I didn't understand that Dislocations make the crystal weak. Even if the crystal is free from dislocation then also we will get lesser CRSS no?.. because still plastic deformation will occur by the same mechanism of extra half plane gliding on slip plane.(extra half plane forming on application of force)
The assumption of a dislocation free crystal is that the crystals remain free of dislocation during the slip process. In this case, the stress required is large. You are imagining a scenario where a dislocation nucleates in an initially dislocation free crystal. In this case, there will be a stress drop as soon as the dislocation nucleates.
Sir can you say 1.Which mechanism is better compared to each? and 2.What are the advantages or uses of these strengthening mechanisms and the last one 3.Will the mechanism be different for different shapes of metals like for a flat surface a particular mechanism and for a cylindrical surface a particular mechanism?
1.cannot say by generalizing, each one has its own disadvantages also like strain hardening reduces the fracture strain, all metals cannot go age/precipitate hardening, solid solution hardening means like making new alloy needs lot of efforts, hardening through grain refinement may be better. 2.advantage is to improve some properties like hardness, yield strength etc. 3.independent of shape
In strengthening mechanisms we study the microstructural causes that increase or decrease the strength of a material. In heat treatment, we study the process of heating or cooling that improves the property of a material. The reason for the change in the property of material due to heat treatment can be one of the strengthening mechanisms. Thus cold working is a process by which strength increases. The mechanism by which this happens involves increase in dislocation density called strain hardening. If a cold-worked material is then annealed, a heat treatment, strength will come down. This is due to reduction of dislocation density.
Hardness measures resistance to plastic deformation under indentation. The stress state under an indenter is complex triaxial stress. Yield stress is a measure of resistance to plastic deformation under uniaxial tension. The stress state is a simple uniaxial tension. Thus both measure resistance to plastic deformation but under different stress conditions. The hardness test is easier to perform than a uniaxial tensile test.
