Investigations On The Influence Of The Chemical Composition On Microdefects In Binary Ni-Al Alloys

By using the positron lifetime and coincidence Doppler broadening instruments,the microdefects and the densities of valence electrons in binary Ni-Al alloy with different chemical compositions have been investigated.The effects of annealing temperature on the microdefects and resistivity of binary Ni-Al alloys have also been discussed.The 3d electrons in Ni have well localized properties and tend to form covalent bonds with Al atoms.This results in the decrease of the number of free electron participating in metallic bonds.Thus,the bonding nature in Ni-Al alloy is a mixture of metallic and covalent.The experimental results showed,the fraction of covalent bonds in Ni₃Al alloy is less than those in NiAl alloy.And the fraction of covalent bonds is increased with Al contents in Ni₃Al and NiAl. The open volume of defects and resistivity of the Ni₇₅Al₂₅is the smallest among those of the Ni₃Al samples.Deviation of this chemical composition will lead to the enlargement of the open volume of defects and the resistivity.And the open volume of defects of the Ni₃Al alloy samples is larger than that of monovacancy or dislocation.It has also been found that the open volume of defects and resistivity of the Ni₅₀Al₅₀is the smallest among those of the NiAl samples.Deviation of this chemical composition will lead to the enlargement of the open volume of defects and the resistivity.And there are microvoids with relatively larger open volume in all NiAl alloys.For the Ll₂-Ni₃Al alloys,as the annealing temperature increasing,the vacancy will recover,and the resistivity of the alloys decrease.However,the microvoids with relatively larger open volume in B2-NiAl alloys could not recover after annealing at temperatures lower than 1200℃,and the variation of resistivity of the alloy with annealing temperature is very small.When positrons are injected into Ni₇₅Al₂₅alloy,the wave function of positron is localized at the octahedral interstitial sites in Ll₂ ordered structure and then annihilated with electrons of Ni.