Lelito Janusz
Faculty of Foundry Engineering, AGH University of Science and Technology, Al. A. Mickiewicza 30, 30-059 Kraków, Poland.
Materials (Basel). 2020 Jun 23;13(12):2815. doi: 10.3390/ma13122815.
This paper presents tests of metallic glass based on MgZnCa 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 MgZnCa alloy. In this model, both Avrami's exponent 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 value at the beginning of the crystallization process has value = 1.9 and at the end of the crystallization process has value = 3.6. The kinetic constant values change in the opposite way as the exponent . At the beginning of the crystallization process the constant has value = 9.19 × 10 s (ln() = -13.9) and at the end of the crystallization process has the value = 6.19 × 10 s (ln() = -18.9). These parameters behave similarly, analyzing them as a function of the duration of the isothermal transformation. The exponent increases and the constant decreases with the duration of the crystallization process. With such a change of the Avrami exponent and the transformation rate constant , 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 at the end of the crystallization process can cause the impingement effect, heterogeneous distribution of nuclei and the diffusion-controlled grain growth in the MgZnCa metallic glassy alloy.
本文介绍了基于MgZnCa合金的金属玻璃的测试。金属玻璃通过感应熔炼制成，并进一步注射到旋转的铜轮上。使用差示扫描量热仪（DSC）研究非晶带的相变。测试在507 K的等温退火温度下进行。采用Kolmogorov-Johnson-Mahl-Avrami-Evans模型分析非晶MgZnCa合金的结晶动力学。在该模型中，分析了阿弗拉米指数和转变速率常数K。这两个动力学参数均作为时间和固相分数的函数进行研究。结晶过程开始时阿弗拉米指数的值为 = 1.9，结晶过程结束时的值为 = 3.6。动力学常数的值与指数的变化方式相反。结晶过程开始时常数的值为 = 9.19 × 10 s（ln() = -13.9），结晶过程结束时的值为 = 6.19 × 10 s（ln() = -18.9）。将这些参数作为等温转变持续时间的函数进行分析时，它们的行为相似。随着结晶过程的持续，指数增加而常数减小。随着阿弗拉米指数和转变速率常数的这种变化，结晶过程由预定晶核上的三维生长控制。因为每种金属玻璃都有非均匀形核的位置，即所谓的预先存在的晶核，在其中形核得到加强且能量势垒降低。这些晶核连同可能的表面诱导结晶，导致在过程开始时快速形核，因此转变分数比纯均匀形核预期的更大。这些位置随时间被利用并饱和，随后主要是均匀形核。此外，结晶过程结束时如此高的阿弗拉米指数值可能导致碰撞效应、晶核的非均匀分布以及MgZnCa金属玻璃合金中扩散控制的晶粒生长。
