Recovery of Creep-Resistant Substructure of Rutile in Air and Vacuum Under Reduced Stress

Abstract

HE present work is a continuation of high-temperature T creep recovery experiments made on rutile single crystals oriented for ( 1 TO) [OOl] def~rmation.'-~ Creep specimens were prepared and tested a s reported previously.' Experiments were conducted in air' and vacuum.In air, creep recovery was studied at IOOO", 1020°, and 1040°C with reduced stresses, up, of 500, 2500, 5000, and 7500 psi. The original creep stress, u~, was 10,000 psi. The variation of recovery index,' n, with recovery time at 1020°C for each reduced-stress level investigated is shown in Fig. 1. For a given relatively shont recovery time, a higher degree of recovery of creep-resistant substructure is obtained at higher reduced-stress levels. For longer recovery times, however, the maximum attainable value of n (n,nc,s) decreases as the reduced-stress level increases (Fig. 2, curve ( A ) ) . No noticeable metallographic changes (dislocation density, dislocation wall spacing) were observed in the plateau region of the reduced-stress treatment, in which n did not vary with recovery time.The activation energy for the recovery of creepresistant substructure decreases as the reduced-stress level increases (Fig. 2 , curve ( B ) ) and strongly suggests that the recovery mechanism is stress-assisted as well as thermally activated. Metallographic examination of etchdpit configurations revealed that dislocation walls became more distinct, the separation of walls increased, and the densities of dislocations between the walls decreased during the portion of the reduced-stress treatment in which the recovery index increased with time. The process primarily responsible for recovery during this treatment is the migration of subboundaries of dislocation walls through the crystal, sweeping up barriers to dislocation motion in their path.In vacuum, creep recovery experiments were conducted under lo-' torr a t lOOO", 1020°, and 1040°C. Compressive specimens' were crept under a stress of 0,,=3000 psi to a strain tc=0.051 early in the secondary stage of creep and then allowed to recover for varying times under a reduced stress of ur=300 psi. Recovery was detected by the increased creep strain which occurred on reapplication of u~. The recovery index,' n, is plotted vs time of recovery, t,, on log-log coordinates in Fig. 3. The mean activation energy for recovery of creep-resistant substructure of vacuum-reduced rutile is 119,-000 cal/mol, whereas that for recovery of creepresistant substructure in air' is 135,000 cal/mol. An error of 5% was estimated for both activation energies for recovery. A stress of 10,000 psi was required to reach a creep strain' of 0.046 for a particular temperature-compensated time, Oc ( 8 , = t exp ( -H,/RT), where H, is the activation energy for creep). In vacuum about the same creep strain was reached under a stress of 3000 psi for the same temperature-compensated time. Creep of Rutile at High Temperatures," ibid., 54 [7] 359-60 (1971). ' W. M. Nirthe and J. 0. Brittain, "High-Temperature Steady-State Creep in Rutile," ibid., 46 [9] 411-1...

Enhanced ionic conductivity in polycrystalline TiO2 by “one-dimensional doping”

“…This shows that the dislocations are persistent in the sample. Bell et al 34,35 studied the recovery rate in creep deformed TiO 2 crystals and observed that the activation energy for recovery at zero applied stress is double the activation energy required to creep. Therefore, they concluded that complete annihilation of dislocations requires a higher temperature and prolonged annealing times (see ref.…”

Creep recovery of CoO single crystals

“…They suggested that the primary recovery mechanism involves the sweeping out of dislocation barriers within the material by the migration of dislocation walls. Krishnamachari et aL [7] have also studied the creep recovery of rutile single crystals in vacuum and under various reduced stresses. They suggested that the basic recovery mechanism in near stoichiometric [6] and vacuum-reduced futile [7] under reduced stress is the same.…”

“…Krishnamachari et aL [7] have also studied the creep recovery of rutile single crystals in vacuum and under various reduced stresses. They suggested that the basic recovery mechanism in near stoichiometric [6] and vacuum-reduced futile [7] under reduced stress is the same. They also suggested that the creep recovery in air is stress assisted.…”

60 years of dislocations in ceramics: A conceptual framework for dislocation mechanics in ceramics

“…In consequence, the yield strength rises with the degree of deformation which is referred to as strain hardening. Strain hardening can be highly pronounced in some materials, for example, TiO 2 , where after a small degree of deformation, the strain rate is limited by the rate at which recovery and creep can take place 207–214 . In other materials, such as for the < 100 > {100} slip system of SrTiO 3 at high temperature, 76 it is hardly observed, see panel Figures A3 and A4.…”