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.