APRIL 2006 | VOLUME 9 | NUMBER 4 9

RESEARCH NEWS

Three-dimensional analysis methods

are essential to understanding a

material’s microstructure. Existing

approaches to tomography either use

transmitted radiation (X-rays,

electrons, or neutrons) or serial

sectioning to reconstruct the material

in three dimensions. Researchers from

the Max-Planck-Institut für

Eisenforschung in Germany are

working to combine these two

approaches in a single system [Konrad

et al., Acta Mater. (2006) 5544, 1369].

Their approach combines a system for

three-dimensional electron

backscattering diffraction (EBSD) with

a focused ion beam (FIB) unit. The use

of a joint high-resolution field-

emission scanning electron microscope

(SEM) with EBSD enables orientation

microscopy of the flat surfaces of a

sample, while FIB is used to take thin

serial sections of the sample. EBSD

provides a plethora of crystallographic

information on the sample, including

the shape of grains, the position and

crystallographic character of interfaces,

defect densities in grains, and texture

evolution, with a resolution of 50 nm

or less. FIB sectioning is highly

controlled, allowing sections as thin as

50 nm to be taken, and fully

automated, enabling large areas (up to

50 µm x 50 µm x 50 µm) to be

investigated. The combination of the

two techniques in a single system

allows the reconstruction of the

original microstructure of the sample

in three dimensions.

The researchers used the novel

technique to investigate the alloy

Fe3Al and the use of Laves particles to

improve mechanical properties. They

found that the crystal orientation of

the soft alloy matrix forms orientation

gradients, with characteristic patterns,

around the hard particles that can

develop into new seed crystals.

Cordelia Sealy

AAllllooyyss iinn tthhrreeeeddiimmeennssiioonnssMICROSCOPY AND ANALYSIS

LLiivvee vviieeww ooff aattoommiicc pprroocceesssseess bbeehhiinndd ccoorrrroossiioonn METALS AND ALLOYS

Corrosion can be an extremely detrimental (and expensive) problem or it can be harnessed in the fabrication of porous materials.

In either scenario, an insight into the structure formation

during the process is essential to its understanding and

control. Now researchers from the Max-Planck-Institut für

Metallforschung, the European Synchrotron Radiation Facility,

and Universität Ulm have used in situ X-ray diffraction (XRD)

to observe the atomic processes that occur during corrosion

as it happens [Renner, et al., Nature (2006) 443399, 707].

“In situ in-liquid scanning tunneling microscopy can reveal

images of the surface during the process, but X-rays can look

deeper in the surface region and reveal the chemical

composition,” says Frank U. Renner of the Max-Planck-Institut

für Metallforschung and European Synchrotron Radiation

Facility. The atomic-scale observations of the surface of a

Cu3Au(111) single crystal alloy during the initial stages of

corrosion in a sulfuric acid solution reveal some surprising

results. After initial Cu dissolution, the researchers found a

Au-enriched single-layer crystal two to three monolayers thick

with an unexpected inverted (CBA) stacking sequence. This

acts as a nanoscale layer protecting against further dealloying.

“This ultrathin initial film has a new crystal structure rotated

by 180°,” says Renner. “This is an important fact for finding

the mechanism involved in dealloying.” At higher potentials,

this protective passivation layer dewets, forming 2.6 nm thick

(12 monolayers) Au-rich islands. These islands form the

templates for subsequent growth of nanoporous structures.

“By influencing the initial structures we should be able to

control the process, either to increase corrosion resistance and

the passivation behavior or to direct the formation of

nanoporous metals,” says Renner.

The researchers believe that their insights into the corrosion

of this single-crystal system should be equally applicable to

other alloys such as stainless steel. They are now looking at

other systems including Ag-Au, Cu-Pd, PtRu, and GaAs.

Cordelia Sealy

Au islands protect the surface of a Cu3Au(111) system in

the passivation regime of an applied corrosive potential

(shown by an ex situ AFM image of 700 x 700 nm). XRD

reveals an ultrathin Au-rich layer of three atomic

monolayers (inset) that is formed before the pure Au

islands are created at elevated potentials.

Coherent thought leads to strong stuff

A hundred-fold increase in yield strength can be

achieved solely by the introduction of coherency

strain, according to a recent study undertaken by Ken

P’Ng and colleagues at the Centre for Materials

Research, Queen Mary, University of London [P’Ng et

al., Philos. Mag. (2005) 8855, 4429]. The discovery could

boost the development of a stronger, more creep-

resistant generation of materials and structures,

eagerly awaited by the aerospace and power

generation industries among many others.

When a thin layer of material is deposited onto a

single-crystal substrate with a different lattice

parameter, thermodynamics favors the creation of

strain in the ‘epitaxial’ layer. This creates a completely

coherent interface in which there is a perfect

alignment of atomic positions between the substrate

and the deposited layer. By depositing an alloy and

fine-tuning the composition, it is possible to choose

the amount of strain produced.

Try to make this layer too thick, however, and misfit

dislocations will form, removing the coherency and

reducing the strain. P’Ng and coworkers overcame this

critical thickness limit by studying superlattices of up

to 74 repeating tensile-compressive bilayers of InGaAs,

supported on a thick InP substrate. Using a standard

three-point bend test at 500°C, they found that the

addition of a 2.5 µm superlattice to a substrate more

than 100 times thicker doubles the sample strength.

The remarkable strength of these superlattices could

be directly applied in a variety of micromechanical

systems, leading to improved cantilevers and more

rigid three-dimensional structures. Furthermore,

coherency strain has particular implications for the

lifetime and mechanical properties of high-

temperature materials. It is the coherent nature of the

γ/γ’ interface in Ni-based superalloys that prevents

coarsening, allowing for the manufacture of turbine

blades with a lifetime measured in days or years,

rather than in seconds. With a proven potential for

strengthening materials at elevated temperatures,

coherency strain will continue to be a hot parameter

for all those involved in developing and characterizing

creep-resistant materials.

Edmund Ward

MECHANICAL PROPERTIES