Solid Solution Strengthening

Line Defects - Dislocations Line Imperfections in the lattice of a crystalline material. Movement of dislocations => material deformation Interference with movement of dislocations => material strengthened Types: Edge Screw Mixed Edge Dislocation Screw Dislocation

“Preventing” Dislocation Movement Slip Impeded by Lattice Imperfections: Point Defects Vacancies Interstitials Substitutional Etc Line type: another dislocation Surface type: Crystal grain boundary Material surface

Some Ways to Strengthen a Metal By Disrupting the Crystal Lattice Alloy: Solid Solution Strengthening Dispersion Strengthening Precipitation Hardening Heat Treatment: Controlled heating and cooling of metals for purpose of altering properties Strain Harden or Cold Work

Three Basic Types of Lattice Imperfections Point Defects Line Defects (Dislocations) Surface Defects Edge Dislocation Screw Dislocation

Defects and Strengthening Mechanisms Strain Hardening Introduces Line Defects Varied Strengths Grain Size Refining Surface Defects Defects and Strengthening Mechanisms Solid Solution Strengthening Single Phase Point Defects Low Strengthening Effect Exceed Solubility Limit Precipitation Hardening Two Phase, Coherent Point and Surface Defects + High Strengthening Effect Dispersion Strengthening Two Phase, Non-Coherent Point and Surface Defect Medium Strengthening Effect

Solid Solution Strengthening Single Phase Point Defects Strengthening Effect Depends on: Difference in size between solute and solvent atoms Amount of solute added

Effect of Solid Solution Strengthening on Properties Hardness Ductility Electrical Conductivity Creep Resistance

Formation of a Solid Solution

Defects and Strengthening Mechanisms Strain Hardening Introduces Line Defects Varied Strengths Grain Size Refining Surface Defects Defects and Strengthening Mechanisms Solid Solution Strengthening Single Phase Point Defects Low Strengthening Effect Exceed Solubility Limit Precipitation Hardening Two Phase, Coherent Point and Surface Defects + High Strengthening Effect Dispersion Strengthening Two Phase, Non-Coherent Point and Surface Defect Medium Strengthening Effect

Dispersion Strengthening Exceeds solubility limit => two solid phases Matrix continuous soft and ductile Precipitate discontinuous strong round numerous

Dispersion Strengthening => Non-Coherent Precipitates One solid phase Another solid phase Phase boundary = surface defect Blocks slip! Non-Coherent Discontinuous Only blocks slip if precipitate lies directly in path of dislocation Does not disrupt surrounding crystal structure

Group Work – Turn in! a b q g Label two phase regions What are the solid solutions and what are intermetallics? Three phase reactions Formation of microstructure of 1% Mg upon cooling Formation of microstructure of 5% Mg upon cooling Where do you have solid solution strengthening only? Where do you have dispersion strengthening? Generate a plot of strength as a function of composition from 0% Mg to 10% Mg at 300 C and explain this graph in terms of crystalline defects and slip. What is the amount and composition of phases present at 200 C for 2 % Mg, 4 % Mg, 6% Mg

a b q s g Eutectic g + q a + q b + g Formation of Microstructures 1% Mg: All L at 700 C a starts to form at ~ 650 C +L at ~640 C All a at ~625 – 0 C (blue line does not really cross solvus line) 5% Mg: a starts to form at ~ 640 C +L at ~620 C All a at ~580 C q starts to form at ~ 240 C q +L at ~240 – 0 C Amount and composition 2% Mg: Ca = 2% Mg and 100% a 4% Mg: Ca = 3% Mg; Cq = 36% Mg Amount a = 36-4/36-3 * 100 = 97% Amount q = 4-3/36-3 * 100 = 3% 6% Mg: Amount a = 36-6/36-3 * 100 = 91% Amount q = 6-3/36-3 * 100 = 9% **what do you notice about the compositions?** **what do you notice about the amounts?** Eutectic g+ L a + L b+ L a b g q s (psi) g + q a + q b + g Solid solutions are a, b, intermetallics are g, q Three phase reactions, eutectic L => g + q, L => g + b Where do you have solid solution strengthening only? In single phase regions. Where do you have dispersion strengthening? In two phase regions. At 0% Mg we have pure material – no defects to block slip, with increasing amounts of Mg we have solid solution strengthening = point defects to block slip at 5% Mg we exceed the solubility limit and now we have two phases, so we have phase boundaries which are surface defects as well as point defects from the solid solution to block slip, the amount of second phase will increase with increasing Mg so the number of surface defects will also increase = more opportunities to block slip (more obstacles in Goldie’s path!!!!!