Compositions Design Of Ti-Ni Based Bulk Metallic Glasses Using Deep Eutectic Related Clusters And Their Properties

Ti-Ni based B2alloys are novel materials showing shape memory effect, super plasticity, excellent corrosion resistance and biological and medical compatibility. Ti-Ni based metallic glasses are promising precursor alloys of micro-grain B2phases. The present work is devoted to the development of B2-related Ti-Ni based bulk metallic glass (BMG) forming alloys, covering a series of issues on composition design, alloys preparation, phase identification, and mechanical and electrochemical property characterizations. Following the guideline of the so-called "cluster plus glue atom" model,[Ni-Ni3Ti9] was derived from the Ti-rich binary eutectic reaction related crystalline phases as the principle atomic cluster for construction of the cluster formulae of multi-component BMGs. Ribbon samples and alloy rods were prepared by means of melt-spinning and copper mold suction casting. X-ray diffraction and thermal analysis were employed to study the glass-forming ability (GFA), phase identification and crystallization kinetics. Microhardness and room temperature uniaxial compression test were used to characterize the mechanical properties of the as-cast alloys. Electrochemical polarization method was adopted to study the corrosion resistance of BMG alloys in various electrolytes. The results are summarized in what follows:1. The occurrence of Ti-rich deep eutectic point was attributed to the existence of close-packed icosahedral atomic cluster [Ni-Ni3Ti19] in the alloy melt. The cluster formula for binary Ti-rich metallic glasses was determined to be [Ni-Ni3Ti9](Ni)3by three Ni glue atoms. Using substitution alloying elements Zr, Cu and Sn, a series of quinary alloy compositions were finally obtained, namely,[Ni3.5Ti8ZrSn0.5](CuxNi3-x).2. Our casting experiment revealed that the alloys [Ni3.5Ti8Zr Sn0.5](Cu2.5Ni0.5)=Ti50Ni25Cu15.625Zr6.25Sn3.125and [Ni3.5Ti8ZrSn0.5](Cu3)=Ti50Ni21.875Cu18.75Zr6.25Sn3.125were good BMG formers, and fully glassy rod samples of2mm diameter were reached. The majority component phase in their3mm diameter as-cast rods, however, was a Ti-Ni based B2phase. The precipitation of B2phase was also found in the2mm diameter as-cast rods of Ti50Ni31.25Cu9.375Zr6.25Sn3.125and Ti50Ni28.125Cu12.5Zr6.25Sn3.125alloys. 3. Upon reheating at constant heating rates, Ti50Ni25Cu15.625Zr6.25Sn3.125and Ti50Ni21.875Cu18.75Zr6.25Sn3.125BMGs exhibited distinct glass transition, followed by two successive crystallization reactions. The activation energies for crystallization were determined to be495kJ/mol and335kJ/mol, respectively for the two alloys.4. The electrochemical polarization curves of the Ti50Ni25Cu15.625Zr6.25Sn3.125and Ti50Ni21.875Cu18.75Zr6.25Sn3.125BMGs in3.5mass%NaCl NaCl and1M NaOH aqueous solutions indicated that stable passivation films formed on top of the alloy surface. The two BMG alloys are highly stable against corrosion in these electrochemical enviroments by showing high puncture potentials and low passivation current densities. Whease in an1M HCl aqueous solution, passivation film was found to be not stable and active corrosion was readily occured in the alloys, instead.5. The mass densities of Ti50Ni25Cu15.625Zr6.25Sn3.125and Ti50Ni21.875Cui8.75Zr6.25Sn3.125BMGs are of6.22and6.14g/cm3, respectively. Their microhardness (Hv) values are measured respectively to be586and579. Under room temperature uniaxial compression, the engineering stress-strain curve of the Ti50Ni25Cu15.625Zr6.25Sn3.125BMG showed a elastic strain of～2%and a fracture strength of2400MPa. The alloy exhibit hardly any plastic strain before fracture.