The Study Of Mg-Based Bulk Metallic Glasses And Their Composites

Mg-based bulk metallic glasses (BMGs) have potential application in precision instruments, micro-forming materials and 3C products due to their high special strength, good corrosion resistance and low cost. However, it is impossible to use such kind of materials because most of Mg-based BMGs exhibit no macro-plastic deformation under compression and the fracture toughness is comparable to the ideal brittle materials, such as SiO₂. Therefore, how to improve the ductility of Mg-based BMGs has become a key issue in this area.By considering mechanical properties and the effect on glass-forming ability (GFA) of choosing elements, glass-forming alloys were designed based on Mg-Cu-Nd, Mg-Ni-Cu-RE (RE=Y, Nd), Mg-Zn-Ca and Mg-Ni-Zn-Y, respectively. As-cast samples with different diameters were fabricated by copper mould casting method. The microstructure, thermal properties related to amorphicity, melting behavior and mechanical properties of as-cast samples were examined by scanning electron microscopy (SEM), X-ray diffraction (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and compression-tension tester, respectively. The main experimental results are as following:The glass-forming region with BMG diameter up to 2 mm was located in Mg-Cu-Nd ternary phase diagram. The addition of Ag element can improve both the GFA and mechanical properties of Mg-Cu-Nd alloys. Furthermore, we found that the GFA reflected by critical cast diameter and plastic strain for Mg₅₇Cu_30.5Nd₁₀Ag_2.5 alloy are as high as 5 mm and 1.5%, respectively.For Mg-Cu-Ni-Nd alloys, at least 5 mm in diameter for glass formation was obtained for the Mg_62.6Cu_10.5Ni₁₄Nd_12.9 alloy. While the Mg₆₀Ni₁₀Cu₁₆Nd₁₄ alloy exhibits not only good GFA, but also excellent plastic strain up to 8.5%, which is the largest strain found up to now according to the literature. Such large plastic strain was attributed the two different compositional amorphous phase existed in matrix, as testified by TEM, which was able to change the shear process and such result in the plastic deformation of the related BMG. The GFA and the mechanical properties of Mg-Cu-Ni-Nd-Y alloys were investigated. The glass formation for Mg_58.5Cu_27.5Ni₃Nd₅Y₆ alloy is as high as 13 mm in diameter. Furthermore, we suggest that the reliability in fracture stress can be evaluated by the relative difference between calculated stress and experimental stress. The smallest value was found for Mg_58.5Cu_27.5Ni₃Nd₅Y₆ BMG, which demonstrates that the fracture stress of this alloy can be reproduced.The GFA for Mg-Zn-Ca alloys was investigated and the best one with composition of Mg₆₈Zn₂₈Ca₄ was located. The fracture stress and plastic strain for Mg₆₈Zn₂₈Ca₄ BMG are 828MPa and 1.28%, respectively. The glass formers and amorphous matrix composites with relative better mechanical properties can be obtained by adjusting Mg and Zn content when Ca content is between 3⁵at.% for Mg-Zn-Ca alloys.The long period ordered (LPO) structure can be formed in the amorphous matrix when Mg, Ni, Zn and Y are in the range of 73-81at%, 8-12% at%, 3-5 at% and 6-10% at%, respectively, which can improve the ductility of Mg-based BMGs. The critical Y/Zn ratio for LPO formation is 0.57 in the composition of alloys, whereas the value of Y/Zn ratio in the LPO is general larger than 1. The fracture stress and plastic strain for Mg₈₁Ni₈Zn₅Y₆ are 550MPa and 21%, respectively. The plastic strain is the largest one found up to now among Mg-based amorphous matrix composites.