Deformation of Metals Introduction • When metals or alloys are stressed, they get deformed • Deformation is the change in dimension or form of matter under the action of applied forces. • Deformation is caused by • Mechanical action of external forces • Various physical and physico chemical process (change in volume of separate crystallites in phase transformation) • Deformation may be wholly temporary – ELASTIC DEFORMATION – which disappears after the deforming load is removed. • Permanent deformation may also occur- PLASTIC DEFORMATION – which remains after the removal of the deforming load. Elastic Deformation • If a metal is loaded, a temporary deformation of the crystals takes place through displacement of the atoms. • As the deforming load is removed, the atoms return to their original stable position and the crystals recovers its original shape. Elastic Deformation • It occurs with comparatively smaller deforming loads, which can keep working stress within the elastic range. • Under tensile deforming load, the metal piece becomes slightly longer as a result of slight elongation of the unit cell in the direction of the tensile load. • This elongation in the metal piece disappears as soon as the deforming load is removed. • When elastic deformation occurs, the strain in metal piece is nearly proportional to the stress, i.e. the condition in the elastically deformed region are expressed by Hooke’s Law Elastic Deformation • Hooke’s law which says in case of the tension test that deformation e is proportional to applied stress: 𝑃𝐿 𝑒= 𝐴𝐸 • • • • P is the applied force L is the length of the metal specimen A is the cross sectional area of the specimen E is modulus of elasticity Plastic Deformation • Plastic deformation follows elastic deformation • If the deforming load is such that the stresses in the metal piece cross the elastic limit, the specimen gets plastically deformed. • If the deforming load, at this stage is removed, the metal piece does not regain its original shape; a permanent (plastic strain) set always remain. Plastic Deformation • When the load is suddenly removed from a plastically deformed specimen, the total strain is immediately reduced by the amount Stress/ Young’s Modulus i.e. the elastic strain Plastic deformation • Next, the anelastic strain disappears gradually after the load has been removed but the plastic deformation remains. • Plastic deformation is the function of • Applied stress • Temperature • Strain rate • It is accompanied by change in both the internal and external state and it is not reversible. It involves distortion of crystal and microstructure. • It is carried out during shaping process such as bending, stamping etc.
