Creep in Polycrystalline Ceramics

These basic relations and factors also apply to polycrystalline materials with the appropriate variations dictated by experimental observations. Historically, Andrade [24] should be considered the father of creep , since it was he who first suggested a unified creep relation. All the many equations describing creep given in the literature follow in the wake of Andrade s concept [24] of this empirical relation. One may express the variation of strain over time as  

According to the symbols and k in Eq. (6.15), it describes beta or kappa creep. When k = 0, the constant creep is obtained  

This represents transient creep, since the creep rate is a decreasing function of time. By differentiating Eq. (6.15), with respect to time, one obtains (6.17)  

This is K creep, describing the stationary stage. Clearly, the strain rate (as obtained from Eq. (6.18)) gives a linear relation, as seen in Eq. (6.19) and, thus, describes steady-state creep  

Andrade postulated that jS creep is related to dislocation glide within the grain, while K flow is related to slip along grain boundaries. Ascribing k flow to grainboundary sliding is known to be in error. Equation (6.18) may also be expressed as  

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Name two mechanisms that give rise to dilfusional creep in polycrystalline ceramics. 

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