Effects Of Zr On The Microdefects And Electronic State In Ni-Al Intermetallic Compounds

Two Ge detectors-two dimension multichannel analyser systemfor measuring coincidence Doppler broadening spectra of positronannihilation radiation has been built in this work. With this setup, thebackground of the spectrum can be reduced remarkably, and we can get aratio of peak to backgroud higher than 10⁵. The signal of coreelectronwith high momentum in an atom can be extracted from the high energypart of the spectrum. It is very sensitive to the d and f electrons oftransition metal and rare earth metal.Positron lifetime spectra and coincidence Doppler broadeningspectra have been measured in some transition metals, Ni-Al intermetalliccompounds doped with ternary elements (shuch as Zr, Nb and Mn). Thesignal of d and f electron can be extracted form the coincidence Dopplerbroadening spectra. The aim of this work is to study the effects ofalloying elements on the microdefects and electronic state in Ni-Alintermetallic compounds. The results for pure transition metals indicate that the probability ofpositron-d electron annihilations increases with the number of 4delectrons of a transition metal atom.The Doppler broadening spectra of polycrystalline Ni₃Al alloys withdifferent Zr content before and after annealing at different temperatureshave been measured. The experimental results show that the open volumeof defects on grain boundary of the Ni₃Al is larger than that ofmono-vacancy or dislocation. Zr atoms are easy to diffuse to grainboundaries and segregated at grain boundaries. As the content of Zr lessthan 1.0at%, the probability of positron -d electron annihilations in Ni₃Alalloys increases with Zr content, and reachs a maximum value at1.0at%Zr. As the content of Zr higher than 1.5at%, the probability ofpositron -d electron annihilations decrease due to the Ni₅Zr phase formedin the alloy. The amount of Ni₅Zr phase increases with the increase ofannealing temperature and Zr content. As the Ni₅Zr phase formed in thealloy, the positron lifetime of the alloy increases, the height of peak ofratio curve of the alloy decreases, and the density of defect in the alloyincreases.In a binary NiA1 alloy, 3d electrons of Ni atoms and 3p electrons ofA1 atoms are localized to form strong covalent bonds due to strong Ni3d-Al 3p hybridization between nearest neighbor Ni-Al atom pairs. The delectron signal in the Doppler broadening spectra for the NiAl alloy is relatively low. The additions of Zr, Nb, Mn, Ti into NiAl alloy lead to thedecrease of the Ni 3d-Al 3p hybridization and the increase of d-dinteractions, and thus the increase of the probability of the positron-delectron annihilations.