V. Srinivas

Structural and electrical properties of AlCuFe quasicrystals

Abstract Structural and electrical transport properties of thermodynamically stable quasicrystalline Al-Cu-Fe alloys have been studied using X-ray diffraction, Mossbauer spectroscopy and d.c. electrical resistivity measurements. These results are compared with measurements on analogous crystalline and rapidly solidified alloys. The electrical resistivity of stable Al-Cu-Fe alloys is anomalously (∼ 3000 μΩ cm) high compared to most disordered alloys. The temperature dependence of resistivity of icosahedral Al63Cu25Fe12 can be described by a structural Kondo effect model.

Structural relaxation in icosahedral Al65Cr10Fe10Ge15

Rapidly solidified Al65Cr10Fe10Ge15 is found to be of the quasi-crystalline structure with icosahedral symmetry. 57Fe Mossbauer effect measurements suggest a disordered arrangement of transition-metal atoms in the as-quenched alloy. Thermal analysis measurements indicate a structural relaxation at approximately 700 K, and X-ray and Mossbauer effect measurements suggest that this represents primarily changes in the chemical short-range order of the alloy, yielding a more ordered structure.

Physical properties of icosahedral Al─Cr─Fe─Ge alloys

Abstract Alloys of the form Al65Cr20Ge15 with up to 15 at.% Fe substituted for Cr have been rapidly solidified from the melt by single-roller melt spinning and have been shown to be single-phase icosahedral quasicrystals. X-ray diffraction measurements show an increase in disorder with increasing Fe content. Thermal analysis measurements indicate structural relaxation of the quasicrystalline phase prior to crystallization. Mossbauer spectra of all alloys exhibit a broadened doublet. These spectra are analysed in terms of models involving multiple transition metal sites and distributions of transition metal environments.

Studies of hydrogen diffusion in metastable TiNiFeSi alloys

Abstract Alloys of Ti 56 Ni 23∼- x Fe x S 16 were prepared by rapid solidification from the melt by quenching on to a single copper roller. For a roller surface velocity of 18 m/s, the alloys are found to have a quasicrystal structure with icosahedral symmetry, and for a surface velocity of 60 m/s they are found to be amorphous. Typically, for short hydrogenation times (∼1 h) an increase in the average interatomic distance is observed but no change in structure for all the alloys. For longer hydrogenation times (∼4 h) precipitation of a crystalline hydride phase is observed for the icosahedral alloy, which is consistent with Mossbauer data. The Mossbauer data for hydrogenation amorphous alloys appear to depend on the concentration of iron.

High‐symmetry transition‐metal sites in Ti56Ni28−xFexSi16 quasicrystals

Mesures structurales, thermiques et magnetiques dans les alliages Ti 56 Ni 28−x Fe x Si 16 (0≤x≤20% at.)

Magnetic and thermal properties of amorphous Al-Gd-TM (TM=Fe,Cu) alloys

Amorphous aluminium-transition metal-rare earth alloys (Al-RE-TM) with compositions Al65Gd15Cu20 and Al1 Gd1 Fe1 have been prepared by melt spinning. X-ray diffraction studies show the average interatomic spacing in these alloys to be about 7% greater than in fcc aluminium and about 20% greater than in amorphous Al-TM-Si alloys. Thermal analysis measurements of the Al65Gd15Cu20 and Al1 Gd1 Fe1 alloys show the crystallization temperatures to be 647 and 945 K, respectively. Magnetization studies show Al65Gd15Cu20 to be paramagnetic with a localized gadolinium moment of 8.0 ± 0.2 μB. Al1 Gd1 Fe1 shows ferromagnetic behaviour, with Tc=275 K and a saturation magnetization of 124 e.m.u. g−1. A room-temperature 57Fe Mössbauer spectrum of Al1 Gd1 Fe1 shows a well-defined quadrupole doublet with a mean splitting of about 0.4 mm s−1. At 77 K, the Mössbauer spectrum shows a well-resolved sextet, corresponding to a mean iron hyperfine field of 96 kOe.

Physical properties of amorphous Al90Fe5Ce5

Amorphous Al90Fe5Ce5, prepared by rapid solidification from the melt, shows unusual tensile strength. The authors have investigated various physical properties of this alloy. X-ray diffraction measurements indicate a mean interatomic spacing comparable to that in FCC aluminium. Thermal analysis studies show an onset of crystallisation at 543 K, corresponding to the precipitation of FCC Al in an amorphous matrix. A second exothermic reaction at 593 K corresponds to the crystallisation of the remaining amorphous component into FCC Al and an intermetallic phase, possible Al(Ce, Fe)3. The electrical resistivity is well described by the Ziman-Faber theory with a Debye temperature of approximately 200 K. Magnetic susceptibility shows the alloy to be paramagnetic and to exhibit combined Curie and Pauli behaviour. 57Fe Mossbauer effect measurements show the short range ordering of this alloy to be similar to that found in transition metal-metalloid glasses.

Effects of Fe on the ferromagnetic properties of Al-Mn-based quasicrystals

Magnetization and Mossbauer‐effect studies of ferromagnetic Al‐Mn‐Si and Al‐Mn‐Cu‐Ge quasicrystals are presented. All alloys show paramagnetic or spin‐glass behavior in addition to ferromagnetism at low temperatures. The effects of Fe substitution on the magnetic properties were investigated. Possible models for ferromagnetic order in quasicrystals are discussed.

Mixed hyperfine interactions in the ferromagnetic quasicrystals Al40Mn25Cu10-xFexGe25 (x=3, 5 or 10)

Mossbauer effect measurements of the ferromagnetic quasicrystals Al40Mn25Cu10-xFexGe25 with x=3, 6 or 10 are reported both above (T=553 K) and below (T=100 K) the Curie temperature Tc. Below Tc, the spectra are best fitted to a model which involves a combined magnetic and quadrupole Hamiltonian. These fits yield the mean hyperfine parameters: hyperfine field H=9 kOe and quadrupole splitting Delta =0.33 mm s-1. These results are essentially independent of Fe content for 3<or=x<or=10 and suggest that the localized Fe moment is small. Above Tc the spectra are suitably fitted to a distribution of quadrupole interactions with a mean value mod Delta mod of 0.36 mm s-1. This is consistent with the value obtained below Tc.

Properties of rapidly solidified Ti—Ni—Fe—Si alloys

Abstract Alloys of Ti56Ni28−xFexSi16 (0≤x≤20) when melt spun at a wheel speed of 18 ms−1 show X-ray diffraction patterns which could be indexed to an icosahedral phase. Alloys quenched at a wheel speed of 60 m s−1 were found to be amorphous. 57Fe Mossbauer spectra of Fe-containing quasicrystals show a singlet; line broadening for samples with x ≥ 7·5 at.% provides evidence that the singlet results from the presence of more than one Fe site. Spectra have been analysed in terms of a distribution of quadrupole splittings. The amorphous Fe-containing alloys show quadrupole-split Mossbauer spectra with a splitting of approximately 0·4 mm s−1.