Effect Of Nb Addition On Microstructure And Mechanical Properties Of CuZrAl Alloys

In this thesis, the microstructure and mechanical properties of stepped-bar alloys with nominal compositions Cu48Zr48-xAl4Nbx(0≤x≤2.0) and Cu46.5Zr46.5Al7-xNbx(0≤x≤2.0) have been investigated systematically. Meanwhile, the relationship between microstructure and the mechanical properties of two kinds of CuZrAlNb alloy systems has been established.For the Cu48Zr48-xAl4 Nbx alloy system, when the Nb content is, the more crystalline phase precipitated in the alloys with 0.3% and 2.0% Nb than in the Nb-free alloy, indicating that Nb addition promotes the nucleation and growth of the second phase in the alloy. The alloys with 0.8% and 1.5% Nb possess better glass forming ability and less second phases. For 1.2% Nb content, volume fraction of the crystalline phase equals to the Cu48Zr48Al4 alloy, showing that Nb addition has a slight effect on glass forming ability. Meanwhile, for the alloys with the diameter of 3mm, when the content of Nb is 0, 0.8% and 1.5%, spherical B2-CuZr phases with uniform size and homogeneous distribution precipitate from the amorphous matrix, consistent with the alloys with 0.8% and 1.5% Nb under a diameter of 4mm. Whereas, for the Cu46.5Zr46.5Al7-xNbx(x=0, 0.3, 0.8, 1.2, 1.5, 2.0 at.%) alloys, the B2-CuZr phases are still appears. The alloy samples with diameters of 3mm and 4mm have fully amorphous structure. As the sample diameter increases up to 5mm and 6mm, the alloys with 0.3% and 2.0% Nb also show amorphous structure, suggesting their excellent the glass forming ability. The samples with 0.8% and 1.2% Nb possess identical volume fraction of crystalline. However, the alloy with 1.5% Nb has high volume fraction of crystalline phase. Furthermore, EDS analyses show that the addition of Nb does not cause the diffusion of elements.For the Cu48Zr48-xAl4Nbx(0≤x≤2.0) alloys and Cu46.5Zr46.5Al7-xNbx(x=0, 0.8-1.5) alloy sample of the same composition and different diameter, the volume fraction of crystalline phase increases with the increase in the sample diameter. The Cu46.5Zr46.5Al7 alloys with 0.3% and 2.0% Nb addition exhibit amorphous structure.The tensile tests results of the Cu48Zr48-xAl4Nbx(0≤x≤2.0) alloys with a diameter of 3mm show that the alloys(x = 0, 0.3, 0.8 and 2.0) exhibit significant work hardening ability. The alloy with 0.8% Nb has both large plastic deformation capacity and high strength. When the Nb content is up to 1.2% and 1.5%, the alloy shows room-temperature brittleness. For the Cu46.5Zr46.5Al7-xNbx(0≤x≤2.0) alloys, when Nb content is 0.8%, the comprehensive mechanical properties of the sample are improved.The mechanism of B2-CuZr phases reinforced metallic glass composites is that B2-CuZr phases hinder the rapid propagation of a single shear band during the deformation. On the one hand, the elastic limit is difference between crystalline phases and matrix. When subjected to tensile loading, the deformation is defined by the interface, causing a great stress concentration at the interface. With further increasing the load, the stress concentration near the interface facilitates the initiation of shear bands, thereby suppressing the formation of the premature crack. On the other hand, the “softer” B2-CuZr phases will transform to the “harder” martensitic phases during the deformation process, which inhibits the strain softening phenomenon. However, only the alloy sample with homogeneously distributed spherical B2-CuZr phases of uniform size can achieve optimizing mechanical properties.The nanoindentation test results of B2-CuZr phase and metallic glass matrix show that microscopic strength of B2-CuZr phases are lower than those of the corresponding matrix in the two kinds of CuZrAlNb alloy systems. The applied loading rate exerts a crucial role on the number of the serrated flows. The macroscopic mechanical properties of the materials in a certain extent can be characterized by the micro-strength and the number of serrated flows from the two phases. The high micro-strength of two phases results in the increasing macroscopic tensile strength of the composite. The more serrated flow the two phases show, the better macroscopic tensile plasticity the composite materials possess.