Mechanical-thermal Coupling Properties And Stress-induced Crystallization Behavior Of Zr-based Amorphous Alloy

Bulk amorphous alloys are also known as bulk metallic glasses.Their structural arrangement is different from that of ordinary crystalline metallic materials in atomic scale.Through a large number of diffraction experiments,it is proved that their atomic arrangement is not periodically long-range ordered as crystals,and there are no common lattice defects such as dislocations and grain boundaries in crystalline materials.Amorphous alloys have good physical and chemical properties because of their structural characteristics.Amorphous materials have broad application prospects in aerospace,biomedicine and other fields.In amorphous systems,Zr-based amorphous alloys have attracted wide attention due to their low cost,high strength and large glass forming ability.They have strong glass forming ability and wide supercooled liquid region.It is easy to use less complex equipment to produce large-sized bulk amorphous alloys.At the same time,the bulk amorphous alloys prepared have high strength,high elasticity and excellent resistance,corrosivity and excellent processing and forming properties have wide application prospects in aerospace and other fields.Therefore,in the past 30 years,the research of Zr-based amorphous alloy system emerged in endlessly,including the formation mechanism of amorphous and the fracture mechanism of amorphous materials,fatigue fracture characteristics and crack tip propagation characteristics.However,Zr-based amorphous alloys are usually manufactured in the form of suction casting.There are many defects in the samples,such as porosity.Under the special service conditions,the mechanical properties of the samples will be abruptly changed due to the mechanical-thermal coupling,especially the fatigue load,which will change the service performance.Therefore,it's necessary to study the oxidation behavior under mechanical-thermal coupling and the crystallization behavior induced by stress.This paper aimed at studying the mechanical behavior of Zr-based amorphous alloys at high temperatures,as well as the oxidation and crystallization behavior of amorphous alloys at different temperatures and mechanical environments.Zr55Cu30Al10Ni5 specimens were tensioned and compressed at high temperature at 100?,200?,300?and 400?.Fracture morphologies at different temperatures were observed by SEM.Fracture morphologies of amorphous alloys at different temperatures were revealed,and the transition from typical vein stripes to melt droplets at the end of shear bands was revealed.In order to study the oxidation behavior under mechanical-thermal coupling,four temperatures below the crystallization temperature of the materials were selected,100?,200?,300?and 400?.The oxidation products at different temperatures were determined by fatigue test and XRD analysis of the fracture samples.Amorphous alloy Zr55Cu30Al10Ni5 has different oxidation behavior at different temperatures.At lower temperatures,that is,in the temperature range of 100-200 ?,Zr55Cu30Al10Ni5 amorphous alloy is oxidized,mainly forming monoclinic Cu O phase,hexagonal Cu Al O2 phase and cubic Cu Al2O4 phase.At higher temperatures,that is,in the temperature range of 300-400 ?,the main oxide is tetragonal Zr O2.The effects of temperature and mechanical load on the oxidation behavior of amorphous alloys were revealed by comparing the oxidation products of fatigue,tension and heat preservation at 400 ? for 30 hours.In view of the crystallization behavior induced by stress of amorphous alloys,micro-defects were added to Zr55Cu30Al10Ni5 amorphous alloy specimens.The fatigue crack initiation position of the tested specimens was controlled while the plane composite stress state was constructed.The elastic modulus and hardness of different micro-zones of the specimens after fatigue fracture were tested by nanoindentation method.It was found that the hardness of the specimens from the internal notch to the fatigue fracture had a significant tendency to increase from small to large,that is,significant hardening under mechanical stress occurs near the fatigue fracture.The morphology of residual indentation in micro-region was observed by SEM,and the crystallization degree of different regions of residual indentation was investigated by FIB and TEM.By comparing the electron diffraction patterns and the bright and dark field images at different regions of inclined surface of indentation,it was found that Zr55Cu30Al10Ni5 amorphous alloy produced plastic deformation under the local force of nano-indentation,which induced the crystallization of amorphous matrix and resulted in the crystallization of amorphous matrix.The closer to the indentation slope,the higher the degree of crystallization is.However,no obvious crystallization phenomenon was found in the FIB sample preparation of tensile fracture specimens at the same location,indicating that the effect of fatigue load on the crystallization of amorphous materials is stronger than that of tensile load.