France-Anne Kuhnast

Transformation kinetics of the Fe73.5Cu1Nb3Si13.5B9 ribbons to the nanocrystalline state

Abstract The transformation kinetics of the as-quenched Fe 73.5 Cu 1 NBb 3 Si 13.5 B 9 ribbons to the nanocrystalline structure has been studied. A differential scanning calorimeter in the continuous-heating and isothermal measuring regimes and a thermobalance with a permanent magnet have been used. The main differential scanning calorimetry (DSC) peak in the continuous-heating regime is wide and asymmetrical, having a long high temperature part and a small transformation enthalpy. The DSC signal in the isothermal regime is monotonically decreasing with a temperature-dependent amplitude, the total transformation enthalpy being slightly changed. Both as-quenched and nanocrystalline samples exhibit two magnetic transformations namely the glass-like and the nanophase-like transformations, the mutual proportion between them being changed with the proceeding transformation. All the observed phenomena are interpreted by the grain growth mechanism of the coarsening of the already existing fine-crystalline structure to the nanocrystalline structure. The experimental results can be interpreted also by the Johnson—Mehl—Avrami nucleation-and-growth crystallization mechanism with an abnormal exponent n ≈ 0.9.

The complex characteristics of crystallization of the Fe75Si15B10 glassy ribbon

The Fe75Si15B10 glassy ribbon was examined in the course of various isothermal and dynamic heat treatments. The number, type and mechanism of formation of the crystallization products formed during two crystallization stages in this alloy were determined by differential scanning calorimetry (DSC), thermomagnetometry, transmission electron microscopy and X-ray diffraction measurements. It is concluded that the first DSC peak is due to the crystallization of α-Fe(Si) or Fe3Si and the composite microcrystals containing Fe3B cores with α-Fe(Si) envelopes. The metastable Fe3B subsequently transforms into the stable Fe2B in the second DSC peak. The remaining amorphous matrix crystallizes by the eutectic reaction also in the second DSC peak forming the Fe3Si and Fe2B eutectic structures. The apparent activation energy E1⊛ decreases during the first crystallization stage from 480 kJ (g atom-1), characteristic of the Johnson–Mehl–Avrami (JMA) nucleation-and-growth kinetics of the as-quenched sample, to 350 kJ (g atom)-1 implying a dominance of the diffusion-controlled growth of iron in the already pre-nucleated sample. Simultaneously, the JMA exponent n1 decreases from 2.5 to 1.5. The apparent activation energy E2⊛ depends on the temperature of the first crystallization stage. It decreases with increasing Ta from 384 to 282 kJ (g atom)-1. This phenomenon was attributed to modification of the chemical composition of the remaining eutectic after the temperature dependent primary crystallization of Fe3B within the composites. The value of n2 is 3.

Complex behaviour of specific heat during structural relaxation of Fe73Co12B15 metallic glass

Abstract The non-isothermal DSC thermograms of the metallic glass Fe73Co12B15 were investigated. The complex analysis of the structural relaxation anomalies of the apparent specific heat of metallic glass samples was performed. The influence of ageing, pre-annealing and more complicated heat treatments, as well as of the heating rate, on the saturation of the structural relaxation enthalpy was demonstrated. The relaxation exotherm is principally connected with the rapid cooling of the sample. The relaxation endotherm represents the retarded approach to a certain heat treatment (non-isothermal crossover effect). All results can be generalized and interpreted by the DNLR (distribution of non-linear relaxation) model.

On structural and thermal relaxation in non-crystalline Zr–Ni–Al alloys

Abstract The differences in the non-crystalline structures and thermal relaxation phenomena of a planar flow-cast (rapidly quenched) and a water-cooled (slowly cooled) metallic glass and a planetary ball-milled (mechanically alloyed) amorphised alloy of the same nominal ZrNiAl composition were studied the actual compositions being slightly different. The differential scanning calorimetry and X-ray diffraction methods were used. It was found that short-range order is qualitatively similar in the bulk and ribbon samples, the former being better ordered within the first coordination shell. In the powder sample, the interatomic correlations decay more rapidly with increasing distance than in the other two samples, pointing to a less ordered packing of basic structural units composed of nearest neighbours. Therefore, the powder sample exhibits a different type of short-range order than the glassy samples. The observed structural relaxation phenomena in both glassy samples were entirely interpreted using the distribution of non-linear relaxation times model (DNLR). The thermal relaxation of amorphised powder consists of several independent processes, the traditional glass-like structural relaxation being the final stage only at the highest temperatures.

Influence of preannealing on the crystallization of Fe75Si15B10 metallic glass

The thermal stability of Fe75Si15B10 metallic ribbon was analyzed. The kinetics of the two-step crystallization of as-quenched and isothermally preannealed samples was studied by differential scanning calorimetry using various heating rates and the Kissinger, isoconversional and Surinach methods. Both crystallization stages are characterized by the Johnson-Mehl-Avrami kinetic law. The first step is dependent on both the heating rate and the preannealing. After sufficient heat treatment, the Avrami exponent decreases from 2.5 to 1.5, and the activation energy decreases to the value characteristic for the crystal growth mode. The second-step Avrami exponent n2 = 3, and the activation energy E2* = 395 kJ (g-atom)-−1, remain constant. The first step was related to the crystallization of αFe(Si) and Fe3Si, during which some of these primary crystals nucleate heterogeneously on the Fe3B cores. The eutectic decomposition of Fe3B into stable Fe2B and αFe was observed during the second step.

Calorimetric and structural analyses of mechanically alloyed and rapidly quenched Zr-Ni-Al alloys

Mechanical alloying and rapid quenching from the melt are used to form Zr-Ni-Al amorphous alloys. An experimental investigation of the amorphous to crystalline transformation is carried out using differential scanning calorimetry, X-ray diffraction and microprobe analysis. The experiments are managed to analyse and correlate the results both for mechanically alloyed powders and rapidly quenched ribbons: (i) a preliminary analysis of the kinetics of relaxation and crystallization, (ii) a connection between the final crystallized phases and the equilibrium ternary phase diagram.

Theoretical analysis of stress relaxation measurements by bend tests for (Fe50Ni50)1−xBx glasses: alloying effects on aging

Abstract We deduce the parameters of the non-linear relaxation distribution model (DNLR) from isothermal measurements. Following this thermodynamic theory, we present a theoretical application of stress relaxation experiments by bend tests of (Fe50Ni50) 1−xBx quenched metallic glasses. From radius of curvature measurements we deduce the three characteristic parameters of the DNLR model: the activation enthalpy ΔH+, the activation entropy ΔS+ and the non-linear parameter Knle. We can thus determine the effect of the boron concentration on the aging of these alloys.

Instability and relaxation of electrodeposited Ni75P25 amorphous alloys: experimental aspects quantitative theoretical predictions of isothermal and anisothermal aging using the cunat non-linear relaxation distribution model

Abstract The aging of electrodeposited Ni75P25 alloys is studied by means of expansion relaxation experiments. The interpretation proceeds within the framework of the non-linear relaxation distribution model. Relaxations appear non-linear and the initial distribution of densities suggests positive and negative excess density regions. For as quenched samples Cunat showed that the initial distribution is related to positive excess densities regions.

Structural Relaxation of Zr55Ni25Al20 Ball-Milled Amorphised Powder

The relation between the structure and thermodynamic state of the amorphised powder produced by planetary ball-milling of a mixture of crystalline single component powders and the same chemical composition planar flow-casted glassy ribbon is discussed. It was found that the powder samples are more structuraly ordered at shorter distances, however, this ordering vanishes more rapidly at larger distances in comparison with the ribbon samples. The relaxation of amorphised powder consists of several independent processes, the traditional structural relaxation being only the final stage at the highest temperature.

Thermomechanical analysis of the relaxation and crystallisation of Fe85−xCoxB15 metallic glasses

Abstract The structural relaxation and crystallization of Fe 85− x Co x B 15 amorphous ribbons was studied by thermomechanical analysis. The role of thermal history on reversibility of sample length change is evidenced. From flexure experiments performed at room temperature after pre-annelaing, we observe a substantial increase of Youngs modulus by 8% for relaxation and 50% for crystallization associated with a small density increase (about 0.08% and 2% respectively). The effect of cobalt content is revealed on the mean thermal expansion coefficient measured between 90 °C and 140 °C on the one hand and on the total residual shrinkage measured at room temperature after complete crystallization on the other hand. In agreement with other measurements, we observe a specific role of x (at.%) = 21 cobalt concentration on thermal stability.