A discussion on plastic flow in metals

A perfect metal crystal is extremely weak, yielding to a vanishingly small shearing force. As deformation proceeds the perfection of the crystal structure is destroyed, and the resistance to shear is increased. After severe cold-working in all directions the metal attains a steady state, with random orientation of the crystal grains, and characteristic mechanical properties. Yield takes plane along definite crystalline planes, and parallel to definite directions in these planes (e. g. [111] and [110] in crystals with a facecentred cubic lattice). A rod under tension develops slip-bands parallel to these planes. Deformation is not uniform, but is concentrated in bands of macroscopic dimensions separated by undeformed portions, more slipbands being formed as extension proceeds.