Physics

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Experimental investigation and theoretical analysis of the structural relaxation in amorphous Fe40Ni40B20.
(1998) Valanathan, Munsami.; Doyle, Terence Brian.
Amorphous metallic alloys are produced by a variety of techniques some of which involve rapid solidification of the alloying constituents. In these methods the solidification occurs so rapidly that the atoms are frozen-in and partially retain their liquid configuration. There are clear structural and other indications from their various properties that amorphous metallic alloys possess short range order but lack long range order. In general, amorphous alloys are not in a thermodynamic equilibrium state and, therefore, relax structurally whenever atoms attain an appreciable mobility. Associated with structural relaxation, many physical properties change; some significantly and others only slightly. Relaxation experiments in amorphous metallic alloys often display approximate In(t) kinetics which can be understood in terms of various models. In the present work the model by Primak (1955), for which the kinetic behaviour of a system depends on processes that are distributed over a range of activation energies, is used as a basis for further development. The Primak model allows, in principle, for the identification of the order of the relaxation reaction and for the determination of an initial activation energy spectrum Po(Єo), where Єo is a characteristic activation energy. Although the model provides for a qualitative explanation of the In(t) law, it has no predictive power as to the quantitative changes accompanying the various relaxing properties. Furthermore, an estimation of Po( Єo), inferred from various isothermal annealing procedures, reveals the approximate shape but does not fix its location on the activation energy axis. These shortfalls are attributed to complications in the frequency factor v, inherent to the Primak model. Also, the Primak model does not include consideration of the entropy involved in a 'configurational jump' of any particular atom during the relaxation process. Inclusion of the configurational entropy through the frequency factor v, in the present treatment, leads to a 'relaxation equation'. Structural relaxation measurements of density (in practice length - from which density can be approximately inferred) and electrical resistivity, in an Fe4oNi40B20 alloy, have been obtained and fitted to this relaxation equation. The fitting parameters are found, within experimental error, to be the same for both length and resistivity relaxation. The initial activation energy spectrum Po(Єo), as inferred from the fits, over the energy range 1.4 to 2.0 eV, reveals roughly three regimes, namely below 1.5 eV, from 1.5 to 1.8 eV, and above 1.8 eV, respectively, over which the initial activation energy spectrum Po( Єo) assumes different approximately constant values. Previous treatments have, however, implicitly assumed that Po( Єo) is constant throughout a temperature range over which In(t) kinetics is observed. The behaviour observed in this work is associated with the intrinsic relaxation mechanism involving consecutive diffusion of the metallic and metalloid atoms, respectively. A configurational entropy change inferred from this work is found to be negative as a consequence of contraction of the spread-out free volume resulting from thermal fluctuations. Within the framework of the 'present model', other related behaviour of amorphous metallic alloys, including the glass transition, crystallization and diffusion, are discussed. Where direct comparison between theory and experiment is possible for the various observed phenomena, the agreement is good and shows an overall consistency in our approach. Finally, the analysis considered here gives an expression which can be easily used to make quantitative predictions about the experimental relaxation behaviour. An immediate understanding of some of the main features of experimental data on relaxation can, therefore, be obtained through application of the present model.
The structural and mechanical properties of the Pt-Ti and Ir-Ti systems.
(2011) Cavero, Miguel.; Pierrus, John.
Ab initio plane wave based density functional calculations within the generalised gradient approximation (GGA) have been carried out on a wide range of phases and stoichiometries for the platinum-titanium (Pt-Ti) and iridium-titanium (Ir-Ti) alloy systems, using the Vienna Ab Initio Simulation Package (VASP) with projector augmented wave (PAW) potentials. For all of the phases in this work, the equilibrium structures were found by performing a full relaxation of the atoms. There were 20 di erent phases considered for varying atomic percentage compositions for each alloy system. Energy-volume calculations and heats of formations were used to determined the equilibrium structures at each atomice percentage composition and to determine if there were high temperature phases at that composition. The elastic constants and elastic moduli are calculated and the electronic structure and density of states (DOS) were considered to understand the hardness and stability properties of the alloys. For the Pt-Ti system, the low and high temperature phases at di erent compositions agreed with previously published results in the literature. Intermediate phases at 50% were also determined, in agreement with previous results. Alloying Pt with Ti resulted in a decrease in the bulk modulus, i.e. not adding strength to the metal. However, the shear modulus increased for most of the alloys compared to bulk Pt and it was found that in general, alloying may increase the resistance to shear. PtTi alloys were found to be ductile in nature, as with both constituent metals in their bulk form. In the Ir-Ti system, bulk Ir was found to have the highest bulk, shear and Young's modulus with each of these values decreasing with increasing percentage Ti in the alloy. IrTi alloys with 66.7% Ir composition or higher were found to be brittle in nature, similar in behaviour to bulk Ir; alloys with a higher percentage concentration of Ti were found to be ductile.
Structure and bi-magnetism of nanocomposites and nanoalloys synthesized by reduction of (Co, Ni) Fe2O4 nano-ferrites.
(2014) Ezekiel, Itegbeyogene Patrick.; Moyo, Thomas.
Structural and magnetic properties of CoFe2O4, NiFe2O4 nano-ferrites, CoFe2O4 /CoFe2, NiFe2O4/NiFe bi-magnetic nanocomposites and CoFe2, NiFe alloys were studied. The nano-ferrites were synthesised using the glycol-thermal method at 200 C for 6 hours. The nanocomposites and alloys were produced by the reduction of the ferrites using di erent amounts of activated charcoal. The reduction reaction was performed at 900 C for 3 hours in high purity owing argon atmosphere. Complete reduction yields the alloy phases while partial reduction produces the nanocomposites. The samples were studied using X-ray di raction (XRD), high resolution transmission electron microscopy (HRTEM), high resolution scanning electron microscopy (HRSEM), 57Fe M ossbauer spectroscopy, LakeShore vibrating sample magnetometer and a mini cryogen free system. XRD measurements of the nano-ferrites showed a single phase spinel structure with average crystallite sizes of about 10 nm. the CoFe2 alloy was obtained at an activated charcoal to ferrite molar ratio nC = 6 while the NiFe alloy was obtained at nC = 5. XRD results of the reduced samples of NiFe2O4 showed the coexistence of bcc -Fe and fcc -Fe lattice structures for NiFe alloy. The HRTEM and HRSEM measurements of all the reduced samples showed clear di erences between the morphology of the parent nano-ferrites and the reduced samples. 57Fe M ossbauer spectra measurements of the reduced samples showed the transformation of the spinel structure to the alloy phase. The presence of mixed phases in the nanocomposites was revealed. A small amount of mixed phase in CoFe2 and NiFe alloys which was not detected by XRD was revealed by M ossbauer measurement for nC = 6 and nC = 5 respectively. The M ossbauer ts showed that Fe3+ was reduced to Fe2+ with lower magnetic hyper ne elds observed in the reduced samples. High eld (50 kOe) magnetization measurements at room temperature for CoFe2O4/CoFe2 nanocomposite show signi cant enhancement of saturation magnetization from 63 emu/g of the parent nano-ferrite to 221 emu/g at nC = 5. In the NiFe2O4/NiFe nanocomposite the in magnetization increased from 57 emu/g to 141 emu/g at nC = 6. Low temperature measurements performed on the nanocomposites exhibited higher magnetizations. The coercivity of the fully reduced samples of the nano-ferrites were observed to be less dependent on temperature.

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