Effect Of Cryogenic Thermal Cycling On The Structure And Properties Of Zr-Based And La-Based Bulk Metallic Glasses

Bulk metallic glasses(BMGs)are a new class of alloy materials that have been developed in recent decades.Because of their unique disordered atomic arrangements,they have more excellent mechanical,physical and chemical properties than the crystalline counterparts with the same compositions,thus arousing great attention in recent years.However,as a metastable alloy,its disordered atomic structure will relax into a relative stable state and thus accelerate the aging and embrittlement when the external conditions such as temperature and pressure increase,which greatly limits the practical application of BMGs.Therefore,how to overcome the relaxation and aging of metallic glasses is the bottleneck of their large-scale application.Recently,it has been found that the cryogenic thermal cycling can increase the overall energy of metallic glasses,showing a rejuvenation effect opposite to the structural relaxation,and increasing the room temperature plasticity of metallic glass to a certain extent.Therefore,it is of great significance to further explore the effect of thermal cycling on the structure and performance of BMGs more systematically.In this paper,the newly developed Zr-Y-Al-Co and La-Al-Ag-Ni-Co BMGs are taken as model systems.The effects of thermal cycling treatment on the structure,free volume,mechanical properties and atomic dynamics of both glasses are studied in detail.The main conclusions are as follows:(1)The Zr42Y14Al22Co22 BMG with phase separation structure,whose critical size is more than 1 mm,has been developed by copper mold suction casting method,and its structure is inhomogeneous.The mechanical properties of the studied metallic glass such as density,hardness,modulus,and three-point bending strength were investigated.It is found that the free volume in the Zr42ZY14Al22Co22 MG gradually increases during the first 20 cycles and within 20 to 200 cycles,part of the free volume in the metallic glass gradually annihilates.During the thermal cycling,there are both structural relaxation and rejuvenation effect in the selected metallic glass.The effect of rejuvenation in the first 20 cycles is obviously stronger than that of structural relaxation in the metallic glass,and the competition between them gradually reaches equilibrium after 20 cycles.Through transmission electron microscopy and synchrotron radiation X-ray diffraction analysis and other tests,it was found that the thermal cycling treatment has a significant effect on the non-affine structure of Zr42Y14Al22Co22 MG.In the first 20 cycles,the nanometer scale heterogeneities of MGs increases,the average distance between atoms increases,the free volume gradually increases,and there are relatively larger electronic density fluctuation in each separate phase.After 20 cycles,the scale of heterogeneities reduces,the average distance between atoms decreases and part of the free volume gradually annihilates.(2)It is found that thermal cycling between room temperature(303 K)and liquid-nitrogen temperature(77 K)has no significant effect on the atomic dynamics of the as-cast and annealed_La62Al14Ag2.34Ni10.83Co10.83 BMGs,including low-temperature boson peak and ?-relaxation.On the contrary,sub-Tg annealing makes the low temperature specific heat and ?-relaxation behavior of BMG sample obviously change.Before 20 cycles,thermal cycling induces significant rejuvenation in the excess free volume,physical and mechanical properties of the as-cast La-based BMG sample,such as density,microhardness,modulus,and the relaxation enthalpy-?Hrel.However,the thermal cycling has no obvious rejuvenation effect on the same properties and atomic dynamics of a fully relaxed La-based BMG sample.The results of synchrotron X-ray diffraction show that the sub-Tg annealing not only reduces the free volume of as-cast BMG sample,but also increases its local structure ordering.In contrast,thermal cycling only increases the free volume of the as-cast sample,but has little effect on its structure.The PALS results indicate that thermal cycling promotes smallest and largest sized open volumes to evolve into intermediate ones referred as flow defects within the first 20 cycles.Thus,our results indicate that the increasing free volume induced by thermal cycling could not be a key factor in controlling the behaviors of boson peak and ?-relaxation,which might more depend on particular atomic packing structures in MGs.