Chapter 8 We can change dislocations by controlling the grain size reducing continuity of atomic planes or strain hardening creating and tangling dislocations or by alloying introducing point defects and more grains to pin dislocation Grain size size of grain has a effect on strength of material o Boundary acts a barrier to dislocation movement o The smaller the grain the shorter the distance atoms can move along a particular slip plane smaller grains improve the strength of a material Strain hardening work hardening or cold working o Increases the number of dislocations by deformation o Makes the metal harder and stronger through plastic deformation o When the metal is plastically deformed the dislocations move and additional dislocations are generated o More dislocations the more they ll interact and become pinned or tangled This makes a strengthening of a material Usually called cold working because the plastic deformation occurs at a low temper that atoms can t rearrange themselves Increasing the strength by cold working will also result in a reduction in ductility When it s hot worked the dislocation can rearrange and little strengthening is achieved Springback elastic strain that is recovered after a material has been plastically deformed Bauschinger effect when a material subjected to tension shows a reduction in compressive strength Percent cold work o Formula o o Ao original cross sectional area of the metal o Af final cross sectional area after deformation Percent reduction in thickness o Formula o o To intial sheet thickness o Tf final thickness o For glasses annealing is a heat treatment that removes thermally induced stresses o Glass that s been treating by heating above the annealing point temperature then cooled slowly to minimize or eliminate residual stresses o A glass obtained by either heat treatment or quenching or by the chemical exchange of Laminated safety glass Two pieces of annealed glass held together by a plastic such as polyvinyl butyral PVB Heat treatment annealing can be used to remove the effects of strain hardening o Three things can occur during heat treatment Recovery recrystallization and grain Annealing Annealed glass Tempered glass ions growth Chapter 8 Three Stages of Annealing o Recovery Low temp annealing heat treatment designed to eliminate residual stresses introduced during deformation without reducing the strength of the cold worked material Polygonised subgrain structure a subgrain structure produced in the early stage of annealing The subgrain boundaries are a network of dislocation rearrange during heating The process of forming new dislocation free grains by heat treating a cold worked material A fine recrystallized grain structure is formed Grains grow larger at elevated temperatures with favored grains consuming o Recrystallization o Grain growth Control of Annealing smaller grains o Recrystallization temperature the temperature at which grains in the cold worked microstructure being to transform into new equiaxed and dislocation free grains Warm working are 3 to 6 times below 3 times are cold working A smaller initial cold worked grain size reduces the recrystallization temp Pure metals recrystallize at lower temps than alloys Increasing the annealing time reduces the recrystallization temp Annealing and materials processing o Deformation processing By taking advantage of the annealing heat treatment the total amount of deformation can be increased o High temperature service When the cold worked metal is placed into service at a high temperature recrystallization immediately causes a catastrophic decrease in strength Hot working temp working o Plastically deforming the metallic material at a temperature above the recrystallization o Lack of strengthening does not occur during deformation by working The amount of plastic deformation is almost unlimited o Elimination of imperfections imperfections may be eliminated or minimized during hot o Anisotropic behavior final properties in hot worked parts are not isotropic o Surface finish and dimensional accuracy the surface finish formed during hot working is usually poorer than that obtained by cold working