Chapter 7 High temperature deformation of crystalline materials-2

Chapter 7 High temperature deformation of crystalline materials-2

Primary creep, Stage I, is a period of decreasing creep rate. Primary creep is a period of primarily transient creep. During this period deformation takes place and the resistance to creep increases until stage II. Secondary creep, Stage II, is a period of roughly constant creep rate. Stage II is referred to as steady state creep. Tertiary creep, Stage III, occurs when there is a reduction in cross sectional area due to necking or effective reduction in area due to internal void formation.

The following stages are observed:1. The incubation deformation. 2. The primary stage. The creep rate decreases when the strain increases.3. The steady-state strain. The plastic strain rate is a constant value. ε˙ = const.4. The tertiary stage. The tertiary creep leads to a rupture.

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Low-temperature plasticity limited by a lattice resistance. The strain-rate is determined by the kinetics of kink nucleation and propagation.

Low-temperature plasticity limited by discrete obstacles. The strain-rate is determined by the kinetics of obstacle cutting.

Diffusional flow by diffusional transport through and round the grains. The strain-rate may be limited by the rate of diffusion or by that of an interface reaction.

Power-law creep involving cell-formation by climb. Power-law creep limited by glide processes alone is also possible.