Study On Crystalline Phases And Plasticizing Mechanism In Metallic Glass Composites

Bulk metallic glasses show unique properties due to their atomic structure.But their strain-softening and lack of plasticity during deformation have affected the applications.So,combining the amorphous matrix with homogeneously distributed spherical B2-CuZr phase has been developed to sovle the problem.In this paper,Cu-Zr based BMG composites were successfully prepared by suction casting using copper mold.The influences of cooling rate and alloying element on microstructures and martensitic transformation in BMG composites were investigated.The influences of different microstructure and martensitic transformation on mechanical properties were further investigated,especially the causes of plasticity and work-hardening behavior.The results show that the glass-forming ability(GFA)of Cu50Zr50 alloys is poor and the Ms of B2-CuZr phase is 190 ?.Most of the B2-CuZr phase transforms into M-CuZr phase,resulting in the lack of plasticity.To keep B2-CuZr phase stable,A1 is added to Cu50Zr50 alloys.The GFA of Cu-Zr-Al alloys increases,the Ms decreases and the stability of B2-CuZr phase increases.When the content of A1 is 5?8%,the crystalline phase is mainly B2-CuZr phase.With the increase of cooling rate,the size of B2-CuZr phase decreases and the distribution is more homogeneous.When the casting diameter is 2 mm,Cu47.5Zr47.5Al5 BMG composite with homogeneously distributed B2-CuZr phase is formed and the Ms is 23 ? which is close to the room temperature.Large compressive plasticity accompanied with strong work-hardening behavior is achieved in the Cu47.5Zr47.5Al5 BMG composites with a diameter of 2 mm,the fracture strength and plasticity of which are up to 1775 MPa and 7%,respectively.The improvements of mechanical properties mainly attribute to the TRIP effect which requires two conditions,including homogeneously distributed B2-CuZr phase and the Ms near the room temperature.The work-hardening behavior is mainly due to the remedy of M-CuZr phase to the strain softening.Both the "blocking effect" of the crystalline phase on shear band and the deformation induced martensitic transformation from B2-CuZr phase to M-CuZr phase are mainly responsible for the improvement of plasticity.To make the size bigger,the fourth component is added to Cu47.5Zr47.5Al5 alloy.The GFA of CuZrAl5M5(M=Ni,Zn,Ti)alloys increases.With the addition of 5%Ni or of 5%Zn,the Ms are 23 ? and 6 ?,respectively.The stability of B2-CuZr phase decreases and the volume fraction of M-CuZr phase increases.With the addition of 5%Ti(Ms=-103 ?)or 5%Ti and 5%Ni,though the crystalline phase is mainly B2-CuZr phase,the energy from deformation is not enough to induce martensitic transformation because of the further decrease of Ms.Compared with Cu47.5Zr47.5Al5 BMG composite,the addition of Ni,Zn,Ti can not significantly improve its plasticity.