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Hardening Metals

Ductility in metals comes from dislocations in the crystal lattices of the individual grains. If a higher yield stress is needed then a way needs to be found to immobilise the dislocations or to dispose of them entirely. Fortunately there are many different methods available depending on the needs of the individual application. Collectively they are known as hardening techniques since increasing the yield stress will also increase the hardness of the metal.

Perfect Crystals

It is possible to grow single crystal whiskers that contain very few or even no dislocations. They have extremely high strengths, close to their theoretical limits, however their small size makes possible applications limited. They should not be confused with single crystal castings; this is where a component, such as a turbine blade, is created in a ‘bridgeman furnace’ that allows solidification to proceed slowly through the casting so that only a single crystal can grow. A blade produced in this way has improved resistance to creep. However single crystal castings still contain dislocations and so do not even approach the theoretical limits of strength.

Grain Boundary Strengthening

In Grain Boundary Modelthe paper models you built you saw that a dislocation could move from one atomic plane to another. At the end of the movement the defect was ready to move on to the next plane even though it consisted of entirely different atoms. Like a wave traveling over the sea the defect travels even thought the individual atoms stay put.

However a dislocation cannot carry on moving forever. Eventually it will run into a grain boundary, the jumbled mess of atoms at the edge of a grain. It is stuck here unless it can jump the boundary into the next grain. Other dislocations pilling up behind the first one can have the effect of increasing the surrounding stresses to the point that the dislocation is forced through the boundary in to the neighbouring grain. This applies to all grains.

If the grain size is decreased though there will be an increase in area of the grain boundary (proportional to the square of the grain size) compared to the number dislocations in the grain (proportional to the cube of the grain size). This decreases the number of dislocations that can pile up in order to force another dislocation across the grain boundary. This in turn increases the yield stress which in turn increases the hardness of the metal.

Grain size can be decreased by rapidly cooling the solidifying metal (quenching). Alternately metallurgists can include additives to the alloy that solidify at temperatures in excess of the melting point of the bulk alloy; these act as seed crystals that trigger the creation of large numbers of crystal grains. Because there are so many grains formed the size is limited.

Work Hardening

Work hardening occurs when a ductile metal is pressed, rolled, drawn, hammered or otherwise forged when it is ‘cold’. Each time the metal is plastically distorted new dislocations are introduced into the individual grains. Initially it seems counter-intuitive since dislocations make ductility possible in the first place. However as the number of dislocations increases they begin to get tangled up with each other. Each dislocation produces a surrounding pattern of stresses and strains and these interact to prevent dislocation movement. It is possible for dislocations to move through each other but this results in distortions to the dislocations called kinks and jogs that make it even harder for the dislocations to move.

Work hardening can be removed by heating the metal, this is called annealing. The higher temperature allows the atoms in the metal to diffuse to their ‘equilibrium positions’. In practice this means that the atoms are free to move small distances in the solid, driven by the stresses introduced by the dislocations. In the process the dislocations are destroyed, restoring the ductility of the metal.

Annealing is necessary for metal that is heavily cold worked since too much cold working will leave it hard to work as well as prone to brittle fracture.

Try It

Piano wire, it is a high carbon steel that has been drawn through a series of dies. The final product has a very high strength but has very little ductility left. The result is that it can support a very high tension but over tightening will cause it to snap in a brittle manner. You can check this by taking two short lengths of thick piano wire (use a plain wire string, not one of the ‘over-wound’ strings used for the deeper notes).

Solid Solutions

This is one of the things that can happen when alloys such as steel, brass and bronze are made. Individual atoms of the alloying element can be dissolved into the crystal structure of the main material. If the atom takes the place of a normal atom then it is called a ‘substitutional defect’; smaller atoms put the crystal lattice in to tension and larger atoms put it into compression. Atoms that are much smaller than the majority atoms can sit in between the lattice points and are called interstitial defects. Either way, the stresses that these defects create in the crystal lattice are ‘pinning points’ that restrict the motion of dislocations and so strengthen the material.

 Phase Diagram 

Care needs to be taken as often only a limited amount of one element can be dissolved in another. Looking at the phase diagram for the lead – tin alloy system you can see that at a temperature of 25 °C there can be a maximum of about 2.5 % tin in the mixture before it starts to come out of solution.

 Phase Diagram 

Element

Diameter (pm)

Crystal Structure

Molybdenum

145

body centred cubic

Nickel

135

face centred cubic

Nitrogen

65

hexagonal

Carbon

70

hexagonal

Hydrogen

25

hexagonal

Aluminium

125

face centred cubic

Magnesium

145

hexagonal

Silver

160

face centred cubic

Precipitation Hardening

This Hardening Diagramis a remarkably clever trick used to strengthen many metallic alloys. However the alloy has to be designed and heat treated very carefully. The process goes roughly like this:

 Phase Diagram 

The islands of B embedded in grains of A act as barriers to the movement of dislocations and this increases the yield stress. The aging process has to be carried out very carefully as the size and spacing of the islands increases with longer aging and beyond an ideal point the strength decreases and the component becomes prone to work hardening.

To some extent the ideal aging time and temperature can be chosen by the design of the alloy. Aircraft panels for instance are usually held together with aluminium alloy rivets. The aging temperature for the rivet alloy is actually around room temperature. As a result after the rivets are made they are stored in dry ice (-78 °C) to prevent them from aging. The aircraft panels are fixed together whilst the rivets are still soft and therefore easy to work. After a few hours in a warm hanger the rivets have aged and become hard, strong and rigid; ideal for their purpose of fixing the panels firmly together.

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