Fundamental Aspects and Modeling Powder Metal Synthesis and Processing: Fundamentals of Powder Consolidation
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Powder Materials Committee
Program Organizers: Paul Prichard, Kennametal; Eugene Olevsky, San Diego State University; Iver Anderson, Ames Laboratory

Tuesday 2:00 PM
February 28, 2017
Room: 16A
Location: San Diego Convention Ctr

Session Chair: Eugene Olevsky, San Diego State University

2:00 PM  Invited
Anisotropy of Mass Transfer during Sintering of Powder Materials with Pore-Grain Structure Orientation: Diletta Giuntini1; Elisa Torresani2; Chaoyi Zhu3; Tyler Harrington3; Kenneth Vecchio3; Alberto Molinari2; Eugene Olevsky1; 1San Diego State University and University of California, San Diego; 2University of Trento; 3University of California, San Diego
    A micromechanical model for shrinkage anisotropy during sintering of metallic powders is proposed and experimentally assessed. The studied powder samples are pre-shaped into their green forms by uniaxial cold pressing before the sintering routine. The samples therefore provide for a directional porous structure and inhomogeneous plastic deformation at the inter-particle contacts. These non-uniformities are considered to be the cause of anisotropic dislocation pipe diffusion, and thus the root cause of the undesired shape distortion during shrinkage. The proposed model describes the shrinkage rates in the compaction and transversal directions, as functions of both structural and geometrical activities of the samples. Dislocation densities can be estimated from such equations, along with dilatometry and image analysis data. The reliability and applicability of this modeling framework is validated by comparing the calculated dislocation densities with nanoindentation data and electron backscattered diffraction based geometrically-necessary dislocation density determinations.

2:40 PM  
Dislocation Density Approach to Understanding Sintering Mechanics: Chaoyi Zhu1; Diletta Giuntini2; Tyler Harrington1; Eugene Olevsky2; Kenneth Vecchio1; 1UC San Diego; 2San Diego State University
    Understanding of how dislocations accumulate in the sintered particles is fundamental to shrinkage kinetics. Based on bulk and pipe diffusivity data from the literature, Olevsky et al.’s theoretical model relates dislocation density to shrinkage rates. Experimentally, nanoindentation allows determination of the total dislocation density, comparable with theoretical predication, including statistically stored dislocation (SSD) and geometrically necessary dislocations (GND). Utilizing Nye’s tensor, we are able to separate out the geometrically necessary part of the total dislocation density present to accommodate lattice curvature using EBSD based computational methods. In this study, GND density data validates the nanoindentation result and reconfirm that the theoretical predication of dislocation density is higher in the compaction plane than the compaction direction of powder compacts. In addition, detailed information about GND dislocation structure can be mapped to the sintered particles, which sheds light on the micromechanics of the sintering process.

3:00 PM  
Effect of Additives on the Densification Kinetics and Microstructure of Hot-Pressed Boron Suboxide: Kristopher Behler1; Cooper Voigt2; Eugene Shanholtz3; Jerry LaSalvia4; Scott Walck1; 1U.S. Army Research Laboratory (SURVICE Engineering); 2U.S. Army Research Laboratory (SEAP); 3U.S. Army Research Laboratory (ORISE); 4U.S. Army Reseach Laboratory
    Isothermal hold experiments have been conducted at various temperatures ranging from 1700 – 1850C in order to investigate the hot-pressing densification kinetics of a boron suboxide (B6O) powder with and without additives. Additives based on Al2O3 and SiO2 (2 – 5 vol%) were acoustically mixed with B6O powder. The densification kinetics and resulting microstructures were found to be significantly influenced by the additives, lowering the onset of rapid densification and activation energy. SEM, EDS, and XRD were utilized to determine the starting powder morphology and composition in addition to the microstructure and phases present within consolidated samples. The starting B6O powder was found to be oxygen deficient, and contained both boron and magnesium borate phases. For the hot-pressed specimens, the presence of Al2O3 and SiO2 additives resulted in the formation of aluminum borate and possibly borosilicate, respectively. Experimental procedures, analyses, and results are presented.

3:20 PM  
Microstructural Evolution during Early Stages of Hot Isostatic Pressing of 316L Austenitic Stainless Steel: Sandeep Irukuvarghula1; Hany Hassanin2; Moataz Attallah3; Michael Preuss1; 1University of Manchester; 2Kingston University; 3University of Birmingham
    The microstructural evolution of 316L austenitic steel during the early stages of powder hot isostatic pressing (HIPping) was investigated by interrupting the HIPping cycle at progressively increasing temperatures, starting from 950˚C. The grain boundary character distribution (GBCD) was studied using electron back scattered diffraction. Our results demonstrate that the microstructure evolves from a random high angle grain boundary distribution in the as-received powder to a twin dominated one in the specimen subjected to full HIPping cycle. Additionally, the fraction of twins (first, second, and third order) and triple junctions with twin boundaries increase as the HIPping temperature increases. Our results suggest that altering the HIPping cycle and/or powder characteristics can potentially change the grain boundary network topology, which affects the performance of the powder HIPped components.

3:40 PM Break

4:00 PM  Invited
Thermodynamics versus Kinetics of Grain Growth Control to Enable Stable Nanocrystalline Materials: Ricardo Castro1; Nazia Nafsin1; 1University of California, Davis
    In this talk we present thermodynamic data on the effect of dopants on the grain boundary energy of a zirconia based system to defend the thesis that grain growth can be virtually stopped by targeting a decrease in the grain boundary energy. By comparatively analyzing both kinetics and thermodynamics of grain growth, we show that although dopants used to inhibit grain growth are typically regarded as kinetic agents allowing drag forces to reduce grain boundary mobility, their roles on decreasing the grain boundary energy can be remarkable and hold the key for effect design of ultra-stable nanocrystalline materials.

4:40 PM  
Grain Growth and Densification of Tungsten Nanopowders: Brady Butler1; James Paramore1; Anthony Roberts1; Jonathan Ligda1; Micah Gallagher1; 1U.S. Army Research Laboratory
    Bulk ultrafine grained materials exhibit unique material properties that are not achievable in their coarse grained counterparts. The synthesis of bulk ultrafine grained materials by powder metallurgical techniques requires a thorough understanding of the grain growth and densification behavior during sintering. Although several models have been developed to describe grain growth in bulk fully dense materials, these models are not sufficient to describe the growth behavior of nanograined compacts. Specifically, the early stages of sintering account for the most rapid growth regime, but this is generally not adequately described by existing grain growth models. This research utilizes high temperature x-ray diffraction to measure the growth behavior of ultrafine grained tungsten during the early stages of sintering to provide a comparison to conventional growth models. Finally, the kinetics of grain growth are evaluated in an effort to understand the active mechanisms during early stages of sintering.

5:00 PM  
Development of Novel Multi-compaction Technique for Fabrication of Hybrid P/M Steels: Minchul Oh1; Hyunjoo Seok1; Byungmin Ahn1; 1Ajou University
    The objective of this study is to develop the multi-compaction processes by simultaneously compacting dissimilar Fe alloy powders side-by-side or inside-and-outside. The concept of this hybrid structure is to balance between high strength and good machinability in addition to lower production cost. In the present study, Fe-1.5Cr-0.2Mo-0.5C alloy powder was selected for high strength part, and Fe-3Cu-0.9C alloy powder was for good machinability and lower price. Fe-Cu-C was pre-compacted into a cylindrical shape under lower compaction pressures. After the pre-compaction, Fe-Cu-C green body and Fe-Cr-Mo-C powder were simultaneously compacted under higher pressures. Multi-compacted P/M alloys were sintered at 1120C in 90%N2-10%H2 gas atmosphere for 60 min and subsequently cooled in the furnace. The interface between dissimilar alloys was investigated by measurement of density and hardness variations and by microstructural characterization of diffusion behavior using OM, SEM, and EDS.

5:20 PM  
Microwave vs Conventional Sintering of Ti Powders: Comparative Analysis: Charles Maniere1; Tony Zahrah2; Eugene Olevsky1; 1San Diego State University; 2MATSYS Inc
    Microwave sintering presents potential benefits such as high heating rates, low power consumption and increase of the sintering rate compared to the conventional sintering. The main challenges associated with this technique are relative temperature instabilities compared to the conventional technique where the heating is long but more stable. A fully coupled ElectroMagnetic-Thermal-Mechanical (EMTM) finite element simulation of the sintering of a Ti powder in a standard microwave cavity is performed and compared to the outcomes of the processing of the same powder specimen in a conventional sintering furnace. The model takes into account both the evolution of the electromagnetic heating parameters and the non-uniform shrinkage of the sample due to the thermal gradients. The comparison of the temperature distribution, the evolution of the sample shape and porosity distribution is made for both microwave (direct/hybrid heating) and conventional sintering techniques.