Materials Week '97: Wednesday AM Session

Focusing on physical metallurgy and materials, Materials Week '97, which incorporates the TMS Fall Meeting, features a wide array of technical symposia sponsored by The Minerals, Metals & Materials Society (TMS) and ASM International. The meeting will be held September 14-18 in Indianapolis, Indiana. The following session will be held Wednesday morning, September 17.

P/M CURRENT RESEARCH AND INDUSTRIAL PRACTICES: Session I: Current Research I

Session Chairperson: Enrique V. Barrera, Rice University, Department of Mechanical Engineering and Materials Science, Houston, TX 77005-1892

OPENING REMARKS AND INTRODUCTIONS: Enrique V. Barrera, former Chair of the Powder Materials Committee

8:40 am INVITED Talk by Current Chair of the PM Committee

SHOCK-INDUCED AND SHOCK-ASSISTED SYNTHESIS OF MATERIALS: Naresh N. Thadhani, Kevin Vandersall, Shantanu Namjoshi, School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245

Shock-compression of powders can lead to chemical reactions and phase transformations resulting in the formation of compounds with modified microstructures, as well as equilibrium and non-equilibrium phases. Two types of processes are possible and can be distinguished on the basis of their respective mechanics and kinetics. Shock-induced processes occurring during the shock-compression state before unloading to ambient pressure, can be utilized for synthesis of high-pressure phases and compounds not possible during ambient conditions. Shock-assisted processes are those that occur after unloading to ambient pressure, in an essentially shock-modified material, with subsequent thermal treatment. In this presentation, our results on "shock-induced" synthesis of carbon nitrides and "shock-assisted" processing of silicides with refined microstructures will be presented.

9:10 am INVITED

DIRECT SELECTIVE LASER SINTERING OF HIGH PERFORMANCE METALS FOR CONTAINERLESS HIP: S. Das, M. Wohlert, J.J. Beaman, D.L. Bourell, Laboratory for Freeform Fabrication, The University of Texas at Austin, Austin, TX 77281

A novel net shape manufacturing method known as SLS/HIP which combines the strengths of selective laser sintering and hot isostatic pressing is presented. Direct selective laser sintering is a rapid manufacturing technique that can produce high density metal parts of complex geometry with an integral, gas impermeable skin. These partscan then be directly post-processed by containerless HIP. The advantages of in-situ HIP encapsulation include elimination of a secondary container material and associated container-powder interaction, reduced pre-processing time, a short HIP cycle and reduction in post-processing steps compared to HIP of canned parts. Results from research conducted on Inconel 625 are presented. This research is funded by DARPA/ONR contract N00014-95-C-0139, titled "Low Cost Metal Processing Using SLS/HIP".

9:40 am

PHYSICAL MODELING OF THE EARLY STAGES OF METAL POWDER COMPACTION AND COMPARISON TO EXISTING ANALYTICAL MODELS FOR HOT ISOSTATIC PRESSING: Henry R. Piehler and David P. DeLo*, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213-3890

Several important mechanisms observed to operate during the early stages of HIPing of powders are not currently included in existing compaction models, which all tend to underpredict strain rates and densification, especially during the early stages of consolidation. A series of interrupted HIP experiments were performed on Ti-6Al-4V powders consolidated in thin-walled containers to characterize early stage behavior using both metallographic sections and stereo pairs of fractured partially consolidated powder compacts. Results using several approaches indicated that particle rearrangement or granular behavior occurred throughout the early stages of consolidation, accounting for somewhere between 20% and 50% of the total densification. These approaches included densification predictions from measurements of center-to-center particle distances on metallographic sections and contact areas on fractured samples, both of which consistently underpredicted the measured Archimedian density levels. Particle size effects, including rigid body motion of larger particles facilitated by the preferential deformation of contiguous small particles, also were observed to contribute to densification by rearrangement. Increases in particle coordination number with densification, again absent in all current consolidation models, were also observed. The inconsistency between the mechanisms and assumptions included in existing HIP models and the early stage consolidation behavior observed here requires new modeling approaches which incorporate this observed behavior in order to improve the accuracy of model predictions. *Now at Wright-Patterson Air Force Base, OH.

10:10 am INVITED

HIGH PRESSURE SINTERING OF NANOCRYSTALLINE CERAMICS: R.S. Mishra, A.K. Mukherjee, Department of Chemical Engineering and Materials Science, University of California, Davis CA 95616

In recent years, nanocrystalline ceramics powder have been synthesized by a number of techniques. Some of these techniques have matured into commercial production of well characterized powders. Sol-gel processing and inert gas condensation techniques are particularly attractive. However, the retention of nanocrystalline microstructure during consolidation of some ceramics phases has proved to be quite difficult. In particular, the processing of nanocrystalline alumina based ceramics has been challenging. One of the ways to circumvent this problem is to use high pressure sintering. We have processed a number of nanocrystalline alumina and alumina based composites at 1 Gpa pressure in a piston-cylinder apparatus. The experimental results agree quite well with the theoretical calculations based on densification by dislocation mechanisms. Use of amorphous precursors provide additional benefits. Amorphous Zirconia-alumina precursor has been sintered to high density at 7000°C. An overview of the advantages of using high pressure sintering in processing nanocrystalline ceramics is discussed.

10:40 am INVITED

CRACK PATH TRANSITIONS IN POWDER PROCESSED CERAMIC MATRIX COMPOSITES: B.R. Patterson, S. Wu, and *P. Bhargava, Department of Materials and Mechanical Engineering, University of Alabama at Birmingham, Birmingham, AL 35284-4461; *Center for Ceramic Research, Rutgers University, Piscataway, NJ

Studies have been performed to better understand the nature of the crack path in particulate reinforced ceramic and glass matrix composites produced by powder processing. A theory has been proposed predicting the combined influence of stresses from thermal and elastic mismatch on crack path. The theory predicts changes in the tendency for crack attraction to or avoidance of the reinforcements with crack tip stress intensity for different combinations of thermal and elastic mismatch. The predictions have been verified by stereological quantification of the crack path in model composite systems. For some systems with thermal and elastic mismatch of opposite sign, the crack path showed reversal in attraction/avoidance tendency from low to high crack velocity, due to a change in dominant mismatch control.

11:10 am INVITED

INFLUENCE OF REINFORCEMENT ACTIVATION ON SINTERED CHAR-REINFORCED Al-Si ALLOY COMPOSITES: J.U. Ejipfor, R.G. Reddy, Department of Metallurgy and Materials Engineering, The University of Alabama, P.O. Box 870202, Tusaloosa, AL 35487-0202

Coconut shell chars dispersed in hypereutectic Al-13.5Si-2.5Mg alloy were studied for lightweight, wear applications. The composites were fabricated by a low-cost, double-compaction reaction-sintering technique. Both primary carbonized and activated chars (under CO₂ atmosphere) were used as the filler phases. The wear, mechanical and thermal behaviors of the composites investigated revealed optimum properties at 0.02 volume fraction of char with improvements when activated char (AC) was used. The wear rate and coefficient of fraction of the alloy reduced by 79 and 55 percent, respectively, when activated char was dispersed. While their tensile properties marginally declined from those of the alloy and the composite containing primary carbonized char (PC), the coefficient of linear thermal expansion fell from 11.7 x 10-6(0C)-1 for PC to 9.8 x 10-6(0C)-1. At 2.5 wt.%Mg, a good interfacial bonding was achieved, and it appeared to be unaffected by activation.