The 3p-3d Electron Interactions In TiAl Alloys Studied By Positron Annihilation Techniques

TiAl alloys retain high strength and resist creep at high temperatures ,especially, it has lower density and higher thrust-to weight ratio than other superalloys; thus, TiAl alloys are considered perfect materials for aircraft industry.TiAl alloys are intermatallic compounds, their brittle behavior appears to be intrinsic in nature . Its brittleness embarrass its present appliancation. How to improve its plasticities was of great importance. The brittle behavior was to be improved by alloying modification. Several groups have studied the effect of ternary additions to binary alloys and found that the poor ductility of binary TiAl alloys is ralataed to low free electron densities in the bulk and the grain boundaries of the alloy. V, Mn ,Cr, Cu, Zr, Nb and Ag additions generally increase ductility of Ti-rich TiAl alloys. Unfortunately, just very limited experiment work has been done on the study of the microdefects and electronic structure in TiAl alloys. In this work, the microdefects and electronic structure of TiAl alloys were studied systematically by positron annihilation technology.The coincidence Doppler broadening spetra were measured by two-detector coincidence system, the background of the spectrum can bereduced remarkably, which allows to obtain the contribution of annihilations with core electrons. The behavior of 3d electron of TiAl alloy has been studied by coincidence Doppler broadening instruments. The microdefects and electron density of TiAl alloy have been studied by positron lifetime spectrum. The effects of alloying elements on the microstructures and plasticities of intermetallie compounds were also discussed. The main contents of this work were as follows:Firstly, binary TiAl alloys with different chemical composition has been investigated by two-detector coincidence system of Doppler broadening of positron annihilation radiation. The results show, in binary TiAl alloys, some of the 3d electrons of Ti atoms and 3p electrons of Al atoms in TiAl alloy are localized to form strong covalent bonds. With the increase of Al content in binary TiAl alloys, it lead to enhance the tendency of the nearest neighbor Ti-Al atom pairs to form strong Ti d-Al p covalent bonds, and weaken the d-d interactions.Secondly, d electron signal for the metals (V ,Mn ,Cr, Cu, Ga, Zr, Nb, Ag, Sn and Ti) and the TiAl-based alloys Ti5oAl48M2 (M is V, Mn, Cr, Cu, Ga, Zr, Nb, Ag, Sn respectively) can be extracted by a two-detector coincidence system from the Doppler broadening of positron annihilation radiation. The d electron signal for the undoped TiAl alloy is relatively low. The additions of V ,Mn ,Cr, Cu, Zr, Nb and Ag to TiAl alloy lead to an increase in the probability of positron annihilation with 3d electrons.Thirdly, positron lifetime measurements were performed in Tis Ti5oAl48V2, Ti5oAl48Ag2 alloys and Ti, Al, Ag, V metals. The electron densities of the bulk and defects in the alloys and metals were calculated by using the positron lifetime parameters. This study was aimed at understanding the effects of V and Ag on defect and electron densities in the alloys.