金属玻璃晶化过程的相形成研究

On Phase Formation in Crystallization Process of Metallic Glass

本文采用差示扫描量热仪(DSC),X射线衍射仪(XRD)及透射电镜(TEM)系统地研究了金属玻璃Ni_(68)B_(21)Si_(11)分别在等温退火和连续加热过程中的晶化动力学与相形成顺序。Ni_(68)B_(21)Si_(11)非晶合金具有较高的热稳定性,其DSC曲线因升温速率不同而呈现两个或三个晶化放热峰。首次发现该金属玻璃在向稳定晶体转变之前,形成一种f.c.C.结构的亚稳相。

The crystallization of metallic glasses is not just asimple transformation from the amorphous state to the final crystallization state. Instead it can be a rather complicated phenomenon taking place in several steps. However current crystallization analyses of metallic glasses are generally limited to cither crystallization kinetics or phase identification[3]. In the present paper, three aspects of crystallization of metallic glass are all considered. Crystallization thermodynamics and kinetics, phase sequence, and morphologies of amorphous Ni_(68)B_(21)Si_(11) alloy have been respectively and systematically investigated by DSC (differential scanning calorimetry), XRD (X-ray diffraction) and TEM (transmission electron microscopy). The five contributions of the paper will be stated and then concisely explained. (1) Correlation between the exothermal peaks and phase formation has been obtained for the first time.This correlation is obtained through examining the crystallization process of amorphous Ni_(68)B_(21)Si_(11) alloy during isothermal annealing and nonisothermal heating. The examination is done by combining DSC and XRD with TEM. (2) The thermal stability of Ni_(68)B_(21)Si_(11) amorphous alloy measured by DSC is found to be much higher than that of Ni_(78)B_(14)Si_8 [3] and Ni_(75)B_(17)Si_8 [4] alloys. (3) It is found that when heating rate is lowered sufficiently, the number of exothermal peaks on DSC curve of metallic glass Ni_(68)B_(21)Si_(11) changes from 2 to 3. Up to now, the numbers of exothermal peaks on DSC curves of the other metallic glasses have already been found to remain unchanged when heating rate varies. (4) For isothermal annealing, the authors find a new stage in the transformation sequence of amorphous Ni_(68)B_(21)Si_(11) : the appearance of a metastable phase with a f.c.c. structure, α= 5.15A within the amorphous matrix. In this new first stage, spherical metastable crystalline 0.1—0.3μm in diameter is formed after annealing 30min at 753K. (5) The polygon nanocrystalline of 30nm in grain size is obtained through so controlling crystallization as to reach a fully stable microstructure.