Isochronal Differential Scanning Calorimetry Research Articles

Phase separation in Nd60−xYxFe30Al10 melt-spun ribbons

The structure of melt-spun Nd60−xYxFe30Al10 (x=0, 10, and 30) ribbons has been investigated by x-ray diffraction, isochronal differential scanning calorimetry (DSC), isothermal DSC, and thermomagnetic measurements. Precipitation of nanocrystalline Nd embedded in an amorphous matrix is found upon quenching. The amorphous phase further separates into Fe-rich regions and Fe-depleted regions. The growth of Nd nanocrystals upon annealing was identified by the characteristic isothermal calorimetric signal. The Fe-depleted amorphous phase crystallizes at 580 K, while the Fe-rich amorphous phase crystallizes at about 740, 779, and 750 K for x=0, 10, and 30, respectively. Phase separation in the amorphous matrix is also revealed by the thermomagnetic measurements, which show ferromagnetic transitions at about 60 and 480 K.

Crystallization Behavior of Mechanically Alloyed Zr-Cu-Al-Ni Glass Composites Containing Second-Phase ZrC Particles

ABSTRACTWe report on mechanical alloying of elemental powders with nominal composition Zr55Cu30Al10Ni5 together with up to 30 vol.% of ZrC particles which results in the formation of a metallic glass matrix in which the carbide particles are homogeneously dispersed and, at the same time, reduced to nanometer size. The thermal stability is investigated by isochronal differential scanning calorimetry and is found to be strongly affected by increasing the ZrC volume fraction. Comparison with calorimetric data for single-phase mechanically alloyed Zr-Cu-Al-Ni-C glassy powders suggests that dissolved carbon, originating from non-negligible ZrC-dissolution processes during mechanical alloying, is responsible for the changes in the glass transition and the crystallization behavior. Isothermal calorimetric data are also presented and discussed in the framework of the Johnson-Mehl-Avrami model. The carbon atoms dissolved in the glassy phase are suggested to slow down the kinetics of overall crystallization. On the other hand, for large ZrC volume fractions nucleation of crystalline phases is promoted by the second-phase particles and changes in the Avrami exponent from n = 4 to n = 3 suggest that the nucleation rate approaches zero after the early stages of crystallization.

Effect of composition on the formation and thermal stability of Ni72(Mo, Co)8B20 metallic glass

The effect of the addition of Co and Mo on the formation and stability of a Ni-B amorphous alloy has been investigated. Metal ribbons were prepared by melt-spinning in vacuum and characterised by X-ray diffraction and isochronal differential scanning calorimetry. When no Mo is present, only crystalline material containing the phases Co3B and Ni is produced. As the content of Mo is increased, the distance between metal-metal nearest neighbours increases, the glass transition and the crystallisation temperatures increase, the enthalpy of crystallisation decreases, and there is a marked increase in the activation energy associated with crystallisation. The explanation for this pattern of glass formation and the increase in thermal stability induced by Mo has been explored by reference to similar systems and to a semi-empirical kinetic model of the thermal stability of amorphous alloys. This model predicts the dependence of crystallisation temperature on composition in this quaternary alloy from the enthalpies of hole formation of its constituent elements with good success.

Solute partitioning in partially crystallized Al-Ni-Ce(-Cu) metallic glasses

A new family of aluminum based metallic glasses show remarkably high tensile strength as well as good ductility. These metallic glasses are based on Al-TM-RE (TM = transition metals, RE = rare earths), containing 80--92 at.% Al. It has been reported that the tensile yield strength of these materials reaches as high as 800 MPa in the amorphous state, and can be increased to 1,500 MPa by partial crystallization. The partially crystallized metallic glass consists of nanoscale fcc particles embedded in an amorphous matrix. The particle dispersion can be controlled by varying the cooling rate, and the yield strength reaches a maximum with a particle volume fraction of approximately 20%. The average grain size can be controlled by substituting Cu and Ag for the transition element. Isochronal differential scanning calorimetry (DSC) has shown that crystallization from the fully amorphous state to the fully crystallized state proceeds in two stages. Lattice parameter measurements suggest that the nanocrystalline fcc particles are the [alpha]-Al phase, but this remains unconfirmed by other techniques. It has also been reported that the crystallization kinetics are very fast and only the growth stage is observed in isothermal DSC. This suggests that certain heterogeneities that may act as more » nucleation sites for crystallization may be present in the melt-spun alloys. Furthermore, it is not understood why nanocrystalline particles are formed by the primary crystallization process, whereas most crystallization from metallic glasses lead to gross dendritic particles. The goal of this study was to examine the solute partitioning during the primary crystallization process form the initial metallic glass state to the partially crystallized state and to understand the mechanism of the nanocrystalline particle formation. In the present paper, the authors report only atom probe analysis results of partially crystallized Al-10Ni-3Ce and Al-7Ni-3Ce-3Cu alloys. « less

The transformation behaviour of the rapidly solidified solid solution of yttrium in magnesium

The evolution of the microstructure of the solid solution of 1.7–8.3 at.% Y in magnesium made by rapid solidification has been analysed by differential scanning calorimetry (DSC), transmission electron microscopy and X-ray diffraction using the Guinier technique. 11 partial reactions were identified to contribute to three exothermic and to two endothermic thermal effects monitored by isochronal DSC analysis. The extended solid solution of 6 at.% Y in magnesium transforms at about 420 K by forming a base-centred orthorhombic β″ phase of lattice parameter a = 6.47 A ̌ , b = 22.42 A ̌ and c = 5.23 A ̊ . The activation energy for microstructural evolution in this temperature regime drops drastically at concentrations of more than 5.5 at.% Y. The formation of the flake-like β phase is triggered by β″ agglomerates at a cell boundary network consisting of the equilibrium β phase Mg 25Y 4 and is completed after 10 h at 563 K. The formation of the equilibrium phases according to isochronal DSC analysis occurs at temperatures of 600 K or higher including crystallization of β from partially amorphous Mg 91.7Y 8.3 at temperatures 30 K higher than for precipitation of β from the corresponding metastable crystalline phase.