Application Of Mechanical Alloying In The Preparation Of Heusler Alloys

This paper mainly explores the application of mechanical alloying in the preparation of Heusler alloys and its influence on the crystal structure,atomic ordering,magnetism and electronic structure in Fe-based and Ni-based Heusler alloys by using first-principles calculations.The Heusler alloy Fe₂CoSi with b.c.c structure was synthesized using mechanical alloying method.However,there was a large difference between the saturation magnetization at 5K and theoretical total moment.This is mainly due to the introduction of atomic disorder to the lattice in the process of ball-milling.By studying the influence of several possible atomic disorder on the magnetism of Fe2CoSi,it was found that the larger saturation magnetization in the milling sample was mainly from the antisite disorder of Fe?A?-Fe?B?-Co-Si.But in Ni₂CoSi a f.c.c phase was obtained after ball-milling.Fe₂NiSi alloy with metastable f.c.c structure has been prepared by mechanical alloying method successfully.But after arc-melting,we obtained Fe₂NiSi Heusler alloy with ordered b.c.c structure.The saturation magnetizations at 5K of Fe₂NiSi with the two structures agree well with the calculated total moments,respectively.The calculation results indicated that the L2₁B atomic disorder can help to stabilize the b.c.c structure in Heusler alloy Fe₂NiSi.Fe₂NiGe alloy with b.c.c structure was fabricated through mechanical alloying method.A rather high field was needed for Fe₂NiGe milling sample to get saturated magnetization,due to the resistance of domain wall shift.The saturation magnetization of as-milling Fe₂NiGe was also lower,which is mainly determined by the large amount of atomic disorder in Fe₂NiGe alloy.Theoretical calculation results showed that the strong covalent hybridization in the electronic structure of Fe₂NiGe can help to stabilize the b.c.c structure in Heusler alloy.Then Fe₂NiGe forms b.c.c phase instead of f.c.c after ball-milling.The site preference of Cu in Ni2CuSn and Ni2CuSb alloys have also been investigated by using the first-principles calculations.The two alloys both crystalize in L2₁B structure.This was different from“valence electrons rule”,which usually determines the site preference in Heusler alloys.Single phase Ni₂CuSn and Ni₂CuSb with L2₁ structure were obtained through arc-melting first.After ball-milling,the Ni2CuSb still formed L21structure,but in Ni₂CuSn the site preference of Cu can be changed by ball-milling.The site preference of X₂NiAl?X=Ti,Zr,Hf?alloys were also investigated.The X₂NiAl formed L2₁ structure due to the strong hybridization between d electrons in X and Ni atoms and the low N?E_F?in the DOS of L2₁ structure.In the low energy L2₁ structure,either Ti₂NiAl or Zr₂NiAl are not half-metallic material.But in L2₁ type Zr₂NiAl,the Fermi level located at an majority energy gap,which leaded to a higher spin polarization ratio.