Mikrostrukturní mechanismus přirozeného stárnutí a raných stadií precipitace ve slitinách Al-Mg-Si a Mg-RE

• Název práce v češtině: Mikrostrukturní mechanismus přirozeného stárnutí a raných stadií precipitace ve slitinách Al-Mg-Si a Mg-RE
• Název v anglickém jazyce: Microstructure mechanism of natural ageing and early stages of precipitation in Al-Si-Mg and Mg-RE alloys
• Klíčová slova: Al-slitiny|přirozené stárnutí|anihilace pozitronů|vakance
• Klíčová slova anglicky: Al-alloys|natural ageing|positron annihilation|vacancy
• Akademický rok vypsání: 2024/2025
• Typ práce: disertační práce
• Ústav: Katedra fyziky nízkých teplot (32-KFNT)
• Vedoucí / školitel: prof. Mgr. Jakub Čížek, Ph.D.
• Konzultanti: doc. RNDr. Martin Vlach, Ph.D.

Zásady pro vypracování

1. Prostudování odborné literatury o přirozeném stárnutí a raných stadiích precipitace ve vytvrditelných slitinách.
2. Stanovení rozpouštěcího žíhání. Výběr vhodných slitin.
3.Výzkum přirozeného stárnutí vybraných Al a Mg slitin a vlivu defektů na přirozené stárnutí.
4. Výzkum vlivu přirozeného stárnutí na precipitační vytvrzení.
5. Vytvoření fyzikálního modelu procesů probíhajících při přirozeném stárnutí a v raných stadiích precipitace.
6. Sepsání disertační práce

Seznam odborné literatury

1. P.Hautojärvi: Positrons in Solids, Topics in Current Physics, Springer-Verlag, Berlin (1979).
2. A. Dupasquier, A.P. Mills, Jr. (eds.): Positron Spectroscopy of Solids, IOS Press, Amsterdam (1995).
3. M.Werinos, H. Antrekowitsch, T. Ebner, R. Prillhofer, P.J. Uggowitzer, S. Pogatscher, Hardening of Al–Mg–Si alloys: Effect of trace elements and prolonged natural aging, Mater. Design 107 (2016) 257.
4. S. Pogatscher, H. Antrekowitsch, M. Werinos, F. Moszner, S. S. A. Gerstl, M. F. Francis, W. A. Curtin, J. F. Löffler, P. J. Uggowitzer, Diffusion on Demand to Control Precipitation Aging: Application to Al-Mg-Si Alloys, Phys. Rev. Lett. 112 (2014) 225701.

Předběžná náplň práce v anglickém jazyce

Solution treatment at elevated temperatures is used to form solid solution of alloying elements in the host matrix. Since the solubility of alloying elements usually drops with decreasing temperature the solution treatment is finished by rapid quenching in order to ‘freeze’ the solutes dissolved in the matrix. In such way supersaturated solid solution (SSS) of alloying element in the host matrix is formed. Since SSS is a non-equilibrium state the alloy tends to lower the free enthalpy by clustering of solutes or formation of solute-rich precipitates. These processes taking place at room temperature or at slightly higher temperatures are generally called early precipitation stages. The kinetics of early precipitation stages critically depends on the concentration of lattice defects mainly vacancies which mediate atomic diffusion.
Early precipitation stages were intensively studied in hardenable Al-alloys [1]. Frequently used precipitation-hardenable aluminium alloys are based on Al-Mg-Si system. Due to their high strength-to-weight ratio, good formability and corrosion resistance combined with a relatively low price, Al-Si-Mg based alloys are widely used as structural materials in automotive and aviation industry. Immediately after quenching Al-Si-Mg alloy from the solutionising temperature to the ambient temperature, dissolved solutes start to cluster by vacancy assisted diffusion [2,3]. This process is called natural ageig (NA) and it leads to pronounced macroscopic effects such as a rise of electrical resistivity and hardness. Moreover NA affects subsequent strengthening processes – artificial ageing (AA) performed typically at 180C for Al-Mg-Si alloys [3]. The solute clusters formed during NA have usually negative effect on subsequent AA [4]. The strengthening process becomes sluggish and the peak hardness can be lower. This effect is known for a long time but the exact dependence of the negative effect on NA temperature and period and on the alloy composition is not yet fully understood. Interestingly for Al-Mg-Si alloys with Mg and Si content below 1 wt.% opposite behaviour was found and NA has a positive effect on the strengthening during AA [5].
Contrary to Al-based alloys natural aging of Mg-based alloys is not common. The Mg-Zn based alloys was the first Mg-based system where NA was reported [6]. An extremely large incubation period (~ 9 weeks) was observed in hardness response to NA in binary Mg-7 wt.% Zn alloy and commercial ZK60 alloy. Natural aging starts after a considerably lower period (~ 100 hours), if specific minor additions as Ti, Cu, Mn, V or Ba are added [6]. Recently a remarkable NA was observed in Mg-Gd and Mg-Tb alloys [7]. The incubation period for NA in Mg-Tb and Mg-Gd alloys is ~ 10 h, i.e. it proceeds significantly faster than in the Mg-Zn alloy. It has been demonstrated that NA has a positive effect on the strengthening during subsequent AA both in Mg-Gd and Mg-Tb alloy [8]. However, there is still a lack of information about the NA phenomenon in most Mg-based alloys.
In the proposed PhD work NA processes in Al-Mg-Si and Mg-RE alloys will be investigated on the atomic scale using positron annihilation spectroscopy (PAS) [9]. Microscopic investigations will be combined with measurement of macroscopic properties (strengthening of alloys) and ab-initio theoretical modelling of processes occurring during NA. The research will be performed in the laboratory of positron annihilation spectroscopy at the Department of Low-Temperature Physics.

References
[1] I. J. Polmear, Light Alloys, Elsevier, Amsterdam (2006)
[2] M. Liu, J. Čížek, C.S.T. Chang, J. Banhart, Acta Mater. 91, 355-364 (2015).
[3] J. Banhart, C. S. T. Chang, Z. Q. Liang, N. Wanderka, M. D. H. Lay and A. J. Hill, Advanced Engineering Materials 12, 559 (2010)
[4] D. W. Pashley, J. W. Rhodes and A. Sendorek, J. Inst. Metals 94, 41 (1966)
[5] C. S. T. Chang, I. Wieler, N. Wanderka and J. Banhart, Ultramicroscopy 109, 585 (2009)
[6] J. Buha, Acta Mater. 56, 3533 (2008)
[7] J. Čížek, B. Smola, I. Stulíková, P. Hruška, M. Vlach, M. Vlček, O. Melikhova, I. Procházka, Phys. Stat. Sol. A 209, 2135 (2012)
[8] O. Melikhova, J. Čížek, P. Hruška, M. Vlach, B. Smola, I. Stulíková, I. Procházka, Defect and Diffusion Forum 333, 51 (2013)
[9] J. Čížek, Characterization of lattice defects in metallic materials by positron annihilation spectroscopy: A review, J. Mater. Sci. Technology 34, 577 (2018)