THE PRECIPITATION HARDENING BEHAVIOR OF TITANIUM-4(+)-DOPED SAPPHIRE

The effects of the thermomechanical working (via the prismatic slip system) of Ti('4+)-doped sapphire on the precipitation of rutile in the crystals was studied by observing the size, shape, and volume fraction of precipitate as a function of aging time at 1300(DEGREES)C. The resulting mechanical properties were measured with major emphasis on the effects of the precipitates on the flow behavior via the prismatic slip system.; The heat treatment of 0.08 and 0.17 wt.% TiO(,2)-doped sapphirein air at 1300(DEGREES)C resulted in the precipitation of rutile (TiO(,2)) needlesoriented in the directions of sapphire, with precipitates(' )nucleating preferentially on dislocations in samples deformed at1600-1800(DEGREES)C prior to aging.; The presence of TiO(,2) precipitates in sapphire increased theroom temperature microhardness in the directions by 5-20(' )percent ((TURN)2350 versus 1900 Kg/mm('2)), with deformed samples aged for 16 hours showing the most significant hardening. The room temperature fracture toughness of the crystals for cracks propagating perpendicular to the basal plane in sapphire was (TURN)50% higher for 0.17 wt.% TiO(,2)-doped sapphire aged for 8 hours (K(,c)(TURN)2.7 MN/m('3/2)) than for undoped sapphire (K(,c)(TURN)1.8 MN/m('3/2)).; The effect of TiO(,2) precipitates on the prismatic {lcub}1120{rcub}(' )^slip behavior was investigated by deforming a-Axis sapphire^filaments, which had been aged in air at 1300(DEGREES)C for times ranging^from four hours to 13 days, in tension at 1450(DEGREES)C. Specimens aged^for three days or less fractured at 1450(DEGREES)C/(epsilon)(' )=(' )6.26 x 10('-4) min('-1)^at stresses higher than the critical resolved shear stress for flow^in undoped sapphire, which indicates that barriers to slip (the^precipitates) allow high enough local stresses to develop to^propagate microflaws unstably instead of initiating deformation.TEM analysis of specimens aged for 10 and 13 days whichdeformed extensively by the prismatic {lcub}1120{rcub} slip system,(' )revealed precipitates sheared by dislocations. Flow stresses were (TURN)10% higher for Ti('4+)-doped and aged sapphire compared to undoped sapphire. The hardening of the prismatic slip system by rutile precipitates was attributed to image forces on gliding dislocations generated by elastic mismatch between rutile and sapphire.