Topic 8: Strengthening Mechanisms in Metals

any plastic deformation increases the strength of a crystalline material → due to dislocation traffic jam

how does plastic deformation affect the strength of a crystalline material

• smaller grains → stronger

• bigger grains → weaker

how does grain size affect strength

• to get small grains → do lots of CW → anneal

• to get big grains → let grain growth happen

how can you manipulate grain size to get small grains

• if dislocation can move easily → weak and ductile material

• to strengthen material → stop dislocation movements

what is the rule of thumb for strengthening mechanisms (for metals)

• solid solutions can be substitutional or interstitial 

• atoms of different elements will have different sizes

• when atoms of another element are inserted into a crystal lattice, there will be some distortion

• smaller solute → greater distortion

• the electrical resistivity of the alloy would also change with differing amounts of alloying component

  • max lattice distortion → max resistivity

solid-solution strengthening

• any boundary can inhibit dislocation movement in the crystal lattice

• a boundary between two phases (phase boundary) will also inhibit dislocation movement

• if you understand how the phase diagram for that alloy works, then you can choose an alloy to maximise the number of boundaries to maximise its strength

  • to maximise σ_y → increase % eutectic solid

• in the eutectic system, there are only two solid phases of interest:

  • α → FCC
    → weak and ductile

  • θ → BCT
    → strong and brittle

multiphase strengthening

• you can further increase the number of boundaries to stop dislocation movements

  • this is done by having lots of small hard phases (ppts) in a ductile matrix

• there are two mechanisms by which precipitates can inhibit dislocation movement and strengthen the alloy:

  • 1. discontinuity in slip system

  • 2. distorting parent lattice

• for the same alloy composition, lots of small precipitates are much more effective at stopping dislocation movements than a few big precipitates

• these microstructures are formed only if slow cooling is allowed

dispersion strengthening/age hardening/precipitation hardening

1. solution heat treatment → heat alloy above solvus to dissolve all Cu atoms

2. quenching → rapidly cool to RT → do not allow diffusion

   • forms super saturated α

   • this is known as the metastable phase

3. aging → heat below solvus temperature → allows diffusion

what are the three stages of heat treatment used for preparing small precipitates dispersed in another phase

• the precipitation of the second phase particles is a diffusion-based process, dependent on BOTH time and temperature

• higher aging T (below solvus)
  → less time to reach max σ_y
  → lower max possible σ_y (and vice versa)

yield-strength - log(aging time) graph

1. phase diagram must show decreasing solid solubility of the strengthening phase with decreasing temperature

   • must be able to quench from a single solid phase region to a 2-solid phase region

2. the parent matrix should be relatively soft and ductile; and the strengthening precipitate phase should hard and brittle and finely dispersed throughout the softer parent phase

3. the ppts should be coherent with the parent matrix and distort it to create strain fields to make dislocation more difficult

4. the alloy should be able to survive the quenching process

   • sudden T change → thermal shock → can cause shape distortions and cracks

what are the four basic requirements before an alloy can be age-hardened

• they can not be welded because they will form hard and brittle θ at joints

• overage

• options for joining:

  • rivets

  • screws

  • glue

why can’t aluminium-based age-hardening alloys be welded together