ABSTRACT
In this study, thin ribbons of amorphous Mg72Zn27Pt1 and Mg72Zn27Ag1 alloys with potential use in biomedicine were analyzed in terms of the crystallization mechanism. Non-isothermal annealing in differential scanning calorimetry (DSC) with five heating rates and X-ray diffraction (XRD) during heating were performed. Characteristic temperatures were determined, and the relative crystalline volume fraction was estimated. The activation energies were calculated using the Kissinger method and the Avrami exponent using the Jeziorny-Avrami model. The addition of platinum and silver shifts the onset of crystallization towards higher temperatures, but Pt has a greater impact. In each case, Eg > Ex > Ep (activation energy of the glass transition, the onset of crystallization, and the peak, respectively), which indicates a greater energy barrier during glass transition than crystallization. The highest activation energy was observed for Mg72Zn27Pt1 due to the difference in the size of the atoms of all alloy components. The crystallization in Mg72Zn27Ag1 occurs faster than in Mg72Zn27Pt1, and the alloy with Pt has higher (temporary) thermal stability. The Avrami exponent (n) values oscillate in the range of 1.7-2.6, which can be interpreted as one- and two-dimensional crystal growth with a constant/decreasing nucleation rate during the process. Moreover, the lower the heating rate, the higher the nucleation rate. The values of n for Mg72Zn27Pt1 indicate a greater number of nuclei and grains than for Mg72Zn27Ag1. The XRD tests indicate the presence of α-Mg and Mg12Zn13 for both Mg72Zn27Pt1 and Mg72Zn27Ag1, but the contribution of the Mg12Zn13 phase is greater for Mg72Zn27Ag1.
ABSTRACT
The influence of heat treatment parameters such as the annealing time and austempering temperature on the microstructure, tribological properties and corrosion resistance of ductile iron have been investigated. It has been revealed that the scratch depth of cast iron samples increases with the extension of the isothermal annealing time (from 30 to 120 min) and the austempering temperature (from 280 °C to 430 °C), while the hardness value decreases. A low value of the scratch depth and a high hardness at low values of the austempering temperature and short isothermal annealing time is related to the presence of martensite. Moreover, the presence of a martensite phase has a beneficial influence on the corrosion resistance of austempered ductile iron.
ABSTRACT
The aim of the study was to analyze the crystallization kinetics of the Mg72Zn28 metallic glass alloy. The crystallization kinetics of Mg72Zn28 metallic glass were investigated by differential scanning calorimetry and X-ray diffraction. The phases formed during the crystallization process were identified as α-Mg and complex Mg12Zn13 phases. Activation energies for the glass transition temperature, crystallization onset, and peak were calculated based on the Kissinger model. The activation energy calculated from the Kissinger model was Eg = 176.91, Ex = 124.26, Ep1 = 117.49, and Ep2 = 114.48 kJ mol-1, respectively.
ABSTRACT
The aim of this study was to analyze the crystallization of the Mg72Zn24Ca4 metallic glass alloy. The crystallization process of metallic glass Mg72Zn24Ca4 was investigated by means of the differential scanning calorimetry. The glass-forming ability and crystallization are both strongly dependent on the heating rate. The crystallization kinetics, during the isothermal annealing, were modelled by the Johnson-Mehl-Avrami equation. Avrami exponents were from 2.7 to 3.51, which indicates diffusion-controlled grain growth. Local exponents of the Johnson-Mehl-Avrami equation were also calculated. In addition, the Mg phase-being the isothermal crystallization product-was found, and the diagram of the time-temperature phase transformation was developed. This diagram enables the reading of the start and end times of the crystallization process, occurring in amorphous ribbons of the Mg72Zn24Ca4 alloy on the isothermal annealing temperature. The research showed high stability of the amorphous structure of Mg72Zn24Ca4 alloy at human body temperature.
ABSTRACT
The aim of this work was to monitor the corrosion rate of the Mg72Zn24Ca4 and Zn87Mg9Ca4 alloys. The purity of the alloying elements was 99.9%. The melt process was carried out in an induction furnace. The melting process took place under the cover of an inert gas (argon). The copper form was flooded by liquid alloy. Then, in order to obtain ribbons, the cast alloy, in rod shape, was re-melted on the melt spinning machine. The corrosion resistance of both alloys has been determined on the basis of the following experiments: measurements of the evolution of OCP (open circuit potential), LSV (linear sweep voltamperometry) and EIS (electrochemical impedance spectroscopy). All corrosion tests were carried out in Ringer's solution at 37 °C and pH 7.2. The corrosion tests have revealed that the zinc alloy, Zn87Mg9Ca4, exhibits significantly higher corrosion resistance in the Ringer solution compared to the magnesium alloy, Mg72Zn24Ca4. Moreover, it has been shown that the cathodic reaction proceeds faster on the surface of ribbons. EIS measurements show that the dissolution of Mg alloy proceeds with two steps: transfer of Mg2+ ions to the Ringer solution and then the formation of the corrosion products, which are deposited on the surface of magnesium alloy. It has been revealed, too, that for both bulk materials, diffusion of chloride ions through the corrosion product's layer takes place.
ABSTRACT
This paper presents tests of metallic glass based on Mg72Zn24Ca4 alloy. Metallic glass was made using induction melting and further injection on a rotating copper wheel. A differential scanning calorimeter (DSC) was used to investigate the phase transformation of an amorphous ribbon. The tests were carried out at an isothermal annealing temperature of 507 K. The Kolmogorov-Johnson-Mahl-Avrami-Evans model was used to analyze the crystallization kinetics of the amorphous Mg72Zn24Ca4 alloy. In this model, both Avrami's exponent n and transformation rate constant K were analyzed. Both of these kinetic parameters were examined as a function of time and the solid fraction. The Avrami exponent n value at the beginning of the crystallization process has value n = 1.9 and at the end of the crystallization process has value n = 3.6. The kinetic constant K values change in the opposite way as the exponent n. At the beginning of the crystallization process the constant K has value K = 9.19 × 10-7 s-n (ln(K) = -13.9) and at the end of the crystallization process has the value K = 6.19 × 10-9 s-n (ln(K) = -18.9). These parameters behave similarly, analyzing them as a function of the duration of the isothermal transformation. The exponent n increases and the constant K decreases with the duration of the crystallization process. With such a change of the Avrami exponent n and the transformation rate constant K, the crystallization process is controlled by the 3D growth on predetermined nuclei. Because each metallic glass has a place for heterogeneous nucleation, so called pre-existing nuclei, in which nucleation is strengthened and the energy barrier is lowered. These nuclei along with possible surface-induced crystallization, lead to rapid nucleation at the beginning of the process, and therefore a larger transformed fraction than expected for purely uniform nucleation. These sites are used and saturated with time, followed mainly by homogeneous nucleation. In addition, such a high value of the Avrami exponent n at the end of the crystallization process can cause the impingement effect, heterogeneous distribution of nuclei and the diffusion-controlled grain growth in the Mg72Zn24Ca4 metallic glassy alloy.
