Effect Of Deep Cryogenic Cycle Treatment On Room Temperature Mechanical Properties Of Zr-based And High Entropy Bulk Metallic Glass

Bulk metallic glasses(BMGs)have many excellent mechanical properties,such as high strength and high hardness.They are expected to be applied in aerospace,military,and other fields,but their lack of plasticity at room temperature seriously limits their practical application.Recently,it has been found that deep cryogenic cycle treatment(DCT)can rejuvenate BMGs and effectively improve the room-temperature compressive plasticity.However,existing research is limited to a few BMG systems,and the rejuvenation degree and effect of plasticity improvement of different alloy systems are different.To investigate the universality of plasticity improvement of BMGs with different systems and enrich the understanding of the toughening mechanism of BMGs during DCT,the classical Zr-based Vit-1 BMG and Ti₂₀Zr₂₀Hf₂₀Cu₂₀Be₂₀ high-entropy bulk metallic glass(HE-BMG)were selected as research objects to explore the influence of DCT on the structure and room-temperature mechanical properties of Vit-1 BMG and HE-BMG.This study included the following aspects.Vit-1 BMG prepared with high purity raw materials was subjected to DCT.After DCT,it maintained its amorphous nature,but it was not rejuvenated obviously.As a result,with increasing number of cryogenic cycles,the hardness first decreased and then increased.The room-temperature compression plasticity first slightly increased and then remained unchanged.After 10 cryogenic cycles,the maximum room temperature compressive plasticity was about 4.33%,and the impact toughness basically remained unchanged.With Vit-1 BMG possesses larger value of fragility index(47–50),as a result,its initial structure is relatively dense,resulting in limited rejuvenation upon DCT Therefore,it is difficult for atom rearrangement to occur during DCT,which leads to limited improvement of the mechanical properties at room temperature.Vit-1 BMG prepared with lowpurity raw materials was also subjected to DCT.After DCT,it maintained its amorphous nature,but it was rejuvenated.With increasing number of cryogenic cycles,the hardness,room-temperature compression plasticity,and impact toughness first increased and then decreased.After five cryogenic cycles,the maximum room-temperature compression plasticity and impact toughness were 5.02%and 4.27 J/cm²,respectively.Compared with the as-cast sample,the impact toughness increased 2.6 times.After DCT,the rejuvenation degrees of Vit-1 BMG prepared from different purity raw materials were different,which led to the difference in the mechanical properties at room temperature.With the addition of Y,which possesses large atomic size,the atom rearrangement of Vit-1 BMG prepared from low-purity raw materials is relatively easy during DCT,so clear rejuvenation occurs.Ti₂₀Zr₂₀Hf₂₀Cu₂₀Be₂₀ HE-BMG was subjected to DCT.After DCT,it maintained its amorphous nature,but it was rejuvenated.With increasing number of cryogenic cycles,the hardness decreased and the room-temperature plasticity and impact toughness of Ti₂₀Zr₂₀Hf₂₀Cu₂₀Be₂₀ HE-BMG gradually increased.After 20 cryogenic cycles,the plasticity of the sample reached 5.38%,which was about 9 times higher than that of the as-cast sample,and the impact toughness was 2.71 J/cm²,which was 67.3%higher than that of as-cast sample.Based on the shear transformation zone(STZ)model,the volume and size of the STZ of HE-BMG were calculated.The volume and size of STZ gradually increased with increasing number of cryogenic cycles,which led to an increase in the plasticity.Owing to the influence of the high-entropy effect and sluggish diffusion,theoretically,HE-BMG is more difficult to rejuvenate than Vit-1 and other single principal BMGs.However,HE-BMG was rejuvenated after DCT,which may be because of its lower fragility index(18.6)than Vit-1 BMG,making atom rearrangement easier during DCT.