Formation And Performance Studies Of Cu-Zr-Zn Bulk Metallic Glasses And Their Composites

Bulk metallic glasses（BMGs）show unique mechanical properties due to their long-range disordering atomic structure and are regarded as one of promising engineer ing materials.But their room-temperature brittleness and strain-softening during deformation have beena stumbling block for practical structural applications.Recently,it has been found that by introducing shape memory phase into the glassy matrix,the fabricated BMG composites show not only a pronounced tensile plasticity but a macroscopic work-hardening ability during deformation at room temperature.Fortunately,it has been demonstrated that the GFA of CuZr-based alloys is relatively large.Additionally,the equiatomic CuZr intermetallic compound has different metastable structures,i.e.the high-temperature B2-CuZr phase with a cubic primitive structure（Pm-3m）,and monoclinic martensites（P21/m and Cm）.It could be a good candidate as bulk metallic glass（BMG）composites materials with transformation-induced plasticity.In this paper,(Cu_0.5Zr_0.5)_100-xZn_x（x = 0,1.5,2.5,4.5,7,10,and 14 at.%）alloy system was studied.Alloys,ribbons and Rods of Cu-Zr-Zn were prepared by arc melting,high frequency furnace,the melt-spinning and injection casting machines.The glass-forming ability（GFA）of(Cu_0.5Zr_0.5)_100-xZn_x（x = 1.5,2.5,4.5,7,10,and 14 at.%）metallic glasses（MGs）were studied.With increasing Zn content,the GFA of CuZr-based MGs gradually increases and then Cu-Zr-Zn bulk metallic glasses（BMGs）and their composites were successfully fabricated.The enhancement of GFA with Zn addition in CuZr-based alloys can be well explained by the λ +（?h）1/2 amorphization criterion,which combines topological instability and electronic criteria.In order to further understand the fabrication of Cu-Zr-Zn BMG composites,the formation of B2-CuZr phase and the crystallization of Cu-Zr-Zn MGs were also investigated.Cu-Zr-Zn MGs during heating devitrified into mainly Cu₁₀Zr₇ and CuZr2 crystals in the glassy matrix,and subsequently some precursor crystals related with B2-CuZr phase precipitated.As the temperature was increased more than 1050 K,the whole crystallization products transform into Cu₁₀Zr₇ and B2-CuZr crystals.This eutectoid transformation temperature is reduced with increasing Zn content,implying that Zn addition can effectively stabilize B2-CuZr phase to lower temperatures.The crystallization kinetics of Cu-Zr-Zn MGs under isothermal conditions shows that the dominated crystallization mechanism is the interface-controlled two-or three-dimensional growth with different nucleation rates.The microstructures and mechanical properties of(Cu_0.5Zr_0.5)_100-xZn_x（x = 0,2.5,4.5,7,10,and 14 at.%）bulk metallic glass（BMG）composites were studied.CuZr martensitic crystals together with a little B2-CuZr crystals and amorphous phase dominate microstructures of the as-quenched samples with lo wer Zn additions（x = 0 and 2.5 at.%）,while B2-CuZr and amorphous phases being accompanying with a little martensitic crystals form in the case of high Zn contents（x = 4.5,7,10,and 14 at.%）.The BMG composites exhibit macroscopically detectable compressive plastic strains and obvious work hardening,which can be due to the formation of multiple shear bands and a deformation-induced martensitic transformation（MT）within B2 crystals.The present BMG composites could be a good candidate as high-performance structural materials.Compositional dependence on the microstructures and martensitic transformation in(Cu_0.5Zr_0.5)_100-xZn_x（x = 1.5,2.5,4.5,7,10,and 14 at.%）alloys were investigated.It was found that CuZr martensites were present in all the investigated alloys.With increasing Zn content,the volume fractions of CuZr marensitic crystals and B2-CuZr phase gradually decrease and increase,respectively.With high Zn additions（i.e.7,10,and 14 at.%）,the matrix is proven to be a eutectic structure.The thermal results show that the initial martensitic transformation temperature（Ms）decreases from 412 ± 5 K to 329 ± 5 K as the Zn content increases from 1.5 to 14 at.%.The values of Ms of Cu-Zr-Zn shape memory alloys are inversely proportional to the number（ev/a）and concentration（cv）of valence electrons,respectively,implying that the martensitic transformation in Cu-Zr-Zn alloys would be of electronic nature.