Jilt Sietsma

The glass transition as a free volume related kinetic phenomenon

Abstract The glass transition, observed in glasses during DSC experiments at a constant heating rate, is described as a kinetic phenomenon caused by the continuous approach of free volume towards equilibrium during the warming up. At a certain temperature below the glass temperature T g , the amount of free volume becomes smaller than the equilibrium value at that temperature. The glass transition is caused by a rather sudden increase of free volume towards equilibrium near T g . This can be calculated by making use of the well known kinetics of free volume annilation and production in some metallic glasses. Calculations performed on this basis are in good agreement with available experimental data, both on structural relaxation and the glass transition.

Structural disordering in amorphous Pd40Ni40P20 induced by high temperature deformation

The influence of plastic deformation on the structural state of amorphous Pd40Ni40P20 is investigated by means of tensile test measurements as a function of temperature, strain rate and pre-annealing time. The structural state of the material after the deformation is investigated by means of differential scanning calorimetry (DSC). It is found that amorphous Pd40Ni40P20, when pre-annealed into its metastable equilibrium state, shows a large strain softening effect when deformed at temperatures close to the glass transition temperature. The strain softening effect is less pronounced or even absent in samples that are only briefly pre-annealed. The DSC traces show that the structural state of the material disorders during the deformation and that the material reaches a strain rate and temperature dependent equilibrium state, which differs from the thermal equilibrium state. It is shown that the strain softening effect is caused by the creation of additional free volume during the deformation.

Magnetic and X-ray diffraction measurements for the determination of retained austenite in TRIP steels

The accurate determination of the volume fraction of retained austenite is of great importance for the optimization of transformation induced plasticity (TRIP) steels. In this work, two aluminium-containing TRIP steels are studied by means of magnetization and X-ray diffraction (XRD) measurements. By fitting the field dependence of the approach to saturation in the magnetization curves, the saturation magnetization is determined, which is linearly related to the volume fraction of retained austenite. Moreover, information with respect to the microstructure can be obtained from the fitting parameters and the demagnetizing factor for the magnetization curve. The volume fractions obtained from the magnetization measurements are compared with data from XRD measurements. A discussion of the data suggests that magnetization measurements lead to more reliable results and a more sensitive detection of the retained austenite than XRD measurements.

Chemical gradients across phase boundaries between martensite and austenite in steel studied by atom probe tomography and simulation

Partitioning at phase boundaries of complex steels is important for their properties. We present atom probe tomography results across martensite/austenite interfaces in a precipitation-hardened maraging-TRIP steel (12.2 Mn, 1.9 Ni, 0.6 Mo, 1.2 Ti, 0.3 Al; at.%). The system reveals compositional changes at the phase boundaries: Mn and Ni are enriched while Ti, Al, Mo and Fe are depleted. More specific, we observe up to 27 at.% Mn in a 20 nm layer at the phase boundary. This is explained by the large difference in diffusivity between martensite and austenite. The high diffusivity in martensite leads to a Mn flux towards the retained austenite. The low diffusivity in the austenite does not allow accommodation of this flux. Consequently, the austenite grows with a Mn composition given by local equilibrium. The interpretation is based on DICTRA and mixed-mode diffusion calculations (using a finite interface mobility).

Ferrite formation in Fe-C alloys during austenite decomposition under non-equilibrium interface conditions

The growth of ferrite during the decomposition of supersaturated austenite in binary Fe-C alloys is studied using a numerical model that allows non-equilibrium conditions at the moving α-γ-interface. A numerical method is presented to account for non-equilibrium interface conditions during diffusional growth. In this model, the driving force for interface migration is given by the difference of the chemical potentials of the austenitic and the ferritic Fe-lattice at the interface. Carbon rearrangement, resulting from the movement of the interface, affects the driving force for interface migration during the transformation, thus establishing a mixed mode of growth control. The mode of control is predicted to vary from essentially a diffusion controlled mode at very low undercooling towards essentially an interface controlled mode at deep undercoolings. The results from this model are in good agreement with several sets of experimental growth rates and the carbon concentration profiles of partially transformed Fe-C alloys.

Dilatometric analysis of phase transformations in hypo-eutectoid steels

Dilatometry is a useful technique to obtain experimental data concerning transformation kinetics in ferrous alloys. This technique is commonly used in cooling experiments to study the austenite decomposition of hypo-eutectoid steel grades. In the standard analysis of the dilatation signal there are two factors that are normally neglected. During the pro-eutectoid ferrite formation the austenite enriches in carbon, resulting in a non-linear temperature dependence of the specific austenitic volume. Furthermore, the specific volume of the formed ferrite is considerably different from that of the formed pearlite. In total not taking into account these two effects can lead to an error in the determined fraction ferrite of up to 25%. A method is presented that takes into account the two above-mentioned factors. In order to determine both the fraction ferrite and the fraction pearlite, in the analysis the temperature range of the transformation is divided into a ferrite-formation range and a pearlite-formation range. Two possible criteria for this division are discussed, and it is shown that the choice does not have an essential influence on the results.

Defect production and annihilation near equilibrium in amorphous Pd40Ni40P20 investigated from viscosity data

Abstract The viscosity of amorphous Pd40Ni40P20 has been measured at temperatures near the glass transition temperature. The results of these measurements were analyzed in terms of the free volume model to find equations for defect production and annihilation during structural relaxation. It was shown that it is not possible to find these equations from viscosity data alone. A description of the viscosity data together with the thermal effects observed in a DSC was, however, more successful in determining the defect relaxation equation. The viscosity data were used to accurately determine the parameters of the kinetic process and the equilibrium defect concentration. The DSC data were used to discriminate between different relaxation equations.

High-frequency permeability of soft-magnetic Fe–Hf–O films with high resistivity

A series of Fe–Hf–O films has been prepared by means of rf sputter deposition. The composition for optimal permeability was found to be Fe55Hf17O28. These films exhibit a combination of soft-magnetic properties (μ>103, Hc 1 mΩ cm) after rapid thermal annealing in flowing argon at 400 °C in a dc magnetic field. The structure of the films consists of α-Fe nanocrystallites, randomly dispersed in an amorphous matrix. The amorphous phase is essential in achieving the advantageous combination of properties. In contrast to conventional soft-magnetic films, the properties of the Fe55Hf17O28 films do not show a strong thickness dependence for thicknesses up to 10 μm. Therefore, the roll-off frequency of the initial relative permeability for 10 μm thick films is high, but not as high as expected from only the resistivity. It is shown that, besides electromagnetic induction, ferromagnetic resonance is also essential in explaining the observed frequency dependence of the ...

Structural relaxation in amorphous Pd40Ni40P20

Abstract Measurements are presented on the change of Youngs modulus during structural relaxation of amorphous Pd 40 Ni 40 P 20 . The data are analysed in terms of the mixed model, considering that structural relaxation involves two sub-processes: Chemical Short Range Ordering (CSRO) and Topological Short Range Ordering (TSRO). To describe the kinetics of CSRO and TSRO the Activation Energy Spectrum (AES) model and free volume model are used, respectively. Pd 40 Ni 40 P 20 is stable enough to bring it to the equilibrium state for both CSRO and TSRO. Therefore reversible relaxation can be shown by performing temperature cycling experiments. Since no theoretical description for the kinetics of TSRO near equilibrium is available, three possible equations are proposed, which were fitted to the Youngs modulus data. The obtained parameters were used to describe the glass transition as observed during a DSC experiment. On the basis of the experimental data a choice for one of the three proposed equations could be made. The kinetics of CSRO is rather well described by the AES model.

The calorimetric glass transition of amorphous Pd40 Ni40 P20

The calorimetric glass transition in amorphous Pd40Ni40P20 was measured in a series of Differential Scanning Calorimetry (DSC) experiments. Especially the exact shape and position of the glass-transition peak were examined after different pre-annealing treatments of the sample and at different heating rates. The experimental results are in good agreement with a kinetic model of the glass transition. In particular, we found strong support for the existence of an equilibrium state when amorphous Pd40 Ni40 P20 is heated above its glass transition, and derived that the shape and position of glass-transition peak of amorphous Pd40 Ni40P20 at a given heating rate depends on a single structure parameter, in this paper designated as the average amount of free volume. The kinetics of free-volume changes are fully in agreement with results from viscosity measurements and speed-of-sound measurements. In order to obtain a good agreement between the model calculations and the experimental DSC curves, heat effects from chemical ordering were included in the calculations and the thermal delay of the measurement device was modelled with a simple RC-model.