Delving Deeper into Materials with STEM
Researchers employ a new ultra high-resolution (S)TEM to explore high performance structural materials.
A large number of economically important industrial segments, including the aerospace and automotive sectors, depend heavily on high performance structural materials. However, financial concerns related to product reliability require a high degree of maturity from materials deployed in commercial applications. Conventional approaches to achieving this maturity require extensive programs of research and development based on laboratory testing and in-use experience, which in turn, result in high costs and long development cycles.
Traditionally, the high cost of materials development has been underwritten in part by government funding, usually through the military. In addition to funding, the military has provided a testing venue that is more willing to accept higher risk in exchange for higher performance. However, there is increasing public pressure to reduce military spending, particularly for long-term development programs that suffer technical setbacks or otherwise fail to meet expectations.
The Center for Accelerated Maturation of Materials (CAMM) at Ohio State Univ., Columbus, for its part, is a collaborative effort between government, industry, and academia, formed to explore new approaches to materials development through the combination of model-based computation and experimental validation. Specifically, it seeks to develop an integrated set of tools which will significantly reduce testing requirements and permit the rapid maturation of new materials with reduced costs and shorter cycle times. The intimate coupling of computational modeling accuracy and experimental characterization at the highest level without artifacts is key to this effort.
One area of focus in the program is microstructural characterization, aimed at providing researchers with an accurate physical picture of the materials they are modeling. To this end CAMM was involved at the early stages in a joint project with FEI Co., Hillsboro, Ore., to develop an advanced S/TEM (scanning/transmission electron microscope) platform, enabling the highest resolution imaging and microanalysis. CAMM was the first U.S. installation of the new tool. The instrument can be fitted with a monochromator as well as aberration correctors, to provide the sub-Ångstrom resolution necessary for atomic- level structural investigations.
In this article we show some preliminary results which are key to further the progress of CAMM, using the system as currently installed without aberration correctors. Correctors will be fitted in the coming months, however even the base platform allows imaging to below 1 Å by utilizing the "TrueImage" focus reconstruction algorithms embedded in the microscope user interface. This allows full exit-wave reconstruction to remove the artifacts caused by the spherical aberration of the objective lens, similar to the effect of an aberration corrector. The important factor for both aberration correctors as well as the reconstruction methods is the information limit of the microscope, which in the new instrument is well below 1 Å.
Strength in metals
The strength of a metal is determined by the forces acting between its constituent atoms. Permanent (plastic) deformation of a metal requires rearrangement of the bonds holding the atoms together. In the simplest analysis, any crystalline structure contains planes along which atoms may slide past each other if enough force is applied. The amount of force required is simply the strength of each atomic bond multiplied by the number of bonds. In this analysis a typical metal should be able to withstand forces much greater than a million pounds per square inch. In practice, metals are not this strong.
A closer look reveals that plastic deformation does not occur by the simultaneous failure of a large number of bonds along a slip plane but rather by the breaking and reforming of individual bonds one at a time. This requires much less force and accounts for the lower strength observed in real metals. Bulk metals are not typically single crystals. They are made up of many grains, each a separate crystal. Moreover, the crystalline structure of each grain contains dislocations where the crystalline structure is not perfect. Plastic deformation occurs as these dislocations move through the grain under applied stress.
A number of methods are used to strengthen metals, all of which depend fundamentally on impeding the movement of dislocations. The first metallurgists discovered that adding tin to copper yielded bronze, a material much stronger than either of its components. We now know that the increase in strength results from distortions in the copper crystalline lattice caused by substitution of larger tin atoms. When a dislocation migrating through the lattice encounters such a distortion it is trapped in place and cannot continue to move.
Other methods for increasing strength include reducing the grain size, since dislocations cannot move across grain boundaries between crystals and reducing grain size increases the boundary to volume ratio. Cold working, permanently deforming the metal by hammering or other means, causes dislocations to pile up against and become entangled with other dislocations, increasing the force required for further deformation. Various heat treatments rely on the introduction of distortions and precipitants as the solubility of constituent metals changes with temperature. For instance, dispersion hardening occurs when fine particles of alloying elements precipitate out as the metal cools from a saturated solution at a temperature high enough to permit recrystallization (annealing temperature).
It should be apparent from this discussion that the ability to see atomic scale
detail in metals is essential to understanding the mechanisms that contribute
to their strength. In much of our research we are concerned with understanding
the processes that degrade their strength leading eventually to failure. For instance,
the blades of a jet turbine are subjected to extreme conditions of heat and stress.
Likewise the frame of an automobile and the components of an automotive engine
are repeatedly heated or stressed in normal use. What are the mechanisms that
lead to failure and how can we generalize an understanding of those mechanisms
to design new materials and accurately predict their performance over time?
High resolution S/TEM
A transmission electron microscope (TEM) illuminates a very thin sample with a broad beam of electrons and forms a real, magnified image from electrons transmitted through the sample-much like a slide projector. A scanning transmission electron microscope (STEM, S/TEM refers to both techniques collectively) scans a finely focused beam over the sample and forms a virtual image that correlates a signal derived from transmitted electrons with the instantaneous position of the beam. Historically the primary limitation on S/TEM resolution has been spherical aberration in the magnetic lenses used to focus electrons. In STEM, the aberration limits the minimum diameter of the scanned beam. In TEM, it introduces blurring and delocalization in the projected image.
Methods exist to remove the effects of spherical aberration. For this reason the resolution capability of an S/TEM is usually specified by two different parameters: the point or image resolution which specifies the resolution that can be directly interpreted from an image, and the information limit, which specifies the resolution achievable if the effects of spherical aberration are removed. While point resolution is controlled by spherical aberration, the information limit is controlled primarily by the mechanical, electrical and thermal stability of the system.
Software corrections for spherical aberration rely on its predictable effects
as a function of the strength of the focusing field. By acquiring a series of
images at various focus settings it is possible to model and remove the aberration
effects from a reconstructed image. Recently, hardware correctors have become
available that correct the electron trajectories for the effects of lens aberration
and permit direct image interpretation with resolution down to the information
limit of the system. The images shown here have been software corrected.
Perhaps the most troublesome effect of lens aberrations for our application is delocalization. High-resolution images of regular crystalline structures showing what appear to be individual atoms have been commonplace for many years. However careful analysis of these images reveals that the contrast observed at any point in the image is actually a composite value including contributions from neighboring atoms-that is, neighboring image points. As long as the crystalline structure is perfect and continuous, delocalization may not be a problem. However, in our applications, it is precisely crystalline imperfections that we are interested in. Delocalization obscures these imperfections. Delocalization also has a deleterious effect where crystalline structure is discontinuous, as at grain boundaries or the boundary of a precipitated particle. For these reasons, the availability of corrections, whether hardware or software-based is critical to our investigations. And given the ability to correct aberration effects, the information limit becomes the primary metric for instrument performance.
Figure 1 demonstrates our use of the new instrument to investigate the composition and crystallography of nano-scaled second phase precipitates. The properties of a material are strongly dependent upon such second phase particles. However, their relationship (e.g., precipitation sequence, morphology, composition and crystallography) to the rest of the micro-structure is often unknown. The figure shows a nano-scaled-lath fully contained within a TiB precipitate in a Ti/TiB metal matrix composite. This particular alloy is interesting given the increased interest in dispersion strengthened titanium alloys based on TiB and TiC. With low amounts of both B and C, dispersions of TiB and TiC form and can significantly strengthen the alloy. Additionally, we have noted that the formation of TiB and TiC can have a pronounced effect on the precipitation sequence in these alloys, although we know little more at this time. The advanced S/TEM platform will allow us to determine the mechanisms involved in the precipitation sequences. It will also provide clear visibility of the interface between the precipitate and the matrix, which plays a crucial role in the modeling accuracy and ultimately determines whether the precipitate strengthens or weakens the material.
Further, the effect of interfacial composition (e.g., atomic partitioning) will be investigated, as that might have a strong influence on the orientation, and resulting strengthening effect, of the particle.
Road to fruition
Accelerating the development cycle for new materials has strategic importance for our ability to compete in the global marketplace. Traditional methods that depend on exclusively on empirical testing are necessarily risky, expensive, and time consuming. The development of accurate models to predict material performance and life expectancy could dramatically shorten the development cycle time.
High-resolution TEM will play a critical role in this effort, first, by providing the atomic scale visibility needed to understand material performance, and second, by providing the means to validate models without having to go to full scale testing. Developments in two key areas are essential in this role. The ability to correct the effects of spherical aberration in electron lenses, whether by software reconstruction or hardware correctors, allows direct interpretation of TEM images down to the information limit of the system. Moreover, careful attention to mechanical, thermal and electrical stability has pushed the information limit down to well below 1 Å. Together these developments have made atomic-scale imaging almost routine.
-Peter Collins,
-Hamish Fraser,
CAMM, Ohio State Univ.
-Jan Ringnalda, FEI Company
A large number of economically important industrial segments, including the aerospace and automotive sectors, depend heavily on high performance structural materials. However, financial concerns related to product reliability require a high degree of maturity from materials deployed in commercial applications. Conventional approaches to achieving this maturity require extensive programs of research and development based on laboratory testing and in-use experience, which in turn, result in high costs and long development cycles.
Traditionally, the high cost of materials development has been underwritten in part by government funding, usually through the military. In addition to funding, the military has provided a testing venue that is more willing to accept higher risk in exchange for higher performance. However, there is increasing public pressure to reduce military spending, particularly for long-term development programs that suffer technical setbacks or otherwise fail to meet expectations.
The Center for Accelerated Maturation of Materials (CAMM) at Ohio State Univ., Columbus, for its part, is a collaborative effort between government, industry, and academia, formed to explore new approaches to materials development through the combination of model-based computation and experimental validation. Specifically, it seeks to develop an integrated set of tools which will significantly reduce testing requirements and permit the rapid maturation of new materials with reduced costs and shorter cycle times. The intimate coupling of computational modeling accuracy and experimental characterization at the highest level without artifacts is key to this effort.
One area of focus in the program is microstructural characterization, aimed at providing researchers with an accurate physical picture of the materials they are modeling. To this end CAMM was involved at the early stages in a joint project with FEI Co., Hillsboro, Ore., to develop an advanced S/TEM (scanning/transmission electron microscope) platform, enabling the highest resolution imaging and microanalysis. CAMM was the first U.S. installation of the new tool. The instrument can be fitted with a monochromator as well as aberration correctors, to provide the sub-Ångstrom resolution necessary for atomic- level structural investigations.
In this article we show some preliminary results which are key to further the progress of CAMM, using the system as currently installed without aberration correctors. Correctors will be fitted in the coming months, however even the base platform allows imaging to below 1 Å by utilizing the "TrueImage" focus reconstruction algorithms embedded in the microscope user interface. This allows full exit-wave reconstruction to remove the artifacts caused by the spherical aberration of the objective lens, similar to the effect of an aberration corrector. The important factor for both aberration correctors as well as the reconstruction methods is the information limit of the microscope, which in the new instrument is well below 1 Å.
Strength in metals
|
A closer look reveals that plastic deformation does not occur by the simultaneous failure of a large number of bonds along a slip plane but rather by the breaking and reforming of individual bonds one at a time. This requires much less force and accounts for the lower strength observed in real metals. Bulk metals are not typically single crystals. They are made up of many grains, each a separate crystal. Moreover, the crystalline structure of each grain contains dislocations where the crystalline structure is not perfect. Plastic deformation occurs as these dislocations move through the grain under applied stress.
A number of methods are used to strengthen metals, all of which depend fundamentally on impeding the movement of dislocations. The first metallurgists discovered that adding tin to copper yielded bronze, a material much stronger than either of its components. We now know that the increase in strength results from distortions in the copper crystalline lattice caused by substitution of larger tin atoms. When a dislocation migrating through the lattice encounters such a distortion it is trapped in place and cannot continue to move.
Other methods for increasing strength include reducing the grain size, since dislocations cannot move across grain boundaries between crystals and reducing grain size increases the boundary to volume ratio. Cold working, permanently deforming the metal by hammering or other means, causes dislocations to pile up against and become entangled with other dislocations, increasing the force required for further deformation. Various heat treatments rely on the introduction of distortions and precipitants as the solubility of constituent metals changes with temperature. For instance, dispersion hardening occurs when fine particles of alloying elements precipitate out as the metal cools from a saturated solution at a temperature high enough to permit recrystallization (annealing temperature).
Al-Mg-Cu alloy. Image: CAMM |
High resolution S/TEM
A transmission electron microscope (TEM) illuminates a very thin sample with a broad beam of electrons and forms a real, magnified image from electrons transmitted through the sample-much like a slide projector. A scanning transmission electron microscope (STEM, S/TEM refers to both techniques collectively) scans a finely focused beam over the sample and forms a virtual image that correlates a signal derived from transmitted electrons with the instantaneous position of the beam. Historically the primary limitation on S/TEM resolution has been spherical aberration in the magnetic lenses used to focus electrons. In STEM, the aberration limits the minimum diameter of the scanned beam. In TEM, it introduces blurring and delocalization in the projected image.
Methods exist to remove the effects of spherical aberration. For this reason the resolution capability of an S/TEM is usually specified by two different parameters: the point or image resolution which specifies the resolution that can be directly interpreted from an image, and the information limit, which specifies the resolution achievable if the effects of spherical aberration are removed. While point resolution is controlled by spherical aberration, the information limit is controlled primarily by the mechanical, electrical and thermal stability of the system.
|
Perhaps the most troublesome effect of lens aberrations for our application is delocalization. High-resolution images of regular crystalline structures showing what appear to be individual atoms have been commonplace for many years. However careful analysis of these images reveals that the contrast observed at any point in the image is actually a composite value including contributions from neighboring atoms-that is, neighboring image points. As long as the crystalline structure is perfect and continuous, delocalization may not be a problem. However, in our applications, it is precisely crystalline imperfections that we are interested in. Delocalization obscures these imperfections. Delocalization also has a deleterious effect where crystalline structure is discontinuous, as at grain boundaries or the boundary of a precipitated particle. For these reasons, the availability of corrections, whether hardware or software-based is critical to our investigations. And given the ability to correct aberration effects, the information limit becomes the primary metric for instrument performance.
Figure 1 demonstrates our use of the new instrument to investigate the composition and crystallography of nano-scaled second phase precipitates. The properties of a material are strongly dependent upon such second phase particles. However, their relationship (e.g., precipitation sequence, morphology, composition and crystallography) to the rest of the micro-structure is often unknown. The figure shows a nano-scaled-lath fully contained within a TiB precipitate in a Ti/TiB metal matrix composite. This particular alloy is interesting given the increased interest in dispersion strengthened titanium alloys based on TiB and TiC. With low amounts of both B and C, dispersions of TiB and TiC form and can significantly strengthen the alloy. Additionally, we have noted that the formation of TiB and TiC can have a pronounced effect on the precipitation sequence in these alloys, although we know little more at this time. The advanced S/TEM platform will allow us to determine the mechanisms involved in the precipitation sequences. It will also provide clear visibility of the interface between the precipitate and the matrix, which plays a crucial role in the modeling accuracy and ultimately determines whether the precipitate strengthens or weakens the material.
Further, the effect of interfacial composition (e.g., atomic partitioning) will be investigated, as that might have a strong influence on the orientation, and resulting strengthening effect, of the particle.
Road to fruition
Accelerating the development cycle for new materials has strategic importance for our ability to compete in the global marketplace. Traditional methods that depend on exclusively on empirical testing are necessarily risky, expensive, and time consuming. The development of accurate models to predict material performance and life expectancy could dramatically shorten the development cycle time.
High-resolution TEM will play a critical role in this effort, first, by providing the atomic scale visibility needed to understand material performance, and second, by providing the means to validate models without having to go to full scale testing. Developments in two key areas are essential in this role. The ability to correct the effects of spherical aberration in electron lenses, whether by software reconstruction or hardware correctors, allows direct interpretation of TEM images down to the information limit of the system. Moreover, careful attention to mechanical, thermal and electrical stability has pushed the information limit down to well below 1 Å. Together these developments have made atomic-scale imaging almost routine.
-Peter Collins,
-Hamish Fraser,
CAMM, Ohio State Univ.
-Jan Ringnalda, FEI Company
Use of this website is subject to its terms of use.
Privacy Policy
