[ RadSafe ] Nanostructured composite material may replace depleted uranium

Thu Feb 1 07:01:44 CST 2007

Comment on ROY HERREN's very interesting posting.

John Taschner, CHP, got a US Navy award in the 1980s for using tungsten
instead of DU.  Before that he led a program that kept radiation
exposures to less than the standard for the general public.   Even so,
he advised that DU would lead to so many PR problems that the advantage
was not worth the cost.

Gordon Riel
(301) 261-7735, FAX 2252
NSWCCD 6301
Bethesda, MD 20817-5700

Message: 12
Date: Wed, 31 Jan 2007 23:43:56 -0800 (PST)
From: ROY HERREN <royherren2005 at yahoo.com>
Subject: [ RadSafe ] Nanostructured composite material may replace
	depleted	uranium 
To: radsafe at radlab.nl
Message-ID: <285262.88749.qm at web81602.mail.mud.yahoo.com>
Content-Type: text/plain; charset=iso-8859-1

http://www.physorg.com/news89466521.html

  Nanostructured composite material may replace depleted uranium  

  Armor-piercing projectiles made of depleted uranium have caused
concern among soldiers storing and using them. Now, scientists at the
U.S. Department of Energy's Ames Laboratory are close to developing a
new composite with an internal structure resembling fudge-ripple ice
cream that is actually comprised of environmentally safe materials to do
the job even better. 

  Ames Laboratory senior scientist Dan Sordelet leads a research team
that is synthesizing nanolayers of tungsten and metallic glass to build
a projectile. "As the projectile goes further into protective armor,
pieces of the projectile are sheared away, helping to form a sharpened
chisel point at the head of the penetrator," said Sordelet. "The
metallic glass and tungsten are environmentally benign and eliminate
health worries related to toxicity and perceived radiation concerns
regarding depleted uranium." 

Depleted-uranium-based alloys have traditionally been used in the
production of solid metal, armor-piercing projectiles known as kinetic
energy penetrators, or KEPs. The combination of high density (~18.6
grams per cubic centimeter) and strength make depleted uranium, DU,
ideal for ballistics applications. Moreover, DU is particularly
well-suited for KEPs because its complex crystal structure promotes what
scientists call shear localization or shear banding when plastically
deformed. In other words, when DU penetrators hit a target at very high
speeds, they deform in a "self-sharpening" behavior. 

"It's very desirable to have this type of behavior together with high
density, so that's why DU is used, but there has been strong global
interest in replacing it since the start of the Gulf War in 1991." said
Sordelet. 

A popular replacement for DU is tungsten because at 19.3 grams per cubic
centimeter, it's a little bit denser than DU. However, tungsten has a
very simple crystal structure known as a body-centered cubic structure.
"If I made the same solid projectile out of tungsten and plastically
deformed it, I'd get a mushroom shape at the impacting face when the
projectile hit the target because tungsten is notoriously resistant to
forming shear bands," explained Sordelet. "It can be compared to taking
a Tootsie Roll and pushing it against something flat and hard -- you get
this mushroom-head effect." 

Sordelet said that researchers have been looking at ways to utilize
tungsten for at least the last 15 years. They've created tungsten heavy
alloys, for example, W-Fe-Ni (tungsten-iron-nickel), in the hope of
forming shear bands during high-rate deformation, but that goal hasn't
been adequately achieved yet. "There are several types of tungsten-based
penetrators, but they don't perform as well as DU," he said. 

In the last few years, Sordelet said research has focused on mixing
tungsten with bulk metallic glasses because glass, as a consequence of
not having ordered planes of atoms, is naturally very susceptible to
shear banding. "The problem is no one has come up with an economically
viable metallic glass that has a sufficiently high density to form a
composite that can compete with DU," he said. "People have made all
kinds of different, interesting structures, but they all have
coarse-grain tungsten of a micron or above in them, and that leads back
to this mushroom-head effect." 

  Sordelet said the ideal approach would be to make the whole penetrator
from a metallic glass matrix composite reinforced with nanocrystalline
tungsten because researchers from the Johns Hopkins University and the
Army Research Laboratory have recently demonstrated that when the grain
size of tungsten is reduced to the nanometer scale, it's propensity to
shear localize is significantly increased. So Sordelet and his Ames
Laboratory co-workers, Ryan Ott, Min Ha Lee and Doug Guyer, decided to
use a mechanical milling approach to reduce the grain size of
coarse-grain tungsten and intimately blend it on a submicron scale with
a metallic glass. 

"We first physically blend the two powders in a tumbler and then
mechanically mill the mixture to synthesize composite particles,"
explained Sordelet. According to him, the composite particles are
composed of alternating nanoscale layers of tungsten and metallic glass
that have an uncanny resemblance to fudge-ripple ice cream. "What was
amazing to us was that in forming the composite powder structure with
this nanolayering, nothing has changed in the two different layers," he
said. "The metals do not blend together -- no alloying is going on
between the two, and the metallic glass structure remains unchanged. The
layer spacings and grain structures are just remarkably small." 

In tests at low strain rates (low rates of deformation), Sordelet's
nanostructured metallic glass+tungsten composite shows susceptibility to
shear localization. "The fact that this occurred at low strain rates is
very remarkable," said Sordelet. "It's extremely suggestive that you
would see it at dynamic deformation rates, as well, which is what's
needed for KEPs." 

Sordelet is optimistic about the potential for the nanostructured
metallic glass+tungsten composites not only for KEPs but also as an
initial step in the development of similar composites for high-precision
machining of advanced materials. But because the density of typical
metallic glasses is fairly low, he knows they must get about 70 volume
percent of tungsten into the composite, which will make it challenging
to extrude in order to achieve a composite density that is acceptable to
his colleagues at the Army Research Laboratory. 

Contemplating that problem, Sordelet wonders, "What if we replace the
glass with something that has a higher density and still might have a
susceptibility to shear localization? The metallic glass is just a
material that's along for the ride because of its strong propensity for
shear localization," he noted. "But work at the Army Research Lab and
Johns Hopkins University has shown that a lot of body-centered cubic
metals have a susceptibility to shear localization if you get the grain
size small enough." That being the case, Sordelet is now looking at a
blend of tungsten and other high-density metals, but that's another
story. 

Source: Ames Laboratory 