Plastic deformation

Describe how plastic deformation occurs in metal.

I know that,
Is the deformation that is permanent after release of applied load.
Point where stress is no longer proportional to strain.

But not sure how you can relate it to metal.

Any help is greatly appreciated.

Don't know of a simple explanation for this one. You need to know a deal about crystal structure.

If you consider the crystal structure and the bonds as vector lines of bonding force, the vector sums add up to different strengths in different directions. There are planes where the bond strength sums are definitely weaker.

Tension exerts shear stress diagonal to the line of the tension. If you examine the crystal structure, after plastic deformation, it should come as no suprise to learn that small failures known as slip planes can be observed along these planes of weakness.

Look up slip planes.