they start to deform. But metals with a small grain
size are also less ductile, which means they can withstand less strain before
breaking. Materials that aren’t ductile won’t bend or stretch much – they just
snap. Conversely, metals with a large grain size are more ductile, but have
lower strength.
The new technique manipulates the grain size to give the
metal the strength of ultrafine-grained titanium but the ductility of
coarse-grained titanium.
The researchers began by using asymmetric rolling to process
a two-millimeter thick sheet of titanium. In asymmetric rolling, the sheet
passes between two rollers that apply pressure to each side of the sheet, but
one of the rollers rotates more quickly than the other. This not only presses
the sheet thinner but, because of the different roller speeds, also creates a
sheer strain in the metal.