Formability, Mechanical And Corrosion Behaviors Of Mg-(Cu,Ni)-(Gd,Nd) Bulk Metallic Glasses

Mg alloys have great potentials in the application for the light weight structural materials. Compared to the conventional cast Mg alloys, Mg-based bulk metallic glasses (BMGs) show better strength and specific strength, which received great interest. For industrial application, glass-forming ability (GFA), mechanical properties and corrosion resistance are three important aspects concerned with the BMGs. Based on the ternary Mg-Cu-Gd alloy system with best glass-forming ability, the Mg-Cu-Ni-Gd BMGs, Mg-Ni-Gd-Nd BMGs and Mg-Ni-Gd-Nd in situ bulk metallic glass composite (BMGC) were developed by adding Ni and/or Nd elements. Copper mold casting method was used to evaluate the glass-forming ability for these alloys and fabricate the bulk metallic glass composite. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, differential scanning calorimetry and differential thermal analysis were employed to characterize the structures and glass nature. Compression test was used to investigate the mechanical properties. Hydrogen evolution , weight loss and electrochemistry tests were used to evaluate the corrosion resistance. The following conclusions are drawn.Using 3D approach, a composition map of Mg-Cu-Ni-Gd alloy system for bulk metallic glass formation was revealed, suggesting a decreasing GFA with increasing the Ni addition into the Mg-Cu-Gd alloys. However, the Mg-Cu-Ni-Gd BMGs with low Ni content exhibited good glass-forming ability with the maximum diameter of glassy rod reaching over 8 mm, fracture strength over 900 MPa and a certain plasticity. Furthermore, based on the Mg-Ni-Gd alloy, a new glass-forming Mg75Ni15Gd10-xNdx (x=0-10) alloy system, which can be casted into glassy rods with the maximum diameter of 5 mm, was developed with the Nd addition. The Mg75Ni15Gd10-xNdx (x=0-10) BMGs exhibited fracture strength of about 870 MPa, plastic strain over 1 % and specific strength up to 2.75×105 Nm/kg.Under compression, the Mg-based BMGs can fail at three different deformation stages. The first stage is within the elastic regime, where the BMGs failed in a cleavage mode without plasticity. The second stage is in the range where the BMGs failed in the cleavage mode combining with the shear mode with a certain plastic deformation. The third stage means the serrated plastic deformation. Within this stage, the BMGs failed in a pure shear mode after the obvious plastic serration. The Mg-based BMGs are sensitive to the cleavage cracks, which is the deformation characteristic of the Mg-based BMGs. Lowing the probability of the nucleation and propagation of the cleavage cracks will favor the transition from the cleavage fracture to the shear fracture so as to reach the third deformation stage, which results in higher plasticity for the Mg-based BMGs.For the Mg-based BMGs, the size-dependent strength and plasticity are correlated with the fracture mode. For the BMG failed in the cleavage fracture mode, such as Mg65Cu25Gd10 BMG, its strength increases with decreasing the sample diameter and however its plastic strain is zero; for the BMG failed in the shear fracture mode, such as Mg75Ni15Gd5Nd5 BMG, its plastic strain increases with decreasing the sample diameter while its strength keeps a relatively constant value. To improve the plasticity of the Mg-based BMG, there are two steps. The first step is to ensure its deformation stage to reach the third stage for performing pure shear plastic flow and the second step is to promote this plastic flow in the third stage.Based on a ternary Mg75Ni15Gd10 metallic glass-former, a new Mg80Ni12Gd4Nd4 bulk metallic glass composite (BMGC) was developed by tailoring the compositions. This BMGC displayed compressive ultimate strength over 900MPa with a total strain to failure of 4.3% and specific strength up to 3.12×105 Nm/kg. The improved mechanical properties were attributed to a"dual phases"structure consisting of ductile Mg solid solution flakes and glassy matrix in the Mg80Ni12Gd4Nd4 BMGC. The homogeneously dispersed Mg flakes reinforcement in the BMGC were characterized as a long-period ordered structure (LPOS) with periodic arrays of six close-packed planes distorted from the ideal hexagonal lattice of 6H-type. This is the first time to report a LPOS in the Mg-Ni-Gd-Nd alloy system. The LPOS-Mg in the composite acted as a soft media to initiate or interact with the multiple shear bands, which resulted in better mechanical properties for the Mg80Ni12Gd4Nd4 BMGC.In the NaCl solution, the Mg-Cu-Ni-Gd BMG with the coexistence of Cu and Ni has better corrosion resistance than Mg-Cu-Gd or Mg-Ni-Gd BMGs with the single transition metal of Cu or Ni, and the Mg-Ni-Gd-Nd BMG has better corrosion resistance than the Mg-Cu-Gd BMG. The corrosion surface film of the Mg-(Cu,Ni)-Gd BMGs mainly consists of Mg(OH)2. The density of the Mg(OH)2 film can be influenced by the size and orientation of the flake-like Mg(OH)2, which is determined by the constituent alloy elments and their contents. Compared to the Mg-Cu-Gd or Mg-Ni-Gd BMGs, the better corrosion resistance of the Mg-Cu-Ni-Gd BMG can be attributed to the more stable and denser Mg(OH)2 film formed on the surface of the Mg-Cu-Ni-Gd BMG .The Mg-Cu-Ni-Gd BMGs, Mg-Ni-Gd-Nd BMGs and Mg-Ni-Gd-Nd BMGC, which exhibit good comprehensive properties as mentioned above, have promising potentials for the application as lightweight engineering materials. The alloy design method in this study can provide a guideline for the development of the Mg-based BMGs and BMGCs.