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Watching a phase transition atom by atom
RESEARCH NEWS
Dislocations in intermetallics in atomic detail CHARACTERIZATION
Z-contrast STEM image of the Laves phase structure of Cr2Hf: Cr atomic columns are red and Hf atomic columns are yellow. (Reprinted with permission. © 2005 AAAS.)
The subangstrom resolution of an aberrationcorrected scanning transmission electron microscope (STEM) has enabled researchers to observe dislocations in an intermetallic compound directly for the first time [Chisholm et al., Science (2005) 307, 701]. This allowed the team from Brown University, Oak Ridge National Laboratory, and UES to confirm the mechanism of dislocation motion in these materials. Many intermetallics can withstand high temperatures, making them desirable for the aerospace, defense, and energy industries. However, they are often hopelessly brittle at room temperature. Plastic deformation is primarily facilitated by dislocation motion, so
the easier it is to move such defects, the less brittle the material is. “The motion of dislocations in response to an applied stress is dependent on the atomic arrangement in the vicinity of the dislocation,” explains Sharvan Kumar of Brown University. “Understanding the local atomic arrangement is a first step in understanding dislocation motion and perhaps modifying it favorably.” Deformation in metals and alloys, which possess simple atomic arrangements, is well understood. Here, dislocations glide between well-separated slip planes. In more complex crystal structures, this may not always be possible. Laves phases, the most common class of intermetallics, are one such case. Z-contrast imaging in the aberrationcorrected STEM was able to reveal the atomic structure of the Laves phase alloy Cr2Hf. Stacking faults and dislocation cores could be observed in atomic detail. The results confirm a mechanism for dislocation motion that was first proposed in the 1950s. Here, dislocation motion occurs through the coordinated movement of atoms in two adjacent planes, or ‘synchroshear’. “The next step is to think about ways to lubricate the synchroshear process by alloying additions so that slip can be enhanced and some much needed plastic deformability provided,” says Kumar.
April 2005
The dentin-enamel junction (DEJ) in human teeth is an exception to the general materials rule that an interface is one of the most susceptible sites for fracture. Researchers at Lawrence Berkeley National Laboratory (LBNL), and the Universities of California at Berkeley and San Francisco are seeking to understand this fractureresistant system to learn a lesson from nature [Imbeni et al., Nat. Mater.
tracked with atomic resolution in real space and real time as the temperature is slowly raised. The researchers used this ability to observe the phase transition as it evolved in a defect-free region of Pb atoms. A transition temperature of 86 ± 2 K was found, and fitted parameters describing the transition are in good agreement with those expected for this type of phase transition. Where point defects existed in the sample, these were found to locally stabilize the low-temperature phase. However, the researchers were able to reject these defects as being the driving force for the phase transition, as had been suggested in some previous reports on the related Sn/Ge(111) system. Jonathan Wood
Patrick Cain
Jonathan Wood
CHARACTERIZATION
12
BIOMATERIALS
(2005) 4 (3), 229] “In general, when you look at implants of any sort, the interface is always the weakest point. If you have a hip implant the first place it will fail is where it’s bonded to the bone,” says Robert Ritchie of LBNL. “Nature does it brilliantly. By understanding the DEJ we can solve this interface problem.” The study found that cracks arrest in the tough inner layer of the tooth, dentin, after penetrating the harder outer enamel layer and the DEJ. Within 10 µm of crack propagation in the dentin adjacent to the DEJ, a series of uncracked ligament bridges develop. These bridges decrease the stress or driving force behind the primary crack, causing it to stop growing. This fracture resistance is believed to originate from a gradual microstructural change between the dentin and the enamel. The researchers quantified the DEJ toughness as 5-10 times higher than the adjacent enamel and 75% lower than the adjoining dentin using interface impingement, scanning electron microscopy, and Vickers hardness testing. The group is applying the findings to design composites for tooth restoration. “If you are trying to do a restoration of the tooth, you could try to mimic nature and this could be a very good way to go,” says Ritchie.
Watching a phase transition atom by atom Researchers at the Universidad Autónoma de Madrid in Spain have watched a phase transition in a layer of Pb atoms on a Si surface as it happens by using a scanning tunneling microscope (STM) [Brihuega et al., Phys. Rev. Lett. (2005) 94, 046101]. The team, led by José M. Gómez-Rodríguez, studied a reversible phase transition in a 1/3 monolayer of Pb on a Si(111) surface. Here, a flat arrangement of Pb atoms with a surface periodicity of (3 x 3) at low temperature changes to a corrugated (√3 x √3) structure at room temperature. The group’s homebuilt ultrahigh-vacuum STM can remain fixed on the same surface area while varying the temperature from 40-200 K. This allows a single surface region 20 x 20 nm2 in size to be
Learning from tough teeth
Dislocations in intermetallics in atomic detail CHARACTERIZATION
Z-contrast STEM image of the Laves phase structure of Cr2Hf: Cr atomic columns are red and Hf atomic columns are yellow. (Reprinted with permission. © 2005 AAAS.)
The subangstrom resolution of an aberrationcorrected scanning transmission electron microscope (STEM) has enabled researchers to observe dislocations in an intermetallic compound directly for the first time [Chisholm et al., Science (2005) 307, 701]. This allowed the team from Brown University, Oak Ridge National Laboratory, and UES to confirm the mechanism of dislocation motion in these materials. Many intermetallics can withstand high temperatures, making them desirable for the aerospace, defense, and energy industries. However, they are often hopelessly brittle at room temperature. Plastic deformation is primarily facilitated by dislocation motion, so
the easier it is to move such defects, the less brittle the material is. “The motion of dislocations in response to an applied stress is dependent on the atomic arrangement in the vicinity of the dislocation,” explains Sharvan Kumar of Brown University. “Understanding the local atomic arrangement is a first step in understanding dislocation motion and perhaps modifying it favorably.” Deformation in metals and alloys, which possess simple atomic arrangements, is well understood. Here, dislocations glide between well-separated slip planes. In more complex crystal structures, this may not always be possible. Laves phases, the most common class of intermetallics, are one such case. Z-contrast imaging in the aberrationcorrected STEM was able to reveal the atomic structure of the Laves phase alloy Cr2Hf. Stacking faults and dislocation cores could be observed in atomic detail. The results confirm a mechanism for dislocation motion that was first proposed in the 1950s. Here, dislocation motion occurs through the coordinated movement of atoms in two adjacent planes, or ‘synchroshear’. “The next step is to think about ways to lubricate the synchroshear process by alloying additions so that slip can be enhanced and some much needed plastic deformability provided,” says Kumar.
April 2005
The dentin-enamel junction (DEJ) in human teeth is an exception to the general materials rule that an interface is one of the most susceptible sites for fracture. Researchers at Lawrence Berkeley National Laboratory (LBNL), and the Universities of California at Berkeley and San Francisco are seeking to understand this fractureresistant system to learn a lesson from nature [Imbeni et al., Nat. Mater.
tracked with atomic resolution in real space and real time as the temperature is slowly raised. The researchers used this ability to observe the phase transition as it evolved in a defect-free region of Pb atoms. A transition temperature of 86 ± 2 K was found, and fitted parameters describing the transition are in good agreement with those expected for this type of phase transition. Where point defects existed in the sample, these were found to locally stabilize the low-temperature phase. However, the researchers were able to reject these defects as being the driving force for the phase transition, as had been suggested in some previous reports on the related Sn/Ge(111) system. Jonathan Wood
Patrick Cain
Jonathan Wood
CHARACTERIZATION
12
BIOMATERIALS
(2005) 4 (3), 229] “In general, when you look at implants of any sort, the interface is always the weakest point. If you have a hip implant the first place it will fail is where it’s bonded to the bone,” says Robert Ritchie of LBNL. “Nature does it brilliantly. By understanding the DEJ we can solve this interface problem.” The study found that cracks arrest in the tough inner layer of the tooth, dentin, after penetrating the harder outer enamel layer and the DEJ. Within 10 µm of crack propagation in the dentin adjacent to the DEJ, a series of uncracked ligament bridges develop. These bridges decrease the stress or driving force behind the primary crack, causing it to stop growing. This fracture resistance is believed to originate from a gradual microstructural change between the dentin and the enamel. The researchers quantified the DEJ toughness as 5-10 times higher than the adjacent enamel and 75% lower than the adjoining dentin using interface impingement, scanning electron microscopy, and Vickers hardness testing. The group is applying the findings to design composites for tooth restoration. “If you are trying to do a restoration of the tooth, you could try to mimic nature and this could be a very good way to go,” says Ritchie.
Watching a phase transition atom by atom Researchers at the Universidad Autónoma de Madrid in Spain have watched a phase transition in a layer of Pb atoms on a Si surface as it happens by using a scanning tunneling microscope (STM) [Brihuega et al., Phys. Rev. Lett. (2005) 94, 046101]. The team, led by José M. Gómez-Rodríguez, studied a reversible phase transition in a 1/3 monolayer of Pb on a Si(111) surface. Here, a flat arrangement of Pb atoms with a surface periodicity of (3 x 3) at low temperature changes to a corrugated (√3 x √3) structure at room temperature. The group’s homebuilt ultrahigh-vacuum STM can remain fixed on the same surface area while varying the temperature from 40-200 K. This allows a single surface region 20 x 20 nm2 in size to be
Learning from tough teeth