Regulation Of The Mechanical Properties Of Fe-based Bulk Amorphous Alloys By Cryogenic Thermal Cycling

Fe-based amorphous alloys exhibit excellent magnetic and mechanical properties,such as high fracture strength and high elastic limit.The former has been widely used in the field of functional materials,but the extremely poor plasticity at room temperature severely limits its practical application as a structural material.In this thesis,typical Fe-based bulk amorphous alloys(BAAs)with good application prospect and low price was selected as the research object,the microstructure of the BAAs was regulated by cryogenic thermal cycling(CTC),which resulted in the improvement of the plasticity.In terms of the study on the effect of upper temperature on the mechanical properties of a series of typical Fe-based BAAs,the best temperature range for the CTC treatment has been proposed.Firstly,the Fe₈₀P₁₃C₇ BAAs with good application prospect and low price was selected as the study object and performed CTC treatment at different upper temperatures with a fixed lower temperature at 77 K.It is shown that the CTC treatment between 77 K and 423 K could modify the room temperature plasticity from 1.7%to 5.4%,while the compressive strength remains about 2.95 GPa.Free volume dynamics includes not only diffusion induced annihilation,but also stress induced creation.Non-affine strain induced by CTC processing will promote the production of the local inhomogeneous stress field and then cause an increase in excess free volume;but a higher upper temperature for the CTC treatment corresponds to a severer structural relaxation,which therefore leads to the annihilation of free volume.The competition between the stress induced creation and the relaxation induced annihilation finally results in the maximum rejuvenation effect at an intermediate upper treatment temperature(about 0.62T_g),which usually corresponds to the best enhancement of plasticity.In order to verify the universality of the upper limit temperature of cryogenic thermal cycling rejuvenation,Fe₇₆Mo₄P₁₃C₇,Fe₆₅Cr₂Mo₉P₁₀C₈B₆ and Fe₆₄Mo₁₄C₁₅B₇ BAAs,which has different glass transition temperature were selected as the study object,using the same process as Fe₈₀P₁₃C₇ BAA.The results show that the above three Fe–based BAAs have the best mechanical properties after CTC treatment in the temperature range of 0.62T_g-liquid nitrogen.This shows that for a typical Fe-based amorphous alloy,the upper limit temperature of its CTC treatment remains basically around 0.62T_g,while the lower temperature of is fixed at 77 K.This provides a process reference for the CTC treatment of other typical Fe-based BAAs.