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LECTURE 11: STRENGTHING METALS AND ANNEALING / WORK HARDENING - Coggle…
LECTURE 11: STRENGTHING METALS AND ANNEALING / WORK HARDENING
INCREASING METAL STRENGTH
Strengthening = increasing yield and tensile strength
4 main methods are:
Increased number of dislocations (work hardening / cold working)
Grain boundaries (grain size strengthening)
Solute atoms (solid solution strengthening)
Second phase particles
Pure metals can only be strengthened by 1 and 2 whereas all 4 can be used for alloys in the right conditions
All methods rely reducing dislocation slips
METHOD 1: WORK HARDENING
Plastically deforming a metal leads it to work harden, it increases strength but reduces ductility and causes an elongated grain structure (produces anisotropic properties)
Occurs when the metal is loaded into the plastic region and then removed
As deformation occurs the dislocations slip and extra dislocations are formed leading to the dislocations interfering with each other preventing them from sliding posts each other and slipping
Occurs during processes such as rolling, forging, drawing, extrusion and forming processes
Can be desirable for final outcome but makes the metal difficult to work with since the ductility is reduced
Interference of Dislocations
A dislocation can interfere with another dislocation in the same plane / direction, they are repelled due to the atoms around the dislocations being in tension or compression causing too much distortion when they approach each other
METHOD 2: GRAIN SIZE STRENGTHENING / METHOD 3: SOLID SOLUTION STRENGTHENING
Method 2: Grain Size Strenghtening
Dislocations cannot normally slip into neighbouring grains since the orientations of the slip planes are different
Grains can be reduced in size to increase the number of grain boundaries and therefore reducing dislocation slip (linked with equation yeild stress = sigma + Ky . D^-1/2 where sigma and Ky are constants and D is average grain diameter)
Grain size can be controlled by inoculants encouraging heterogenous nucleation
Method 3: Solid Solution Strengthening
Solute atoms in a solvent metal hinder dislocation slips therefore increasing yeild and tensile strength, hardness and creep resistance
Reduces ductility and electrical conductivity
Often solute atoms segregate the dislocations therefore reducing the distortion (slip takes place = increased distortion = unfavourable process)
METHOD 4: SECOND PHASE PARTICLES
Using second phase particles in a solid solution to help prevent dislocation slips (usually in intermediate compounds)
DISPERSION STRENGTHENING
Precipitation (age) hardening is an improved form of this that uses a heat treatment to further disperse the second phase particles
Caused by the precipitation of a second phase material (continuous phase = matrix, second phase = precipitate)
In a eutectic lamellar layers of alpha and beta restrict slips
PRECIPITATION HARDENING
Dispersion hardening enhanced by heat treatment
In a 2 phase region of an alloy, it can be used to form a more highly dispersed second phase resulting in the reduction of slips
Alloy at room temperature composed of grains plus large areas of P2
Alloy heated and enters single phase area of phase diagram so P2 dissolves and P1 changes composition (comp. A1 to comp. A2)
Cooled rapidly with quenching, freezing in single phase structure as there is no time for diffusion (supersaturated solid solution)
Heated gently so it remains in the 2 phase region (sometimes left at room temperature) so the P2 atoms can reform within the P1 matrix in a much more finely dispersed manner (ageing)
At higher temperatures the particles grow too large and the effect of preventing dislocation slips lessens - means that these alloys cannot be used in high temperatures (second phase can also redisolve in high temperatures)
ANNEALING
Heating the metal to increase the ductility (esp. after cold working) and remove the effects of strain hardening
There are 3 stages, recovery, recrystallisation and grain growth
RECOVERY
Recovery aka stress relief anneal doesn’t change the mechanical properties and returns the electrical and thermal properties
The high number of tangled dislocations can move with the slight heat, rearranging themselves to minimise interactions with each other (high temp. = diffusion + vacancy formation)
RECRYSTALLISATION
Annealing at a higher temperature and for longer leads to the formation of lower energy ordered regions leading to a finer grain structure with fewer dislocations (more similar to pre cold work levels)
Reduced strength and increased ductility
Recrystallisation temperature depends on amount of cold work done, OG grain size, alloy comp. and melting temp. And annealing time
GRAIN GROWTH
Grain growth is driven by a reduction in grain boundaries as the temp. Increases; grains grow in size at the expense of their neighbours
Usually an undesirable process
Must consider impacts of annealing in high temp. Working conditions and metal joining processes such as welding that would potentially cause local recrystallisation