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Alloys in three dimensions
RESEARCH NEWS
Alloys in three dimensions MICROSCOPY AND ANALYSIS 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) 54, 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 fieldemission 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
Coherent thought leads to strong stuff MECHANICAL PROPERTIES
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) 85, 4429]. The discovery could boost the development of a stronger, more creepresistant 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 hightemperature 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
Live view of atomic processes behind corrosion
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) 439, 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
(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.
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
“By influencing the initial structures we should be able to
results. After initial Cu dissolution, the researchers found a
control the process, either to increase corrosion resistance and
Au-enriched single-layer crystal two to three monolayers thick
the passivation behavior or to direct the formation of
with an unexpected inverted (CBA) stacking sequence. This
nanoporous metals,” says Renner.
acts as a nanoscale layer protecting against further dealloying.
The researchers believe that their insights into the corrosion
“This ultrathin initial film has a new crystal structure rotated
of this single-crystal system should be equally applicable to
by 180°,” says Renner. “This is an important fact for finding
other alloys such as stainless steel. They are now looking at
the mechanism involved in dealloying.” At higher potentials,
other systems including Ag-Au, Cu-Pd, PtRu, and GaAs.
this protective passivation layer dewets, forming 2.6 nm thick
Cordelia Sealy
(12 monolayers) Au-rich islands. These islands form the
Au islands protect the surface of a Cu3Au(111) system in the passivation regime of an applied corrosive potential
templates for subsequent growth of nanoporous structures.
APRIL 2006 | VOLUME 9 | NUMBER 4
9
Alloys in three dimensions MICROSCOPY AND ANALYSIS 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) 54, 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 fieldemission 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
Coherent thought leads to strong stuff MECHANICAL PROPERTIES
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) 85, 4429]. The discovery could boost the development of a stronger, more creepresistant 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 hightemperature 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
Live view of atomic processes behind corrosion
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) 439, 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
(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.
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
“By influencing the initial structures we should be able to
results. After initial Cu dissolution, the researchers found a
control the process, either to increase corrosion resistance and
Au-enriched single-layer crystal two to three monolayers thick
the passivation behavior or to direct the formation of
with an unexpected inverted (CBA) stacking sequence. This
nanoporous metals,” says Renner.
acts as a nanoscale layer protecting against further dealloying.
The researchers believe that their insights into the corrosion
“This ultrathin initial film has a new crystal structure rotated
of this single-crystal system should be equally applicable to
by 180°,” says Renner. “This is an important fact for finding
other alloys such as stainless steel. They are now looking at
the mechanism involved in dealloying.” At higher potentials,
other systems including Ag-Au, Cu-Pd, PtRu, and GaAs.
this protective passivation layer dewets, forming 2.6 nm thick
Cordelia Sealy
(12 monolayers) Au-rich islands. These islands form the
Au islands protect the surface of a Cu3Au(111) system in the passivation regime of an applied corrosive potential
templates for subsequent growth of nanoporous structures.
APRIL 2006 | VOLUME 9 | NUMBER 4
9