Research On Deformation Behavior Of Zr₅₀Cu₄₀Al₁₀and Fe₇₈Si₉B₁₃Amorphous Alloys At High Temperature

Recently,Zr-based and Fe-based amorphous alloys have been widely studied for excellent properties,such as high strength and high hardness.However,it is found that Zr-based and Fe-based amorphous alloys do not exhibit ductility at room temperature.Meanwhile,deformation behavior is a single shear band mode,resulting in macro-brittle fracture.Thus,the wide application as high strength engineering materials is seriously restricted.In the supercooled liquid region,the amorphous alloys exhibit viscous flow and superplastic deformation,but its strength decreases significantly.The strength of amorphous alloys can be improved at high temperature when the propagation of single shear band is hindered or the viscosity of amorphous alloys is increased during deformation.Previous studies have shown that strain hardening of primary crystallized Al-based amorphous alloys occurs at high temperature tensile testing,but the effect of crystallization mode on mechanical properties of materials at high temperature has not been systematically elucidated.Therefore,Zr₅₀Cu₄₀Al₁₀ polycrystalline amorphous alloy and Fe₇₈Si₉B₁₃3 primary crystallization amorphous alloy were investigated in this thesis.The amorphous ribbons were prepared by a single roller melt spinning technique.The high temperature deformation behaviors of the materials were studied by changing the temperatures and initial strain rates in the tensile testing.The fracture behavior of Zr₅₀Cu₄₀Al₁₀ and Fe₇₈Si₉B₁₃3 amorphous alloy were investigated by structural characterization and microscopic morphology.For Zr₅₀Cu₄₀Al₁₀ amorphous alloy at high temperature tensile testing,the results show that strain hardening was not found in the stress-strain curves.The plastic deformation occurs below the glass transition temperature（T_g=420℃）.A small amount of plastic deformation decreases the stress concentration at 350℃,resulting in an increase of strength to 830 MPa.There is typical vein shape in fracture morphology.The number of veins increases in the fracture morphology with the increase of temperature.The serrated flow occurs in Zr₅₀Cu₄₀Al₁₀ amorphous alloy at high temperature tensile testing.With the increase of temperature,the shear bands are easier to be activated,which leads to increase in the number of serrations and decrease in the stress fluctuation（Δσ）.The multiple shear bands initiated during the deformation process can delay the tensile fracture behavior and inhibit the fracture along the main shear band,thus increasing the plasticity of the sample.By comparing the Zr₅₀Cu₄₀Al₁₀ amorphous alloy protected in argon with those in air at the same loading conditions,it is found that a number of ZrO₂ and Al₂O₃ oxides of samples were precipitated in air,which resulted in the decrease of Zr element.The decrease of Zr in the residual amorphous matrix results in the decrease of strength and the increase of plastic deformation capacity.For Fe₇₈Si₉B₁₃ amorphous alloy at high temperature tensile testing,the results show that the tensile strength decreases and the plastic deformation increases with the increase of temperature at constant initial strain rate.The fracture strain is 0.272 at 490℃.At constant temperature,the complete the amorphous state of the sample is maintained with the increase of initial strain rates,and thus the fracture strength increases gradually.The shear steps and vein-like fracture morphology appeare in the tensile samples at 250℃-350℃.The strain hardening occurs in tensile testing at 510℃,6.67×10^-55 s^-11 and 1.67×10^-44 s^-1.Deformation in tensile testing is conducive to the crystallization of the samples,and thus the crystal volume fraction difference between the deformed ribbons and the undeformed ribbons is about 8.12%at 510℃.The dynamic deformation of primary crystallized Fe-based amorphous alloys breaks the solute concentration formed during the preferential precipitation ofα-Fe in the crystallization process.This result promotes the further crystallization and precipitation of theα-Fe phase,which makes strain hardening phenomenon in the stress-strain curves.