Question: Explain Creep mechanism in detail

Subject Material Technology

Topic Failure Mechanism

Difficulty Level HIgh

mt(63) • 707 views
modified 10 months ago  • written 10 months ago by gravatar for Atharv Inamdar Atharv Inamdar650
  1. The Mechanism of creep depends on stress as well as temperature.

  2. And under this dislocation conditions various creep mechanism occurs which are as follows:

    i) Bulk diffusion

    Here dislocation glide creep does not involve atomic diffusion. The lateral sides of the crystal are subjected to a tensile stress, and the horizontal sides to a compressive stress. The atomic volume is altered by applied stress: it increases in regions under tension and decreases in regions under compression. This type of creep is strongly temperature dependent. For lattice diffusion of atoms to occur in a material, neighbouring lattice sites or interstitial sites in the crystal structure must be free. A given atom must also overcome the energy barrier to move from its current site to the nearby vacant site

    ii) Climb Creep

    This creep mechanism is observed at high temperature. Initial creep rate is larger than steady state one. When the applied stress is not enough to for a moving dislocation to overcome the obstacle on its way via dislocation glide alone, the dislocation could climb to a parallel slip plane by diffusional processes, and the dislocation can glide on the new plane.

    iii) Grain boundary Diffusion

    The atoms diffuse along grain boundaries to elongate the grains along the stress axis. as the temperature increases so does the grain boundary diffusion. However, since the number of nearest neighbours is effectively limited along the interface of the grains, and thermal generation of vacancies along the boundaries is less prevalent, the temperature dependence is not as strong as in Diffusion creep.

    iv) Solute drag creep

    Solute drag creep is one kind of mechanisms for power law creep (PLC), involving both dislocation and diffusional flow. Solute drag creep is observed in certain metallic alloys. Their creep rate increases during the first stage of creep before a steady-state, which can be explained by a model associated with solid-solution strengthening. The size misfit between solute atoms and edge dislocations could restrict dislocation motion. The flow stress required for dislocations to move is increased at low temperatures due to immobility of the solute atoms. When the applied stress becomes sufficiently large, the dislocations will break away from the solute atoms since dislocation velocity increases with the stress. After breakaway, the stress decreases and the dislocation velocity also decreases, which allows the solute atoms to approach and reach the previously departed dislocations again, leading to a stress increase. The process repeats itself when the next local stress maximum is obtained. So repetitive local stress maxima and minima could be detected during solute drag creep. So these were the major Mechanism for creep to occur in a material at high temperature.

written 10 months ago by gravatar for Atharv Inamdar Atharv Inamdar650
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