Effects Of Boron Content On Environmental Embrittlement Of Ordered Intermetallic Compound Ni₃Fe Alloys

Ni₃Fe alloy is a typical L1₂ type intermetallic compound.There is the order-disorder transition in Ni₃Fe alloy.The environmental hydrogen embrittlement induced by moisture does not exist,but that induced by hydrogen in environment exists in ordered Ni₃Fe alloy.Boron atom can effectively suppress the environmental embrittlement induced by hydrogen in the ordered Ni₃Fe alloy.But the effect of high boron content（C_B>0.06 wt.%）on mechanical properties and environmental hydrogen embrittlement of the ordered Ni₃Fe alloy is still unknown.In order to completely understand the action mechanism of boron content on the environmental embrittlement induced by hydrogen in the ordered Ni₃Fe alloy,the effects of the high boron content on microstructure,mechanical properties,hydrogen diffusion law and susceptibility to hydrogen embrittlement of the ordered Ni₃Fe alloy have been investigated systematically in this thesis.The results can provide experimental basis for further understanding the mechanism of boron atom suppressing environmental hydrogen embrittlement of the ordered Ni₃Fe alloy.To reveal the effect of high boron content on susceptibility to hydrogen embrittlement of the ordered Ni₃Fe alloy in hydrogen,the boron content and distribution,especially at grain boundary of the ordered Ni₃Fe alloy,and their influence on microstructure,catalytic cracking ability of hydrogen molecule and diffusion of hydrogen atom in the alloy were studied in this thesis.We have further understanded the law of boron atom suppressing the environmental embrittlement of the ordered Ni₃Fe alloy induced by hydrogen,and also revealed a controlling factor of the environmental embrittlement induced by hydrogen in the ordered Ni₃Fe alloy.The following main conclusions are obtained through experiments and research.1.The quantitative results of atom probe technology show that boron atoms are segregated obviously at the grain boundary of the ordered Ni₃Fe alloy.Boron atom significantly reduces the grain size and improves the tensile properties of the ordered Ni₃Fe（Ni-24Fe and Ni-25Fe,at.%）alloys doped with 0.06 wt.%B.Both ordered Ni-24Fe-0.06 wt.%B alloy and ordered Ni-25Fe-0.10 wt.%B alloy have the best tensile properties when tested in vacuum and gaseous hydrogen.2.The results obtained by computational material science show that the boron atoms will be segregated at the grain boundary of the ordered Ni₃Fe alloy,but hydrogen atoms will be distributed uniformly in the ordered Ni₃Fe alloy in thermodynamics.When boron atom competes with hydrogen atom for the occupation of interstitial site near the grain boundary of the ordered Ni₃Fe alloy,it is found that boron atoms are more likely to segregate at the grain boundary than hydrogen atoms according to thermodynamic energy.3.There is a correlation between the stoichiometry of the alloy and the effect of boron content suppressing the environmental embrittlement induced by hydrogen in the ordered Ni₃Fe alloy.For Ni-24Fe and Ni-25Fe alloy,the boron content thresholds of completely suppressing the environmental embrittlement induced by hydrogen are0.06 wt.%and 0.10 wt.%,respectively.4.The chemisorption capacity of hydrogen atom decreases with the increase of boron content in the ordered Ni-24Fe alloy doped with boron content from 0 to 0.06wt.%.5.At the same hydrogen charging temperature,the diffusion coefficient of hydrogen in the ordered Ni-24Fe alloy decreases with the increase of boron content when the boron content C_B≤0.06 wt.%.However,the diffusion coefficient of hydrogen is not sensitive to change of boron content when C_B>0.06 wt.%.With the increase of boron content,the diffusion activation energy of hydrogen in the ordered Ni-24Fe alloy increases and then decreases.The diffusion activation energy of hydrogen reaches the maximum value for Ni-24Fe-0.06 wt.%B alloy.The controlling factor of the susceptibility to hydrogen embrittlement of the ordered Ni-24Fe alloy is hydrogen diffusion in the alloy.