Toshio Nasu

Solid state amorphization of FeC alloy by mechanical alloying

Abstract Alloys prepared by mechanical alloying four kinds of crystal FeC powder mixtures were examined by X-ray diffraction. The amorphization reaction was confirmed in the case of Fe 75 C 25 at 25.2 × 10 5 s of milling time.

EXAFS study on amorphous and nanocrystalline M–W (M=Fe,Ni) alloys produced by electrodeposition

Abstract Ductile amorphous and nanocrystalline M–W (M=Fe,Ni) alloys were produced by electrodeposition. Structural studies of these alloys were made by using EXAFS and SAXS methods. The crystal structure of the electrodeposited Ni–W alloy was similar to that of the Ni4W crystal phase. The nearest Ni–Ni distance was determined to be 2.49 A; the Ni–W distance was determined to be 2.50 A. The atomic distances in the nanocrystalline Ni–W alloys increases with the tungsten concentration. The average crystal grain size of the Ni–W alloy was estimated at about 2.5 nm from a Guinier plot of the SAXS spectra. The electrodeposited amorphous Fe–21.6 at.% W alloy has only short range order, and no medium range order.

An EXAFS study of structural changes induced by mechanical milling of the Ni3Al ordered intermetallic compound

Extended X-ray absorption fine-structure (EXAFS) data from the Ni3Al ordered intermetallic compound during structural evolution induced by mechanical milling (MM) are presented and discussed. X-ray diffraction and transmission electron microscopic observations confirm that the L12Ni3Al ordered intermetallic compound changes to a disordered fcc form of Ni3Al at an early stage of milling and then changes to a two phase nanocrystalline and amorphous microstructure. Least-squares-fitting of the EXAFS data shows that both the NiNi and the NiAl atomic distances increase slightly with milling time. During the first 5 h of milling, the NiNi coordination decreased while the NiAl coordination increased. This observation is also indicative of an order-disorder transition. Subsequent milling produced a remarkable increase in the NiNi coordination number as a nanocrystalline structure developed. Simultaneously, the NiAl coordination number decreased markedly, the total Ni coordination always remaining roughly constant. These EXAFS results suggest that segregation of Al from the Ni3Al occurred during the MM process, and eventually a partly amorphous structure evolved. After the longest milling time, a Ni-rich nanocrystalline phase was surrounded by an Al-rich amorphous phase. The Al segregation may be at the nanocrystalline grain boundaries and/or within the more extensive amorphous regions observed by transmission electron microscopy.

EXAFS study on mechanical deformation for PdSi amorphous alloy

Abstract Extended X-ray absorption fine structure (EXAFS) spectra were measured in situ under several tensile strains to study the atomic-scale process of mechanical deformation in Pd83Si17 amorphous alloy using synchrotron radiation at KEK. With an increase in the tensile strain, the PdSi distance shifts to higher R space with little change in the coordination number while PdPd distance is displaced towards lower R space values with a large reduction in the coordination number. During the unloading process, the values of the PdSi and PdPd distances do not return to their initial values. From these facts, the deformation mechanism is discussed in terms of the atomic-scale structure.

Positron Annihilation in Plastically Deformed Ni-P Amorphous Alloy

The positron life time and the angular correlation of positron annihilation radiation (ACPAR) from a plastically deformed Ni–P amorphous alloys were measured. The counting rate around θ=0 increased, the counting rate around θ=10 mrad decreased and the life time of positron increased, as a result of plastic deformation. It is interpreted that a slight growth of the free volume in the specimen by plastic deformation was detected. “Work softening” behaviour of the amorphous alloy is interpreted by this growth of the free volume.

EXAFS study on mechanically alloyed Fe-B powder mixtures

Abstract The main purpose of this work is to utilize EXAFS to observe the atomic-scale structural changes during the MA (mechanical alloying) process of two Fe-B powder mixtures: one is Fe80B20 (which is amorphized by rapid quenching (RQ but not by MA) and the second is Fe50B50 (which is amorphized by MA, but not by RQ). In the MA process of Fe80B50, the dissolution of B atoms into the α-Fe crystal was observed. In contrast to this, in the MA process of Fe80B20, the B atom dissolution into the α-Fe crystal was not observed.

EXAFS study on solid state amorphization process of NiNb alloy by mechanical alloying

Abstract The main purpose of this work is to investigate the micromechanism of the solid state amorphization of metals by mechanical alloying. The changes of radial distribution functions around the atomic species of a NiNb mixture powder through ball milling were studied by means of EXAFS. The atomic structural change during crystalline to amorphous phase was discussed.

A positron annihilation study of the evolution of amorphization in Nb3Sn by mechanical milling

Abstract In an attempt to probe the effect of vacancy-related defects on the crystalline-to-amorphous phase transformation, positron annihilation lifetime measurements were performed on Nb 3 Sn with the A15 structure as a function of mechanical milling. The lifetime spectra were deconvoluted into two components. Both components increased during the initial stage of milling, consistent with an increase in the concentration of the vacancy-type defects and void-type defects formed as a result of milling. The formation of these types of defects plays an important role in the amorphization reaction of A15 Nb 3 Sn.

Core-level and valence-band electron states in Zr-based bulk metallic glass

A Zr-based bulk metallic glass of Zr70Ni20Al10 has an excellent glass-forming ability (GFA), and can form a massive bulk glass by simple water-quenching. In order to clarify the role of electronic structure in this excellent GFA of Zr70Ni20Al10 glass by comparing to the Zr70Ni30 reference glass, having a worse GFA, valence-band photoemission spectra were measured with changing the incident photon energy using synchrotron radiation at BL7 of HiSOR. We have also measured Ni 3p, Zr 3d and Zr 4p core-level photoemission spectra of the Zr70Ni20Al10 glass, and compared them to those of the Zr70Ni30 reference glass. The shifts of the binding energies in every core-level are observed, indicating changes of the chemical nature by replacing the Ni atoms with Al atoms.

Structural changes in B2 CoZr compound during amorphization process by mechanical milling

Abstract A structural study of the B2 CoZr compound during amorphization reaction by high energy ball milling was done using extended X-ray absorption fine-structure (EXAFS) measurement to discuss the micromechanism of amorphization. The B2 CoZr compound powder was prepared by are melting and annealing. The mechanical milling was done using a vibrating mill. A change in the radial distribution function, provided by the Fourier transform of EXAFS signal, around the Co atom shows that the Zr atom at the first-nearest neighbor around a Co atom is replaced by Co atom at the second-nearest neighbor, simultaneously the crystalline structure disintegrates and the amorphization reaction occurs. When the accumulated strain energy, e.g. the nanocrystal grain boundary energy, is comparable with the difference to drive the crystalline-to-amorphous phase transformation in CoZr, the amorphization reaction starts.