High temperature creep in a 2-3-4 garnet: Ca3Ga2Ge3O12

High temperature plastic deformation in a single crystal of a 2-3-4 garnet, Ca3Ga2Ge3O12, was investigated. A Czochralski-grown single crystal of Ca3G a2Ge3O12 was deformed in compression in air along (100) or (110) at tempera tures of 1472 to 1573 K (T/T-m = 0.90-0.96). The samples show higher resist ance to creep than other 3-3 garnets, namely the flow stress at the strain- rate of 4 x 10(-6) s(-1) is similar to 200-400 MPa in this temperature rang e. The TEM observations of dislocation microstructures show little evidence of climb and plastic deformation in this garnet appears to occur exclusive ly by dislocation glide, using mostly the 1/2 (111){110} slip systems. Disl ocations with b = (100) are frequently observed but they are interpreted as products of dislocation reactions among 1/2 (111). The single crystal used contained a number of precipitates that grew during annealing and also dur ing deformation. These precipitates act as sources for dislocations but no evidence for their significant effects on creep strength is observed. The n ormalized flow law of Ca3Ga2Ge3O12 is similar to other 3-3 oxide garnets (e .g., YAG, GGG), but in contrast to 3-3 garnets, the more stable and hence l ess mobile dislocations have a large edge component. (C) 1999 Kluwer Academ ic Publishers.