Fatigue Behavior And Mechanism Of Zr₆₁Ti₂Cu₂₅Al₁₂ Bulk Amorphous Alloy

Bulk metallic glasses(BMG)are now competitive candidate engineering structural materials due to their exceptional strength and fracture toughness.For example,Zr₆₁Ti₂Cu₂₅Al₁₂ BMG has relatively low cost and its combination of strength and damage tolerance extends beyond the benchmark ranges established by the strongest and toughest bulk materials.However,the fatigue properties of materials are crucial for engineering structures under cyclic loading.In order to find an effective method to improve the fatigue properties of BMGs,it is necessary to clarify the fatigue mechanism of BMGs,which still remains elusive.In the present work,Zr₆₁Ti₂Cu₂₅Al₁₂ bulk amorphous alloy was used as the model material.Based on the characterization of its tension-tension fatigue and fatigue crack propagation,the physical mechanism and structural origin of fatigue were unraveled to shed light on its application in engineering structures.In the present work,Zr₆₁Ti₂Cu₂₅Al₁₂ alloy rods and plates with a diameter of 6 mm and a thickness of 3 mm,respectively,were prepared by arc melting using high purity elemental ingots.The tension-tension fatigue samples were fabricated by using alloy rods.S-N curve was measured in the tension-tension fatigue test,and the fatigue limit was determined to be 195 MPa,which was larger than the fatigue limit of structural steel with high fracture toughness(about 150 MPa).This indicates that Zr₆₁Ti₂Cu₂₅Al₁₂BMG is a kind of structural material with great potential for engineering application.Shear band flocks were found on the surface of fatigue specimens tested with stress amplitude which was higher than the fatigue limit,and the direction of each shear band in the shear band flocks is perpendicular to the loading direction.The fracture morphology analysis showed that the fatigue crack of Zr₆₁Ti₂Cu₂₅Al₁₂ amorphous alloy originated from one of the shear bands,that is to say,fatigue crack initiated through opening of one of the shear bands.In the present work,the da/d N-?K curve of Zr₆₁Ti₂Cu₂₅Al₁₂ BMG was measured by using three-point bending specimen to further investigate its fatigue crack propagation behavior.The obtained stress threshold value?K_th is 3.9 MPa?8),which is higher than that of all BMGs as far as we know.Crack morphology in both plain-stress region and plain-strain region were investigated by using SEM.A lot of shear bands were observed in the third fatigue crack propagation stage,while no shear band were discovered near the crack tip at other fatigue crack propagation stages,and cavitations were observed near the fatigue crack in Paris region.In order to further investigate the fatigue crack propagation mechanism,the fatigue crack tips at different propagation stages were investigated by using nano-CT.We found that fatigue crack propagated through coalescence of cavitations around crack tip in the first and the second fatigue crack propagation stages.The density of the material around cavitations were improved.Therefore,a crack propagation mode of cavitation initiation,growing and coalescing at the crack tip was proposed,instead of Laird-Smith model which suggests a repetitive blunting and resharpening mechanism for fatigue crack advance as reported in literatures.The above viewpoints were further confirmed in TEM investigation.In the first and the second stages of fatigue crack propagation,the ligaments between adjacent cavitations were necking until the final break.The phenomenon indicated that a large plastic rheological process occurred during crack propagation in of the tough BMG.The autocorrelation analysis was conducted on the crack tip material and informed that the change of atomic clusters and the decrease of atomic order was produced through the local unstable energy dissipation during the crack growth process.However,in the instability stage of rapid crack growth,no cavitations were observed,the fatigue cracks propagated along shear bands.High density nanocrystals with grain size of 2?5 nm was found in the shear band from the position which is about 8 times crack tip radius away from the crack tip to the end of the shear band,because the accumulated energy in this stage is greater than the crystallization energy barrier.This was different from the first and the second fatigue stages in which the crack propagated through cavitation;the third stage dissipates energy in the way of crystallization.The material in the shear band between crack tip and nanocrystal region was amorphous and its atomic arrangement order was reduced.Nevertheless,crystallization was not found in the wake of the crack at any crack growth stage,because the melting and subsequent cooling occurred in this region due to the release of fracture energy during abrupt crack advance,thus making the region more disordered.This indicated that shear band was produced by high?K,nanocrystals generated during cyclic loading,nanocrystals was melted by the released elastic energy during crack propagation.In summary,through investigating tension-tension fatigue behavior of Zr₆₁Ti₂Cu₂₅Al₁₂ BMG,we found that fatigue crack initiated from a shear band in a shear band flock generated through shear softening.In the study of fatigue crack propagation,a crack propagation way including cavitation formation,growth and coalescence was found in the first and the second crack propagation stage.However,the cracks propagated in shear bands in the third stage.The shear band generated under high?K,crystallized during cyclic loading,and disordered during abrupt crack advance.