Dr. Adrian JIANU

Four Fe-Mn-Si alloys, Fe(62)Mn(32)Si(6), Fe(62)Mn(20)Si(5)Cr(8)Ni(5), Fe(62)Mn(16)Si(5)Cr(12)Ni(5) and Fe(65)Mn(9)Si(7)Cr(10)Ni(9), were obtained by the melt-spinning method. The samples were structurally, magnetic and shape memory effect (SME) investigated, both ''as quenched'' and thermally treated. The Mn-rich compositions show different phase, magnetic behavior and SME in comparison with Mn-poor compositions. The thermal treatments generate transformation between the two existing majority phases (alpha and gamma), related magnetization and SME behavior. The features are derived from the corroboration of structural, magnetic interaction and magnitude of SME data. (c) 2008 Elsevier B.V. All rights reserved.

Exchange-coupled Nd2Fe14B/alpha-Fe nanocomposite magnets receives considerable research interest, because these are regarded as promising candidate materials for permanent magnets, with an adequate performance while keeping the content of relatively expensive Nd low. The advantage of these new magnetic materials are the lower rare earth content (and consequently, cheaper magnets with better corrosion resistance). The Nd2Fe14B/alpha-Fe nanocomposites were obtained by recrystallisation treatment from an amorphous phase prepared by melt-spinning. For Fe contents, between 83 and 85 at. %, we have obtained the hardening of the alpha-Fe phase by exchange interactions between the hard Nd2Fe14B phase and the soft alpha-Fe phase. The evolution of the structure and main magnetic characteristics after different annealing conditions was studied. For all isotropic permanent magnets obtained from the prepared nanocomposites was measured a remanence ratio higher than 0.5, which confirm the existence of the mentioned exchange interactions.

Melt spun ribbons of FexCu100-x were obtained by different procedures, in order to maximize the dispersion of the Fe atoms in the Cu matrix. Subsequent thermal and mechanical treatments were used for a controlled crystallization process of the soft magnetic phase. A detailed analysis of structural aspects and crystallization dynamics and their relation with the magnetic behavior of the Fe-Cu ribbons has been obtained via Mossbauer spectroscopy, X-ray diffraction (XRD) and magnetic measurements. In order to obtain composite permanent magnets with shape anisotropy, the optimal conditions and the suitable processing were established. © 2004 Elsevier B.V. All rights reserved.

The equiatomic composition of the FeRh alloy was studied by in situ high pressure energy dispersive X-ray experiments. Pressures as high as 19 GPa were generated using a diamond anvil cell. An incomplete bcc --> fcc phase transition was evidenced, both bcc and fcc phases coexisting above 10.0 GPa. The transition is accompanied by a decrease of the lattice parameter of the remaining bcc phase by approx. 1.5%. A substantial low lattice parameter was observed for the newly formed fee phase, reflecting a rather compressed structure. (C) 2004 Elsevier B.V. All rights reserved.

The Fe spin configurations in both the soft- and hard- magnetic phases of nanocomposite Fe/RE2Fe14B exchange-spring ribbons have been analysed by Mossbauer spectroscopy. New criteria for evaluating the inter-phase exchange-coupling strength are proposed. The coupling strength is discussed in relation to the annealing conditions and the relative content of the soft magnetic phase.

Using nuclear inelastic scattering of synchrotron radiation and inelastic neutron scattering we have investigated the vibrational properties of Fe and Co impurities in NiAl intermetallics, and compared them with the density functional theory calculations. The Fe phonon spectra show two sharp peaks appearing in the frequency gap between the Ni and Al phonon bands, and originating from the local modes of Fe residing in Ni, or Al sublattices. Co, which replaces Ni, vibrates within a narrow frequency interval occurring close to the upper edge, but still within the Ni phonon band. The ab initio calculations show that these effects are expected when impurity-host force constants differ from host-host force constants causing a dynamic mismatch between guest and host vibrations.

The influence of Zr and Ti substitutions on the formation of a nanocomposite magnet was studied by high-temperature X-ray diffraction and magnetic measurements. It has been shown that Ti and especially Zr additions favorably change the crystallization temperatures and the sequence of phase transformations which leads to an exchange coupled magnet, even in a composition range that promotes the precipitation of the detrimental Nd2Fe23B3 phase. (C) 2003 Elsevier B.V. All rights reserved.

The catalytic chemical vapor deposition (CCVD) method in which the outer furnace is replaced by the high frequency induction heating (IH) has been used for synthesis of carbon nanostructures. Using different catalysts, various types of carbon nanofibers were obtained, with the absence of the catalyst particles at their tip as a common characteristic. The IH mode combined with the CCVD method seems attractive for the synthesis of carbon nanostructures, allowing a significant decrease of energy consumption and of the overall reaction time as compared with the heating mode with outer furnace. (C) 2003 Elsevier Ltd. All rights reserved.

The intermetallic compounds in the Au-Al system were studied by Au-197 Mossbauer spectroscopy. The observed isomer shifts and electric quadrupole splittings are compared with the electron densities and electric field gradients at the gold nuclei obtained by LAPW calculations using the WIEN97 computer code. This provides insights into the electronic structure of Au-Al alloys and yields values for the groundstate quadrupole moment of Au-197 and for the change of the nuclear charge radius for the 77 keV transition of Au-197. (C) 2002 Elsevier Science B.V. All rights reserved.

The magnetic interactions between the nanograins in heterogeneous granular Cu-(SmCo5)-Fe ribbons were evaluated with a classical mean field model to obtain the exchange coupling as well as the magnetic moment per nanoparticle. The samples were subjected to various thermal treatments and a significant magnetoresistive effect was obtained when the optimal annealing conditions had been achieved. (C) 2001 Elsevier Science B.V. All rights reserved.

Alloys with nominal composition Al94-xMn6Cex (x = 1-4 at.%) and Al99-yMnyCe1 (y = 6, 8 and 10 at.%) were prepared by melt-spinning in protective Ar atmosphere and examined using transmission and scanning electron microscopy (TEM/SEM), selected-area electron diffraction (SAED) and high energy-resolution synchrotron radiation powder diffraction. Depending on the alloy composition, the average size of the icosahedral phase grains could be varied between 2-5 nm up to several hundred nm. The observed grain-size refinement effects open a way for the microstructure control of these novel quasicrystal-based nanocomposite materials and for the fine-tuning of their macroscopic behaviour.

The formation of quasicrystalline (QC) single-phases at compositions close to Ti53Zr27Ni20 was investigated using transmission electron microscopy (TEM), selected-area electron diffraction (SAED) and high resolution powder diffraction using synchrotron radiation. The primary Ti-Zr-Ni alloys were melt-spun in protective Ar atmosphere using peripheral wheel speeds omega = 20 m/s and omega = 25 m/s. TEM and SAED investigations revealed that significant microstructural differences occur even for rather small changes of the rapid quenching rate. Grain refinement and improved material homogeneity were found to be responsible for the observed gain in the ductility of the as-quenched ribbons.

Quaternary Ti53Zr21Ni13Fe13 alloys were prepared by melt-spinning using peripheral wheel speeds omega =19.5, 24.2 and 30m/s. The icosahedral (i-) phase formation was studied using high resolution synchrotron radiation powder diffraction. An amorphous phase component is present irrespective of the rapid solidification conditions, its volume fraction decreasing towards lower quenching speed values. The amorphous phase coexists with a b.c.c. rational approximant phase (W-phase). For omega below 20 m/s the formation of an i-phase was also noticed. Transmission Fe-57 Mossbauer spectroscopy measurements indicate that the number of asymmetrical Fe positions decreases with decreasing w in agreement with the improved structural perfection of the W-phase and with the formation of an additional i-phase. (C) 2000 Elsevier Science B.V. All rights reserved.

The laser ablation (LA) synthesis of ultra-fine-grained powders with dominant icosahedral structure has been attempted using Al-Cu-Fe and Al-Pd-Mn targets. The structure of the Al-Cu-Fe LA powders was examined using synchrotron radiation diffraction experiments which revealed the presence of the icosahedral Al-Cu-Fe phase. The Al-Pd-Mn powders were studied using scanning electron microscopy (SEM) and energy dispersive synchrotron radiation diffraction. For Al-Pd-Mn, the icosahedral phase is also a constituent phase of the LA alloy powders. Our results indicate that LA may be successfully used for obtaining quasicrystalline-based ultra-fine grained powders and/or surface coatings, while also providing a basis for the powder metallurgy processing of these materials. (C) 2000 Elsevier Science B.V, All rights reserved.

The room-temperature compressibility and stability of single-phase icosahedral Ti-Zr-Ni alloys were investigated for pressures up to 30 GPa, using synchrotron radiation energy dispersive diffraction. The icosahedral Ti-Zr-Ni single phase is stable over this pressure range, no phase transition being noticed. The pressure-volume data were least-squares fitted to a Birch-Murnaghan equation of state to obtain the zero-pressure bulk modulus B-0 = (173+/-5) GPa and its pressure derivative B-0' = 2.3+/-0.5.

The formation of icosahedral phases in Ag-substituted Ti-Zr-Ni alloys was investigated by in situ annealing diffraction experiments using synchrotron radiation. The as-quenched (Ti1-xAgx)-Zr-Ni alloys (x=2, 5, 7 and 10 at.%) are amorphous and/or in a mixed nanostructured state. Upon heating, the Ti-Zr-Ni-Ag alloys transform to either a crystalline or a quasicrystalline state, depending on the Ag content. Differential scanning calorimetry (DSC) studies were performed to identify the nature and sequence of the phase transitions. (C) 2000 Elsevier Science B.V. All rights reserved.

Single-phase icosahedral phases (i-phases) of TiZrNi and AlCuFe alloys were obtained by single roller melt-spinning in protective argon atmosphere. The high-pressure (up to 25 GPa) compression properties of i-TiZrNi and i-AlCuFe were investigated by in situ energy-dispersive synchrotron radiation (SR) diffraction experiments using a gas-membrane diamond-anvil cell. In contrast to i-AlCuFe, reproducible deviations from the Birch-Murnaghan equation of state were noticed in the form of hysterezis loops during the first compression cycles of i-TiZrNi alloys. This response of quasicrystalline matter to hydrostatic pressure conditions is here reported for the first time. Plastic deformation processes and effects related to the small grain size of the rapidly-quenched TiZrNi alloys are tentatively considered as possible explanations. (C) 1999 Elsevier Science B.V. All rights reserved.

Structural and magnetic properties of melt-spun Cu-Co-(Fe-Si) ribbons, as-quenched and after appropriate annealing, are investigated and correlated with their magnetoresistive properties. The effect of structural refinement of the magnetic phases and of the coexistence of multiple magnetic phases on the magnetoresistance is discussed. The existence of an optimum in the annealing conditions for obtaining a significant magnetoresistive effect, is inferred. (C) 1999 Elsevier Science B.V. All rights reserved.

Melt-spun Cu80Co10Fe10 ribbons are shown to exhibit a nanogranular structure with magnetic alpha-Fe(Co) grains segregated from the Cu-rich matrix after appropriate annealing. The Cu matrix purification as well as the magnetic properties of the alpha-Fe(Co) grains existing in a range of sizes are studied using X-ray diffraction, Mossbauer spectrometry and magnetic measurements. There is an optimum annealing for obtaining a high magnetoresistive effect. (C) 1999 Elsevier Science B.V. All rights reserved.

The magnetic properties of the amorphous and nanocrystallized Fe-Cu-Nb-Si-B and Fe-Gd-Cu-Nb-Si-B ribbons are studied using temperature dependence of the specific magnetization. A two-phase ferromagnetic behaviour is obtained for the magnetization curves. Gd addition tends to stabilize the interfacial (grain boundary) phase by forming a metastable Gd-Fe-B phase, leading to the coexistence of two phases up to thermal treatments as high as 993K.

High-resolution synchrotron-radiation powder diffraction experiments were performed to observe structural changes induced by hydrogen loading in rapidly-quenched Ti-Zr-Ni alloy ribbons with dominant icosahedral character. Lattice expansion effects due to hydrogen storage in Ti-Zr-Ni quasicrystals as well as phonon and phason disorder coefficients are obtained from an analysis of diffraction linewidths.

Crystallized Fe-(Cu)-Sm-B ribbons were investigated by means of X-ray diffraction, differential scanning calorimetry and Mossbauer spectrometry. The bcc alpha-Fe, tetragonal Fe3B and tetragonal Fe2B phases were found to crystallize after appropriate annealing. The relative proportion of alpha-Fe decreases with increasing Sm content, in agreement with the increase of the crystallization point. The metastable Fe3B phase decomposition into alpha-Fe and Fe2B was observed within a wide range of annealing temperature. The Sm ions randomly accommodate the Fe sites in the Fe3B lattice. After cumulative annealing at higher temperatures, the tetragonal Sm1.1Fe4B4 paramagnetic phase is found.

The influence of Gd addition on the structural properties of Fe-Cu-Nb-Si-B nanocrystallized and amorphous alloys is studied. The crystallization temperature increases and the microstructure of the annealed samples changes. Gd addition induces the formation of Gd-Fe-B phases. In fully crystallized Fe-Gd-Cu-Nb-Si-B alloys the alpha-Fe(Si), Fe-Nb-B, Gd3Fe62B14 and Gd1.1Fe4B4 phases are observed. The evolution of the microstructure is followed as a function of the cumulative effects of annealing time and temperature. The results suggest the transformation of the metastable Gd3Fe62B14 phase into Gd1.1Fe4B4 and alpha-Fe. The hyperfine parameters of the Gd3Fe62B14 Mossbauer contribution are reported. (C) 1997 Elsevier Science S.A.

Au-substituted Al-Cu-Fe icosahedral phases, Al62Cu25.5-xAuxFe12.5 (x less than or equal to 7) were prepared by melt spinning. EDS data showed that Cu as well as Al atoms are effectively substituted for with Au. The hypercubic lattice parameter of as-quenched samples slowly increases with x, while phonon-and phason-type disorders are not affected by Au substitution. Experimental structure factors for 'sum' lines were used to derive an average radius of the atomic hypersurfaces. Comparison of experimental versus calculated ratios of structure factors for different models yielded information about the Au distribution over the icosahedral sites. It was inferred that a fraction of the gold shapes its Al surrounding in a AuAl2-type configuration. Evidence was found for d-sp hybridization of Au-Al bonds, on the basis of the 'white-line' area at the Au L-3 absorption edge.

Metastable phases can appear in metallic alloys if their surface response function (liquidus temperature versus cooling rate) is less steep than that of the stable ones. in this case, rapid solidification procedures can result in their formation. On the other hand, metastable alloys are expected to yield special microstructures when the equilibrium phase composition is restored by their transformation or decomposition. These microstructures depend on the thermal history of the sample (e.g., the heating rate and temperature). Metastable phase was obtained by melt-quenching in the Ag1-xGex (x = 0.2, 0.35). X-ray diffraction revealed in Ag-Ge samples a metastable h.c.p. The h.c.p. Ag-Ge phase is the Hume-Rothery type. Besides, an alpha-Ag-Ge solid solution was present. Amounts of non-reacted Ge were also found, with a lattice constant close to the ideal value. Differential scanning calorimetry (DSC) runs revealed stability of the Ag-Ge metastable phase up to temperatures of 521 K (x = 2.0) and 626 K (x = 0.35), where exothermic peaks appeared with relatively high heat release (46.6 and 48.8 J g(-1), respectively). Work is in progress on the inhomogeneous lattice distortions in the metastable phases, which could be induced in the rapid solidification process and are expected to affect their stability. (C) 1997 Elsevier Science S.A.

The crystallization kinetics of the amorphous alloy Fe77Gd3B20 were examined under isothermal flash annealing conditions both in air and in a vacuum. Magnetic measurements were used to determine the reaction order (n) and activation energy (E(a)), and Mossbauer spectroscopy (MS) was used to identify some resulting crystalline phases.

The gradual substitution of Cu by Ag in the Al62Cu25.5-xAgxFe12.5 icosahedral phase (x less than or equal to 10) is investigated by X-ray diffraction from the viewpoint of the induced structural changes. The phonon and random phason disorders, estimated from diffraction linewidths, increase with x, the latter showing, however, an induction range of Ag concentrations. EXAFS results at the CuK and FeK edges show. for low Ag contents, and enhanced disorder in the first coordination sphere, partly relaxed for higher x values. The 6D hypercubic lattice parameter is found to be independent of the Ag content, despite the large diameter of monovalent Ag. The latter facts suggest a d-sp hybridization of the Ag bonds to the surrounding Al, taking place above a certain x value.

The results of the potentiostatic polarization studies taken on some Al-Mm-MT metallic glasses in reducing acid media are presented. The rest potentials development and the electrochemical parameters evaluated from the anodic and cathodic curves were correlated with the alloys composition, and related to those of the alloying components in crystalline state. Our results indicate that: alloying with rare earths does not affect interface processes; the transition metals interfere with the hydrogen reduction kinetics determining its acceleration by increasing Ni content in the alloy composition; substitution of Ni by Fe contributes to corrosion rate reduction, to passive state extension and stabilization, so that Al-Mm-Fe alloy behaves better than Al-Mm-(Fe, Ni) and Al-Mm-Ni alloys.

Electrochemical laboratory investigations were used as a screening tool for selection of metallic materials applicable to concrete reinforcement. Simultaneous examination of FeCrXP9C11 (X = 4; 7; 9), FeB13Si9, FeB13.5Si3.5C2 as compared to classical carbon steels aas conducted in simulated concrete corrosive electrolyte: Ca(OH)(2) sat. +/- NaCl +/- y Na2SO4 +/- CO2 sat. (x = 5; 50; 100 g/L, y = 1; 10 g/L). Indications are that FeCrXP9C11 amorphous alloys are highly resistant materials. Their resistance to the most aggressive media is 100-1000 times greater than of FeBSi(C) alloys and carbon steels.

Icosahedral alloys with the composition Al(62)Cu(25.5)TM(12.5) (TM=V, Cr, Mn, Fe, Co, Ni) were prepared by melt spinning. Prevailing icosahedral phases were found for TM=Cr, Mn and Fe. Phonon and random phason disorder were evaluated by an analysis of X-ray diffraction linewidths. A marked chemical trend was noted for phonon-type disorder, which increases in the sequence Fe<Mn<Cr. The parameter of the hypercubic lattice for the Mn-based icosahedral phase allows information to be derived on the metallic valence of Mn and its local coordination via the effective atomic radius.

A sequential algorithm, leading to three-dimensional polytetrahedral dense random packing structures is employed to simulate the atom-by-atom growth of relatively small clusters, under a large variety of physical assumptions. The computer model developed can provide information about the influence of experimental growth conditions on the topological and energetical parameters of the cluster : average coordination number, atomic pair distribution function, the degree of local tetrahedral perfection and specific bonding energy as a function of cluster size. The degree of icosahedral order built-in during the simulated growth process can be estimated from the pair distribution function and is of particular interest, due to its relevance to quasicrystalline thin films investigation.

Mossbauer measurements performed on Fe100-xGdx thin films and on Fe80-xGdxB20 both as thin films and ribbons show a dependence of the spins orientation and [H-hyp] versus temperature, Gd content and preparation conditions. Increasing the Gd content, the initial low anisotropy disappears and [H-hyp] decreases. A sharp increase of the anisotopy with temperature in ribbons with low Gd concentration is evidenced.

Phonon- and random-phason-type structural disorder was measured in Al-transition metal (Tm) ternary alloys, prepared with Al excess by rapid quenching. The disorder parameters correlate with the calculations on the energy stability of Al12TM icosahedra. Thus, substitution of 'unstable' elements (Fe and Co) for 'stable' elements (Cr and Mn) causes a strong increase in both types of disorder, leading finally to crystallization. This chemical trend is modulated around special compositions of the TM sublattice. The degree of structural order correlates with the thermal stability of icosahedral phases.

The icosahedral metastable Al-Mn phase was obtained in thin films prepared by the high temperature sequential vapor deposition method. Analysis of X-ray diffraction line profiles allows an estimate of the random phason strain, revealing the presence of a highly disordered icosahedral fraction. Its concentration is correlated with the degree of crystallographic perfection of the Al substrate. Thereby, the crucial role of Al diffusion in the formation of the icosahedral phase is evidenced.

Magnetic and Mossbauer experiments were performed on Fe100-xGdx thin films and on Fe80-xGdxB20 both in thin films and ribbons for x less-than-or-equal-to 20. The ferrimagnetic order is perturbed by the presence of B and Fe-interactions are.different. For small Gd content, the low anisotropy is in the film plane, while for greater Gd the anisotropy goes out of the plane giving typical features of hysteresis loops and strong decrease of Mossbauer hyperfine fields.