Effects Of Chemical Composition And Alloying Elements On Microdefects And D Electrons In Fe-Al Intermetallic Compounds

The long-range ordered intermetallic compounds based on FeAl (B2) and Fe₃Al (Do₃) are considered as a good candidate of high-temperature structural material, since this material offers a wide range of attractive properties including good oxidation resistance, sulfidation resistance, low density and potentially lower cost than many high temperature structural materials. However, limited ductility at room temperature and a sharp drop in strength above about 873k have been major deterrent to their acceptance for structural applications. Many attempts to increase the room-temperature ductility and high-temperature strength of Fe₃Al and FeAl alloys by means of alloying method have been made by several investigators. But, the micromechanism for chemical composition and alloying elements affecting the mechanical properties of Fe₃Al and FeAl alloys has not been clarified yet by now.In the present work, the Doppler broadening measurements of positron annihilation radiation have been performed in binary Fe-Al alloys of differentchemical composition, Fe₃Al doping with Mo, Nb, Cr or Si and FeAl containing Zr, B or Si .The 3d-shell electrons behavior in Fe-Al alloys can be extracted by a tow-detector coincidence technique. The positron lifetime spectra parameters are used to study the microdefects and electron densities in Fe-Al Alloys, and then reveal the micromechanism of the mechanical properties of binary and ternary Fe-Al alloys. The results are shown in the following:1. When Fe atoms and Al atoms are combined to form binary Fe-Al alloys, the 3d electrons of Fe atoms and 3p electrons of Al atoms are localized to form strong covalent bonds due to strong Fe 3d-Al 3p hybridization between nearest neighbor Fe-Al atom pairs. Fe₃Al and FeAl are also the intrinsic brittle alloys.The increase of Al content in binary Fe-Al alloys will enhance the 3d-3p interactions and weaken the d-d interactions; as a consequence the probability of positron annihilation with the 3d high-momentum electrons of Fe atoms drops sharply. At the same time, the concentration and open volume of structure vacancies in binary Fe-Al alloys increases rapidly with the increase of Al content.2. The addition of Cr to Fe₃Al leads to an increase in the probability of positron annihilation with the 3d electrons compared with the binary Fe₃Al alloys .It indicates that the Cr addition can enhance plasticity by weakening the 3d-3p interactions and enhancing the d-d interactions.The addition of Nb, Mo or Si to Fe3Al results in the decrease of the probability of positron annihilation with the 3d electrons compared with the binary Fe3Al alloys. Namely the Nb, Mo or Si addition into the Fe3Al alloy will enhance the p-d covalent bonds and weaken the d-d metallic bonds.3. The addition of Zr into FeAl alloy gives rise to the 3d electron signal in the spectrum of the alloy. It shows Zr addition can enhance the d-d metallic bonds and weaken the p-d covalent bonds; it is beneficial to the ductility of the FeAl alloy.When B atoms are added into FeAl alloy, B atoms are dissolved interstitially into the bulk or segregated to the grain boundaries, they increase the electron densities in the bulk and grain boundaries .B and Zr are found to be beneficial elements for improving the brittleness of FeAl alloys.Whereas, if FeAl is alloyed with Si, the 3d electron signals in the alloys decline sharply, it shows that the alloying element Si make the p-d interaction stronger and the d-d interaction weaker, and then the alloy is brittler because of the stronger covalent bonds.