Small second phase particles (Hard) distributed in a ductile matrix are a common source of alloy strengthening.
Precipitation hardening is produced by solution treating (heating to a single phase region) and quenching an alloy in which a second phase is in solid solution at the elevated temperature but precipitates upon quenching and aging at room temperature (natural aging) or at slightly higher temperature (artificial aging, 100-200degrees). for this to occur the second phase must be soluble at an elevated temperature but must exhibit decreasing solubility with decreasing temperature. coherency between the precipitates and matrix is essential. The hardness increases with the formation of GP zones (Guinier-preston) and the intermediate transition precipitates.. A peak in hardness results due to critical dispersion of GP zones , further aging leads to decrease in hardness due to coarsening of precipitates (incoherent). This is called over-aging. eg:Al-Cu (aerospace industry) and Cu-Be (sparking tools in coal-mines).
The peak hardness depends on
1. Average particle size (fine particles)
2. Number of Particles (more finer particles)
3. Inter particle distance (less)
Methods of studying precipitation
1.Mechanical properties During aging as the amount of precipitate increases with time which increases the strength or hardness of the alloy. tension test or hardness measurement can be used to know the changes in mechanical properties.
2. Electrical resistivity During aging the excess solute comes out gradually hence strains in the crystal lattice decreases hence resistivity decreases.
3. X-ray diffraction Its application is to measure strain in the crystal. strains in the lattice will decrease with time.
4. Electron Microscopy as precipitates are very small in size (few nano meters) we have to use electron microscopy to observe the precipitation.
3. Inter particle distance (less)
Methods of studying precipitation
1.Mechanical properties During aging as the amount of precipitate increases with time which increases the strength or hardness of the alloy. tension test or hardness measurement can be used to know the changes in mechanical properties.
2. Electrical resistivity During aging the excess solute comes out gradually hence strains in the crystal lattice decreases hence resistivity decreases.
3. X-ray diffraction Its application is to measure strain in the crystal. strains in the lattice will decrease with time.
4. Electron Microscopy as precipitates are very small in size (few nano meters) we have to use electron microscopy to observe the precipitation.
The fraction of second phase is limited by solubility limit. Higher supersaturation causes faster precipitation. The degree of super saturation decreases with the increase of aging temperature resulting in lower peak hardness at high temperatures. since the amount of second phase is less and inter particle distance is high.
Reasons for hardening in precipitation-hardening
1. Internal strain-hardening by elastic coherency strains around GP zones.
2. Chemical-hardening due to precipitates being sheared (cut) by moving dislocations.
3. Dispersion-hardening due to formation of loops of dislocations around precipitates.
The requirement of a decreasing solubility with temperature places a limitation on the number of useful precipitation-hardening alloy systems. Precipitation hardened alloys can't be used at higher temperatures precipitates dissolve in the matrix at higher temperature. To overcome these difficulties a dispersion strengthening is developed.
Dispersion hardening The hard and strong foreign particles are dispersed in a metal/alloy (matrix). Powder metallurgy is the best route to consolidate these dispersion alloys. These particles are oxides, carbides, nitrides etc. Such alloys are called dispersion strengthened alloys. The second phase alloys has very little solubility in the matrix even at elevated temperatures. No coherency between the second-phase particles and the matrix. These alloys are much more resistant to recrystallization and grain growth than single-phase alloys. Second phase particles donot dissolve even at high temperature.
Advantages
1. can be used for High temperature applications
2. we can use this for any alloy system
3. No limitation on the fraction or amount of dispersoid