Fracture Behavior Of Zr₆₁Ti₂Cu₂₅Al₁₂ Bulk Metallic Glass With Low Hf Content

Because of its high strength,bulk metallic glasses have potential applications in engineering structural materials.However,their brittle fracture is an important problem that must be solved before putting them into engineering applications.We choose Zr₆₁Ti₂Cu₂₅Al₁₂2 bulk metallic glass with the highest combination of strength and fracture toughness in this work to further improve its fracture toughness by alloying technique.The as-cast plate with Zr₆₁Ti₂Cu₂₅Al₁₂2 composition was prepared by using high purity zirconium elemental pieces with low Hf content.XRD and DSC examinations indicated that the as-cast samples were fully amorphous.Toughness testing samples were taken from the bottom of the as-cast plates.Single notch three-point bending toughness testing samples were prepared according to the ASTM E399 standard.Fatigue pre-crack was prepared and fracture toughness was tested at room temperature.The fracture toughness of our sample was 147?±7?MPa_·m^1/2,which is 31%higher than the reported value(112?±6?MPa_·m^1/2)of the same nominal composition.An unusual inflection point was found after the general plastic deformation stage on the stress intensity factor K_I-crack opening displacement COD curve.After the inflection point,the dependence of load on displacement increases much more rapidly that the dependence before the point.We found that circular arc shear bands were produced on the side surface of the sample after the inflection point through in-situ microscope investigation,and these shear bands cutting through the whole sample from the tip of the fatigue pre-crack to the other end of the sample.Pure shear fracture occurred along the outermost arc shear band in the plastic zone.As far as we know,pure Mode II fracture under Mode I loading conditions has not been reported.Three-dimensional images of cracks ahead of the tip of the fatigue pre-crack in fractured sample were reconstructed by using synchrotron X-ray Nano-CT.Mode I cracks were found in the sample.Some of the cracks were sharp without cavitation,and some of the cracks were blunted by cavitation.This indicated that the three-dimensional hydro static tension ahead of the crack did not cause catastrophic fracture of the sample because of the blunting effect,while catastrophic fracture of reported bulk amorphous alloy was induced by cavitation.Our theoretical analysis showed that with the blunting effect,the growth rate of the three-dimensional hydro static tension with COD were slowed down,while that of shear stress accelerated,which leads to the catastrophic shear fraction in Mode I Loading.Standard three-point bending specimens were prepared by using the whole amorphous alloy plate.The effect of cooling rate on the fracture toughness our samples were investigated by testing the fracture toughness of different position on the alloy plate.Cooling rate increases from the top of the as-cast plate to the bottom.The results showed that fracture toughness of the specimens decreased with decreasing cooling rate,the size of plastic zone and the number of shear bands decreased significantly,and the fracture morphology changes from viscous vein pattern to dimple,indicating that the fracture toughness of amorphous alloy would be highly sensitive to cooling rate.Amorphous alloy engineering structures would be applied at various temperatures.We also tested fracture toughness of our samples at cryogenic and elevated temperatures.Test results showed that the samples have the maximum fracture toughness at room temperature.The fracture toughness increased with decreasing temperature bellow room temperature.At the same time,the fracture toughness decreased significantly with increasing temperature at elevated temperatures.The lowest fracture toughness obtained in all tests was better than that of Al alloy,and was comparable with Ti alloy.Therefore,Zr₆₁Ti₂Cu₂₅Al₁₂2 bulk amorphous alloys with low Hf content would be applied in engineering structures,especially at room temperature and low temperature environment.