M. Mori

Amorphization in an immiscible CuV system by mechanical alloying and structure observed by neutron diffraction

Abstract Copper and vanadium atoms are immiscible even in the liquid state and the system is characterized by a positive heat of mixing. A mixture of elemental copper and vanadium powders was subjected to mechanical alloying. The mixing process down to an atomic level during mechanical alloying was studied by neutron diffraction and differential scanning calorimetry measurements. We take full advantage of the negligibly small scattering length of the vanadium atom in the neutron diffraction measurements. Some particular Bragg peaks of the f.c.c. copper crystal diminish quickly whereas the remaining peaks become broadened with increasing milling time. A halo pattern dominates after 60 hours of milling, though the first peak remains still rather sharp. The peaks of RDF(r) corresponding to those of the f.c.c. copper crystal are broadened with increasing milling time and approach gradually the characteristic of an amorphous state.

Use of periodic approximants in the structure refinement of icosahedral AlCuFe

Abstract By exploiting the periodic cells which are approximants to the icosahedral phase, the problem of structure determination of the icosahedral phase can be converted to the standard crystallographic structure refinement problem. As larger approximants are used, structures are obtained which converge to that of the icosahedral phase. We refine the structure of the (hypothetical) 1/1 cubic approximant to icosahedral AlCuFe using this technique, with the aim of determining the structure of the icosahedral phase.

Effect of Ambient Temperature on the MA and MG Processes in Ni-Zr Alloy System

The mechanical alloying and mechanical grinding were carried out at three representative temperatures, i.e., one at low temperatures -180<T<-120, the second at 25°C and the third at a high temperature of 200°C, using the mixture of pure Ni and Zr powders at 50 at.% proportion and the NiZr2 intermetallic compound, respectively. The higher the ambient temperature, the faster the amorphization proceeds in the case of the MA process. This is attributed to the increasing interdiffusion rate at the interface of the two metals. An increase in ambient temperature is found to be effective also in the amorphization due to the MG process. The defects, which are produced and equilibrate under the milling at high temperatures, are the likely cause for the energization and amorphization of the intermetallic compound NiZr2.

Differential scanning calorimetry study of various intermetallic compounds subjected to mechanical grinding

Abstract Two intermetallic compounds NiZr 2 and Cu 5 Zn 8 with good and poor glass-forming abilities, respectively, were subjected to mechanical grinding (MG) and the resulting structural changes have been studied by x-ray diffraction and DSC measurements. Similar analyses were also made for Cu 30 Ta 70 powders subjected to mechanical alloying (MA), the system of which is known to possess a positive heat of formation. The total heat ΔH tot accumulated by the ball-milling, the heat ΔH x associated with the crystallization of an amorphous phase and the crystallization temperature T x were deduced from the DSC spectra. We found that the behavior of ΔH tot , ΔH x and T x plotted against ball-milling time is characteristic of the respective systems and can be discussed consistently in terms of the thermodynamic consideration.

Phase transition and diffuse scattering studies in the Al-Cu-Fe ternary system

The reversible phase transformation between the crystalline multidomain structure of the rhombohedral phase to the perfect icosahedral quasicrystalline state at high temperature has been studied by in situ X-ray diffraction on a four-circle diffractometer using synchrotron radiation. The transient state known as the 5-fold modulated state has been observed and its evolution with temperature is characterised. Diffuse scattering located close to the Bragg reflections is observed in the studied temperature range. Its evolution with temperature shows a reversible transformation with an important change of shape in agreement with the proximity of a 5-fold phason instability.

Stability of the F2-(Al-Pd-Mn) phase

The stability of the F2-(Al-Pd-Mn) phase has been studied by in-situ neutron diffraction on a single quasicrystal with composition Al69.8Pd21.4Mn8.8. We find that the F2 phase is not stable and corresponds to a transient state in the process of the transformation of the icosahedral phase to the F2M phase. The icosahedral-to-F2Mphase transition occurs at around 715oC. In the F2 phase a large amount of diffuse scattering is located close to the icosahedral Bragg reflection in place of the S 1 superstructure reflections characteristic of the F2m phase.

Crystallographic characterization of a single quasicrystal of AlCuFe

Abstract Large, single-quasicrystalline grains, both cooled from the melt and annealed on cooling at ≈825°, were ground approximately into spheres with a resultant diameter of ≈0.2 mm. These crystal were then studied at the NSLS where, using λ = 1 A , integrated intensities on more than 1300 (inequivalent) reflections were collected out to Q⊥ greater than 1.7 A−1 (Q|| units). A crucial aspect of this work has been the careful characterization of crystal quality. For example, Bragg radial (φ−2λ) scans reveal a nearly perfect Gaussian profile while omega (ω) scans show essentially a pure Lorentzian shape. The Q-width (FWHM) of the radial scans increases from ≈0.006 to 0.009 A −1 as Q|| increases from 1 to 11 A−1. The FWHM in ω is characteristically about 0.065° with a mean variation of ≈0.001°, and its lineshape appears to be associated with a true mosaic spread as it does not depend in any essential way on angle or on Q⊥ or Qλ. Through depth-dependent measurements we show that the Lorentzian profile is substantially enhanced by mechanical grinding, although an unground crystal fragment still displays Lorentzian wings. TEM on unground samples from both sources reveals sharp diffraction spots with ideal icosahedral symmetry and images thet indicate an icosahedral phase rather than an ensemble of approximant domains.