Z. Altounian

Crystallization characteristics of Ni‐Zr metallic glasses from Ni20Zr80 to Ni70Zr30

We present the results of a crystallization study in glassy Ni‐Zr. The alloys were prepared by melt spinning over the continuous range Ni20Zr80 to Ni70Zr30 including for the first time the region between Ni40Zr60 and Ni60Zr40. As expected we find that the glasses are most stable at and around the eutectic compositions, except that at the extreme Zr‐rich eutectic the glasses are destabilized by the premature precipitation of ω‐Zr. The products of crystallization indicate that the current crystalline phase diagram is incomplete: a new peritectoid phase must be added at the composition Ni2Zr.

The influence of oxygen and other impurities on the crystallization of NiZr2 and related metallic glasses

The role of oxygen and other impurities on the crystallization characteristics of Ni‐Zr glasses near the composition NiZr2, as well as for FeZr2, CoZr2, and NiHf2, has been investigated. For NiZr2 glasses with 1 at. % oxygen, the first crystallization product is the metastable E93 structure with a =1.227 nm instead of the equilibrium C16 structure. A similar effect is found for samples containing ≳3 at. % B. For FeZr2, CoZr2, and NiHf2 the first crystallization product is also E93 structure, even with very small levels of oxygen (≤0.2 at. %). The formation of the E93 structure is always accompanied by an increase in the electrical resistivity, an increase which transmission electron microscopy shows is intrinsic to the phase and unrelated to crystallite size. For Ni36.5Zr63.5 and Ni42Zr58 the crystallization is also accompanied by an increase in electrical resistance and the evolution of a crystal structure similar to the E93 structure in the size of the unit cell and packing fraction but with a different...

Crystallization characteristics of Cu‐Zr metallic glasses from Cu70Zr30 to Cu25Zr75

The crystallization characteristics of glassy Cu‐Zr has been studied by differential scanning calorimetry, x‐ray diffraction, and measurement of electrical resistance and magnetic susceptibility. Our results allow us to remove many discrepancies between previously published data; they also suggest that the equiatomic Cu‐Zr does not exist as a single phase in the crystalline state. We observe parallel changes on crystallization between the magnetic susceptibility and the electrical resistance, which we interpret through changes in the d density of states.

Formation, structure, and crystallization of Al‐rich metallic glasses

The formation, structure, and the crystallization of Al85YxNi15−x are studied using x‐ray diffraction and differential scanning calorimetry. The results show two distinct glasses depending on composition. Y‐rich glasses (x≥8) are homogeneous with a well‐defined glass transition. The x‐ray diffraction patterns have a single main peak. These glasses crystallize through a nucleation and growth process. Y‐poor glasses (x<8) do not show a glass transition and have a shoulder on the high‐angle side of the main peak in their x‐ray diffraction patterns. We show that the shoulder peak is due to quenched‐in Al nuclei. These glasses are shown to crystallize through the growth of these nuclei. Y‐rich glasses (x≥8) are more stable as demonstrated by the presence of the glass transition and their higher crystallization temperature, enthalpy, and activation energy. The occurrence of a prepeak for all compositions is attributed to Y‐Y pairs.

Temperature dependence of coercivity in MnBi

In contrast to most ferromagnetic materials, the low‐temperature phase of MnBi exhibits an increased coercivity, Hc, with temperature. μ0Hc has a value of 0.2 T at room temperature, and rises dramatically to a maximum value of 1.9 T at 550 K. In the temperature region near its maximum value, Hc is much larger than that of Nd‐Fe‐B and has a very‐low‐temperature coefficient. For this reason, MnBi has a great potential as a permanent magnet material at high temperatures. To describe the temperature dependence of Hc, we develop a hybrid domain‐wall pinning model which combines Hilzinger and Kronmuller’s scaling theory for an anisotropic domain wall with Gaunt’s theory of thermal activation. The hybrid model gives an excellent fit to the temperature dependence of Hc and provides good estimates for the domain‐wall energy and thickness over the temperature range studied.

The formation of single-phase equiatomic MnBi by rapid solidification

The low temperature phase (LTP) of MnBi, which is of interest because of its large magnetic anisotropy, has been obtained in almost single-phase form (>95%) by rapid solidification, followed by thermal annealing. X-ray and electron microscope studies indicate that the melt-spun ribbons are amorphous, contrary to the accepted rules for glass formation. The transformation of the amorphous phase is a complex process. The amorphous phase crystallizes first into Bi, Mn 3 Bi, and ferrimagnetic MnBi. The formation of the LTP begins immediately after the eutectic melting of these phases around 540 K.

The structure and large magnetocaloric effect in rapidly quenched LaFe11.4Si1.6 compound

The structure and magnetocaloric effect (MCE) of rapidly quenched LaFe11.4Si1.6 compound were investigated by means of x-ray diffraction analyses and magnetic measurements. The rapid quenching greatly facilitates the formation of the cubic La(Fe,Si)13 (1:13 structure) in LaFe11.4Si1.6 alloy. Rietveld analyses indicate that about 22 wt% of the 1:13 phase is formed in rapidly quenched LaFe11.4Si1.6 alloy. A subsequent very brief annealing (1273 K/20 min) of the as-quenched alloy is sufficient for the formation of a single-phase 1:13 structure. Magnetic behaviour near TC indicates the occurrence of a field induced magnetic transition above TC, which is responsible for the large magnetic entropy change ΔS in annealed rapidly quenched LaFe11.4Si1.6 alloy. Comparing with annealed arc melted alloy, the annealed rapidly quenched alloy shows a smaller ΔS at or slightly below TC, where the magnetocaloric effect mainly originates from the temperature induced magnetic transition. On the other hand, the peaks of ΔS above TC for the two alloys are almost identical, which is mainly due to the field induced magnetic transition. The results indicate that ΔS due to a temperature induced magnetic transition is sensitive to the microstructure while the MCE related to a field induced magnetic transition is not.

Crystallization characteristics of Co‐Zr metallic glasses from Co52Zr48 to Co20Zr80

We present a crystallization study of melt‐spun Co‐Zr metallic glasses. The primary crystallization products are all metastable or unstable except around the equiatomic composition where the stable phase CoZr is produced. Alloys at the composition Co25Zr75 show a crystallization characteristic that is dependent on heating rate.

Structure and magnetic properties of bulk nanocrystalline SmCo6.6Nb0.4 permanent magnets

The crystal structure and magnetic properties were studied for the bulk nanostructural permanent magnet SmCo6.6Nb0.4 prepared by spark plasma sintering method. The magnet crystallizes in a single phase with a hexagonal TbCu7-type crystal structure after the sintering process. Rietveld fitting results show that the alloying element of Nb prefers to occupy the 3g site. Microstructure observation indicates that the average crystal grain size is about 30nm. The coercivity decreases almost linearly from 2.8to0.5T with increasing temperature from 300to773K. A remanence enhancement resulting from intergrain exchange coupling among the fine grains was also observed.

Hydrogen in amorphous Ni--Zr: Pressure concentration isotherms, site occupation, and binding energies

Measurements of pressure-concentration (p-c) isotherms of hydrogen in a wide variety of amorphous Ni--Zr alloys are presented. The measurements are complemented by an analysis of hydrogen sites in computer-generated amorphous clusters. The binding energy of these sites has been calculated using an effective medium theory. Filling these sites randomly in order of decreasing binding energy with a nearest-neighbor exclusion leads to a chemical potential in agreement with what is found from the (p-c) isotherms. In the pressure range required in a hydrogen-storage device (1--10 atm) the hydrogen occupies interstitial sites surrounded by 3Zr, 1Ni atoms or 2Zr, 2Ni atoms.