Altering Material Properties Flashcards
Why does grain boundary act as a barrier to dislocation motion
• Since the two grains are of different orientations, a dislocation passing into a second grain
will have to change its direction of motion; this becomes more difficult as the crystallographic
misorientation increases.
• The atomic disorder within a grain boundary region will result in a discontinuity of slip
planes from one grain into the other.
Are fine grain or coarse materials grain harder and stronger
Fine grain due to greater total grain boundary area
Solid solution hardening
Alloying with impurity atoms that go into either
substitutional or interstitial solid solution.
How does solid solution hardening increase tensile and yield strength
Impurity atoms impose lattice strains on the surrounding host atoms. lattice strain field interactions between dislocations and these impurity atoms mean dislocation movement is restricted. Smaller impurity atoms causes tensile strain on surrounding crystal lattice and larger cause compressive. Solute atoms tend to segregate around dislocation to reduce overall strain energy (cancel it out). Ie smaller impurity atoms tend is located where its tensile strain will partially nullify some of the dislocations compressive strain
Strain hardening
When a ductile metal becomes harder and stronger as it is plastically deformed. Trade of is that ductility decreases. Dislocation density in a metal increases due to formation of new disc locations or dislocation multiplication. Motion of dislocation is hindered by other dislocations (dislocation to dislocation strain interactions are repulsive).
Percentage cold work
When a ductile metal becomes harder and stronger as it is plastically deformed. Trade of is that ductility decreases. Dislocation density in a metal increases due to formation of new disc locations or dislocation multiplication. Motion of dislocation is hindered by other dislocations (dislocation to dislocation strain interactions are repulsive).
How to remove the effects of strain hardening
Annealing heat treatment
Precipitation hardening
Formation of extremely small uniformly dispersed particles of a second phase within original phase matrix by appropriate heat treatment.
Solution heat treating (stage 1 to achieve precipitation hardening)
Solute atoms dissolved to form single phase solid solution. Heat to a temperature
within the α phase field-say, T0-and waiting until all the β phase that may have been present is
completely dissolved. This
procedure is followed by rapid cooling or quenching to temperature T1 which for many alloys is
room temperature, to the extent that any diffusion and the accompanying formation of any of the
β phase are prevented. So nonequilibrium solution exists where a-phase solid is saturated with B atoms.
Precipitation heat treating (stage 2 to precipitation hardening)
the supersaturated α solid solution is heated to an intermediate temperature in α + β two-phase region. B precipitate phase begins to finely disperse, process termed as aging. Finally alloy called, rate of cooling not important, to lock in structure. (Rest is like how alloy is stronger than pure- lattice strains impeded fracture movement)
What influences character of b particles formed during precipitation heat treatment
Precipitation temperature and aging time at this temp. . Usually happens at room temp over extended time. With increased time strength and hardness increases and reaches a max. Reduction in strength is called overaging
Non equilibrium phases
Equilibrium transformations are driven by the diffusion of atoms, if cooling is too fast non equilibrium is formed
How to form coarse Pearlite (equilibrium)
Full annealing (flow furnace cooling)
How to form finer Pearlite (non equilibrium)
Normalising (faster air cooling). Less time for atoms to diffuse so they diffuse a shorter distance lead8mg to finer structure.
Properties of fine Pearlite
Greater strength than steel but less ductile that coarse pearlite
What is bainite
Consists of ferrite and cementite like Pearlite but bainite is very fine. Forms at lower temps that Pearlite via transformation from austenite
How is martensite formed
Quench pure iron in the γ field so no time for the Fe atoms to diffuse to change
from γ-Fe (FCC) to α-Fe (BCC), so γ-Fe (FCC) persists. γ-Fe is so
unstable that small regions within crystals change by displacement, not diffusion, to a distorted BCC
form known as body-centred tetragonal (BCT). ame basic transformation can occur in steel, although the transformation temperatures depend
on the carbon content.
Properties of martensite
Hard and strong but brittle
Tempered martensite
Martensite can be tempered to restore ductility. e. reheated to a certain
temperature and held to allow some diffusion of atoms and stress relief to occur.
Heat treatments
Involve no equilibrium processes to alter microstructure so that the desired mechanical properties are attained
How are time temperature transformation diagrams formed
Taking large no of identical samples, super cooling them to diff temps, holding them isothermally and measuring what phases form and when.
Why are TTT diagrams helpful
Tell us what phases or structures form and when
How does presence of C impact marsensite
C causes more lattice disortion so the marsensite becomes harder and more brittle. If too much c, temp for martensite transformation drops below ambient so marsensite transformation does not go to completion.
Types of cast iron
gray, nodular, white, malleable
and compacted graphite iron
