Fundamental Aspects and Modeling Powder Metal Synthesis and Processing: Field-assisted Processing
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 8:30 AM
February 28, 2017
Room: 16A
Location: San Diego Convention Ctr

Session Chair: Eugene Olevsky, San Diego State University

8:30 AM  Invited
Mechanisms of Pore Formation in High-temperature Carbides: Case Study of TaC Prepared by Spark Plasma Sintering: Olivia Graeve1; James Kelly1; 1University of California, San Diego
    The compositionally diverse TaC1-x phase is an interstitial carbide having a rocksalt crystal structure and mixed covalent, metallic, and ionic bonding. Therefore, this material has an interesting combination of properties that in some respects are intermediate between typical ceramics and metals. In this study, a detailed analysis of microstructure and pore evolution of TaC has been studied and three pore-forming mechanisms have been discovered: (1) evolution of oxygen impurity, (2) evolution of excess carbon, and (3) incongruent sublimation of TaC. The evolution of oxygen impurities is the predominant mechanism and can be a consequence of the high amount of oxygen impurity typical of nanopowders. We propose that the latter two mechanisms can be facilitated by local hot spots and discuss a possible source. The root cause of the three mechanisms indicates that a limiting sintering temperature (~2173 K) and carefully controlled chemistry are essential for producing TaC without trapped porosity.

9:10 AM  
A Numerical Tool to Master the SPS Densification of TiAl Complex Shapes: Martins David1; Estournes Claude2; Sallot Pierre1; Bellet Michel3; Mocellin Katia3; 1SAFRAN; 2CIRIMAT; 3CEMEF
    Spark Plasma Sintering (SPS) is a process belonging to the powder metallurgy family. It consists in applying simultaneously a load and a high intensity pulsed direct current on graphite tools containing powder to sinter this latter. The very fast temperature increase is driven by the Joule’s effect and the grain growth is almost suppressed. To use such device at industrial scale, its modelling to predict densification and temperature profile depending on the sintering cycle is needed. In this study, a fully coupled numerical computation model has been developed to model a TiAl 48-2-2 powder densification. The numerical model was implemented in the finite element code FORGE and simulations are compared with results obtained from experimental tests performed to densify simple shape specimens. The comparison based on the shape evolution and relative density distribution in samples is in good agreement with experimental observation in either simple or complex shape specimens.

9:30 AM  
Influence of Loading Modes in Spark Plasma Sintering: Xialu Wei1; Eugene Olevsky1; 1San Diego State University
    Two loading modes, free-forging (upsetting with free lateral surface) and rigid die pressing are employed in spark plasma sintering (SPS) system to consolidate porous ceramics. It is shown that these two loading modes provide different levels of densification under the same axial pressure applied during the SPS process. Theoretical studies have been carried out to investigate the densification kinetics induced by the loading schematics. The studies suggest that the above mentioned loading modes can be properly combined to achieve the highest final relative density. The derived densification expressions also promote a way to evaluate the creep parameters of the utilized material under SPS conditions. The reliabilities of the modeling results are confirmed by comparing them to the outcomes of a series of specially designed experiments. The developed modeling-based concept of optimal SPS pressure mode is used as a basis for innovative SPS tooling design facilitating higher efficiency of consolidation.

9:50 AM  
Modeling and Optimization of Hierarchical Porous Structures during Spark Plasma Sintering: Diletta Giuntini1; Eugene Olevsky1; 1San Diego State University
    Nanosized powders are becoming widely used in powder processing techniques, and especially in Spark Plasma Sintering, thanks to their high potential for the production of materials with outstanding mechanical properties and advanced components. Some examples are the high surface area components used in supercapacitors, rechargeable batteries and gas absorbers. Nanopowders are nevertheless very prone to agglomeration issues, which lead to the formation of hierarchical porous structures and non-uniform densification. A combined analytical and FEM model for the study of shrinkage kinetics and mechanical properties of agglomerated powders is proposed. Both densification kinetics and mechanical properties are derived as functions of the different-size porosities, and tools are developed to individuate the optimal thermal regime to achieve in-situ de-agglomeration. Comparisons with experimental data are provided.

10:10 AM Break

10:30 AM  Invited
Predicting (1) Activated Sintering of Refractory Metals and (2) Flash Sintering of Oxides: Jian Luo1; 1UC San Diego
    This talk will review our recent studies in two areas. FIRST, a series of studies extend the bulk CALPHAD methods to grain boundaries (GBs). In particular, GB diagrams have been computed to predict activated sintering in W- and Mo-based refractory alloys [J. Am. Ceram. Soc. 95: 2358 (2012)]. This work has been extended to multicomponent alloys [Acta Mater. 91:201 (2015); Current Opinion, in press]. In a broader perspective, the computed GB diagrams (as extensions to bulk phase diagrams) are a useful tool for the Materials Genome Initiative. SECOND, if time permits, flash sintering of oxides will also be discussed. A model has been developed to forecast the onset flash temperatures [Acta Mater. 94:87 (2015)]. Guided by this model, ZnO specimens have been sintered to >97% densities in seconds at furnace temperatures of <120C [Scripta Mater. 106:26 (2015)]. Further studies investigate the fast densification mechanisms and various electric field/current effects.

11:10 AM  
Optimization of Temperature Regime of Spark Plasma Sintering of AlON Powder: Yingchun Shan1; Xialu Wei2; Xiannian Sun1; Geuntak Lee2; Jiujun Xu1; Eugene A Olevsky2; 1Dalian Maritime University; 2San Diego State University
    Using bimodal pure aluminum oxynitride (AlON) powder synthesized by carbothermal reduction and nitridation method, including 0.5% Y2O3 as a sintering additive, AlON ceramics are fabricated by spark plasma sintering (SPS) at different sintering temperatures. Relative density of ≥99.86% is achieved for all the obtained AlON ceramics samples sintered at 1400-1550C after holding for 10min under 40MPa. The observed phase assemblages of sintered samples strongly depend on the sintering temperature due to the phase transformations between AlON, α-Al2O3 and AlN. Single phase AlON ceramics are obtained when sintering temperature is greater than 1500C. The measured maximum infrared transmittance for 1.2mm thick samples is 70.2%, which is similar to the one obtained in samples pressureless sintered for 30min at 1880C . Therefore, the conducted study indicates that sintering temperature and holding time needed to fabricate transparent AlON ceramics by SPS can be lowered by 380C and 20min, respectively.

11:30 AM  
On the Role of Electric Current in Spark Plasma Sintering of Conductive Powders: Geuntak Lee1; Eugene Olevsky1; Joanna McKittrick2; 1San Diego State University; 2University of California, San Diego
    Previous studies indicate that heat conduction via the Joule heating directly influences the densification of conductive powders during spark plasma sintering (SPS) process. However, the effect of electric current on SPS was not clarified mainly due to the difficulty of the deconvolution of the Joule heating and possible non-thermal field phenomena. In the present work, three different experimental setups are utilized to overcome this obstacle. One setup restricts the electric current flow through the powder, and the other two setups allow small or large amount of the electric current passing through the powder specimen. All the three setups are employed under the same SPS temperature regime. The constitutive equations for the hot pressing of powders are modified to take into account the electric current effect on SPS. We found that electric current accelerates mass transfer during SPS, affecting the activation energy and densification mechanism of powder consolidation.