Work-hardening of metals

Minute amounts of added material can change the strength greatly. For example, added carbon atoms in iron can act as dislocation traps and halt the gliding motion. Work-hardening of metals is a process whereby many of the dislocations intersect and collide with one another, thereby becoming partially immobilized. The movement of dislocations can be studied at various temperatures and the activation energy found. At 1500°C plastic flow can be seen, even in diamond. 

Y.V. Milman unpublished results A. Seeger The temperature dependence of the critical shear stress and of work hardening of metal crystals, Philos. Mag. 7, 771 (1954)... 

During plastic deformation, existing dislocations serve as nucleation sites for new dislocations to form hence, the dislocation density of the material increases significantly. Whereas the dislocation density (in units of dislocation distance per unit volume mm/mm or mm ) of pure metallic crystals is on the order of 10 mm , the density may reach 10 mm in heavily deformed metals. It should be noted that line defects may not always be detrimental. As we will see in Chapter 3, the interactions among neighboring dislocations are responsible for work hardening of metals. 

It is thought that the increased hardness (Tab. 6.11) is due in part as a result of the reduced pore density and the increase in dislocation density caused by work hardening. It is well known that raising the temperature of a metal work hardens that metal and it is thought that the implosion of cavitation bubbles dose to the electrode raises the microscopic temperature. In addition collapse will produce surface impacts by solvent also generating work hardening. 

In fact m (Eqn 12-52) is greater than 2.5. Anderson (Reference 21) ascribes this to work hardening of the metal by plastic deformation. 

The plastic deformation of a metal between opposed dies while it is held above its crystallization temperature thus avoiding work hardening of the workpiece. 

The upper roll of the three-roU assembly can be adjusted vertically. The radius of the bend is controlled by the roll adjustment. Premature failiire will occur if the contour radius is decreased too rapidly however, too many passes through the rolls may cause excessive work hardening of the work metal. Several trial parts must sometimes be made in a new material or shape to establish suitable operating conditions. 

More uniform results may be expected if a substantial layer of metal is removed from the specimens to ehminate variations in condition of the original metaUic surface. This can be done by chemical treatment (pickling), electrolytic removal, or grinding with a coarse abrasive paper or cloth, such as No. 50, using care not to work-harden the surface. At least 2.5 X 10 mm (0.0001 in) or 1.5 to 2.3 mg/cm (10 to 15 mg/iu") should be removed. If clad alloy specimens are to be used, specif attention must be given to ensure that excessive metal is not removed. After final preparation of the specimen surface, the speci-... 

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