Cryogenic Treatment On The Mechanical Properties Of Amorphous Alloys

Bulk metallic glasses (BMGs) have attracted great interests due to theirexcellent mechanical properties. It is found that BMGs do not have structuressuch as grain and grain boundary like their crystalline counterparts. When theplastic deformation occurs far below the glass transition temperature, the stressconcentration cannot be released through dislocations slip and other energydissipation mechanisms. As a result, the plastic deformation of BMGs is highlylocalized into shear bands. The propagation of monolithic shear band leads tocompletely brittle fracture of materials. The lack of plastic deformation farbelow the glass transition temperature strongly limits the application of BMGsin engineering. In order to improve their plasticity, many researchers have madeattempts to promote the propagation of shear bands by many methods toimprove their mechanical properties.It has been found that cryogenic treatment can improve the mechanicalproperties of conventional metallic materials. This raises a question whether themechanical properties of BMGs are improved due to cryogenic treatment. In thiswork, Zr60Al15Ni25metallic glass was chosen to tune its structure by immersionof specimens in liquid nitrogen for different times. The micro-hardnessmeasurements and subsurface indentation tests were conducted by VickersIndenter. The morphologies of shear bands formed due to indentation wereinvestigated by scanning electron microscopy. The structures and thermalstability of the specimens were investigated by X-ray diffraction and differentialscanning calorimetry. Moreover, the relaxation annealing below the glasstransition temperature (sub-Tg) after cryogenic treatment was employed to tunethe structures of specimens in this work.It is found that the number of shear bands under indentation increases; thespace between adjacent shear bands decreases and the morphologies of shearbands become smooth after cryogenic treatment. The changes of the free volumeand atomic viscosity indicate that the plasticity of Zr60Al15Ni25metallic glass is improved. The micro-hardness increases after cryogenic treatment. The spacebetween the adjacent shear bands becomes smaller, and the atomic configurationbecomes more ordered due to isothermal anneal after cryogenic treatment. As aresult, the plasticity is further improved and the micro-hardness further increases.Moreover, the sub-Tganneal temperature is higher, the atomic configuration ismore stable, which benefits the mechanical properties of BMGs.