Precipitation Hardening of Metallic Materials - Essay Example

Precipitation Hardening of Metallic Materials Precipitation Hardening of Metallic Materials Introduction Precipitation hardening is currently one of the most widely used techniques for strengthening metal alloys. Generally, it is widely believed that the hardness and strength associated with certain metal alloys could be enhanced by forming an additional phase of adversely small and evenly dispersed particles to the initial phase matrix1. Attaining this requires correct heat treatment. This process is referred to as precipitation hardening and it happens it takes place in three unique stages: solution treatment which is intended to reduce segregation within the alloy, quenching in order to come up with an extremely saturated solution and finally, aging which is intended to help in forming coherent precipitates that interfere with dislocation movement in order to strengthen the alloy2. A number of the aluminum-based alloys could be strengthened through the process of age hardening. This paper critically describes how precipitation hardening is normally carried out based on the use of aluminum copper based alloy systems with particular focus to the effects of aging time on the various mechanical properties of the alloy, theories explaining such effects as well as the estimation of the residual fatigue resistance through application of the traditional fracture mechanics approach. The Process of Precipitation Hardening Using aluminum-copper alloy as an example, In order to strengthen the aluminum-copper alloy via precipitation, there should be some terminal solid solution featuring a solid solubility that decreases with a decrease in temperature. The phase diagram shown in figure 1 below is for the Al-Cu phase and offers a better illustration of this decrease3. The decrease is noted along the solvus found from within the range α to α + Ɵ. AS pertain to the figure, a 96wt%Al-4wt%Cu alloy is chosen because of a significant decrease in terms of the solid solubility associated with the solid solution α when temperature diminishes from 550°C – 75°C. In this illustration, the process of precipitation hardening follows three stages: solution treatment, quenching then aging. Solution Treatment This is the first phase of the process of precipitation hardening. Here, the alloy is heated at a temperature level that surpasses the solvus temperature followed by soaking till production of a uniform solid solution (α). Besides, all Ɵ precipitates will be dissolved at this phase along with the reduction of any segregation featured in the initial alloy. Quenching This is the second stage during which the solid α is subjected to rapid cooling in order to form an overly saturated αss solid solution, which constitutes excess copper and it is never an equilibrium structure. At this phase, the atoms never have an opportunity of diffusing to possible nucleation sites thus evading the formation of the Ɵ precipitates. Aging This is the third phase involving heating of α and αss at temperature levels that are less than the solvus temperature in order to yield finely dispersed precipitates. At this phase, atoms only diffuse for short distances. Considering that the overly saturated α is never stable, the additional copper atoms will have an opportunity of diffusing to a number of nucleation sites thus permitting the growth of precipitates. In age hardening precipitation, a general demompositon model is usually given followed by the precipitation sequence details in at least 4 alloy systems namely Al-Cu-Mg, Al-Cu, Al-Zn-Mg and Al-Mg-Si. For example, using Al-Cu system, the decomposition can be given as: a0 (SSSS) → GP zones → θ → →θ → θ        or, more fully: a0 (SSSS) → α1 + GP zones → α2 + θ → α3 + θ → α4 + θ The Effect of Ageing Time Ageing brings about a 0.5% increase in yield stress and ultimately a heightened tensile strength at its initial stages, attains a maximum value then begins to diminish with any further increase in the ageing duration. In another sphere, the strain-hardening factor, total elongation as well as uniform elongation reduce in response to additional aging duration soon after the material in question attains the expected maximum strength (typically T6 conditions) then hits a minimum value then raises again4. The area reduction that pertains to the final fracture also diminishes to a minimum value, and this occurs at the same time with the maximum value of the final tensile strength. This then increases as the ageing time increases. Theories There are a number of theories that have been put forward to explain the impact of precipitation on the mechanical properties that pertain to the precipitation-hardened alloys. One of these theories is the theory of homogenous nucleation. Basically, this theory is based on the existence of Guinier Preston (GP) zones within alloys5. The GP zones plus their associated strain field work together to prevent slip through dislocation movement. Moreover, the theory hints that the zones increase following an increase in the ageing time and making the slip to become more difficult thus hardening the alloy age. In case of more elaborated ageing times, clusters coarsen then the slip becomes easier thus softening the alloy. Figure showing the age hardening curve and the ageing sequence Another theory is the dislocation theory based on the dislocation looping mechanism proposed by Orowan. This theory advocates for use of non-deformable particles for dispersion strengthening. Whenever any dislocation meets a hard and non-deformable particle, it will end up looping around the individual particles. It is noteworthy that every attempt to arrive at a more physical method of direct hardening of strains from this theory entails a comprehension of the spatial arrangement regarding the dislocation microstructure along with its strain dependence. So widespread are the models that have been proposed to tackle of dislocation patterning and the models have generally strayed from the equilibrium thermodynamics to more physical approaches that entail non-linear dynamics that are not any close to equilibrium6. Precipitation and Hardening by Grain Refinement Compared There are a number of differences and similarities between precipitation and hardening by grain refinement. Wadhwa & Dhaliwal (2008) particularly describe hardening by grain refinement as a process that focusses on reducing the sizes of grains in order to increase the resultant number of grain boundaries inside the specimen. This move suppresses the idea of the dislocations gliding past the established grain boundaries with ease. Thus, through refining the structure of the grain, dislocation movement is retarded hence improving the strength associated with the specimen. On the other hand, precipitation hardening focusses on the formation of a finely-dispersed precipitate inside the alloy. The idea of formation of the precipitate is founded on the idea that solid solutions that are stable at heightened temperatures could turn supersaturated and unstable (with regard to the solute) at lower temperature levels. This precipitate then suppresses dislocation movement through its ability to force the dislocations to go around or cut through the precipitated particles7. It is restriction of dislocation movement at deformation phase that leads to strengthening of the alloy. Generally, precipitation hardening focuses on the formation of a finely-dispersed precipitate inside the alloy while grain refinement is often largely concerned with reducing the sizes of grains in order to increase the resultant number of grain boundaries of the particular metal or material. Part B Fracture mechanics approach can effectively be applied in the estimation of residue fatigue resistance among other important parameters involved in the alloy development. This is particularly based in conventional stress analysis. For example, the crack can effectively be modeled thereby allowing for the estimation of stress intensity as:?K = Fe Fs Fw Fg ?e ?a Where, a= Crack length Fs=free surface effects Fw=finite width Fg=non uniform stress on the crack Fe=crack shape E=equivalent constant stress. Final Evaluation Overall, the learning materials I was provided with excellently explain how precipitation hardening helps in hardening of metal alloys. I am also impressed by the materials giving details of other hardening options. However, I have not come across practical business cases on the hardening approaches present. Thus, I suggest that the materials should include business cases associated with each of the hardening approaches. In this way, firms that refer to the material will be able to make choices that best suit their hardening needs. Conclusion In conclusion, precipitation hardening is one of the most effective techniques for strengthening metal alloys. The process particularly occurs in three unique stagesnamely, solution treatment which is intended to reduce segregation within the alloy, quenching in order to come up with an extremely saturated solution and finally, aging which is intended to help in forming coherent precipitates that interfere with dislocation movement in order to strengthen the alloy. Finally, there are a number of differences and similarities between precipitation and hardening by grain refinement some of which include the fact that precipitation hardening focuses on the formation of a finely-dispersed precipitate inside the alloy while grain refinement is often largely concerned with reducing the sizes of grains in order to increase the resultant number of grain boundaries inside the specimen. References 1. Callister, W.D. (2010). Fundamentals of Materials Science and Engineering. New York: Wiley & Sons. 2. Jae-Ho, J. A. N. G., Dae-Geun, N. A. M., Yong-Ho, P. A. R. K., & Ik-Min, P. A. R. K. (2013). Effect of solution treatment and artificial aging on microstructure and mechanical properties of Al–Cu alloy. Transactions of Nonferrous Metals Society of China, 23(3), 631-635. 3. Munitz, A., Cotler, C., & Talianker, M. (2000). Aging impact on mechanical properties and microstructure of Al-6063. Journal of materials science, 35(10), 2529-2538. 4. Wadhwa, A. S., & Dhaliwal, H. S. (2008). A textbook of engineering material and metallurgy. New Delhi: University Science Press. Read More