Simultaneous high strength and large ductility are always desirable for metallic materials. However, while the strength of metals and alloys can be easily increased by five to 15 times through simple plastic deformation or grain refinement down to the nano-scale, the gain in strength is usually accompanied by a drastic loss of uniform ductility. Ductility depends strongly on the work hardening ability, which becomes weak in materials with high strength, especially in a single-phase material.