Study On Microstructure, Mechanical Properties And Thermal Stability Of Zr_56.2Ti_13.8Nb_5.0Cu_6.9Ni_5.6Be_12.5 Bulk Metallic Glasses Composites

The Zr_56.2 Ti_13.8 Nb_5.0 Cu_6.9 Ni_5.6 Be_12.5 (LM2) bulk metallic glasses composites containing ductile 13 dendrite were prepared via an in situ method by rapidly water-cooling. The Microstructure, mechanical properties and thermal stability of the cast composites were investigated by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electrical microscopy (SEM), transmission electrical microscopy (TEM), dynamic mechanical analysis (DMA) and tested in uniaxial compression with INSTRON testing machine. The main works of the paper were following:1) The LM2 bulk metallic glasses composites were prepared via an in situ method by rapidly water-cooling. The microstructure, solidification rule and compressive properties of LM2 composites were investigated. The results show that the relationship between secondary dendrite arm spacing (λ) and cooling rate (ν) can be expressed by empirical formula:λ=αx(ν)^-n, a=29～461μm·Kⁿ·s^-n, n=0.46±0.04, there are a lot of twins in the dendrite for all the cast LM2 composites, moreover, Intermetallics Zr₂Cu were found in theΦ20,Φ25 casting samples and quasi-crystalline is existed in theΦ13-1 cast sample(raw materials with low purity). The yield strength decrease from 1645MPa (3mm sheet) to 1343MPa (Φ25) and the plasticityis between 5.3% and 16% with the dendrite size increasing, forΦ20 cast sample, the plasticity is. up to 16%. As a fragile phase, the Intermetallics Zr₂Cu have no intrinsic effect on the plasticity,, while the quasi-crystalline is greatly affecting the mechanical properties, the casting sample fracture abruptly, when loaded.2) The microstructure relaxation of LM2 composites was investigated by isothermal/ isochronous annealing. The results show that the twins disappear and theβ-Zr in the dendrite transform toα-Zr in the isothermal/ isochronous annealing process, meanwhile, an unknown phase appears in the amorphous matrix. The results of isochronous annealing between room temperature and 543K show that the twins in the dendrite gradually disappear and the unknown phase yields in the amorphous matrix, the yield strength and fracture strength of LM2 composites are improved, the plasticity of LM2 composites become badly with temperature increasing; when the temperature is between 583K and 653K, the LM2 composites have no plasticity and fracture abruptly, the fracture strength is gradually decreasing with isothermal annealing temperature increasing. After isothermal annealing at 583K (sub-T_g), the LM2 composites with different times annealing have no plasticity, the fracture strength gradually improved with annealing times increasing. The results of all the works show that the residual stress can be one of mainly reasons improving the plasticity, moreover, the ductileβdendrite can improve the plasticity through dislocation slip or/and twinning and phase transformation by itself; on the other hand, the ductileβdendrite can hinder and restrict the single shear band to yield and extend, the nucleated shear band propagate and interact with other dendrite arms and shear bands.3) The visco-elasticity of LM2 composites were investigated by dynamic thermal mechanical analysis (DMA), the thermal stability of composites is given by DMA The research shows that the storage modulus(E')and loss modulus(E") of LM2 composites are different form the monolithic bulk metallic glasses, there are two plateau in the storage modulus and two coupled peaks in the loss modulus, which are called a-relaxation andβ-transformation, the a-relaxation is caused by the amorphous microstructure changing, while the 13-transformation is caused by the phase transformation ofβ-Zr. The a-relaxation is fit with the VFT equation (f=f₀exp (B/(T-T₀))with parameters f₀=2.3E19Hz, B=15771K, T₀=332K, the fragile index of glasses-forming liquid is 56, as a strong liquid as Vitl, Vit4 bulk metallic glasses; theβ-transformation is endothermic process and fit with Arrhenius equation(f= f₀exp(E/ R T)), f₀=5.6×10^-61Hz, EA=8.1Ev. The isothermal spectrum of relaxation frequencies shows that the frequency of peak shift to high frequency with the isothermal temperature increasing, the single theoretical /empirical model can't describe the super-positioned process of a-relaxation andβ-transformation. Bio-KWW modelφ(t)=K exp(-t /Ï„₁)^β₁ + (1 - K) exp(-t /Ï„₂)^β₂ is used to simulate the experimental data, can separate theα-relaxation andβ-transformation and basically fit with the experimental data at 653K,663K. This suggests that theα-relaxation andβ-transformation can affect each other and are two approaching processes at 653K, 663K.