Inquiry 2016, Issue 1

Inqui r y I s sue

| 2016

Inqui r y I s sue

| 2016

including melt spinning, injection casting, and high-pressure

gas atomization (covered separately on pages 6-7) to produce

small-grained and amorphous materials.

MELT SPINNING

This technique involves shooting a stream of molten

material onto a spinning copper wheel where it solidifies

quickly, forming a ribbon of metal. The copper wheel is

typically water cooled and depending on the speed at which

it spins, up to 30 meters per second, the molten metal is

quenched up to 10

Kelvins per second.

“There are limits to the process,” said Ames Laboratory

scientist and ISU associate professor of materials science

and engineering Jun Cui. “The copper wheel must be

perfectly balanced to spin at such high speeds. And

beyond a certain point, the material no longer flows in a

ribbon but breaks apart.”

There is also a variation in the process where the copper

wheel has small grooves cut across its surface. These grooves

intentionally break the cooled metal into short strips, which

Cui said are easier to work with in some applications.

INJECTION CASTING

As the name implies, injection casting forces the molten

material into a copper mold, typically a small cylinder that

will produce short rods one to four millimeters in diameter.

The mold is held inside a larger water-cooled copper mold

providing quench rates fast enough to produce amorphous

(glassy) samples in some alloys.

“Small samples—usually less than five grams—are

placed in a graphite or quartz nozzle and rapidly heated by

induction to several hundred degrees above the melting

point,” said Matt Besser, Ames Laboratory scientist and

manager of the Laboratory’s Materials Preparation Center.

“We then drop it out of the heating zone and pressurize the

system so the material squirts into the mold.”

By using different shaped molds, material can be cast in

plates, or wedges. Besser said thermocouples can be placed

along the length of the wedge to measure the difference in

cooling rates from the fastest at the thin tip to the slowest at

the thicker end.

“We’re able to fabricate samples to fit specific needs,”

Besser said, “and it’s convenient because we can produce

small samples, especially when the alloy contains

expensive materials.”

One of the most common and robust ways to create a

new material, particularly a metallic alloy, is to melt two or

more constituent materials, mix them in the liquid state,

then freeze or “solidify” them under certain controlled

conditions. While seemingly simple, solidification processing

can produce an incredible variety of material structures

with important features on scales from nanometers to

centimeters, giving rise to a host of remarkable properties

ranging from enhanced strength and stiffness to unusual

magnetic, thermal, electrical, and photonic properties.

But the make-up and structure, and therefore the

properties, of that end result can vary greatly depending on a

variety of conditions present as the material transitions from

liquid to solid. Ames Laboratory scientist Ralph Napolitano

works to explain and predict what takes place at that liquid-

solid interface and how those various interactions result in

certain structures, chemistries and properties.

“When a material goes from a liquid to a solid phase, a lot

of things must happen as part of that transformation,” said

Napolitano, who is also an Iowa State University professor of

materials science and engineering. “Nominally speaking, an

amorphous or non-crystalline liquid phase has to reconfigure

itself into some kind of crystalline packing. But many other

simultaneous events are taking place to make that happen.

Indeed, it is the way that the different transport processes

and different structural entities enter into that equation that

really influences what that final structure may look like.”

If equilibrium yields the normal or expected result,

there are all kinds of deviations that can shift the result

from equilibrium. Some of them are very small deviations,

such as slightly different chemical compositions or

slightly different concentrations of different kinds of

crystalline defects. Deviations can also be very large—

completely different crystalline packing or composition

or even an array of multiple phases that you might never

see closer to equilibrium.

“What dictates how far away from the final equilibrium

state you might be is what happens along that pathway

from the equilibrium liquid to this far-from-equilibrium

structure,” Napolitano said. “Varying the composition of

a material and the rate at which we cool it has dramatic

influence over final phase or assembly.”

“Beyond just the phase—the particular crystalline

structure—conditions during freezing greatly influence the

growth morphology,” he continued. “Any given phase will

growwith a certainmorphology that is dynamically optimized

with respect to all of the different processes—such as the

redistribution of heat, chemical species, and configuration

of crystalline defects—making the overall transformation

more efficient. Composition and cooling rate, along with

the phase itself and the energies of the crystal defects

and interfaces, all play a role in this collective dynamical

optimization, ultimately resulting in the selection of the final

state, which may look nothing like the equilibrium state.

“This far-from-equilibrium

synthesis provides a portal

or pathway to structures,

chemistries, and properties

that aren’t accessible through

conventional

methods,”

Napolitano said.

To complicate matters,

these pathways may include

several other steps —before

and after solidification, so

that the complex freezing

structure may only serve

as some intermediate stage,

along the way to a desired

structure.

Cooling rate provides a high level of control in certain

windows. At the low (slow) end, cooling rate can be

controlled very carefully, and even cooling rates from

isothermal treatments to 100 degrees per second can be

controlled reasonably well.

“We can go to cooling rates of 10

to 10

degrees per

second with techniques like melt spinning, but within that

window, process control is challenging and local variations

exist,” Napolitano said. “We have investigated such

variations, and our understanding has certainly increased.

Even so, with relatively few ‘process knobs’ to turn (e.g. melt

temperature, wheel speed, wheel material, injection rate

and stream diameter) precise quantitative control remains

a real challenge.”

Seeking explanations for

solidification puzzles

Jacob Fischer, undergraduate research assistant, loads a

sample into the injection casting system. The melted mate-

rial is injected into water-cooled copper molds.

As a strategy to reveal a clearer picture of the complex

behaviors, Napolitano’s group has chosen to focus on a few

select two-component or “binary” systems. In particular,

binary systems, such as copper-zirconium and aluminum-

samarium, provide great opportunities to investigate far-

from-equilibrium transformation. These systems exhibit

complex competitive solidification, glass formation, and

crystallization, forming a host of non-equilibrium phases and

multi-scale growth structures. At the same time, with only

two components, analytical and computational treatment of

the thermodynamics and kinetics become more tractable,

compared with multi-component systems.

“With both of these systems, there’s a composition

range over which the liquid forms a glass rather easily so

you can cool it at rates that are achievable experimentally,”

Napolitano said. “Once the alloy is glassy, other treatments

can be used to crystalize the material at low temperature.

In this regime, conditions can be controlled carefully, and

reactions can be slowed substantially, even permitting in-

situ real-time investigation. Of course, having an accurate

and comprehensive picture of the system thermodynamics

is critical. Whether you’re solidifying the material directly

from a liquid, or first quenching to a glass and then

heating to crystallize the material, you still have that same

thermodynamic playing field.”

The aluminum-samarium work is being expanded to a

larger range of binaries, including other aluminum-rare earth

alloys. In general, those systems are expected to exhibit

similar behaviors, though Napolitano warns that very subtle

effects can dramatically tip the balance between phases

and growth structures. Very small energetic differences

exist between the competing phases. Under high driving

forces, these differences are often negligible and the kinetic

pathways control the outcome. Even changes in chemical

composition on the order of a percent or less can dramatically

change the final state.

“This kind of study is only possible by bringing together

many different approaches in theoretical condensed matter

physics, materials science, computational thermodynamics,

materials synthesis, and state-of-the-art characterization,”

Napolitano said. “There is no doubt that this work requires

the full gamut of experimental and computational capability

and a team of investigators with a broad range of expertise.”

To that end, the new electron microscopy equipment at

Ames Laboratory’s Sensitive Instrument Facility (SIF) will

play a vital role.

“It’s important not only in terms of spatial resolution,

but some of the in situ capabilities as well,” he said. “Hot-

stage transmission electron microscopy with atomic scale

resolution will allow us to look at some of the early stage

dynamics that really are watershed events that tend to send

the material down a whole different trajectory. So absolutely,

the SIF is critical to moving forward in this area.”

Ralph Napolitano