The previous section described the different regions of a creep curve and their corresponding characteristics. Mechanistically, the creep curve is a result of the changes occurring in a material at a microstructural level.

The creep curve is basically the outcome of the competition between the processes of strain hardening and recovery. Materials usually strain harden during plastic deformation due to dislocation multiplication. The strain hardening is a kind of ‘defense’ mechanism in response to an applied stress. Further plastic deformation can occur only if the applied stress exceeds the increase in flow stress of the material due to strain hardening. Alternatively, deformation can proceed at the initial applied stress if the material soft­ens. The mechanism of recovery acts to soften a deformed specimen thus allowing further plastic deformation. In the primary stage, the rate of strain hardening is greater than the rate of recovery. This is due to the formation of a more resistant creep substructure. The substructure could be the forma­tion of dislocation networks or the arrangements leading to the formation of subgrains. In the secondary stage of creep, the rate of strain hardening is balanced by the rate of recovery due to dislocation annihilation and defor­mation occurs at a constant strain rate. In the tertiary stage of creep, the increase in applied stress due to a reduction in specimen cross-sectional area surpasses the increase in flow stress due to strain hardening. The reduc­tion in specimen cross-sectional area can be due to necking or internal void formation. The tertiary creep is often associated with metallurgical changes such as the coarsening of precipitate particles, recrystallization or diffusional changes in phases present, void formation and so forth.

Figure 3.2 depicts four types of creep curves that have been generally observed.4 The shape of the creep curve is dependent on the initial condition

of the material prior to deformation. Curve A is a typical creep curve observed in several materials. The curve consists of a normal primary stage characterized by a decreasing strain rate, a secondary stage where defor­mation proceeds at a constant rate and a tertiary stage where the material deforms at increasing strain rates with time/strain leading to eventual fail­ure. Such creep curves are usually exhibited by annealed metals and cer­tain alloys (known as class-M or class-II type). In comparison to curve A, curve B depicts a very small primary creep stage. In fact, it appears as if the material enters the steady state immediately. Such a type of curve is obtained when the substructure pertaining to creep remains constant such as in some alloys (known as class-A or class-I type). Curves of type C are obtained from materials that have been previously crept at a higher stress. The increasing creep rate over the primary creep stage is due to the recov­ery of the substructure corresponding to the previous steady-state condi­tion. The sigmoidal type of creep curve (curve D) suggests the nucleation and spread of slip zones until a steady state is achieved. Such creep behavior has been exhibited by certain dispersed phase alloys.

In certain cases, it is possible that the total creep curve is in the primary stage, and the secondary and tertiary creep stages are not attained at all. Such a curve has been seen for materials tested at low temperatures (T < 0.3 TM) where effects due to diffusion are suppressed (no annealing or recov­ery) and the entire deformation is due to work hardening (dislocation con­trolled). The primary creep strain rate tends towards a value of zero at long periods. The strain hardening due to long range dislocation interactions pre­cludes a constant rate of creep deformation. The absence of recovery pro­cesses due to the low test temperatures allows the strain hardening process to be the creep-controlling mechanism. This eventually leads to an increase in strength of the material to a value greater than the applied stress value. Further deformation can only occur under the application of a higher stress or in the presence of a higher temperature. Such a behavior is described as an exhaustion creep behavior and the creep curve can be described by a logarithmic creep equation.4