CHAPTER 5 : DISLOCATION & METAL STRENGTHENING MECHANISMS

TOPIC 1) INTRODUCTION 2) ELASTIC & PLASTIC DEFORMATION 3) SLIP & TWINNING 4) STRAIN HARDENING Beginning course details and/or books/materials needed for a class/project.

INTRODUCTION Deformation : Changing of shape of metal under the action of force Give desired shape Deformed metals – more superior than cast metal Operation of deformation – rolling, forging, spinning. Materials may experience 2 kinds of deformation : Elastic deformation Plastic deformation Objectives for instruction and expected results and/or skills developed from learning.

ELASTIC & PLASTIC DEFORMATION Elastic Deformation Plastic Deformation Defined as process of deformation which appears & disappears with the application & removal of stress Defined as process of permanent deformation &exist after removal of stress At the beginning of the deformation progress Takes place after elastic deformation has stopped a) Original state b) Elastic deformation with vertical force applied c) Elastic deformation with diagonal force applied d) The beginning of plastic deformation upon a slip plane e) Deformation Proceeding upon further slip plane.

Modes of Plastic Deformation There are 2 basic modes : 1) slip 2) twinning Slip Twinning Due to sliding of atomic planes over the others Due to change of orientation of a crystal with other Crystal orientation above & below slip plans is the same after deformation Twinned portion is the mirror image of the original lattice Atomic movements are over large distance. Atomic movements are over a fraction of atomic space. Require lower stress for atomic movement Require higher stress for atomic movements Occurs on widely spaced planes Occurs on every atomic planes involved in twinned region

STRAIN HARDENING Defined as – process deforming ductile metal to become stronger & harder. Also called ‘work hardening’ or ‘cold working’. Deformation takes place at temperature that is ‘cold’ relative to melting temperature. Below its recrystallization temperature. During cold working : Certain amount of work done is stored internally in form of strain energy The energy produces internal stress Can lead to cracking Relieve metal from internal stress by annealing – metal is heated (below melting temperature) Metal loses its stored energy – come back to strain free condition

- metal losses its stored energy in 3 stages : Recovery Recrystallization Grain growth

Remove internal stress Recovery Defined as – process of removing internal stress in metal by heating it to a relatively low temperature (below melting temperature) Remove internal stress Does not affect – grain structure - hardness & strength Increase ductility During cold working : Dislocation pile up at grain boundary During recovery : Dislocation number starts reducing & rearrange themselves

Recrystalization Defined as : Process of forming a new set of strain free grains in metal by heating it to a temperature known as recrystallization temperature. The new strain free & equiaxed grains: Have low dislocation density Characteristics of precold-worked condition Driving force : The difference in internal energy between strained & unstrained material New grains form as very small nuclei & grow to replace parent material Involved short range diffusion Take place through 3 process : nucleation Primary grain growth Secondary grain growth

Recrystallization produces : Grain Growth Defined as – process of forming strain-free grains larger in size by heating a metal to a temperature above recrystallization temperature. Recrystallization produces : Strain-free new grain smaller in size but of equal shape Grain will continue to grow if temperature is increased Grain growth does not need to be preceded by recovery & recrystallization May occur in all polycrystalline material (metal& ceramic) Growth rate –rapid & becomes slow when temp increased During growth , combination of individual grains takes place.

Reducing grain boundary area Reduction in total energy Grains becomes stable Driving force for grain growth