Composition Design For Ti-Cu Based Bulk Metallic Glasses Based On Cluster-plus-glue-atom Model

Ti-based amorphous alloys usually have low mass densities and high strength with potential application for biomedical due to the good biocompatibility and corrosion resistance of Ti element.But the glass formation ability is mainly dominated by the addition of Be,Ni,Pd or Ag.etc.The corresponding biotoxic and high cost limits its application for biomedical.The Ti-based amorphous alloys without thses elements usually have small critical diameter(lower than 3mm)and low content of ETM elements(lower than 55%).Using the so-called "Cluster-plus-glue-atom”model,bulk metallic glasses are interpreted by simple composition formulas[cluster](glue atom)I or 3,with the valence electron number per unit formula(e/u)being universally 24,and a few bulk metallic glasses were developed.However,the definition of clusters in crystalline structure and the calculation of cluster radius should be reappraised.And this single-phase model is naturally disabled in the analysis of crystallization behavior,which is always related to galss formation ability.Hence in the work,the definition of clusters in crystalline structure and the calculation of cluster radius are modified,and a more versatile procedure for composition design is proposed and applied in Ti-Cu system.The influence of alloying element on the crystallization behaviors is investigated to find its correlation with glass formation ability.And the influence of alloying element on corrosion resistance and mechanical property is investigated to develop Ti-based bulk metallic glass for biomedical.The main results obtained in this study are summarized as follows:1?With considering the features of ETM-LTM systems,the cut-off distance can be determined corresponding to the radial disatance of second neiborghing in the structure of single substance,and the cluster radius could be calculated by Goldschmidt radius.Then a more versatile procedure for composition design is proposed and applied in Ti-Cu system,and[Ti-Cu5.45Sn0.55Ti6.2Zn1.8]Cu3(5mm)and[Cu-Ti6.4Zr1.6Cu4]Cu0.5Sn0.5(4mm)was developed.The analysis of the correlation between e/u and glass forming ability shows that the e/u close to 24 is a necessary but not sufficient condition for the formation ability of high glass alloy,which can effectively guide the composition design,but can not quantify the addition of alloying elements.2?It is found that when one elment content as the only variation either the thermal parametes or e/u values are not in accompany with the change of GFA.The effects of Zr and Sn addition on the thennal stability of the crystalline phase are then studied.It is revealed that the enhancement of GFA by alloying ends when a new precipitation replaces the repressed one.And in Ti-Cu system,this kind of replacement usually accours at the second or third exothermic peak,not the first one,which is the reason to the disaccompany with GFA.Also the expression of original cluster fonnula is not suitable after that,hence the e/u values are not meaningful when excess alloying.The application of this conclusion is appkied to the alloys[Ti-Cu6-ySiTi6.2Zr1.8]Cu3 and[Cu-Ti6.4Zr1.6Cu4-ySiy]Cu and the experimental results agree with the prediction.3?The increase of Mo content promotes the migration of CuTi2 to the low temperature zone,which leads to the decrease of the glass formation ability,and the precipitation of the solid solution phase of the beta-Ti base is found in the cast alloy samples when overdose addition.It can be concluded that the local component separation in the alloy is caused by the very low solid solubility of Mo in Cu,and the segregation of the early transition element is the reason that CuTi2 phase is easy to precipitate.The crystallization behavior of Nb element did not show obvious regularity,but the formation ability decreased rapidly with the increase of the content.The potentiodynamic polarization experiment shows that the addition of trace Mo and Nb can effectively improve the corrosion resistance of the alloy in PBS solution.The electrochemical impedance spectroscopy results show that the low frequency part should be related to the "liquid zone" of amorphous alloys.The addition of Mo and Nb also plays an important role in this region,and could improve the stability of the "liquid like zone",thus improving the corrosion resistance of the alloy.It is possible to speculate that the mechanism of the segregation of Mo and Nb in the "liquid like zone" leads to the enrichment of the early transition elements,thus improving the stability of the passivation film.The plastic deformability can be improved by adding Fe elements with extremely low solid solubility in Cu,and the plastic strain of the alloy[Cu-Ti6 4Zri.6Cu4]Cu0.25Fe0.25r0.5 is about 2.44%.Finally,the[Cu-Ti6.2Nbo.2Zr1.6Cu3.85Si0.15]Cu0.4Fe0.25Sn0.35 alloy was designed.The experimental results show that the alloy has a critical size close to 5mm,and has a certain plastic deformation ability,and shows good corrosion resistance.4?The application of cluster model in multiphase system is preliminarily explored.In several ETM-LTM systems,it is found that the average coordination number of clusters of the two solids in the phase transition reaction is different,while the difference of the two solid phases in the peritectic reaction is smaller,and larger for that of the eutectic reaction.Using cluster-plus-glue-atom model and e/u=24,the ideal amorphous composition in the Mg-Cu and Mg-Cu-Y systems are deciphered,and the eutectic point Mgs8Cu42 is interpreted by dual cluster formula model,that is,[Mg-Cu4Mg11]Cu2Mg+[Cu-Cu6Mg6]Cu=Mg19Cu14?=Mg57.58Cu42.42.This expressions is similar to SNDP-GC alloy,and the formation of SNDP-GC alloy can be regarded as a kind of "Eutectic" reaction in the broad sense.Using the cluster model of solid solution as the structural unit,the ordered-disorder transition of AuCu3 is investigated.When a Au atom alternates one of its nearest neighboring Cu,the corresponding order parameter can be calculated.The results show that the order parameter is-0.157,and close to the experimental value of-0.152 when the transition occurs at the critical temperature.And the corresponding enthalpy and entropy change can be calculated,and the results are close to the experimental values.