Preparation And Properties Of Zr-Based Bulk Metallic Glass Composites

Zr-based bulk metallic glasses (BMGs) possess unique mechanical properties, which endow them more potential predominance on new promising high-performance structural materials. However, most of them macroscopically exhibit obvious brittleness on compression or tension tests at room temperature, which limit their development to a great extent. Accordingly more intensive effort are devoted to prepare BMG matrix composites, introducing the second phase into the matrix by ex-situ or in-situ process, in order to enhance the abilities of deformation. To date, the critical casting thickness of composites are usually small, and the size of samples performed the mechanical properties is 2 mm or 3 mm, what are unable to satisfy the demand of application of those materials. So how to design and prepare BMG matrix composite with improved mechanical properties and critical casting thickness compatibly is always a hot topic in the field of those materials study.Zr_50.5Al₉Ni_4.05Cu_36.45 BMG was chosen as the matrix alloy in this dissertation. Ex-situ SiC particle, in-situ spherical B2 CuZr phase and martensite CuZr phase was respectively introduced into the matrix to synthesize the composite by copper mould injection casting method. The structures of three composites were employed by x-ray diffractometer, transmission electron microscope and scanning electron microscope.The compressive properties of them were measured by mechanical tester. The effect of interfacial cohesion, chemical composition and cooling rate on the structure and compressive properties of composites are analysed. The present research mainly includes the following aspects. Ex-situ SiC particle reinforced Zr_50.5Al₉Ni_4.05Cu_36.45 BMG matrix composite was prepared. The effect of interfacial cohesion, volume fraction and particle size of SiC particle on compressive properties are discussed. The results show limited interfacial reaction is helpful for the plasticity of composite. When the size of SiC particle is 70μm, and volume fraction is 6 %, the interfacial production of ZrC layer is composed of particles, and the width of layer is about 100 nm. Such productions can effectively improve the interfacial cohesion, transfer the loading, accordingly hinder the localization of dense shear bands (SBs), and induce the multiplication of SBs, which contribute to improve the compressive fracture strength and plasticity of composite. When the size of SiC particle is 70μm, and volume fraction is 6 %, the fracture strength and plastic strain of composite with a diameter of 5 mm is 2002 MP and 2 % respectively. Contrarily, ZrC phase is almost lumpy, and the width of ZrC layer is about 600 nm when the volume fraction of SiC particle is increased to 10 %. Because more SiC particles induce the difficulties of mixing of process by arc melting. The excessive interfacial reaction weakens the bonding between SiC particle and the matrix so that the cracks propagate along the interface unobstructed and rapidly, and consequently the compressive properties show nearly brittle. Increasing the size of SiC particle can contribute to improve the wetting ability with the matrix, easily melt the mixture of particle and ingot of matrix, and limit the interfacial reaction, accordingly obviously improve the mechanical properties. So the fracture and plastic strain of composite with diameter of 5 mm is 2045 MP and 4.6 % respectively when the size of SiC particle is 150μm, and volume fraction is 10 %.In-situ spherical micro-level B2 CuZr phase reinforced Zr_50.5Al₉Ni_4.05Cu_36.45 BMG composites were prepared by alloy method, partially substituting Zr with Nb or Ta element. The substitutions decrease the glass forming ability of alloy, and accordingly induce B2 CuZr phase to precipitate from the matrix during solidification. The compressive properties of Zr_49.5Al₉Ni_4.05Cu_36.45Nb₁(Ta₁) composite obviously show enhanced plasticity and work-harden behavior. The yield strength, fracture strength, and plastic strain of composite with a diameter of 5 mm is 1500 MPa, 2000 MPa, and 8 % respectively. The enhanced compressive properties of composite contribute that ductile B2 CuZr phase can transform to martensite CuZr phase induced by stress, what conduce to improve the work hardening and plasticity. Another hand, the similar Young modulus and strong interfacial cohesion between B2 CuZr phase and the matrix ensure to transfer the loading reciprocally efficiently. So spherical particles of B2 CuZr phase can effectively hamper the localization of SBs by deflecting them at the interface, simultaneously induce the seeding and branching of SBs during deformation. In addition, the particles are distributed uniformly, and accommodate the strain more easily on the whole sample. So more numerous SBs form simultaneously, accordingly avoid the crack occur or spread at the region of segregation. The effects of cooling rate and chemical composition on structure and compressive properties of Zr_50.5-xAl₉Ni_4.05Cu_36.45Ta_x (x as 0.6, 1, 1.5, 3, 6) alloy are also discussed. The volume fraction of B2 CuZr phase for Zr_50.5-xAl₉Ni_4.05Cu_36.45Ta_x alloy with a diameter of 3 mm is less than the fraction for the alloy with a diameter of 5 mm respectively, because the cooling rate of Zr_50.5-xAl₉Ni_4.05Cu_36.45Ta_x alloy with a diameter of 3 mm is higher than which of alloy with a diameter of 5 mm, and consequently restrain the precipitation of B2 CuZr phase relatively strong. Carefully tailoring the content of Ta of Zr_50.5-xAl₉Ni_4.05Cu_36.45Ta_x alloy can promote the precipitation, and offset the decrement of B2 CuZr phase for the alloy with a diameter of 3 mm. Such as, when the content of Ta is increased from 1% to 1.5%, the volume fraction of B2 CuZr phase for Zr₄₉Al₉Ni_4.05Cu_36.45Ta_1.5 alloy with a diameter of 3 mm is similar as that of the fraction for Zr49Al9Ni4.05Cu36.45Ta1 alloy with a diameter of 5 mm, as well as the better compressive properties.In-situ new micro-level martensite CuZr phase reinforced Zr_50.5Cu_27.45Ni_13.05Al₉ BMG matrix composite was successfully synthesized by partially substituting Cu with Ni element based on Zr50.5Cu36.45Ni4.05Al9 BMG. The plastic strain and fracture strength of composite with a diameter of 3 mm or 5 mm is improved to 6 % and 2190 MPa respectively. The properties are attributed to spherical particles of martensite CuZr phase distributed uniformly in the matrix. The martensite CuZr phase can hold numerous lath-like (021) typed twins, and afford coherent twin boundaries with rather low free energy, especially several nano-level sized laths with considerable coherent twin boundaries accommodating dense dislocations. All those contribute to the compressive properties.