Structural Changes Of Bulk Metallic Glasses Induced By Heating, Ball Milling And High-current Pulsed Electron Beams

The applications of bulk metallic glasses (BMGs) as structural materials are currently limited due to the absence of macroscopic plastic deformation. Recent researches have shown the enhancement of plasticity can benefit from the partial crystallization or the inhomogeneous surface microstructure by surface treatment. To this end, non-equilibrium processing such as shot penning, electron beam irradiation, electropulsing, and mechanical milling have been used to introduce nanostructures into the amorphous matrix. For the same purpose, the present work investigated the structural changes of Zr-Al-Ni-Cu BMGs induced by thermal annealing, ball milling and high-current pulsed electron beam (HCPEB) treatments and, particularly, charactered the nanostructures and nanocrystallization during such treatments. It is noticeable that it is the first beneficial finding of the HCPEB effects on the surface of BMGs.Two stable Zr-Al-Ni-Cu BMGs in this family, Zr65Al7.5Ni10Cu17.5 and Zr58Al16Ni11Cu15, were crystallized by annealing, ball milling and pulsed electron beam, respectively. The initial and treated samples were characterized by combination with transmission electron microscopy (TEM), scanning electron microscope (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD) and differential scanning calorimetry (DSC), in a complementary manner, to analyze accurately the microstructure evolution encountered in the materials.The thermal analyses reveal that both Zr-Al-Ni-Cu BMGs have large glass-forming abilities together with large supercooled liquid regions. The crystallization kinetics shows that Zr58Al16Ni11Cu15 BMGs exhibit the higher thermal stability than Zr65Al7.5Ni10Cu17.5 BMGs, due to the higher activation energy. In addition, both Zr-Al-Ni-Cu BMGs show a multi-step crystallization reaction. The tetragonal Zr2Cu and hexagonal Zr6Al2Ni phases are the crystallization products of Zr65Al7.5Ni10Cu17.5 BMGs. Zr58Al16Ni11Cu15 BMGs undergo a primary crystallization reaction with a cubic primary phase formation. This phase is transformed into the big cubic Zr2Ni, hexagonal Zr2Al,α-Zr, and to an unknown phase.In the case of ball milling, an interesting feature we have observed is that the amorphous structures of Zr-Al-Ni-Cu BMGs are not stable under ball milling at room temperature. Ball milling can lead to precipitation of a nano-sized FCC ZrMxNy (M=Al,Ni,Cu) phase from the metallic glass matrix at room temperature. In the milled Zr65Al7.5Ni10Cu17.5 alloys, the abnormal precipitates of nano Cu particles were also observed. The mechanical crystallization of Zr-Al-Ni-Cu BMGs is attributed to the competition between the mechanical-deformation effects and the stability of amorphous structures together with the influence of the contamination of nitrogen.For HCPEB irradiation, the induced special phenomena are related to the pulse numbers. We found that the low-energy bombards induces the segregation of elements via formation of crater structures. And crater density is strongly dependent on the pulse numbers. Also, a multi layer structure is created on the top surface due to the electron irradiation. After repeated pulsing treatments, nano-sized FCC Cu solid solution, FCC Zr2(Cu,Ni) phases as well as nano twins are formed in the irradiated area. The formation of such special nanostructures after HCPEB treatment is related to the induced dynamic thermal-stress field, electron irradiation, and contamination effects.The nanocrystallization behavior of ball mill and HCPEB is significantly different from the observations of thermal annealing. The significance of this research is the advancement of the fundamental understanding of mechanical crystallization and the electron irradiation-induced crystallization. The finding of this research may be used for the design of BMG-related composite structures that are potential for the structural applications.