Quenched-in vacancies in Fe-Al alloys

Intermetallic Fe-Al alloys are perspective materials for industrial applications at elevated temperatures. A high concentration of vacancies is a specific feature of such alloys. The equilibrium concentration of thermal vacancies in these alloys is considerably higher than in pure metals and may be as high as several at.%. Moreover, a high concentration of thermal vacancies existing at elevated temperatures can be quenched-in down to room temperature. Hence, a detailed investigation of vacancies and other vacancy-like defects in Fe-Al based alloys is necessary for a deeper understanding of the physical properties of these materials.

Quenched-in vacancies in Fe-Al alloys were investigated in this work employing three complementary techniques of positron annihilation: Doppler broadening spectroscopy performed using a variable energy slow positron beam (SPIS = slow positron implantation spectroscopy), coincidence Doppler broadening (CDB) spectroscopy and positron lifetime (LT) measurements. It was found that quenched alloys exhibit a high concentration of vacancies. Although a free positron component cannot be resolved in our LT spectra, the concentration of quenched-in vacancies can still be determined from the shortening of the positron diffusion length measured by SPIS. Hence, SPIS is a very important technique for the investigation of defect-rich materials, like quenched Fe-Al alloys.

In the present work, the concentration of quenched-in vacancies was determined in Fe-Al based alloys at various Al content ranging from 24 up to 45 at.-%. The lowest concentration of quenched-in vacancies was found in the stoichiometric Fe3Al alloy, i.e. an alloy with an Al concentration of 25 at.-%. This concentration increases with an increasing degree of non-stoichiometry with respect to Fe3Al. Hence, the concentration of quenched-in vacancies increases both in alloys with under-stoichiometric as well as in those with over-stoichiometric Al content. However, the increase in concentration is more pronounced in the Al-rich alloys, i.e. alloys containing more than 25 at.% of Al. CDB revealed that quenched-in vacancies are surrounded predominantly by Al atoms which indicates that they occupy predominantly the Fe sub-lattice.