Additive Manufacturing: Advanced Characterization with Synchrotron, Neutron, and In Situ Laboratory-scale Techniques: Poster Session
Sponsored by: TMS: Additive Manufacturing Committee
Program Organizers: Fan Zhang, National Institute of Standards and Technology; Tom Stockman, Los Alamos National Laboratory; Tao Sun, Northwestern University; Donald Brown, Los Alamos National Laboratory; Yan Gao, Ge Research; Amit Pandey, Lockheed Martin Space; Joy Gockel, Wright State University; Tim Horn, North Carolina State University; Sneha Prabha Narra, Carnegie Mellon University; Judy Schneider, University of Alabama at Huntsville
Monday 5:30 PM
February 24, 2020
Room: Sails Pavilion
Location: San Diego Convention Ctr
A-18: HEX, A New High Energy Beamline in the Making: Michael Drakopoulos1; Zhong Zhong1; Andy Broadbent1; 1Brookhaven National Laboratory
The mission of the High Energy Engineering X-ray Scattering (HEX) beamline at the National Synchrotron Light Source II (NSLS-II) is to help advance clean energy technologies and creative energy storage solutions. This facility will provide world-leading x-ray imaging capabilities to support engineering materials development in NY State and beyond. HEX will operate in the 30-250 keV energy range and will be capable of delivering monochromatic and white beam. Samples to be studied include fully assembled and functioning batteries, photovoltaic materials, fuel cells, and engineering materials under extreme conditions. The scientific techniques will consist of imaging and diffraction. Imaging includes fast radiography and tomography in attenuation and phase contrast mode. Diffraction can be monochromatic powder-methods on area detectors or white light energy-dispersive methods to separate small sample volumes from surrounding substances.
A-19 (Invited): The Effects of Chemical Composition on Precipitate Evolution in an Additively Manufactured Nickel Base Superalloy: James Zuback1; 1Pennsylvania State University
Small variations in minor alloying element concentrations lead to remarkable differences in the grain structure, precipitate morphology, and tensile properties of Inconel® 625 fabricated by laser based directed energy deposition additive manufacturing. The secondary phases, which contribute to these different properties, originate from a complex interplay between the initial iron, silicon, and titanium contents of the powder feedstock. Relatively high amounts of silicon promote the formation of Laves phase, while high amounts of titanium favor MN-type nitrides. Ex-situ characterization revealed that the type, composition, and morphology of these precipitates evolve during hot isostatic pressing, although their total volume fraction remained virtually unchanged. The precipitate evolution was elucidated by in-situ high energy synchrotron x-ray diffraction measurements during a heat treatment replicating the thermal history of hot isostatic pressing. It is proposed that precipitate morphology can be controlled by careful selection of powder with appropriate amounts of minor alloying elements.
A-20: Accessing Nano-scale Structure and Dynamics During 3D-Printing by Operando X-Ray Photon Correlation Spectroscopy: Maria Torres Arango1; Yugang Zhang1; Gregory Doerk2; Ruipeng Li1; Chonghang Zhao3; Yu-chen Karen Chen-Wiegart4; Andrei Fluerasu1; Lutz Wiegart1; 1National Synchrotron Light Source II, Brookhaven National Laboratory; 2Center for Functional Nanomaterials, Brookhaven National Laboratory; 3Stony Brook University; 4National Synchrotron Light Source II, Brookhaven National Laboratory. Stony Brook University.
Despite the increasing interest in printing technologies for additive manufacturing (AM), important challenges remain, mainly associated with the printing method’s inherent anisotropy, out-of-equilibrium processing stages (extrusion and subsequent relaxation), and material-specific contributions. Therefore, accessing the nano-scale dynamics and structural evolution during printing is key for understanding fundamental processing-properties relationships, affecting the materials self-assembly, defect formation/propagation, and ultimately performance in devices/components. We conduct operando x-ray photon correlation spectroscopy (XPCS) studies during 3D-printing, at the Coherent Hard X-Ray (CHX) 11-ID beamline, National Synchrotron Light Source II. We investigate the nano-scale spatio-temporal evolution during the out-of-equilibrium stages of extrusion-based direct ink writing of viscoelastic colloidal systems, exploring different formulations and contributions of shape retention mechanisms to the printing process. We expect our investigations to shed light on the material’s geometric fidelity and self-assembly characteristics, contributing to the theoretical understanding and modelling of AM processes, and opening opportunities for engineering materials with novel properties.
A-21: Building a Novel Heat Exchanger with Haynes 230 Alloy and Using Data Science to Characterize Rheological and Microstructural Properties in Additive Manufacturing: Srujana Rao Yarasi1; Andrew Kitahara1; Ziheng Wu1; Anthony Rollett1; Elizabeth Holm1; 1Carnegie Mellon University
Additive Manufacturing has enabled novel designs, without restrictions in their complexity, to be produced using superalloys such as Haynes 230. Laser Powder Bed Fusion method is used to fabricate a novel Heat Exchanger using the Haynes 230 superalloy, after optimizing build parameters to minimize defects. There is immense potential in the use of computer vision and machine learning tools in the additive manufacturing domain. This ranges from the quantitative investigation of qualitative factors like powder morphology, which affects the flowability in powder bed fusion processes, to the characterization and analysis of microstructures. Flowability is measured through rheological experiments conducted with the FT4 rheometer and the GranuDrum. The use of Convolutional Neural Networks (CNN) to generate hypercolumn descriptors is proposed as part of a framework that describe characteristics of the powder feedstock such as particle size distribution, sphericity, surface defects, and other morphological features as well as microstructural features.
A-22: Characterizing the Deformation in Single Cell Ti-5553 Lattice Structures: Maria Strantza1; Nathan Johnson2; Donald Brown2; Jenny Wang1; Jefferson Cuadra1; David Macknelly1; John Carpenter2; Manyalibo Matthews1; 1Lawrence Livermore National Laboratory; 2Los Alamos National Laboratory
Additive manufacturing (AM) brings new possibilities to the production of lightweight, high stiffness lattice structures with various densities. The load bearing properties, the strength-to-weight ratio and the tailored shock absorption are the main mechanical benefits of the lattice structures. However, their deformation behavior is complex and challenging to be predicted. In this investigation, we perform in-situ diffraction experiments during the deformation of Ti-5553 lattice structures using synchrotron X-ray diffraction. Prior to the in-situ diffraction experiments, the density of the AM samples was characterized using CT imaging. Our aim is to use the lattice parameter of the material in order to understand the behavior of single cell lattice structures under compression, which will permit improved design optimization of the lattice structures for specific applications. Our results are also used to validate computational models for the prediction of the deformation behavior of the lattice structures. Prepared by LLNL under Contract DE-AC52-07NA27344.
A-23: Creep Behavior of a AlSiMg Alloy Produced by Additive Manufacturing: Chiara Paoletti1; Stefano Spigarelli1; Marcello Cabibbo1; Emanuela Cerri2; 1Università Politecnica delle Marche; 2Università di Parma
The present study aims at investigating the effect of the microstructure of additive manufactured samples on the creep beavhior of a AlSiMg alloy. Constant load creep experiments were carried out between 150, 175 and 205°C on an AlSiMg alloy produced by Powder Bed Fusion AM. The samples were mostly strained up to rupture, although in some cases the test were interrupted at the early onset of the tertiary region. Analyzing the time-to rupture,, in the different load and temperature conditions, as a function of the applied stress, it can be clearly seen that the alloy produced by AM is substantially comparable, in terms of time to rupture, with an alloy of similar composition, tested in the die-cast state. The high values of the stress expontent suggest that the creep behavior is strongly affected by the presence of secondary-phase particles.
A-24: Effect of Laser-matter Interaction on Molten Pool Flow and Keyhole Dynamics: Nadia Kouraytem1; Xuxiao Li1; Ross Cunningham2; Cang Zhao3; Niranjan Parab3; Tao Sun3; Anthony Rollett2; Ashley Spear1; Wenda Tan1; 1University of Utah; 2Carnegie Mellon University; 3Argonne National Laboratory
The dynamics of laser-induced keyholes, pertaining to laser-based additive manufacturing, are not fully understood. In this study, the state-of-the-art dynamic x-ray radiography (DXR) technique is used to observe, in-situ, laser-induced keyholes for a combination of laser powers and scanning speeds in SS304. The observations from DXR are used to populate a processing map of keyhole morphology for SS304. A multiphase, multiphysics numerical model is then used to predict keyhole morphology and is subsequently validated using the experimentally observed keyhole morphologies and fluctuation. After validation, the simulation results are further leveraged to elucidate the details of both the formation and dynamics of laser-induced keyholes. Particularly, three typical morphologies of keyholes are discussed in terms of laser-matter interaction (specifically, the spatial distribution and temporal variation of laser absorption), thermofluidic flow, keyhole-wall motion, and major competing forces. The combination of experimental and modeling techniques provides a powerful, unprecedented tool for elucidating keyhole dynamics.
A-25: Evaluating the Effectiveness of Compliant Substructures at Controlling Residual Stress in Additively Manufactured Components: Donald Brown1; Maria Strantza2; Bjorn Clausen1; Rishi Ganeriwala2; Lyle Levine3; Thien Phan3; Wayne King2; 1Los Alamos National Laboratory; 2Livermore National Laboratory; 3National Institute of Standards and Technology
Additive manufacturing can produce complex and lightweight structures with a high level of flexibility and minimal waste. However, the thermal history of the additively manufactured (AM) components can result in the development of unwanted and, frequently, detrimental residual stresses. Establishing the ability to predict and control residual stresses in AM parts could lead to the design of beneficial residual stresses, which could improve the resultant properties. Various strategies to mitigate and/or control the residual stresses in AM components have been attempted. For instance, building the component on a mesh substructure changes both the path for heat to escape the component and the mechanical constraints on the component. In this work, we use high-energy X-ray diffraction to evaluate residual stresses in AM’ed bridge shape components made from Ti-5553, built directly on a solid substrate and with substructures of varying stiffness.
A-26: High Speed X-ray Diffraction: Niranjan Parab1; Seunghee Oh2; Joseph Aroh2; Joseph Pauza2; Sidi Feng2; Rachel Lim2; Christopher Kantzos2; Robert Suter2; Chihpin Chuang3; Cang Zhao3; Tao Sun3; Anthony Rollett2; 1Argonne Natl. Laboratory; 2Carnegie Mellon University; 3Argonne National Laboratory
Recent developments in additive manufacturing (AM) allow extremely complex parts to be produced that are not attainable with traditional techniques. Using a set-up similar to that used to observe particle dynamics during laser melting, an in situ high-speed high-energy synchrotron X-ray dynamic diffraction technique at the 1-ID beamline at the Advanced Photon Source allows us to monitor diffraction patterns at high-speed frame rate, of order 250 Hz. This technique allows us to quantify transient behaviors including phase transformations, thermal effects and microstructural evolution during the laser melting and re-solidification of metals. A 500 W fiber-laser beam coupled with a scan head was used to produce micro-weld beads. Temperature can be inferred via thermal expansion. Residual strain development appears as peak broadening. Unexpected precipitation behavior has been observed in, e.g., IN 718.
A-27: Identifying the Formation of Laser Powder Bed Fusion Defects In-situ by Coupling High Speed X-ray and Infra-red Imaging: Benjamin Gould1; Sarah Wolff2; Niranjan Parab1; Cang Zhao1; Aaron Greco1; Tao Sun1; 1Argonne National Laboratory; 2Texas A&M University
Laser powder bed fusion has become increasingly popular over the past decade. However, the reproducibility of parts manufactured by this technique is still a problem, overcoming which requires the understanding of the keyhole and melt pool dynamics during the process. In this work, we synchronize the high-speed X-ray imaging with high-speed infra-red imaging to study the laser powder bed fusion processes in real time. Using this technique, we demonstrate the simultaneous observation of many dynamic physical phenomena that affect the quality of the additively manufactured parts. Additionally, the formation of observable print defects in the X-ray images were correlated to distinct features observable via high speed Infra-red imaging of the surface of the print.
A-28: In-situ Characterization and Quantification of Melt Pool Variation Under Constant Input Energy Density in Laser Powder-bed Fusion Additive Manufacturing Process: Qilin Guo1; Cang Zhao2; Minglei Qu1; Lianghua Xiong1; Luis I. Escano1; S. Mohammad H. Hojjatzadeh1; Niranjan D. Parab2; Kamel Fezzaa2; Wes Everhart3; Tao Sun2; Lianyi Chen1; 1Missouri University of Science and Technology; 2Argonne National Laboratory; 3Honeywell FM&T
Size and shape of a melt pool play a critical role in determining the microstructure in additively manufactured metals. However, it is very challenging to directly characterize the size and shape of the melt pool beneath the surface of the melt pool during the additive manufacturing process. Here, we report the direct observation and quantification of melt pool variation during the laser powder-bed fusion additive manufacturing process by synchrotron x-ray imaging. We show that the melt pool can undergo different melting regimes and both the melt pool dimension and melt pool volume can have orders-of-magnitude change under a constant input energy density. We found that energy absorption changes significantly under a constant input energy density, which is an important cause of melt pool variation. Our further analysis reveals that the significant change in energy absorption originates from the separate roles of laser power and scan speed in depression zone development.
A-29: In-situ Full-field Mapping of Melt Flow Dynamics in Laser Metal Additive Manufacturing: Qilin Guo1; Cang Zhao2; Minglei Qu1; Lianghua Xiong1; Luis I. Escano1; S. Mohammad H. Hojjatzadeh1; Niranjan D. Parab2; Kamel Fezzaa2; Tao Sun2; Lianyi Chen1; 1Missouri University of Science and Technology; 2Argonne National Laboratory
Melt flow plays a critical role in laser metal additive manufacturing, yet the melt flow behavior within the melt pool has never been explicitly presented. Here, we report in-situ characterization of melt-flow dynamics in every location of the entire melt pool in laser metal additive manufacturing by populous and uniformly dispersed micro-tracers through in-situ high-resolution synchrotron x-ray imaging. The location-specific flow patterns in different regions of the melt pool are revealed and quantified. The dominating physical processes at different locations in the melt pool are identified. The full-field melt-flow mapping approach reported here opens the way to study the detailed melt-flow dynamics under real additive manufacturing conditions. The results obtained provide crucial insights into laser additive manufacturing processes, and are critical for developing reliable high-fidelity computational models.
A-30: In-situ X-ray Characterization of Keyhole Dynamics in Laser-based Additive Manufacturing of Aluminium Alloys: Hongze Wang1; Yu Zou1; 1University of Toronto
To reveal the role of the dynamic keyhole in laser manufacturing, we used the x-ray phase contrast method to observe the characteristics of the keyhole and molten pool when a stationary 140 μm diameter laser was irradiated to the surface of three representative aluminium alloys (A1050, A2024, and A5083). Violent fluctuation was observed in both the depth direction and the radial direction of the keyhole, and the non-uniform distribution of the metal vapor in the keyhole may account for this fluctuation. Two representative mechanisms for the bubble formation were observed, the first one was due to the fluctuation of the keyhole along the depth direction, and the second one was due to the fluctuation along the radial direction. This study concluded that the keyhole could appear first before the molten pool at some cases due to the fast evaporation of the solid metal.
A-31: Influence of Alloy Composition on Cell Formation in Additively Manufactured Stainless Steels: Joseph Aroh1; Seunghee Oh1; Rachel Lim1; Chihpin Chuang1; Niranjan Parab1; Cang Zhao1; Tao Sun1; P. Pistorius1; Anthony Rollett1; 1Carnegie Mellon University
Metal additive manufacturing introduces rapid solidification resulting in nonequilibrium microstructures. The as-built microstructure in 316L stainless steel consists of submicron cellular structures that improve both strength and ductility over conventional material. Characterization of AM 316L repeatedly shows higher Cr and Mo concentrations at the cell boundaries. To understand the effect of alloy composition and micro-segregation, we produced three Fe-Ni-Cr ternary alloys with different amounts of Cr and Ni with the intent of varying the balance between ferrite and austenite. In-situ high-speed synchrotron x-ray diffraction experiments were carried out on each alloy at the Advanced Photon Source under conditions representative of laser-based AM processes. We discuss the effect of composition and cooling rate on undercooling, segregation, and phase selection at the cellular length scale in Fe-Ni-Cr alloys.
A-32: Influence of Powder Recyclability on the Defect Density of Components Fabricated with Nickel-based Alloys Characterized with Micro X-ray CT: Curtis Frederick1; Edson Santos1; Michael Kirka2; Pradeep Bhattad1; Paul Brackman1; 1Carl Zeiss; 2Oak Ridge National Laboratory
Development of a material and additive manufacturing process can span many years. Once the process is defined and a part reaches serial production, quality is not guaranteed because of many instabilities in the process chain. Micro X-ray CT makes it possible to perform 3D characterization of powder as it changes over time from the first to last print. Defects then observed in the printed parts can be linked to the powder characteristics. With crack prone nickel alloys produced using the selective electron beam melting process defect density is of great importance as each defect can disrupt the grain structure and final part properties. This presentation will show how X-ray CT can be used to assure quality after printing by characterizing powder at multiple stages of the additive manufacturing process.
A-33: Investigating the Behavior of Ti-5553 Octet Lattice Wedges Under Compression: Jenny Wang1; David Macknelly2; Stephen Knaus1; Kyle Klein1; Mary LeBlanc1; Jeffrey Florando1; Manyalibo Matthews1; 1Lawrence Livermore National Laboratory; 2Atomic Weapons Establishment
Laser-powder bed fusion (L-PBF) additive manufacturing enables many diverse applications including the design and fabrication of light-weight metallic lattice structures. However, the deformation behavior of metallic lattice structures under complex loading conditions has not been thoroughly investigated, which leads to questions regarding the qualification and certification of printed parts. In this investigation, Ti-5553 octet lattice wedges, processed by L-PBF, are compression tested under an angled load-path using a custom-built fixture that is compatible with Digital Image Correlation (DIC). DIC is used to track the strain field within the lattice during compression. These experiments are used to validate computational models and topology optimization (TO) algorithms that can be applied to improve part performance. The models predict and compare the failure points of non-TO lattice samples against samples that have undergone TO through the Livermore Design Optimization (LiDO) software code. Prepared by LLNL under Contract DE-AC52-07NA27344.
A-34: Laser Additive Manufacturing of Dissimilar Metals: Xuan Zhang1; Wei-Ying Chen1; Cang Zhao1; Chihpin Chuang1; Tao Sun1; Meimei Li1; 1Argonne National Laboratory
In nuclear power plants, dissimilar metal welded joints are widely existing, and those welds are indicated to be vulnerable points against the safe operation of the plants. In this study, we explored the laser additive manufacturing as a potential alternative to produce the components with dissimilar metals joints. We combined a suite of experimental techniques, including synchrotron x-ray based ultrafast imaging and high-speed diffraction, electron microscopy, and nano-indentation, to study the effect of laser parameters on the joining process, microstructures, and mechanical properties of dissimilar metal systems. We studied two systems relevant to nuclear applications, including the G91 steel + 316 stainless steel, and Alloy 690 Ni-based alloy + 316 stainless steel. Our preliminary study shows that the novel experimental approach used in this study can accelerate the development of additive manufacturing for nuclear applications.
A-35: Measurements and Predictions of Residual Stresses in AM Ti-6Al-4V NIST Challenge Specimens: James Sobotka1; Matthew Kirby1; Sheng-yen Li1; 1Southwest Research Institute
In 2018, the National Institute of Standards and Technology (NIST) presented additive manufacturing (AM) benchmark tests as a challenge for the modeling and simulation community to develop predictive models of the AM build process. These tests included a residual stress/distortion challenge problem built from Inconel 625 and 15-5PH stainless steel using laser powder-bed fusion machines. For this challenge, NIST introduced a bridge structure geometry with twelve variably sized legs. In 2019, we followed NIST specifications to produce the same geometry using Ti-6Al-4V in a Renishaw AM250 machine. This presentation describes the build process, shows 3D micro-CT scans pre/post-sectioning, and presents distortion measurements following sectioning of the specimen from the build plate. Furthermore, this presentation describes a process-modeling framework to predict residual stresses using a sequential thermo-mechanical approach driven by laser scan strategy. This presentation closes with results from sensitivity studies and validation predictions that showcase predictive capabilities.
A-36: Microstructural Development and Mechanical Properties of Selective Laser Melted Co-Cr-W Dental Alloy: Leonhard Hitzler1; Jonas Von Kobylinski1; Robert Lawitzki2; Christian Krempaszky1; Ewald Werner3; 1Technical University Munich; 2University of Stuttgart; 3Technical University, Munich
Dental restorations are unique and patient specific in their designs, while inhabiting complex shapes and serving multi-functional purposes. Thus, they represent a predestined area for additive manufacturing technologies such as selective laser melting. Comparing the traditional fabrication route of dental restorations via investment casting associated with notoriously slow cooling rates, the rapid solidification rates achieved in SLM lead to a fine-grained microstructure with improved mechanical properties. However, this fine-grained microstructure possesses a face-centered cubic crystal lattice structure, lowering its respective Young’s modulus when compared to the predominantly hexagonal close-packed crystal structure present in coarse-grained cast parts.Heat-treatments proved to be a viable option to increase the elastic stiffness up to 40-50 GPa, to safely meet dental standards. The improvement in stiffness, however, is not due to the phase-transformation from fcc to hcp, but seems to be caused by a precipitate formation similar to that in Ni-base alloys after creeping.
A-37: Real Time Observation of Binder Jetting Printing Process Using High-speed X-ray Imaging: Niranjan Parab1; John Barnes2; Cang Zhao1; Ross Cunningham3; Kamel Fezzaa1; Anthony Rollett3; Tao Sun1; 1Argonne National Laboratory; 2The Barnes Group Advisors; 3Carnegie Mellon University
A high-speed synchrotron X-ray imaging technique was used to investigate the binder jetting additive manufacturing (AM) process. A commercial binder jetting printer with droplet-on-demand ink-jet print-head was used to print single lines on powder beds. The printing process was recorded in real time using high-speed X-ray imaging. The ink-jet droplets showed distinct elongated shape with spherical head, long tail, and three to five trailing satellite droplets. Significant drift was observed between the impact points of main droplet and satellite droplets. The depth of disturbance in the powder bed from movement and ejection was defined as interaction depth, which was found to be dependent on the size, shape, and material of the powder particles. For smaller powder particles (diameter less than 10 microns), three consecutive binder droplets were observed to coalesce to form large agglomerates. The observations reported here will facilitate the understanding of underlying physics that govern the binder jetting processes.
A-38: Residual Strain Gradients in Thin Walled Additively Manufactured Stainless Steel Pressure Vessels: Bjorn Clausen1; Rishi Ganeriwala2; Donald Brown1; Robert Ferencz2; John Carpenter1; 1Los Alamos National Laboratory; 2Lawrence Livermore National Laboratory
Additive manufacturing (AM) facilitates producing structural components with complex shapes and features, but often results in parts with significant residual stresses. This can be exacerbated in closed structures as in the present example of a thin walled 304L stainless steel pressure vessel with built in mounting points manufactured using powder bed fusion metal AM. We employed high spatial resolution neutron diffraction strain measurements to map residual stresses in both thin- and thick-walled parts of the pressure vessel and used the results to validate and guide the development of a fully coupled thermomechanical multiphysics finite element based process model of the powder bed fusion technique. The predictions of the model are generally in good agreement with the measured data, including significant through thickness gradients found in some thin walled sections as well as in thicker sections near the built in mounting points.
A-39: Residual Stress Measurement Techniques for Additive Manufacturing Parts: Adi Benartzy1; Kahraman Demir1; Jack Peterson1; Grace Gu1; Peter Hosemann1; 1University of California, Berkeley
There are different sources for residual stresses in additive manufacturing (AM) parts including thermal expansion leading to micro plasticity during heating, thermal contraction during solidification of each laser pass, and volume change due to phase transformations. Currently, residual stress measurement techniques are destructive or semi destructive tests based on releasing the stresses by drilling or slitting the sample while X-ray or neutron techniques are non-destructive but require powerful beams. In this work, a novel Non-Destructive Testing technique to measure residual stresses is established using Femto laser surface trenching up to 100μm depth. Residual stresses are applied to a carbon steel ring using 250W YAG laser and measured by strain-gages during Femto laser trenching. The results are verified by macro slitting of the ring and spreading measurement. Both micro and macro stresses are in good agreement with numerical analysis of the stresses that described the residual stresses using first principles.
A-40: Strengthening Effect and Thermal Stability of Sub-grain Solidification Structures in L-PBF Stainless Steel 316L: Thomas Voisin1; Jean-Baptiste Forien1; Y. Morris Wang1; 1Lawrence Livermore National Laboratory
316L stainless steels (316L SS) made via the laser powder bed fusion (L-PBF) technique has shown an outstanding combination of high strength and ductility. Using a suite of in situ and post mortem characterization techniques such as TEM, SEM/EBSD, high energy XRD we investigate the effect of the sub-grain solidification cells and precipitates on the tensile properties and the their thermal stability up to 1200C. We observe a drop in strength at annealing temperatures above 600C, when sub-grain structures become unstable. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.