Effect Of Compression Deformation On Structure And Properties Of Zr-based Bulk Metallic Glasses

Bulk metallic glasses（BMGs） have attracted increasing attention in recent years due to their unique properties such as high strength, large elastic limit, excellent soft magnetic properties, and good corrosion resistance. However, the deformation of metallic glassesat room temperature usually is highly localized in shear bands and propagates rapidly due to work softening, resulting in catastrophic failure along a single dominating shear band and a limited plastic strain for metallic glasses. Therefore, their application as structural materials is limited severely by their poor room-temperature ductility. Shear bands possess enhanced free volume or reduced atomic density relative to undeformed metallic glass matrix. With the introduction of shear bands,enhanced free volume will decrease the strength or hardness of metallic glasses, and the interaction of shear bands will increase the strength or hardness of metallic glasses. However, how both free volume and interaction of shear bands affect the hardness or strength of metallic glasses is still unclear. Hence, investigation on the evolution of shear bands and free volume is essential for understanding deformation of deformed metallic glasses and exploring the relationship between structure and mechanical properties for deformed metallic glasses. Meanwhile, it will provide insight for overcoming the brittleness of metallic glasses. In addition, decreasing shear band spacing is a promising approach to overcome the brittleness of metallic glasses. Hence, it is of significance to introduce a high density of uniformly distributed shear bands into metallic glasses. Finally, good overall properties of metallic glasses are of significance for broadening their potential applications. Minor addition is an effective way to explore new metallic system with outstanding properties in conjunction with good glass forming ability. Hence, it is essential to investigate the effect of minor addition on the structure, glass forming ability, and properties of metallic glass. Based on these problems, this dissertation has carried out the following three parts:（1） The evolution of shear bands and free volume in Zr_64.13Cu_15.75Ni_10.12Al₁₀ BMG during compressive deformation were studied. The effect of free volume and shear band interaction on the hardness of deformed Zr_64.13Cu_15.75Ni_10.12Al₁₀ BMG was analyzed. The structure of the Zr_64.13Cu_15.75Ni_10.12Al₁₀ BMG was characterized by X-ray diffraction（XRD） and transmission electron microscopy（TEM）. The evolution of shear bands including the density of shear bands,average spacing between shear bands, distribution of shear band spacings, and intersectionsbetween shear bands in a large plastic strain range was investigated by scanning electron microscopy（SEM）. The variation of absolute free volume content in deformed Zr_64.13Cu_15.75Ni_10.12Al₁₀ BMG was determined quantitatively by using differential scanning calorimetry（DSC）. The hardness of the deformed Zr_64.13Cu_15.75Ni_10.12Al₁₀ BMG exhibits a tendency of initial decrease and subsequent increase with plastic strain. This variation tendency of hardness was studied by focusing on the evolution of shear bands and free volume for the deformed BMG samples with different plastic strains. As the plastic strain increases from 6% to83%, the density of shear bands increases from 0.04 to 0.92 μm-1, the average spacing between shear bands decreases from 25.4 to 1.1 μm. Meanwhile, the distribution of shear band spacings gradually narrows, and the number of intersections between shear bands increases. When the plastic strain is larger than 47%, a lot of fine shear bands form, the number of intersections increases faster with plastic strain. Free volume content in the deformed Zr_64.13Cu_15.75Ni_10.12Al₁₀ BMG at room temperature increases continuously with plastic strain increasing from 6% to 83%.Both free volume induced softening and shear band interaction induced hardening may affect the hardness of the deformed Zr_64.13Cu_15.75Ni_10.12Al₁₀ BMG. In the plastic strain range of 0% to 47%,free volume induced softening mainly contribute to the initial decrease in hardness with plastic strain. In the plastic strain range of 47% to 83%, shear band interaction induced hardening may result in the increase in hardness with plastic strain, after counteracting free volume induced softening. Repeated compression cycle tests confirmed that enhanced shear band interaction can result in hardening behavior in Zr_64.13Cu_15.75Ni_10.12Al₁₀ BMG.（2） The inhomogeneous plastic deformation of the plastic Zr_64.13Cu_15.75Ni_10.12Al₁₀ BMG with arc-shaped edges was performed by compressive deformation. Dense shear bands with an average spacing 520 nm are observed, and the minimal shear band spacing of about 110 nm is achieved. In addition, dense shear bands were also introduced into brittle Cu₆₀Zr₃₀Ti₁₀ BMG.（3） We studied the effect of Ti addition on the structure, glass forming ability, crystallization behaviors, and properties of Zr_64.13Cu_15.75Ni_10.12Al₁₀ BMGs.(Zr_64.13Cu_15.75Ni_10.12Al₁₀)100-x Tix（x=0-7）BMGs were prepared by water-cooled copper mold casting method. XRD results indicate that the structure of these alloys is amorphous. The reduced glass transition temperature increases with Ti addition, indicating that the glass forming ability increases with Ti addition. Compared with that of one crystallization step for the sample without Ti addition, the Ti（x≥2） containing alloys exhibit a two-step crystallization behavior. Icosahedral quasicrystal phase was found to precipitate in the amorphous matrix in the first crystallization step, implying that strong icosahedral short-range order may exist in the Ti（x≥2） containing alloys. Moreover, the(Zr_64.13Cu_15.75Ni_10.12Al₁₀)_100-xTi_x BMGs exhibit an excellent ductility, i.e., a plastic engineering strain larger than 80% for x≤6. Tiaddition can also enhance the corrosion resistance of the Zr-Cu-Ni-Al BMG. Hence, the Zr-Cu-Ni-Al-Ti BMGs possess large room-temperature ductility in a large composition range. In addition, glass forming ability and corrosion resistance are enhanced by Ti addition.