Age hardening

Slides on Age Hardening phenomena

1. Age Hardening By Bedanta Lath Department of Metallurgical Engineering GANDHI INSTITUTE OF ENGINEERING & TECHNOLOGY

2. Contents Introduction Basic Requirements Steps in Age Hardening Treatment Precipitation Sequence During Ageing Hardening Mechanism Conclusion

3. Introduction In 1906, Alfred Wilm, a German engineer, discovered by accident, the phenomenon of natural ageing. He found that a quenched alloy of aluminum with copper and magnesium-called Duralumin, increased its hardness with ageing(time). This is called age hardening or precipitation hardening, because the hardness of the quenched alloy increases as a function of ageing time. This mechanism has made possible the use Duralumin as air-craft-frame-materials since the Second World War.

4. Basic Requirements The main requirement of a precipitation-hardening alloy system is that the solid solubility limit should decrease with the decrease in temperature. i.e. the phase diagram should show a solvus as shown in the figure below.

5. Basic Requirements The precipitates of the second phase should be coherent in nature. Example – Alloy systems Mg- Pb , Al-Mg, Al- Mn show decrease of solubility with the decrease of temperature, but coherent precipitates are not formed. Alloys of such system can not be age hardened.

6. Steps In Age Hardening SOLUTIONIZING It is the process of heating the alloy just above the solvus temperature to obtain a single phase solid solution

7. 2. QUENCHING The solutionised alloy is cooled fast to retain the high temperature single phase solid solution at room temperature as metastable supersaturated solid solution(SSSS).

8. 3 . AGEING It is the process of controlled decomposition of SSSS to form finely-dispersed-precipitates usually at one and and sometimes at two intermediate temperatures for a suitable time period. Artificial ageing is process of ageing by holding the alloy at slightly higher temperature than room temperature

9. Precipitation Sequence The decomposition of SSSS during ageing is a complex process. The equilibrium precipitate, Ө , normally does not form directly from the SSSS. The precipitation occurs in steps involving several transition temperature. Example- The following is the sequence of precipitates if an Al-4.5% Cu alloy is aged after obtaining SSSS by quenching from 550 C. GP Zones Ө // (GP Zone 2) Ө / Ө (CuAl 2 ) This alloy system exhibits the greatest number of intermed -ate stages in its precipitation at low temperature ageing.

10. 1 . GP Zones Guinier-Preston zones, earlier called GP1 zones-the early stage of ageing, are named thus, after the man who first independently studied their formation by X-Ray diffraction. GP zones are plate like clusters predominantly of copper atoms segregated on to {100} planes of the aluminum lattice. The plates have diameter of about 100 o A and thickness of only 3-6 o A. GP zones, although also called precpitates , are actually coherent precipitates. GP zone formation occurs by diffusion of copper atoms aided by the quenched in vacancies over relatively short distances.

11. 2. Ө // Precipitate It was earlier called GP-2 zone, but it has a definite but different crystal structure than matrix, it is more appropriate to call it a coherent intermediate precipitate with a symbol Ө // . Ө // precipitate is in plate form of maximum thickness 100 o A and up to a maximum diameter of 1500 o A. It has tetragonal crystal structure with a= 4.04 o A and C=7.68 o A , i.e. , it fits well with the aluminum unit cell in two directions but not along ‘C’ axis. Ө // precipitate has ordered arrangements of copper and aluminum atoms.

12. 3 . Ө / Precipitate This transition precipitate is large enough to be resolved under the optical microscope. It has tetragonal structure with a=4.04 o A and C=5.8 o A. The composition is slightly different than Ө . The disc shaped Ө / precipitates are semi coherent. The elastic strain around these precipitates is small as the long range strain fields of its dislocations and the precipitate largely cancel. Thus, the formation of Ө / structure leads to the softening of the alloy. Ө / precipitate form heterogeneously.

13. 4. Ө Precipitate The equilibrium precipitate, Ө (CuAl 2 ) has tetragonal structure(a=6.07 A o , C=4.87 o A).It is fully incoherent precipitate and thus, its formation always leads to softening as coherency strains disappears. It nucleates heterogeneously and it is more easily formed while ageing at higher temperatures. Ө are the ultimate result of overageing .

14. GP zone structure Al-Cu Precipitates structures

15. Kinetics of Precipitation The entire process of precipitation is very complex as it depends on a large number of factors such as temperature and time of ageing, nature of alloy, the composition of alloy etc. It is difficult to make quantitative derivation. However, Newkirk has made qualitative generalizations The rate of precipitation is faster at higher temperature of ageing. The rate of precipitation is faster in alloys of widely dissimilar metals. Impurities in soluble or in insoluble state invariably increase the rate of precipitation. Plastic deformation, just before ageing, increase the rate of precipitation.

16. Hardening Mechanisms Their could be at least three reasons of hardening by ageing: 1. Internal strain –hardening by elastic coherency strains around zones, 2. Chemical-hardening due to precipitates being sheared(cut ) by moving dislocations, 3. Dispersion-hardening due to formation of loops dislocations around precipitates

17. Internal strain hardening due to coherency strains around zones :- The coherency strains (around coherent, or even semi-coherent precipitates) act as barriers to the movement of moving dislocations. For a dislocation to pass through such regions of internal stress, the applied stress must be least equal to average internal stress.

18. Conclusion

19. Resources Phase transformations in metals and alloys, D.A. Porter, & K.E . Easterling, Chapman & Hall. Materials Principles & Practice , Butterworth Heinemann, Edited by C. Newey & G. Weaver. Mechanical Metallurgy , McGraw-Hill, G.E. Dieter, 3rd Ed. Hull , D. and D. J. Bacon (1984). Introduction to Dislocations . Oxford, UK, Pergamon. Courtney, T. H. (2000). Mechanical Behavior of Materials . Boston, McGraw-Hill