The Behavior Of Microdefects And 3d Electron In Ni-Based Alloys Studied By Positron Annihilation Spectroscopy

By using the positron lifetime and Doppler broadening instruments, we have studied the structures of microdefects and the behavior of 3d electron of Ni-Al alloys. The effects of alloying elements on the microstructures of intermetallic compounds were also discussed. The Doppler broadening spetra were measured by two-detector coincidence system, the background of the spectrum can be reduced remarkably, which allows to obtain the contribution of annihilations with core electrons.The microstructure of binary Ni-Al alloys with different chemical composition has been investigated by two-detector coincidence system of Doppler broadening of positron annihilation radiation. The experimental results show, in binary Ni-Al alloys with different chemical composition, some of the 3d electrons of Ni atoms and 3p electrons of Al atoms in NiAl alloy are localized to form strong covalent bonds. With the increase of Al content in binary NiAl alloys, it leads to enhance the Ni d-Al pinteractions, while weaken the d-d interactions.The 3d electron signal for the transition metals (Cr, Co, and Ni) and the NiAl-based alloys (NiAl, Ni₄8Al₄5Cr₇, and Ni₄8Al₄5Co₇) can be extracted by a two-detector coincidence system from the Doppler broadening of positron annihilation radiation. The 3d electron signal for the undoped NiAl alloy is relatively low. The additions of Cr and Co to NiAl alloy lead to an increase in the probability of positron annihilation with 3d electrons. The Co addition is more effective in enhancing the d-d interactions in NiAl alloy as compared with the Cr addition.Positron lifetime spectra have been measured in binary Ni-Al alloys with different chemical compositions and NiAl alloys alloying with ternary elements. The results show that some of the valance electrons of Ni and Al atoms are localized to form covalent bonds in B2-NiAl and Ll₂-Ni₃Al alloys, which leads to a lower free electron density of the alloy. The mean free electron density of the binary Ni-Al alloy decreases with increasing Al content. The open volume of defects on grain boundary of the M₃Al is larger than that of monovacancy or dislocation. Structural vacancies and microvoids are found in B2 NiAl alloys with Al content above 45% (mole fraction), and the concentration of the vacancies and open volume of microvoids will increase with higher Al content.The addition of the third element M (such as Cr, Fe, Zr) into NiAl results in the increase of the density of free electrons and the fraction ofmetallic bonds in the bulk of the alloy when M substitutes for Al atom, this also decreases the ordering energy of the alloy and the open volume of the defects in grain boundary. Furthermore, the density of valence electrons in grain boundary increases due to the appearance of M atoms. However, the densities of free electrons in bulk and grain boundary will simultaneously decrease if NiAl is alloyed with Mg element.The Doppler broadening spetra of single and polycrystalline Ni₃Al alloys were measured. Positrons injected into Ni₃Al alloy are mainly trapped by the octahedral interstitial sites in Ll2 ordered structure and then annihilated with electrons of Ni. In a polycrystal of Ni3Al, the defects on grain boundaries with relatively large open volume are deeper traps of positrons. The probability of positron annihilation with 3d electrons of Ni increases with B concentration in polycrystal of Ni3Al due to B segregating to the grain boundaries, while decreases with the increase of B concentration in single crystal of Ni3Al due to B occupying the octahedral interstitial sites.