A. Lucci

A computer method to determine the kinetic law of solid-state reactions from DSC curves

Abstract A computer program has been worked out to evaluate the activation energy, the Arrhenius pre-exponential factor and the mechanism of solid-state reactions from non-isothermal measurements. A univocal determination of the reaction mechanism is obtained by the simultaneous application of two selection criteria: (a) activation energy value by the Ozawa method; (b) degree of linearity in the A modification of the Rogers and Smith method was then used to obtain empirically complete agreement between kinetic expression and experimental results when the kinetic law has to be improved to account for a whole experimental curve. The accuracy of the computer method has been checked through a calibration of the program by means of 16 theoretical functions proposed by

Methods of non-isothermal kinetic analysis by DSC curves: An investigation on lattice reorganization in copper

Abstract Various analytical methods of non-isothermal kinetics have been applied to DSC curves to determine the kinetic parameters ruling the lattice reorganization (primary recrystallization) of prestrained pure copper. The techniques by Borchardt and Daniels, and Rogers and Smith have been modified to make them suitable for complex phenomena other than nth-order reactions. The results supplied by the new formulas are consistent with the data previously obtained under isothermal conditions. The same applies to the other methods tested, provided an accurate temperature calibration be made to correct the thermal lag of the calorimetric sensors. Such a calibration is of prime importance when DSC peak shifts are analyzed as a function of heating rate.

A DSC study of structural relaxation in metallic glasses prepared with different quenching rates

Abstract Sets of amorphous Fe-Ni-Cr-P-B, Fe-Ni-Mo-B, Fe-B-C alloys have been prepared by melt spinning using different rotation speeds of the wheel. The enthalpy of relaxation, the glass transition temperature Tg, the crystallization temperature are studied as a function of quenching rate. Structural relaxation takes place continuosly in a wide temperature range from 420–450 K up to the Tg region (650–670 K) and it is governed by a spectrum of activation energy described by the heat evolution curve at constant heating rate. The chemical and topological contribution to structural relaxation are separated by means of suitable thermal treatments.

Influence of preannealing on crystallization kinetics of some metallic glasses

Abstract DSC measurements and X-ray analysis have been carried out on Fe 40 Ni 38 Mo 4 B 18 (Met-glas® 2826 MB) and Fe 29 Ni 49 P 14 B 6 Si 2 (Metglas® 2826 B) glassy alloys after quenching at increasing temperatures in correspondence with significant points of the thermogram. The activation energy of crystallization and crystalline phases present after such annealings have been determined to reveal the effect of the latter on crystallization. A possible insight in the dimensional nature of the growth process has been looked for through the determination under non-isothermal conditions of the n exponent of the Avrami rate equation.

A statistical investigation of normal and abnormal grain growth in iron

The evolution of the statistical distribution of linear grain sizes during isothermal grain growth in unstrained or slightly strained pure iron specimens was investigated. A log normal distribution was confirmed to fit the data well for normal growth, and was shown to be applicable in the initial and final stages of abnormal growth. Analysis of statistical parameters of the size distributions proves that the coefficient of variation /gs¯d significantly distinguishes between abnormal and normal growth processes.

A study of primary crystallization in Fe40Ni38Mo4B18 glassy alloy by small angle x-ray scattering

Abstract The primary crystallization of the Fe40Ni38Mo4B18 glassy alloy was followed with a Kratky SAXS camera. By using SAXS conventional treatments of the experimental data, an interconnected structure was found distinctive of the underlying texture in the early stage of the phase decomposition. At higher temperature treatments and well inside the DSC peak relevant to the solid solution appearance, the structural inhomogeneities reach a more defined morphology with a clear interphase boundary. Nevertheless, during this nucleation stage no distinctive crystalline states were detectable in the wide angle patterns as remarked by the authors in a previous work [4]. The particle size evolution points out that a coarsening mechanism starts with the achievement of the steady value of the SAXS integrated intensity and concurrently with the long range order establishment.

Glass-forming ability and crystallization kinetics of amorphous palladium-boron alloys

Abstract Palladium-boron alloys have been prepared to check their ability to produce metallic glasses when spun from the melt. An amorphous alloy with 31.5 at.% boron obtained in the form of ribbon has been submitted to both isothermal and non-isothermal DSC tests. The isothermal crystallization kinetics have been analyzed according to the Avrami law for phase transformations in solids. The n exponent of the law has been determined to obtain information on the geometrical features of the growing crystals. A confirmation of the calculated n value has been sought through an analysis of non-isothermal DSC peaks

RDF analysis of structural evolution during the primary crystallization of the Fe40Ni38Mo4B18 glassy alloy

Abstract The changes in the atomic structure of amorphous Fe40Ni38Mo4B18 during the crystallization process were studied by X-ray diffraction. The radial distribution functions of various specimens frozen at significant points of the relative DSC thermograms, were obtained from the interference functions by means of the usual Fourier analysis. It appears from the results that the onset of crystal formation implies the simultaneous occurrence of atomic rearrangements involving the basic structural units already present in the unannealed sample; a five-fold symmetry and icosahedral structure were found for these units. An effort was made to correlate the progress in the crystalline organization with the significant features of the DSC related thermograms.

Effect of transition elements (Ti, V, Cr, Mn, Co, Ni) on thermal stability of Fe-B metallic glasses part I: Activation energy for crystallization

Abstract An investigation of the activation energy E c for crystallization in iron-boron-based metallic glasses submitted to partial crystallization has been undertaken. The hypothesis of quenched-in nuclei is put forward when E c remains unchanged. The contribution of activation energy for nucleation is evaluated when both nucleation and crystal growth act.

Activation energy for nucleation and crystal growth during the crystallization of palladium-based metallic glasses containing boron and beryllium

Abstract Completely or almost completely amorphous ribbons have been obtained by melt spinning around the eutectic composition of Pd-B and Pd-Be. Activation energy for crystallization Ec and the n exponent of the Avrami law which governs the kinetics of such a transformation, have been evaluated by DSC tests on Pd76B24 and Pd72Be28 glassy alloys. These measurements have been carried out both on as-quenched specimens and after different steps of preheating which bring about increasing amounts of crystalline fractions. Ec and n are lowered by the presence of preformed nuclei and tend to reach constant values that would correspond to a condition of zero nucleation rate. Such values are considered to represent the activation energy for crystal growth Eg and the geometrical contribution b contained in the n Avrami exponent, respectively. A relationship connecting the three activation energies Ec, Eg and En (this latter for the nucleation process) is used to evaluate En.