Study On Microstructure And Energy State Of Ti-Based In-Suit β-Ti Bulk Metallic Glass Composites

Amorphous alloys have unique properties due to their long-range disordered structural characteristics,but their wide application is limited by their lack of obvious macroscopic tensile plasticity at room temperature.Excellent mechanical properties are obtained by precipitating the crystalline phase in the amorphous phase to form an amorphous endogenous composite.The thermal stability and phase transition of amorphous composites are also important research contents of non-equilibrium metal materials,which are of great significance for the practical application of such materials.In this paper,(Ti_0.471Zr_0.34Cu_0.06Be_0.129)_100-xCo_x（x=3,1）amorphous composites（i.e.BT48-Co3 and BT48-Co1）were used as the research object.The cast microstructure of Ti-based bulk metal glass composites was systematically studied,as well as the thermal stability and high-temperature microstructure morphology during heating.By preparing Ti-based amorphous endogenous composites of different sizes and compositions,the influence of cooling rate and composition on their microstructure was studied.When the cooling rate is high,the size and volume fraction of the dendrite phase in the amorphous endogenous composites increase with the decrease of the cooling rate,and the content of stableβ-Ti elements in the dendrites will also decrease.However,when the cooling rate is low,the supercooled liquid phase and the dendrite phase will reach quasi-equilibrium between the two phases,and the dendrite phase will continue to mature,but its volume fraction and composition will no longer change with the change of cooling rate.The thermal stability of amorphous composites with different cooling rates and Co content was compared with the characteristic temperatures and components inβ-Ti.With the decrease of cooling rate,the precipitation temperature of medium-temperatureω-Ti of amorphous composites decreases,and when the Co content decreases,the transition temperature will also decrease significantly,which indicates that reducing the cooling rate and Co content will reduce the stability ofβ-Ti.Through the analysis of the energy changes of BT48-Co3 and BT48-Co1 amorphous composites with different cooling velocities at different heating stages,it is found that the cooling rate and Co content have a great influence on the energy changes.The cooling rate of amorphous endogenous composites was reduced,and the enthalpy of relaxation,enthalpy of crystallization and enthalpy of melting were reduced,and the amorphous composites tended to a more stable state with the decrease of cooling rate.The decrease of Co content will lead to the increase of relaxation enthalpy,crystal enthalpy and melting enthalpy of amorphous composites,which is conducive to the energy storage of amorphous composites.The microstructure characteristics of amorphous composites at high temperatures were explored by heating BT48-Co3 and BT48-Co1 amorphous composites to the temperature at the end of each phase transition,and characterizing their microstructure in a series of microstructures.Studies have shown that as the temperature increases,before the glass transition temperature,theβ-Ti in such amorphous composites will first be converted toω-Ti without significant changes,and thenω-Ti will be converted back toβ-Ti again during subsequent heating processes.In the crystallization stage,theβ-Ti in the amorphous composites will partially transform into needle-like and lamellarα-Ti,and the amorphous matrix will also precipitate Cu₁₀Zr₇ intermetallic compounds,which are more stable at high temperatures.As the temperature continues to rise,α-Ti will be converted back toβ-Ti,and Be₂Zr intermetallic compounds will be precipitated from the amorphous matrix at higher temperatures.The effect of BT48-Co3 on structural reorganization and viscosity was studied by comparing BT48-Co3 under different heating rates and different cooling rates.For BT48-Co3,in the process of slow heating rate,it will first undergo glass transformation to form a supercooled liquid phase region,and then the amorphous phase will crystallize to form a crystalline phase,and finally reach the melting point and become liquid.The glass transition temperature and the initial crystallization temperature move in the direction of high temperature with the increase of the cooling rate.