Vacancy-induced hardening in Fe-Al alloys


Vacancy-induced hardening in Fe-Al alloys

Lukac, F.; Cizek, J.; Prochazka, I.; Jiraskova, Y.; Janickovic, D.; Anwand, W.; Brauer, G.

Iron aluminides are perspective materials for high temperature structural applications due to a high mechanical strength and excellent corrosion resistance. Hardness of Fe-Al alloys shows non-trivial dependence on chemical composition and thermal treatment of samples and cannot be fully explained by consideration of intermetallic phases formed according to the equilibrium phase diagram of Fe-Al system. Hardening caused by quenched-in non-equilibrium vacancies was proposed to explain rise of hardness in quenched Fe-Al alloys [1].
In the present work the concentration of quenched-in vacancies in Fe-Al alloys with various Al content cAl ranging from 18 to 49 at.-% was determined by means of two techniques of positron annihilation spectroscopy (PAS): (i) positron lifetime spectroscopy was employed for investigation of samples with vacancy concentration less than 2 x 10-4 at.-1 representing a limit for saturated positron trapping; (ii) in samples containing more vacancies than 2 x 10-4 at.-1, the vacancy concentration was determined using back-diffusion measurement of monoenergetic slow positrons. It has been demonstrated that both these methods give mutually consistent results [2].
Non-equilibrium vacancies were detected in all alloys studied after quenching from 1000°C. The concentration of quenched-in vacancies strongly increases with increasing Al content from 10-5 at.-1 in the alloy with cAl = 18 at.-% up to 10-1 at.-1 in the alloy with cAl = 49 at.-%. Comparison of the vacancy concentration and the Vickers microhardness measured on quenched samples revealed that hardness indeed increases with increasing concentration of quenched-in vacancies. In alloys with cAl > 30 at.-% the concentration of quenched-in vacancies exceeds 10-4 at.-1 and the hardness was found to be proportional to square root of the vacancy concentration in agreement with solution hardening model proposed by Chang et al. [1]. Subsequent annealing of samples at 520 °C causes recovery of quenched-in vacancies. This is accompanied by a drop of microhardness, but only in alloys where the initial concentration of quenched-in vacancies was least 10-4 at.-1. Hence, vacancies have a measurable effect on hardness of Fe-Al alloys when their concentration becomes 10-4 at.-1 or higher.
References
[1] Y. A. Chang et al., Intermetallics 1, 107 (1993).
[2] J. Čížek et al., Physica B (2012) doi:10.1016/j.physb.2011.12.122.

Keywords: Fe-Al alloys; vacancy-induced hardening; positron annihilation spectroscopy

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