Introduction The precipitation hardening is a heat treatment process that improving the strength of material by impeding the motion of the dislocation

The precipitation hardening is a heat treatment process that improving the strength of material by impeding the motion of the dislocation. The particles in small sizes are formed after precipitation hardening heat treatment process and distributed uniformly throughout of an alloy. The particles are called precipitates which are impede dislocation motion through the alloy (Michael Pfeifer, n.d.). There are only certain alloy can be precipitation strengthened such as Al-Cu, Al-Mg-Si and Cu-Be (Michael Pfeifer, n.d.). The precipitation hardening processes undergo solution heat treatment, cooling, then finally followed by aging. There are four precipitate structure which are GP zone, ??’?^’precipitate,? ??^’ precipitate and ? percipitate. During aging process, the peak aged is more preferred and the overaged and underaged are avoided.
The martensitic formation is a transformation occurs in a diffusionless and military manner (Anon., n.d.). Due to diffusionless process, the composition don’t changes, that is only crystal structure changes. The martensite formation is done by undergo high temperature and cooling down as fast as possible. The microstructure of the martensite is normally in lath and plate size. The transformation only occurs when the martensite is in metastable stage.

Similarities between precipitation hardening and martensitic formation
Both precipitation hardening and martensitic formation improve the mechanical properties of a material. Both processes need to undergo a heat treatment at a specific range of heating temperature. The cooling processes are needed for both heat treatment processes. Both processes will change the structure of material after heat treatment. Both of them also having nucleation and growth of new phases.
– Having phase transformation

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Differences between precipitation hardening and martensitic formation
Process (heating and cooling)
Precaution or recommendation
There are four requirements needed to being fulfilled by alloy before doing precipitation hardening process. Firstly, the solid solubility of alloy must decreasing when temperature decreasing and this trend must able performed in the phase diagram. Secondly, the matrix of the alloy must have soft and ductile properties, however the precipitate must have hard and brittle properties. Next, the alloy must be quenchable because the thermal shock might happen if the alloy cannot cooled rapidly. The distortion of the part might happen due to rapid cooling. To reduce the residual stress, the alloy are quenched in hot water about 80°C. Finally, after aging process, microstructure of alloy must be a coherent precipitate. Thus, without fulfilled these four requirement, the alloy cannot get the desired strength after precipitation hardening.

In my opinion, the requirement for the martensite formation is the percentage of carbon content. If the carbon is needed in the formation of martensite. The hardening of iron cannot be success due to low carbon content. For example, for iron carbon alloy minimum percentage of carbon needed for the formation of martensite is 0.3% (Biswas, 2016). This is because the cooling rate is no enough fast to gain martensitic structure when the percentage of carbon is below 0.3% for iron carbon alloy.

Figure 1: Three TTT diagram with different percentage of carbon content of plain carbon. (Manna, n.d.)
Based on Figure 1 (b), the percentage of carbon in plain carbon steel is lower than 0.8%C and from Figure 1 (d), the plain carbon steel is having percentage of carbon steel than higher than 0.8%C. The TTT curve for both Figure 1(b) and (d) are more shifted to the left compare to the TTT-curve for eutectoid steel which having 0.8% carbon. The hypoeutectoid steel and hypereutectoid steel are more difficult to get martensite because the nose of the curve is too close to the y-axis, this might cause the difficulties in the cooling rate. The cooling rate must controlled in very fast rate in order to get the martensite. The cooling rate for eutectoid steel is slower but still can get the martensitic structure, this is due to the nose of the curve in TTT-diagram is more further away to y-axis. For hypoeutectoid steel and eutectoid steel, the Ms and Mf is above the x-axis, therefore maximum 100% percentage of martensite can be obtained. However, for hypereutectoid steel, only Ms and M50 are available while Mf is not available, this cause the 100% of martensite should not be obtained. Therefore, the percentage of carbon for plain carbon to get fully martensitic structure is below 0.8%C. The retained austenite and partial martensite will be gained for plain carbon that having more than 0.8%C (hypereutectoid). Thus, the percentage of carbon is very important to get a fully martensitic formation.

There are some differences in process between two heat treatment. Firstly, the process of precipitation hardening has three steps.
The first step is solution heat treatment or homogenization (Anon., 2009). Refer to Figure 1, the alloy with composition of x1 is heated at temperature of T1. The alloy should heat at temperature between solvus and solidus line. Then, the temperature is maintained or hold for until all the solute dissolved into ? phase and finally an unsaturated solid solution structure is formed.
Then, the second step is quenching which is cooling rapidly across the solvus line to exceed the solubility limit. The alloy is cool down rapidly to a lower temperature, T3 normally is room temperature. The cooling agent is water at room temperature. Quenching process is needed to prevent the immediate diffusion of solute. The supersaturated solid solution is formed. Quenching processes is use to maintain the uniform solid solution structure of alloy below the homogenization temperature (Anon., 2009).
Finally, the last step of the precipitation hardening process is aging. The supersaturated solution is heated to a certain temperature to induce precipitation and the temperature is hold to let it aged. During aging, the time is adjusted to get a peak aged precipitation. After aging, the supersaturated solid solution will form into incoherent precipitation. When aging done at room temperature, this process is called natural aging. Artificial aging is the aging process that having temperature above room temperature. Normally, the artificial aging is done in the industry to save more time.

Figure 2: The binary phase diagram for two metals A and B. (Anon., 2009)

The metal is heated to austenite temperature, and the temperature is hold to formed austenite. Then, the quenching is done as fast as possible to prevent the interruption with the nose of the TTT-curve. The martensite start to form during the metal is quenched to Ms temperature and this formation will be ended during Mf temperature. The Ms and Mf temperatures are depending on the percentage of carbon. The less the carbon, the higher the Ms and Mf temperature.

Formation of structure
For precipitation hardening, the precipitate formed only during aging process. There are four stages of precipitation.
First, the disc-shaped GP zone is the first stage of precipitate. GP zone is the nucleation homogeneously from supersaturated solid solution. The disc faced are perfectly coherent with the matrix as shown as Figure 3. The disc edge also coherent to the matrix but with a large coherency strain as shown as Figure 3.

Figure 3: The coherent precipitate in GP zone. (S, n.d.)
Then, the second stage is ?^”precipitate growth. Some of the GP zone grow to form ?^”precipitate. The remaining GP zones dissolve and transfer solute to the growing ?” by diffusion through the matrix. Disc faces are perfectly coherent. Disc edges are coherent, but the mismatch of lattice parameters between the ?” and the matrix generates coherency strain.
Next, the precipitates called ?’ nucleated at matrix dislocations. The ?” precipitates all dissolved and transfer solute to the growing ?’. Disc faces are still perfectly coherent with the matrix. But disc edges are now incoherent.
Finally, equilibrium ? nucleates at grain boundaries and at ?’-matrix interfaces. The ?’ precipitates all dissolve and transfer solute to the growing ?. The ? is completely incoherent with the matrix. Because of this it grows as rounded rather than disc-shaped particles.

Figure 4: The sequence in the formation of precipitation hardening alloy. (a) aluminium, (b) ?”, (c) ?’, (d) ? (S, n.d.)

Martensite is formed from austenite, ? which is a faced centred cubic (FCC). The martensite is a body centred tetragonal (BCT) which is metastable.

Figure 5: Bain Distortion (S, n.d.)
During this transformation, the percentage of carbon remain unchanged. Figure 5 (a) shows that the FCC austenite alternate choice of cell (b) is come from two combination of FCC austenite. The tetragonal martensite (c) is formed by having 20% of contraction of c-axis and 12% of expansion of a-axis.


Figure 6: The ageing time for precipitation.
The precipitate start to form in GP zone but the nucleation is not enough due to small size and underdeveloped precipitation. Then, the aging time is increasing to get an optimum size and distribution of precipitation for strengthening. The precipitate growth until a peak aged structure is gained. The aging process should end here to prevent the overage happen. The precipitate continue to coarsening and this structure will decrease the hardness of alloy. The hardness and strength is obtained at peak aged precipitation.
The microstructure of the martensite is needle-shaped features as shown in Figure 7 below. The microstructure appears in plates or laths shapes. When martensite forms, there is no time for the formation of cementite and the austenite transforms to a highly distorted form of ferrite which is super saturated with dissolved carbon. The combination of the lattice distortion and the severe work hardening resulting from the shear deformation processes necessary to achieve the transformation cause martensite to be extremely strong but very brittle.

Figure 7: The microstructure of the martensite.

Harderning Mechanism
Four separate hardening mechanisms are at work during the aging process :
Solid Solution Hardening
At the start of aging the alloy is mostly strengthened by the 4 wt% of copper that is trapped in the supersaturated ?. But when the GP zones form, almost all of the Cu is removed from solution and the solution strengthening virtually disappears.
Coherency Stress Hardening
The coherency strains around the GP zones and ?” precipitates generate stresses that help prevent dislocation movement. The GP zones give the larger hardening effect.
Precipitation Hardening: a) Cutting Stress and b) Bowing Stress
The precipitates can obstruct the dislocations directly. But their effectiveness is limited by two things: dislocations can either cut through the precipitates, or they can bow around them. Resistance to cutting depends on the shearing resistance of the precipitate lattice. In fact the cutting stress increases with aging time. Bowing is easier when the precipitates are far apart. During aging the precipitate spacing increases from 10 nm to 1 ?m and beyond. The bowing stress therefore decreases with aging time.

Figure 8: Four separate hardening mechanisms are at work during the aging process

Figure 9: (a) Hardening by cutting stress
(b) hardening by bowing stress

The high strength of martensite implies that there are many strong barriers to dislocation motion in this structure. There are two structures in quenched iron-carbon alloys. The conventional martensite has a plate structure with a unique habit plane and an internal structure of parallel twins each about 0.1 ?m thick within the plates. The other type of martinsite structure is a block martensite containing a high dislocation density of (10¹¹) to (10¹²) dislocations per square centimeter. Thus, part of the high strength of martensite arises from the effective barriers to slip provided by the fine twin structure or the high dislocation density. The second important contribution to the strength of martensite comes from the carbon atoms. On rapidly transforming from austenite to ferrite in the quench, the solubility of carbon in iron is greatly reduced. The carbon atoms strain the ferrite lattice and this strain can be relieved by redistribution of carbon atoms by diffusion at room temperature. One result is that a strong binding is set up between dislocations and carbon atoms (Al-Ethari, n.d.).
There are two reasons for a martensite hardness :
A tetragonal lattice doesn’t have a slip planes such in a cubic lattice. A martensite is a super saturated solid solution ( from carbon atoms which is confined in a body center tetragonal crystal lattice, since a cubic lattice of an austenite grain may be deform to transfer to the tetragonal lattice; this deformation lattice (B.C.T) may be the essential reason of very high hardness of martensite. the occurring expansion in a crystal lattice because of martensite formation will generate a high local stresses which cause a plastic deformation in a basic phase therefore the martensite appear as an acicular (Al-Ethari, n.d.).
Interstitial carbon atoms will prevent the slip. The solute atoms (carbon atoms) occupy interstitial sites in the solvent lattice (iron) and that lead to distort the crystal lattice and form a stress fields around the solute atoms, this stress fields will interacts with the stress fields of dislocations. This interact reduces the dislocation velocity, in the other hand, this interact will forms an obstacles to the movement of dislocations (Al-Ethari, n.d.).

The precipitation hardening is a diffusion process while the martensitic formation is a diffusionless process. In precipitation hardening, the quenching process is as fast as possible to prevent the diffusion by maintain the stability of the supersaturated solid solution. However, during the ageing, the diffusion is occurs to let the precipitate growth and coarsening from GP zone to ?’ which has the optimum size of precipitates (peak aged). The time of aging is long enough just only for the precipitation growth. However, the martensitic formation doesn’t have any diffusion process occurs during whole process. The percentage of carbon remain unchanged before and after the process. There are only crystal structure change during process.

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