Investigation of Amorphization of Nb-Si Alloys by Mechanical Alloying

Amorphous Nb_(62.5)Si_(37.5) and Nb_(75)Si_(25) alloys were produced from mixtures of elemental Nb and Si powders by mechanical alloying(MA).The structural changes of mechanically alloyed powders were monitored by x-ray diffraction.It was found that NbSi_(2),intermetallic compound was formed in the initial stage of MA of or Nb_(62.5)Si_(37.5) or Nb_(75)Si_(25) powders.With continued milling,the mixtures of intermetallic NbSi_(2) and elemental Nb transformed into amorphous Nb-Si alloys(Nb_(62.5)Si_(37.5) or Nb_(75)Si_(25)).The slow interdiffusion of Nb and Si and the slower diffusion of silicon atoms to niobium matrix than that of niobium atoms to silicon matrix were suggested to be responsible for the formation of intermetallic NbSi_(2).The amorphization of 3NbSi_(2)+7Nb mixtures(or NbSi_(2)+5Nb mixtures)is believed to be controlled by material transfer,which moves composition of line intermetallic NbSi2 off stoichiometry,and causes the matrix of elemental Nb deformed.

Nano-structure of Iron Formed by Mechanical Grinding

Nano-crystalline elemental iron powder was successfully formed by high-energy ball grinding.The x-ray diffraction,transmission electron microscopy and Mdssbauer spectroscopy were employed to follow the structure changes in the process of mechanical grinding with milling time.The thermal stability of nanocrystalline iron powder was also studied by differential scanning calorimetry.The results showed that the grain size reduces drastically at the initial stage of the milling and reaches 6nm after 40h milling.The broadening of Mdssbauer spectrum is attributed to the large fraction of iron atoms existing at the interfaces for the powder milled.

Amorphization of Co-Nb Magnetic Alloys by Mechanical Alloying

Formation of amorphous Co-Nb system by mechanical alloying(MA)in the composition range of Co_(25)Nb_(75)-Co_(70)Nb_(30) was studied by X-ray diffraction.Magnetic properties of the mechanically alloyed Co_(70)Nb_(30) powder were measured.The variation in H_(c) curve with milling time can be explained by two-stage mechanism involved in the milling process.The results show that the amorphous powder prepared by MA is of good soft magnetic properties.

FORMATION OF AMORPHOUS Cu_(60)Zr_(40)AND Ni_(50)Zr_(50)POWDERS BY MECHANICAL ALLOYING

Amorphization of Cu-Zr and Ni-Zr systems by mechanical alloying(MA)was studied by X-ray diffraction.The results show that the amorphization is not merely a simple interdiffusion by thermal activation;in addition,the strain induced by MA also accelerates this process.

FORMATION OF AMORPHOUS Al_(80)Fe_(20)BY MECHANICAL ALLOYING

Amorphous Al_(80)Fe_(20) powder which can hardly be obtained using conventional rapid quenching technique was directly prepared from crystalline Fe and A1 powders by means of mechanical alloying process.The amorphization process was examined by X-ray analysis and Mssbauer spectroscopy,and the thermal behaviour was studied by DSC.The results show that the rate of amorphization is controlled by the diffusion reaction at the interfaces and changes with the milling time.The effect of milling conditions on amorphization by mechanical alloying was al- so investigated.

NEUTRON-IRRADIATION EFFECTS ON CRYSTALLIZATION OF Cu-Ti GLASSES

The effect of neutron-irradiation on crystallization of Cu-Ti glasses of four compositions was systematically studied by differential scanning calorimetry(DSC)and internal friction measurements.The results show that the susceptibility of the glasses to irradiation is quite composition dependent.

Deformation induced structural evolution in bulk metallic glasses

The structural behavior of binary Cu50Zr50 and ternary Cu50Zr45Ti5 bulk metallic glasses (BMGs) under applied stress was investigated by means of in-situ high energy X-ray synchrotron diffraction. The components of the strain tensors were determined from the shifts of the maxima of the atomic pair correlation functions (PDF) in real space. The anisotropic atomic reorientation in the first-nearest-neighbor shell versus stress suggests structural rearrangements in short-range order. Within the plastic deformation range the overall strain of the metallic glass is equal to the yield strain. After unloading,the atomic structure returns to the stress-free state,and the short-range order is identical to that of the undeformed state. Plastic deformation,however,leads to localized shear bands whose contribution to the volume averaged diffraction pattern is too weak to be detected. A concordant region evidenced by the anisotropic component is activated to counterbalance the stress change due to the atomic bond reorientation in the first-nearest-neighbor shell. The size of the concordant region is an important factor dominating the yield strength and the plastic strain ability of the BMGs.