In Situ Study Of Structure Evolution Of Severe-Plastic-Deformed Zr-Based Bulk Metallic Glasses During Heating

Metallic glasses,also known as amorphous alloys or liquid metals,are non-equilibrium alloys with liquid-like structures formed by rapid cooling techniques.For a long time,the structural origin and structural response of metallic glass plasticity have been unresolved.The deformation of metallic glass does not depend on the dislocation slippage mechanism,and the structure-property correlation mechanism during the process of plastic deformation is still unclear.However,in situ non-destructive characterization of atomic-scale structural response during the deformation of amorphous materials encountered difficulties in current research.In this thesis,Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅（BAM 11）metallic glass samples after severe plastic deformation were prepared by high-pressure torsion（HPT）method and traditional cold rolling（CR）method.State-of-art characterization techniques and analysis,including differential scanning calorimetry,in situ X-ray synchrotron radiation scattering,real-space structure analysis,scanning electron microscopy,mechanical property tests,were employed to investigate the characteristics of structures and energy states in BAM 11metallic glass after severe plastic deformation,the structure evolution during heating and.Main content of the research and results are as follows:（1）Using HPT and CR plastic deformation methods,the BAM 11 samples after severe plastic deformation with improved energy state and structural inhomogeneity were prepared.Differential scanning calorimetry analysis found that the enthalpy of the BAM 11 samples after severe plastic deformation significantly increases,and the crystallization temperature（T_x）,glass transition temperature（T_g）,and supercooled liquid region（ΔT_x）decreases,which indicates that although the energy state keeps higher the thermal stability of the glasses decreases.Through the SEM characterization of microscopic morphology,we can observe that the BAM 11 sample had a higher number density of shear bands after the HPT and CR method,indicating that the plastic deformation method has a significant effect on the inceasing structural inhomogeneities.The TEM characterization result of severely-deformed sample suggests that the structure of shear band is probably looser in comparison to the matrix.（2）The changing trend of the structural factor S（Q）of the BAM 11 sample after severe plastic deformation during the heating process has been analyzed in the reciprocal space.The first sharp diffraction peak position（FSDP）Q₁of S（Q）shows that the Q₁ of the as-cast（AC）sample has a linear downward trend before T_g,the downward trend of the HPT sample obviously becomes gentler from about 550 K to T_g,and the downward trend of CR sample has also been found to be milder from about 620 K and T_g.The integration of Q₁ peak intensity revealed that the Q₁ peak intensity integration of AC sample increases slightly in a small range before T_g,that of HPT sample increases significantly from about 550 K to T_g,and that of CR sample has an identical trend,but the starting temperature was around 620 K and the increased amplitude is between AC and HPT samples.The above results indicated that the medium-range order structure of the HPT and CR samples has an ordering behavior and the packing density increases,and the HPT method behaves more effectively.（3）Using the in situ real-space analysis method,the changing trend of the distribution function G（r）of the BAM 11 sample after severe plastic deformation during the heating process was investigated.The integration results of the intensity of the first to the sixth shell（r₁-r₆）on G（r）pattern revealed that the changing trend of the integrated intensity of r₆ for the AC sample and the samples after the deformation treatments were significantly ahead of the previous shells.The trend of the r₆ integrated intensity of the three kinds of samples is as follows:that of AC sample increases from 650 K to T_g,that of CR sample increases from620 K to T_g,and that of HPT samples increases from 550 K to T_g.The results show that the ordering behavior of the medium-range order structure of the CR and HPT samples is much stronger than that of the short-range order,which dominated the structure evolution process during heating before T_g.In addition,the connectivity analysis of the medium-range order atomic clusters illustrates that the proportion of the more ordered 3-and 4-atom clusters in the HPT sample increases while that of 1-and 2-atom clusters decreases before T_g,thus promoting the structure ordering behavior of the medium-range order.（4）The contrast between the microhardness and nanoindentation results of the BAM 11samples after severe plastic deformation and the as-cast sample has been analyzed.The microhardness results showed that the hardness of the HPT and CR sample is lower than that of the as-cast state,and the HPT sample is softer than CR sample.The nanoindentation test demonstrates that with the same load and indenter,the indentation depth and indentation area of AC,CR,and HPT samples increased sequentially,which also showed the softening effect of the HPT method.In addition,the load-displacement curve of the AC sample shows some small"pop-in"phenomena,and the CR sample still have some very slight"pop-in"events.For the HPT sample,the“pop-in”basically disappeared,indicating that its plastic deformation ability has been improved.In summary,this thesis illustrated the atomic structural evolution mechanism of the severe plastic deformation process,and found that the ordering behavior of the medium range order structure after the severe plastic deformation process by HPT is significantly enhanced,which proves the excellent engineering ability of the HPT plastic deformation method on the structure and performance of the BAM 11 metallic glasses.This thesis provides a reference for the medium-range order structure-property correlation of BAM 11 metallic glass and is of great significance for the further development of Zr-based metal glasses with good plasticity.