Fabrication Of Zr-and Cu-based Metallic Glassy Composites By SPS And The Study Of Strengthening And Toughening Mechanism

Bulk metallic glasses （BMGs） exhibit particular advantages, such as high strength,high hardness, high wear-resistance, superior corrosion resistance and magneticproperties. However, they are unsuitable for structural applications, because most ofBMGs display very limited dimension and plasticity at room. Therefore, how toimprove the plasticity of BMGs with high strength is a key issue for the study ofBMGs.The plasticity of BMGs can be improved by introducing second phases into themetallic glassy matrix. For the methods of fabricating bulk metallic glassy composites（BMGCs）, casting techniques were adopted by most of the researchers, which limits thedimension and affects the glassy formation ability （GFA） of BMGCs. Therefore, thedevelopment of appropriate techniques to fabricate large-size BMGCs is necessary.Spark Plasma Sintering （SPS） is regarded as an appropriate method of fabricatingBMGCs, because BMGCs can be obtained by SPS at low temperature and shortsintering time. On the other hand, although a number of works focus on the research ofBMGCs, the deformation mechanism of BMGs reinforced by the second phases isinsufficient. Especially, the study on the effect of the properties of reinforcement andglassy matrix on the mechanical properties of BMGCs is limited.In this study, TiNb/Zr₅₅Cu₃₀Al₁₀Ni₅, ZrO₂/Zr₅₅Cu₃₀Al₁₀Ni₅andTiNb/Cu₄₆Zr₄₂Al₇Y₅bulk metallic composites were fabricated by SPS. The structuresand thermal properties of the BMGCs were studied. Meanwhile, the interface betweenthe reinforcement and the glassy matrix was observed. Finite element analysis wasadopted to study the stress field and strain field of the reinforcement, the glassy matrixand the interfaces between them during compression deformation. The effects of volume fraction, particle size and properties of reinforcements as well as the fracturetoughness of the glassy matrix on the mechanical properties of BMGCs were studiedsystematically. The strengthening and toughening mechanism of BMGCs withreinforcements were revealed. The results are summarized as follows.The influence of sintering temperature on the densities and strength of BMGswere studied. The results indicated that the density and strength of BMGs arenon-linear related to the sintering temperature. There is the optimum sinteringtemperature. When the sintering temperature is below this temperature, the strength anddensity increase with sintering temperature. However, when the sintering temperature isover this temperature, the strength and density of BMGs decrease with sinteringtemperature, because much more defects and crystalline phases are generated.Cu₄₆Zr₄₂Al₇Y₅metallic glass with99.85%relative density was obtained by SPS with adiameter of15mm, which was larger than the largest size of10mm for the as-castspecimen. The fracture strength of the sintered specimen reached1803MPa. Andlarge-size Zr₅₅Cu₃₀Al₁₀Ni₅bulk metallic glass with fracture strength1677MPa wasobtained at623K.Based on the process of fabricating Cu₄₆Zr₄₂Al₇Y₅and Zr₅₅Cu₃₀Al₁₀Ni₅specimens,TiNb/Zr₅₅Cu₃₀Al₁₀Ni₅, ZrO₂/Zr₅₅Cu₃₀Al₁₀Ni₅and TiNb/Cu₄₆Zr₄₂Al₇Y₅bulk metallicglassy composites were fabricated. And the densities, structures, thermal properties andthe interfaces of the composites were studied. The results indicated that the BMGCswith nearly full density can be obtained by SPS. Meanwhile, the amorphous structureand thermal stability of the glassy matrix were not influenced by the reinforcements. Agood bonding interface between the reinforcement and the glassy matrix was observedby TEM and nano-indentation.The relationships of the mechanical properties of BMGCs to volume fraction andparticle sizes were studied. For the BMGCs reinforced by TiNb, the plasticity strainincrease with TiNb volume fraction.11%plasticity strain was achieved for30vol.%TiNb/Zr₅₅Cu₃₀Al₁₀Ni₅during compression. However, for the BMGCs reinforced byZrO₂, the largest plasticity strain was achieved with10-15vol.%ZrO₂particles. Both ofBMGCs with TiNb and ZrO₂, larger plasticity was achieved with smaller particle size.Finite element analysis was adopted to study the stress fields and stress fields ofthe reinforcement, glassy matrix and the interface between them. The results showed that the stress concentration is introduced due to the strain misfit betweenreinforcement and the glassy matrix during compression, which results in the initiationof shear bands in the glassy matrix. Based on the point mentioned above, that largevolume fraction reinforcement and smaller particle size mean more stress concentrationbeing introduced. Therefore, larger plasticity strain was obtained in BMGCs with largervolume fraction or smaller particle size.The mechanical properties of BMGCs with ductile metal （TiNb） and hard ceramicphase （ZrO₂） were compared. In order to explain the influence of properties ofreinforcements on the mechanical properties of BMGCs, the computational simulationwas adopted. The results indicated that since there are lower yield strength and largerplasticity of TiNb than that of ZrO₂that a larger degree of misfit strain and larger stressconcentration are obtained in the composites with TiNb particles. Thus, the plasticmisfit strain played a key role for the BMGCs toughening by reinforcement.The fracture toughness of Cu₄₆Zr₄₂Al₇Y₅and Zr₅₅Cu₃₀Al₁₀Ni₅were tested. And theinfluence of the fracture toughness of the glassy matrix on the mechanical properties ofBMGCs was studied. For the Cu₄₆Zr₄₂Al₇Y₅glassy matrix, it is sensitive to the crack,due to the lower fracture toughness. Once a crack is formed in the glassy matrix, it willdevelop quickly and result in failure of the specimen. However, for the Zr₅₅Cu₃₀Al₁₀Ni₅glassy matrix with higher fracture toughness, when crack is initiated in the glassymatrix, shear bands can be formed at the front of crack due to stress concentration. Itwill decrease the energy of the crack, so the development of crack is hindered.Therefore, the difference of the fracture toughness of the glassy matrixes leads to thedifferent effect of TiNb on toughening BMGs.In summation, bulk metallic glassy composites with high strength and largeplasticity were fabricated. And strengthening and toughening mechanism ofreinforcement in BMGCs were revealed. The influences of the properties ofreinforcements and the glassy matrix on the mechanical properties were studied. Thestrengthening and toughening mechanisms studied in this work provide a guideline forthe design of high strength and large plasticity bulk metallic glassy composites.