Strengthening Mechanisms The ability of a metal to plastically - PowerPoint PPT Presentation

Kasetsart University 213211: Strengthening Strengthening Mechanisms The ability of a metal to plastically deform depends on the ability of dislocations to move. Restricting or hindering dislocation motions renders a material harder and stronger. 1. Grain Size Reduction 2. Solid-Solution Strengthening 3. Precipitation Hardening 4. Strain Hardening Dr.Peerapong Triyacharoen Department of Materials Engineering 135

Kasetsart University 213211: Strengthening Grain Size Reduction • Grain boundaries are barriers to slip. slip plane B • Barrier "strength" n i a r increases with g misorientation. grain A grain boundary • Smaller grain size: more barriers to slip. σ yield = σ o + k y d − 1/2 • Hall-Petch Equation: Note: not valid for both very large grain and extremely fine grain materials. Dr.Peerapong Triyacharoen Department of Materials Engineering 136

Kasetsart University 213211: Strengthening Example: Grain Size Strengthening • 70wt%Cu-30wt%Zn brass alloy grain size, d (mm) • Data: 10-1 10-2 5x10-3 200 σ yield(MPa) 150 0.75mm ky 100 σ yield = σ o + k y d − 1/2 1 50 0 0 4 8 12 16 [grain size (mm)]-0.5 Dr.Peerapong Triyacharoen Department of Materials Engineering 137

Kasetsart University 213211: Strengthening Solid Solution • Impurity atoms distort the lattice & generate stress. • Stress can produce a barrier to dislocation motion. • Smaller substitutional • Larger substitutional impurity impurity A C D B Impurity generates local shear at A Impurity generates local shear at C and B that opposes disl motion to the and D that opposes disl motion to the right. right. Dr.Peerapong Triyacharoen Department of Materials Engineering 138

Kasetsart University 213211: Strengthening Example: Solid Solution Strengthening Nickel in Copper Dr.Peerapong Triyacharoen Department of Materials Engineering 139

Kasetsart University 213211: Strengthening Precipitation Strengthening • Hard precipitates are difficult to shear. Ex: Ceramics in metals (SiC in Iron or Aluminum). precipitate Large shear stress needed Side View to move dislocation toward precipitate and shear it. Unslipped part of slip plane Dislocation Top View “advances” but precipitates act as “pinning” sites with S spacing S. Slipped part of slip plane σ y ~ 1 • Result: S Dr.Peerapong Triyacharoen Department of Materials Engineering 140

Kasetsart University 213211: Strengthening Strain Hardening (Cold Working) • Room temperature deformation. • Common forming operations change the cross sectional area: force -Forging -Rolling roll die Ad Ao Ao Ad blank roll -Drawing force -Extrusion Ao die Ad container die holder tensile force Ao ram Ad billet extrusion force die die container %CW = A o − A d x100 A o Dr.Peerapong Triyacharoen Department of Materials Engineering 141

Kasetsart University 213211: Strengthening Impact of Cold Work • Yield strength ( σ y ) increases Stress • Tensile strength (TS) increases • Ductility (%EL;%RA) decreases % cold work Strain Dr.Peerapong Triyacharoen Department of Materials Engineering 142

Kasetsart University 213211: Strengthening Effect of Heating after %CW Dr.Peerapong Triyacharoen Department of Materials Engineering 143