Altering Material Properties Flashcards
What is grain size reduction and how does it work? (Long explanation)
Grain size reduction is when the different grains of a metal are changed in size in order to make dislocation between the boundaries harder i.e. the atomic disarray between the two boundaries makes it harder for one grain to slip into the other and the lack of orientation between the two means that dislocation would require one of the offending particles to change its direction of motion, making the dislocation much harder to carry out
What is the result of grain size reduction? (Short version)
Dislocation is harder i.e. the material becomes stronger and tougher
How can grain size be controlled?
By controlling the rate of solidification from the liquid phase or by plastic deformation followed by an appropriate heat treatment
What is solid solution hardening?
The hardening and strengthening of a material by introducing an alloying material that disrupts its atomic structure in much the same way as grain size reduction. The alloying impurities impose lattice strain field interactions between themselves and the host atoms and thus dislocation movements are restricted.
What is strain hardening?
The phenomenon whereby a ductile material becomes harder and stronger as it is plastically deformed. As the last two increase, the ductility does the inverse. The process is also sometimes called cold working.
How does cold working work?
As the material is deformed, the amount of dislocations increases. As dislocation-dislocation interactions are repulsive, they will not tend to react well to each other. This means that as the dislocation density within the material increases, there are more of these dislocation-dislocation interactions, making the material harder to deform without it just fracturing.
How can cold working effects be removed?
Through an annealing heat treatment
What is precipitation hardening?
This is when the material is hardened through the dispersion of a much smaller phase of the same material scattered throughout the original phase matrix. This is carried out either by specific heat treatments (solution and then precipitation heat treating) or sometimes passively as the material ages.
What is solution heat treating?
Solution heat treating is when solute atoms are completely dissolved in order to make a single phase comprised of two materials. This is then rapidly cooled in order to stop the atoms to moving, forcing them to stay in the same place and resulting in a soft, non-equilibrium metal. This is the first part of the precipitation hardening process.
What is precipitation heat treating?
The second part of the precipitation hardening process, the single phase solution is heated regularly to an intermediate temperature, allowing for diffusion and a second phase to appear in small precipitates. This solution is then cooled back down to room temperature. If the material is aged (left at the moderate temperature) for too long the hardness and strength actually starts reducing again.
Name 4 non-equilibrium phases and structures of steel
Bainite, fine pearlite, martensite and tempered martensite
How is fine pearlite formed and what are its properties?
Through the normalising (fast air cooling) of steel. Greater strength but less ductile than coarse pearlite (the equilibrium state)
How is martensite formed and what are its properties?
Formed when pure iron is quenched very quickly in water so the atoms can’t diffuse through the structure. Can be done in steel as well, but the carbon content effects the temperatures required. Also often forms in needles like bainite. Hard and strong, but very brittle.
How is tempered martensite formed and what are its properties?
When a martensitic structure is reheated for anc amount of time, some of the ductility can be recovered, resulting in a much improved balance between the ductility and the hardness and strength.
How is bainite formed and what are its properties?
Like pearlite consists of cementite and ferrite, but forms in incredibly small (electron microscope required) needles or pines at lower temperatures than pearlite
