3-D Tomography and Ultrafast Laser-Material Interactions

The development of high fidelity material property models often requires three-dimensional information on the distribution of phases, grains or extrinsic defects. Concurrently, information on orientation and spatial distribution of elements may also be essential. Acquisition of this information in appropriate representative volume elements is a major challenge. We have developed femtosecond lasers for rapid layer-by-layer ablation provides new capabilities in terms of the volume of material that can be sampled as well as new opportunities for multimodal analysis.

The high pulse frequency (1 kHz) of ultra-short (150 fs) laser pulses can induce material ablation with virtually no thermal damage to the surrounding area. This technique has been demonstrated ex-situ with optical imaging and an example of a 3-D dataset where the distribution of micron-scale nitrides within mm3-scale volumes of a steel have been characterized is shown.

More recently, we have developed an in-situ “TriBeam” approach that combines the femtosecond laser within a focused ion beam platform to permit high resolution imaging, as well as crystallographic and elemental analysis, without intermediate surface preparation or removal of the sample from the chamber. The TriBeam system, shown schematically below, combines the high resolution and broad detector capabilities of the DualBeam FEI DB235 microscope with the high material removal rates of the femtosecond laser, allowing 3D datasets to be acquired at rates 4 to 6 orders of magnitude faster than 3D FIB datasets. The TriBeam platform couples the laser and electron optics systems and employs a positioning of a stage that can quickly and automatically be located to the multiple analysis positions. This system also allows for acquisition of “multimodal” datasets wherein structural information as well as chemical information (EDS) and crystallographic information (EBSD) can be quickly acquired for each material slice.

A first demonstration of the capability of this system is shown below, where multiple slices of a pure polycrystalline Ni sample have been acquired for analysis of grain size, shape and misorientation within the TriBeam system.

Selected Publications

M.P. Echlin, A. Mottura, C.J. Torbet and T.M. Pollock, “A New TriBeam System for Three Dimensional Multimodal Materials Analysis, Rev. Sci. Inst. 83, 023701, (2012).

M. Echlin, N.S. Husseni, J. Nees and T.M. Pollock, “A New Femtosecond Laser-Aided Tomography Technique for Multiphase Materials”, Advanced Materials, 23, 2339 - 2342 (2011).

A. Kumar and T.M. Pollock, “Mapping of femtosecond laser-induced collateral damage by electron backscatter diffraction", J. Appl. Phys, 110, 083114, (2011).

J. Madison, J.E. Spowart, D.J. Rowenhort and T.M. Pollock, “Three Dimensional Reconstruction of the Dendritic Structure at the Solid-Liquid Interface of a Ni-Base Single Crystal”, JOM, Vol. 7, 26 – 30 (2008).

J. P. McDonald, S. Ma, T. M. Pollock, S. M. Yalisove, “Femtosecond pulsed laser ablation dynamics and damage morphology of nickel based superalloy CMSX-4’, Journal of Applied Physics, 103, 093111 (2008).

S. Ma, B. Tryon, J.P. McDonald, S.M. Yalisove and T.M. Pollock, “Femtosecond Laser Ablation Regimes in a Single Crystal Superalloy”, Metallurgical and Materials Transactions A, 38A, 2153 – 2161, (2007).

Intermetallic Bond Coats and Thermal Barrier Coatings

Our research aims to discover, design and synthesize new material systems to overcome the fundamental barriers to higher temperature operation with requisite durability in future gas turbines. Because of the complexity of the system and phenomena involved, a systems perspective is essential, i.e. the research must not only the individual layers of the system but also their interplay as they evolve over time, and their implications for material discovery and design. The systems perspective demands an understanding of both intrinsic and extrinsic failure modes within these complex, multilayered systems under a range of thermomechanical cycling conditions. We are developing models for damage growth under cyclic conditions that can guide development of strain compatible coating systems and exploring new compositional domains for failure-resistant coatings for high temperature applications.

Selected Publications

C. Mercer, K. Kawagishi, T. Tomimatsu, D. Hovis and T.M. Pollock, “A Comparative Investigation of Oxide Formation on EQ and NiCoCrAlY Bond Coats under Stepped Thermal Cycling”, Surface and Coatings Technology, 205, 3066 - 3072 (2011).

T.M. Pollock, B. Laux, C.L. Brundidge, A. Suzuki and M. He, “Oxide Assisted Degradation of Ni-Base Single Crystals during Cyclic Loading: The Role of Coatings”, Journal of the American Ceramic Society, 94, S136 – S145, (2011).

A. Suzuki, M.F.X. Gigliotti, B.T. Hazel, D.G. Konitzer and T.M. Pollock, “Crack Progression during Sustained Peak Low Cycle Fatigue in René N5, Metall. Mater. Trans. 41A, 948 – 956, (2010).

D.K. Das, B. Gleeson, K.S. Murphy, S. Ma and T.M. Pollock, “Formation of Secondary Reaction Zone in Ruthenium-Bearing Nickel Base Superalloys with Diffusion Aluminide Coatings”, Materials Science and Technology, Vol. 25, 300 – 308, (2009).

D.K. Das and T.M. Pollock, “Femtosecond Laser Machining of Cooling Holes in Thermal Barrier Coated CMSX4 Superalloy, J. Mater. Proc. Tech., 209, 5661 - 5668, (2009).

F. Cao, B. Tryon, C.J. Torbet and T.M. Pollock, “Microstructural Evolution and Failure Characteristics of a NiCoCrAlY Bond Coat in “Hot Spot” Cyclic Oxidation”, Acta Materialia 57, 3883 – 3894, (2009).

D.K. Das, J.P. McDonald, S.M. Yalisove and T.M. Pollock, “Depth Profiling Study of a Thermal Barrier Coated Superalloy using Femtosecond Laser-Induced Breakdown Spectroscopy”, Spectrochemical Acta B 63 27 – 36, (2008).

D.K. Das, J.P. McDonald, S.M. Yalisove and T.M. Pollock, Femtosecond pulsed laser damage characteristics of 7%Y2O3-ZrO2 thermal barrier coating, Appl. Phys. A 91421 – 428, (2008).

A.G. Evans, M.Y. He, A. Suzuki, M. Gigliotti, B. Hazel and T.M. Pollock, “The Mechanism Governing Sustained Peak Low Cycle Fatigue of Coated Superalloys”, Acta Materialia, 57, 2969 – 2983, (2009).

D.K. Das, K.S. Murphy, S. Ma and T.M. Pollock, “Approaching Non-destructive surface chemical analysis of CMSX-4 superalloy with double-pulsed laser induced breakdown spectroscopy”, Spectrochemical Acta B 63 561 - 565, (2008).

B. Tryon, K.S. Murphy, C.G. Levi, J. Yang, and T.M. Pollock, “Hybrid Intermetallic Ru/Pt-modified Bond Coatings for Thermal Barrier Systems, Surface and Coatings Technology 202, 349 – 361, (2007).

Ni-base Single Crystals

Nickel base single crystals are among the major materials achievements of the twentieth century. Among all structural materials, nickel-base alloys are unique for their ability to operate at 90% of their melting point with substantial mechanical loads in chemically severe environments. Design advances in these materials have been synonymous with advances in the performance and efficiency in aircraft and power-generation turbines, rocket engines, and nuclear power plants. Our research focuses on the design of new alloys that are compatible with advanced thermal barrier coating systems, development of new single crystal solidification processes that can produce physically large single crystals for power generation systems, development of rejuvenation approaches to extend the lifetimes of these high value materials and for predicting their behavior during processing and in service environments.

Selected Publications

J. Madison, J. Spowart, D. Rowenhorst, L.K. Aagesen, K. Thornton and T.M. Pollock, “Fluid Flow and Defect Formation in the 3-Dimensional Dendritic Structure of Nickel-Base Single Crystals”, Metallurgical and Materials Transactions 43A, 369, (2012).

C.L. Brundidge, J.D. Miller and T.M. Pollock, “Development of Dendritic Structure in the Liquid-Metal Cooled Directional Solidification Process”, Metallurgical and Materials Transactions 42A, 2723 – 2732, (2011).

J. S. Van Sluytman, A. La Fontaine, J.M. Cairney and T.M. Pollock, “Elemental Partitioning of Platinum Group Metal Containing Ni-base Superalloys using Electron Microprobe Analysis and Atom Probe Tomography”, Acta Materialia 58 1952 – 1962, (2010).

J. Madison, J. Spowart, D. Rowenhorst, L.K. Aagesen, K. Thornton and T.M. Pollock, “Modeling Fluid Flow in Three-Dimensional Single Crystal Dendritic Structures”, Acta Mater. , 2864 – 2875, (2010).

A.J. Heidloff, J. Van Sluytman, T.M. Pollock and B. Gleeson, “Structural Stability of Platinum Group Metal Modified g + g’ Ni-base Alloys”, Metallurgical and Materials Transactions 40A, 1529 – 1540, (2009).

N. Zhou, C. Shen, P.M. Sarosi, M.J. Mills, T.M. Pollock and Y. Wang, “g’ Rafting in Single Crystal Blade Alloys – A Simulation Study”, Materials Science and Technology, Vol. 25, 300 – 308, (2009).

L.J. Carroll, Q. Feng and T.M. Pollock, “Interfacial Dislocation Networks and Creep in Directionally Coarsened Ru-Containing Nickel-Base Single Crystal Superalloys”, Metallurgical and Materials Transactions 39A, 1647 - 1657, (2008).

N. S. Husseini, D.P. Kumah, J. Z. Yi, C. J. Torbet, D.A. Arms, E. M. Dufresne, T. M. Pollock, J. W. Jones and R. Clarke, “Mapping Single Crystal Dendritic Microstructure and Defects in Ni-Base Superalloys with Synchrotron Radiation”, Acta Materialia, 56, 4715 - 4723, (2008).

T.M. Pollock and S. Tin, "Nickel-Based Superalloys for Advanced Turbine Engines: Chemistry, Microstructure and Properties", AIAA J. Propulsion and Power, 22, 2, (2006), pp. 361 - 374.

New L12-Containing Cobalt-Base Alloys

New structural and functional materials enable a multiplicity of paths to improved efficiency in energy generation, storage, transmission and conversion. While alternative energy technologies are highly desirable, for the foreseeable future fossil fuels will be a primary energy source. This motivates discovery of new structural materials that can increase the operating temperatures within energy generation systems and provide critically needed improvements in the efficiency of power generation.

The discovery of a stable ternary Co3(Al, W) intermetallic compound with an ordered L12 structure has provided a pathway for development of a new class of load-bearing cobalt-base high temperature alloys. The thermodynamic coexistence of the γ -Co solid-solution phase with fcc structure and the γ ' - Co3(Al, W) phase and the similarity of their lattice parameters permit establishment of a two-phase structure with a high degree of coherency. This structure consists of submicron regularly aligned cuboidal γ ' precipitates embedded in a continuous γ matrix phase. This structure is morphologically identical to the microstructure of Ni-base superalloys. Our recent research on ternary and quaternary variants of these two-phase L12-containing systems indicates a potential temperature benefit of up to 150˚C relative to current nickel-base alloys. This translates to a very substantial benefit in energy efficiency for advanced turbine systems. To achieve these benefits, a broader range of composition space must be explored in an efficient manner to optimize properties and concurrently identify compatible environmental and thermal barrier coating systems.

Selected Publications

T.M. Pollock, J. Dibbern, M. Tsunekane, J. Zhu and A. Suzuki, “New Co-based γ – γ’ High Temperature Alloys”, JOM, 58, (2010).

M. Tsunekane, A. Suzuki and T.M. Pollock, “Single Crystal Solidification of New Co-Al-W-base Alloys”, J. Intermetallics 19. 636 - 643, (2011).

M. Titus, A. Suzuki and T.M. Pollock, “Creep Deformation and Directional Coarsening in Single Crystal L12-Strengthened Cobalt-Base Alloys”, Scripta Mater., in press, (2012).

M. Titus, A. Suzuki and T.M. Pollock, “High Temperature Creep of New L12-Containing Co-base Alloys”, submitted to Proc. 12th International Symposium on Superalloys, (2012).

A. Mottura, A. Janotti and T.M. Pollock, “A First Principles Study of the Effect of Ta on the Superlattice Instrinsic Stacking Fault Energy in L12 – Co3(Al,W), submitted to J. Intermetallics, (2011).

A. Mottura, A. Janotti and T.M. Pollock, “Alloying Effects in the γ’ Phase of Co-Based Superalloys, Proc. 12th International Symposium on Superalloys, Superalloys, (2012).

M. Knop, B. Laux and T.M. Pollock, “Cyclic Oxidation Behavior of New γ’ Strengthened Co-Base Alloys”, to be submitted to Metallurgical and Materials Transactions, (2012).

N Vermaak, A. Mottura and T.M. Pollock, New γ’ Strengthened Cobalt-base alloys: MCrAlY Coating Interactions”, submitted to Surf. Coatings Technology, January, (2012).

A. Suzuki and T.M. Pollock, “High Temperature Strength and Deformation of γ-γ’ Two Phase Co-Al-W Alloys”, Acta Materialia, 1288 – 1297, (2008).

A. Suzuki, G.C. DeNolf and T.M. Pollock, “Flow Stress Anomalies in γ/ γ’ Two-phase Co-Al-W Base Alloys”, Scripta Materialia, 56, pp. 385 – 388, (2007).

Ultrahigh Cycle, High Frequency Fatigue

We have developed a unique capability for high temperature fatigue testing in the kHz frequency range for characterization of the refractory-rich metallic and CMC materials. The high temperature fatigue apparatus consists of a load line connecting a series of components tuned to 20 kHz, including an ultrasonic converter that provides a controlled sinusoidal displacement and an ultrasonic horn that amplifies the axial displacement. This assembly is located within a hydraulic load frame that can superimpose static loads. Testing can be conducted on conventional fatigue specimens or on small sheet samples using a carrier specimen that resonates, imposing a displacement on the thin specimen attached. This system has been utilized for high cycle fatigue testing of Ni-base single crystal specimens at temperatures up to 1000˚C with samples as thin as 150 μm. Various heating approaches have been utilized, including induction heating for larger samples and microtorch approaches for small sheet samples.

Selected Publications

J. Miao, T.M. Pollock and J.W. Jones, “Microstructural Extremes and the Transition from Fatigue Crack Initiation to Small Crack Growth in a Polycrystalline Nickel-base Superalloy”, Acta Materialia, in press (2012).

L. Liu, N. Husseini, C. J. Torbet, D. Kumah, R. Clarke, T. M. Pollock and J. W. Jones, In-Situ Imaging of High Cycle Fatigue Crack Growth in Single Crystal Superalloys by Synchrotron Radiation”, ASME Journal of Materials and Engineering Technology, 130, 021008-1 to 6, (2008).

J.Z. Yi , C.J. Torbet, Q. Feng, T.M. Pollock and J.W. Jones, “Ultrasonic Fatigue of a Single Crystal Superalloy at 1000˚C, Materials Science and Engineering A, 443. 142 - 149 (2007).

A. Shyam, C. J. Torbet, S. K. Jha, J.M. Larsen, M. J. Caton, C. J. Szczepanski, T. M. Pollock, J.W. Jones, "Development of ultrasonic fatigue for rapid high temperature fatigue studies in turbine engine materials," in Materials Damage Prognosis, TMS, Warrendale, PA, pp. 247-252, (2005).

Non-linear Acoustics and Damage Development in Materials

A new methodology for in-situ characterization of evolving damage using nonlinear ultrasonic measurements via analysis of the feedback signal of an ultrasonic fatigue system. Nonlinear ultrasonic studies explore the generation of second and higher harmonics of the fundamental frequency due to distortion of sinusoidal ultrasonic waves as they propagate through a nonlinear or anharmonic solid. We have observed for the first time that the ultrasonic nonlinearity during fatigue cycling increases with evolution of dislocation structure, crack initiation and growth of damage.

Selected Publications

A. Kumar, R.R. Adharapurapu, J.W. Jones and T.M. Pollock, “In-situ Damage Assessment in a Cast Magnesium Alloy during Very High Cycle Fatigue”, Scripta Materialia 64 65 – 68, (2011).

A. Kumar, C.J. Torbet, T.M. Pollock and J.W. Jones “In Situ Characterization of Fatigue Damage Evolution in a Cast Al Alloy via Nonlinear Ultrasonic Measurements”, Acta Mater. 58, 2143 – 2154, (2010).

A. Kumar, C.J. Torbet, J.W. Jones and T.M. Pollock, “Nonlinear Ultrasonics for In-situ Damage Detection during High Frequency Fatigue”, J.Appl. Phys, 106, 024904 (2009).

Materials for Hypersonic Flight

Hypersonic flight vehicles could enable a range of future U.S. defense, aviation and space missions. However, the extreme environmental conditions associated with high Mach number flight pose a major challenge for vehicle materials and structures, particularly within the hybrid ramjet/scramjet/rocket propulsion systems envisioned to date. In propulsion systems such as those utilized in commercial and military gas turbine engines, the performance requirements are sufficiently well understood that property goals for materials development can be set and new materials developed and tested in isolation from the propulsion design process with reasonable certainty of success. The complexity of hypersonic propulsion systems, however, requires closer coupling of design principles with new materials development to achieve expanded levels of performance and structural durability.

The overarching goal of this research is to define new strategies for development of structural materials for air-breathing hypersonic propulsion systems. The approach includes (a) development of a suite of materials models that integrate key material properties with design models, (b) creation of new characterization and test techniques that support model development and (c) employment of these new modeling and characterization techniques to direct development of emerging materials and for discovery of new materials systems.

Selected Papers

S. J. Pérez-Bergquist, N. Vermaak and T. M. Pollock, “High Temperature Performance of Actively-Cooled Vapor Phase Strengthened Nickel-Base Thermostructural Panels”, AIAA J., 49, 1080 – 1086, (2011).

R. Rhein, M. D. Novak, C. G. Levi and Tresa M. Pollock, “Bimetallic Low Thermal Expansion Panels of Co-Base and Silicide Coated Nb-Base Alloys for High Temperature Structural Applications”, Materials Science and Engineering A528, 3973 – 3980, (2011).

S.J. Johnson, B. Tryon and T.M. Pollock, “Post-Fabrication Vapor Phase Strengthening of Nickel-Based Sheet Alloys For Thermostructural Panels”, Acta Materialia, 56, 4577 – 4584, (2008).

Multiphase Nano-scale Materials

Remarkable progress in the synthesis of nanoparticles along with their functionalization and assembly into “multi-material” architectures has enabled new domains of optical, electronic, and magnetic material properties to be accessed. A remaining grand challenge is to build these unique properties into inorganic “hard” materials that can be fabricated in sufficient quantities, and that possess the requisite stability for operation in engineering environments. Several collaborative projects in this area aim to develop a unifying set of experimental and theoretical techniques that enable the design, creation and control of the structure of biphasic functional materials. Unique properties will be achieved via directed, spontaneous synthesis paths that result in robust, morphologically stable two-phase nanoscale architectures that exhibit unusual thermal, magnetic, or radiation-resistant properties.

Integrated Computational Materials Engineering

The convergence of new computational capabilities, advanced characterization techniques and the ability to generate and harness large-scale data enables new pathways for the discovery, development and deployment of advanced materials systems. Historically, the lack of an integrated predictive tool set has resulted in strongly experimentally-driven approaches to the development of new materials. While this approach has yielded viable high performance systems, the large experimental matrices demand substantial financial resources and are often too slow to address barriers encountered late in development, either in processing when shortcomings in higher order properties are uncovered or when unanticipated failure modes are encountered upon introduction into a realistic service environment. Given the magnitude of the challenge for a fully computational approach, for the foreseeable future, we are developing approaches wherein new experimental and computational tools are coupled for rapid, iterative discovery and development.

Selected Publications

W. Tu and T.M. Pollock, “Deformation and Strain Storage Mechanisms during High Temperature Compression of a P/M Ni-based Superalloy”, Metall. Mater. Trans. 41A, 2002 - 2009 (2010).

R.W. Kozar, A. Suzuki, W.W. Milligan and T.M. Pollock, “Strengthening Mechanisms in Polycrystalline Multimodal Ni-base Superalloys”, Metallurgical and Materials Transactions 40A, 1588 – 1603, (2009).

S. J. Pérez-Bergquist, N. Vermaak and T. M. Pollock, “High Temperature Performance of Actively-Cooled Vapor Phase Strengthened Nickel-Base Thermostructural Panels”, AIAA J., 49, 1080 – 1086, (2011).

“Integrated Computational Materials Engineering: A Transformative Discipline for Improved Competitiveness and National Security”, National Academies Press, (2008).

Lightweight Magnesium Alloys

The compelling need for lightweight, energy-efficient, environmentally benign engineering systems is driving the development of a wide range of structural and functional materials for energy generation, energy storage, propulsion, and transportation. These challenges motivate widespread use of magnesium—the eighth most common element in the earth’s crust and also extractable from seawater. In addition, the ease of recycling, compared with polymers, makes magnesium alloys environmentally attractive. Importantly, with a density of 1.74 g/cm3—about 30% less than aluminum, one-quarter that of steel, and nearly the same as many polymers—magnesium is attractive for lightweight structural systems and, notably including, automotive systems. Major challenge for this class of materials remains in the definition of alloys with sufficient combinations of strength and ductility and processing paths that can produce these materials in sufficient quantities. Our research explores new Mg systems and aims to develop new tools for the development of new alloys and new processing approaches.

Selected Publications

T.M. Pollock, “Weight Loss with Magnesium Alloys”, Science 238, 5981, 986 - 987, (2010).

T.D. Berman, W. Donlon, R. Decker, J. Huang, T.M. Pollock and J.W. Jones, “Microstructure Evolution in AZ61L during TTMP and Subsequent Annealing Treatments”, Magnesium Technology 2011, TMS, Warrendale, PA, (2011).

N.D. Saddock, A. Suzuki, J.W. Jones and T.M. Pollock, “Grain-scale Creep Processes in Mg-Al-Ca Alloys: Implications for Alloy Design”, Scripta Materialia Viewpoint Paper, 692 - 697 (2010).

J. R. TerBush, N. D. Saddock, J. W. Jones and T.M. Pollock, “Partitioning of Solute to the Primary α-Mg Phase Composition in Mg-Al-Ca-based Cast Alloys”, Metallurgical and Materials Transactions 41A, 2435 – 2442, (2010).

A. Suzuki, N.D. Saddock, J.R. TerBush, B.R. Powell, J.W. Jones and T.M. Pollock, “Precipitation Strengthening of a Mg-Al-Ca-Based AXJ530 Die-cast Alloy”, Metallurgical and Materials Transactions 39A, 696 – 702, (2008).

A. Suzuki, N.D. Saddock, J.W. Jones and T.M. Pollock, “Solidification Paths and Intermetallic Phases in Mg-Al-Ca Ternary Alloys, Acta Materialia 53, 2823, (2005).