The Crystallization Behavior And Mechanical Properties Of Zr₄₅Cu₄₅Ag₁₀ And Zr-Al-Co-Cu Bulk Metallic Glasses

Bulk metallic glasses (BMGs) exhibit high strength, high elasticity and high wear and corrosion resistances. The room temperature plastic deformation of BMGs is very limited due to the occurrence of shear localization, i.e., the majority plastic strain is concentrated within a single or a few shear bands. The deformation of these materials are characterized by work softening. The application of BMGs has been greatly limited by the poor room temperature plasticity. The fabrication of BMGs and their composites associated with low material cost, high glass-forming ability (GFA), and large room temperature plasticity is vital for their practical application.The present study is focused on a special class of Zr-based BMGs which are composed of one or two constituents exhibiting positive enthalpies of mixing with Cu. The crystallization behaviors of several BMGs were first investigated by means of X-ray diffraction (XRD), Differential scanning calorimetry (DSC), Differential thermal analysis (DTA), and Transmission electron microscopy (TEM). And by manipulating the alloy compositions, it is expected that BMG composites containing nanocrystals or dendrite phases can be reached in the large dimension scale samples of Zr-Al-Cu-Fe-Nb system. The room temperature mechanical properties of several BMG alloys were also studied using 3 mm diameter samples.The significant experimental results are summarized as follows:(1) The crystallization kinetics of a Zr45Cu45Ag10 BMG were examined in the courses of isochronal and isothermal anneals. An Avrami exponent 3.4-4.1 has been revealed in the isothermal crystallization process of this BMG, and the enthalpy of crystallization and the apparent activation energy for crystallization were determined to be ΔHave=5.45 kJ/mol and Ec= 283±20 kJ/mol, respectively. The isochronal crystallization process, however, delivered a similar apparent activation energy for crystallization Ep=263±5 kJ/mol, which reveal the dynamic nature of glass decomposition.(2)TEM study revealed that the Zr45Cu45Ag10 glass crystallized into two different unknown phases with large unit cells when annealing at a rate of 40K/min to a temperature 600 ℃, namely, a simple orthogonal phase (a=0.64 nm,b=1.03 nm, c=1.24nm) and a body centered tetragonal phase (a=1.15 nm, c=1.1 nm). The Zr55.8Al19.4Co17.4Cu7.4 glass crystallized into two different phases with large unit cells when annealing at 600℃, namely, a known Al2CoZr6 type phase (a=0.79 nm, c=0.6849 nm) and a base centered orthogonal phase (a=1.32 nm,b=0.81 nm, c=0.69 nm).(3)The Zr55.8Al19.4Co17.4Cu7.4 alloys has been found to exhibit the largest GFA in the Zr-Al-Co-Cu system (Dc=12 mm, Tg=744 K, Tx=S09 K,Trg=0.578). Room temperature mechanical tests showed that the alloy have a fracture strength of 1850 MPa. Especially, the 3 mm diameter BMG specimen exhibits a large plastic strain exceeding 3.0%.(4) The newly developed 3mm diameter Zr65Al10Fe5Cu15Nbs BMG-dendrite phase composite exhibits a fracture strength of 1700 MPa and a large plastic strain of～2.0%.