Chapter 5 - Strengthening Mechanisms Flashcards
In order to strengthen a metal you need to make it harder for “_____” to move?
Dislocations.
What are the two methods of strengthening a pure metal?
Work Hardening. Grain Size.
How do you reduce the grain size for a metal?
Cold work the metal, then anneal it to the point of recrystallisation. The higher the amount of cold work the smaller the grain size. If you anneal it too long then the grains will grow again though.
What are the methods that can strengthen alloys?
Solid Solution Strengthening. Multiphase Strengthening. Dispersion Strengthening (A.K.A. age hardening).
What does solid solution strengthening refer to?
As you replace solvent atoms with solute atoms in the crystal lattice the crystal lattice bends and distorts. This makes it harder for dislocations to move.
When strength increases ductility…?
Decreases.
When strength increases resistivity…?
Increases.
What does multiphase strengthening refer to?
Multiphase strengthening refers to increasing the number of solid phases, as the border between solid phases stops dislocations from moving.
In a copper-aluminium phase diagram what kind of crystal structure is the alpha? Does that make it strong or ductile?
FCC. Ductile.
In a copper-aluminium phase diagram what kind of crystal structure is the theta? What does that mean for its slip systems?
BCT (Body Centred Tetragonal). There are no slip systems the metal is hard and brittle.
What composition of an alloy in multiphase strengthening will be the strongest?
Eutectoid Alloy. This maximises the number of layers and stops the formation of solid grains.
What is the issue with using hypereutectoid aluminium-copper alloy?
This will form lots of brittle grains of theta material which is extremely unpredictable. AVOID THIS MICROSTRUCTURE.
What does dispersion strengthening involve?
Precipitation hardening creates a fine dispersion of a harder second phase in small precipitates throughout a material.
What is the first stage of age-hardening an alloy?
First you must heat the alloy to above its solvus line, till a single solution of alpha is formed.
What is the second stage of age-hardening an alloy?
Rapidly cool the alloy (quenching) this causes the material to turn into a metastable supersaturated solid solution rather than turning into its normal stable form.
