1. Solid Solution Strengthening

2. 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

3. “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

4. 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

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

6. 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

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

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

9. Formation of a Solid Solution

10. 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

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

12. 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

13. 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

14. 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!!!!!