Study On Ultrafast Consolidation Of Bulk Nanocrystalline Titanium Alloy Through Ultrasonic Vibration

With the development of modern industry,more stringent requirements on the performance and cost of fabrication in titanium alloy is the main aim for the research.Nanocrystalline materials are polycrystalline materials with grain sizes about of nanometers?<100 nm?.Due to the grain boundary volume fraction in the structures increases greatly,nanocrystalline materials exhibit many excellent physical and chemical properties.However,crystal materials prepared by sintering,microwave sintering,self-propagating sintering techniques,due to the limitation of sintering speed,the grain coarsening during sintering.In view of this,we invented a method of ultrafast consolidation of bulk nanocrystalline titanium alloy,using titanium-based amorphous alloy as raw material,the bulk crystal material was prepared under the ultrasonic vibration,and the sintering mechanism,mechanical properties and crystal size were studied.First,Ti₆₆Nb₁₃Cu₈Ni_6.8Al_6.2 amorphous alloy powder was prepared by mechanical alloying.There are three stages in the mechanical alloying process:?1?cold welding dominates;?2?particle deformation causes cracking,propagation until particle breakage;?3?stabilization phase.The powders were subjected to DSC?Differential Scanning Calorimeter?tests using different heating rates.It was found that the powder crystallization activation energy was the highest and the thermal stability was the best at 100 h.The microstructure of the sized particle powder was observed by TEM?Transmission Electron Microscope?,the small Particle powder was an amorphous alloy,and the large particle powder was ultrafine crystal.Thereafter,the Ti₆₆Nb₁₃Cu₈Ni_6.8Al_6.2 amorphous alloy powder was consolidated under different cylinder pressures by ultrasonic vibration sintering method.It was found that the phase composition of the bulk composite material by the consolidated amorphous alloy powder at different pressures was the same: ?-Ti and?Cu,Ni?Ti2 phase.However,the volume fraction of the phase in the bulk composite material under different cylinder pressures is different.As the gas pressure increases,the volume fraction of?Cu,Ni?Ti₂ phase increases continuously.Finally,the microstructure of the near fully dense specimen consolidated under 400 k Pa,it is observed that some nanoscale particles are precipitated in the matrix by scanning electron microscopy.At the same time,it also can be found that well-defined interface layer with thickness of hundreds nanometers is located between the precursor powders,suggesting that some kind of fusion occurs at the adjoin region during consolidation.Based on the analysis,it can be considered that the ultrasonic vibration sintering process can be divided into four stages: rearrangement of powders,frictional heating,viscous flow induce by the flash heat,and crystallization of the specimen.In summary,in the ultrasonic vibration sintering process,the amorphous alloy powder is brought to the T_g point due to the friction between the powders,and is transformed into the supercooled liquid phase region,and viscous flow occurs,filling the gap between the powders,and accelerating the densification.At the same time,high heating rate can change the crystallization mechanism of the amorphous alloy powder,promote the nucleation rate,and inhibit the growth of crystal grains.Therefore,ultrasonic vibration sintering of amorphous alloy powder is a promising method for fabrication bulk nanocrystalline materials.