Reaction sintering of shock-compressed Ti + C powder mixtures
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Fig. 1--Schematics of (a) the cylindrical implosion system used for shockcompression experiments and (b) a hot-press assembly used for reaction sintering of Ti + C powder mixtures (all dimensions in millimeters).
efficient term, and using Shewmon's 1261analysis, similar to the treatment adopted by Whittenbergel~271 for ball-milled powder mixtures, it has been estimatedI25~that the time for compound formation in shock-modified intermetallic forming powder mixtures ( e . g . , Ni-AI of 25/zm average particle diameter) is about 30 seconds at approximately threefourths of the melting temperature of A1.12~1 Unshocked powder mixtures, on the other hand, require 8 hours for complete solid-state diffusion1251 at the same reaction temperature. Shock compression can thus be used to produce highly activated microstructurally modified powder mixture compacts, which can undergo solid-state diffusion reactions at temperatures far below the melt temperatures, yielding products free from defects associated with liquid-state reactions. The objective of the present work was to use shock compression as a densification process to make Ti + C powder mixture compacts which can be reaction sintered to produce bulk TiC ceramics with ultrafine grain microstruc1750~VOLUME 27A, JULY 1996
EXPERIMENTAL PROCEDURE
Commercial purity Ti (99.9 pct) and graphite (99.99 pct) powders were mixed (in 1Ti:0.95C ratio) under an argon atmosphere in a slow-speed V-shaped mechanical blender. The respective average particle sizes of Ti and C were 20 /~m (-325 mesh) and 2 ~m. A cylindrical implosion system, shown schematically in Figure l(a), was used for shock compression of the Ti + C powder mixtures. The implosion system consisted of the explosive ANFO (ammonium nitrate mixed with 6 wt pct of fuel oil) having a detonation velocity of 3.4 mm//xs, surrounding a 73-mmdiameter and 152-mm-long steel powder container tube (4.8-mm thick) and contained in a 152-mm-diameter PVC pipe. The blended Ti + C powder mixture was packed in the steel container at - 6 5 pct theoretical maximum density (TMD). A 6-mm-diameter solid titanium mandrel was also placed in the center of the powder container to avoid formation of the Mach stem. Both ends of the powder container tube were sealed by epoxying 25-ram-thick steel plugs. Upon initiation of the explosive, a detonation wave propagates vertically downward, producing a calculated implosion pressure of - 2 GPa transmitted into the powder mixture. After shock compression, cylindrical compacts were recovered by machining the steel container tube and sectioned into triangular-shaped samples for subsequent reaction sintering. The sintering experiments were performed by embedding the triangular compacts in SiC powder in an induction-heated graphite die (75-mm i.d.) placed in a hot press, as sho
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