Corrosion Behavior,mechanical Properties And Superplasticity Of Ti Based Bulk Amorphous Composites

Room-temperature brittleness and strain-softening during deformation of bulk metallic glasses,and limited processability of shape memory alloys have been stumbling blocks for their advanced functional structural applications.In this paper,the Ti-Ni alloy with strong glass forming ability and shape memory effect were selected.Simultaneously,3mm diameter shape memory/amorphous composite material having a continuous tissue gradient prepared by the levitation suspend melting-water cooled Cu mold process.The functionally graded composites with continuous gradient microstructure can be obtained by controlling temperature gradient during solidification.The composite material has a functional gradient,the surface layer is a fully amorphous structure formed by rapid cooling,and the core is an austenite phase with good plasticity and toughness.Therefore,the shape memory alloy TRIP effect is applied to the plastification of metallic glass.On this basis,the superplastic deformation behavior of metal-glass composites in supercooled liquid region was investigated.The research contents include:corrosion behavior of materials in different corrosive media and effect of adding Zr element on its corrosion resistance;rheological behavior of Ti-Ni based metallic glass composite at room temperature;superplasticity of Ti-Ni based amorphous composite in supercooled liquid region.The materials were analyzed and characterized using x ray diffraction analyzer,universal testing machine,nanoindentation,transmission electron microscope,three-electrode system electrochemical workstation,Gleeble3500 thermal simulation test machine,differential scanning calorimeter,scanning electron microscope and energy spectrum analyzer.The conclusions are as follow:?1?In artificial seawater and simulated body solution?PBS?,compared with crystalline TC4 alloy,this alloy showed passive behavior.That is,the alloy exhibits high free corrosion potential that shows low corrosion thermodynamics tendency and owns low corrosion current density as well as high polarization resistance,indicating low dynamic corrosion rate.The surface morphology after electrochemical etching shows that the oxide film structure in the edge region of the alloy is compact and distributed evenly.Due to the low activity anion concentration in PBS solution,the alloy exhibited more excellent corrosion-resistance than in artificial seawater.No pitting corrosion pits and corrosion products were found after electrochemical corrosion.The oxide film in the edge area is more compact and uniform than in core area as well.The alloy,Zr element content is 0.02,0.04,0.06,exist a sharp crystal phase above the amorphous diffuse peak in 2?=35-50°,indicating that its structure is amorphous composite structure.And the crystal phase is different shape memory crystal phases.The addition of Zr increases the martensite content in the alloy,and the Zr=0.04 alloy has the strongest amorphous forming ability.It exhibits excellent corrosion resistance in artificial seawater and PBS solution.The addition of Zr element leads to the enrichment of Ti element.?2?The microstructure of(Ti_0.5Ni_0.5)₈₀Cu₂₀0 alloy is consists of amorphous matrix and shape memory crystal phase,forming random dense pile of amorphous structure in the edge area;precipitation phase for undercooling-austenite phase in the core area.At the same time,the alloy exhibits excellent mechanical properties at room temperature under compressive stress?yield strength 1000 MPa,fracture strength2750 MPa,plastic strain 25%?as well as work-hardening and serrated plastic flow at the plastic stage.The plastic-sawtooth rheology of the macroscopic plastic stage and the microstructure of the microscopically different plastic structures indicate that the serrated rheology of the composite is mainly based on the amorphous matrix in the initial stage of deformation.With the increase of external stress,the strain is increased.Therefore,the pinning effect and the dislocation removal effect between Cu atoms and dislocation is significant.At the same time,the stress field generated inside the dislocation material inside the crystal phase interacts with the stress field produced by the amorphous shear band,which makes the shear band of the amorphous matrix proliferate,making the pop-in phenomenon is the most remarkable.Therefore,the serrate flow of the material in the process is crystals interact with the amorphous matrix.At the later stage of material deformation,the limited presence of the austenitic phase in the alloy so that the TRIP effect is weakened and the solute atoms do not contribute to the effective pinning of mobile dislocations,making the sawtooth less sparse than the mid deformation.Meanwhile,the proliferative process of the matrix shear zone is also completed.Therefore,the sawtooth rheological phenomenon is weakened,eventually fracture happened.?3?The BMGC of(Ti_0.5Ni_0.5)₈₀Cu₂₀0 alloy exhibits good fluidity and stability in the supercooled liquid region.The best superplastic deformation temperature is 560?.This temperature is in the intersection of T>0.5Tm and?Tx.And the optimum deformation rate is 5×10^-4s^-1.At the same time,the macroscopic morphology of the deformation is the"single drum"morphology of anti"N"and oblique or normal shape.The material produces near Newtonian rheology at high temperature and low strain rate.In addition,the energy dissipation rate of the amorphous composite at560?and compression rate of 5×10^-4s^-1indicates that the material has the best processability under this condition,indicating that the region is suitable for a large number of deformation superplastic processing.When the deformation rate is 5×10^-4s^-1,the superplastic flow behavior of the BMGC,which needs about 40 atoms'place to complete a jump of atoms in flow process,depends strongly on the test temperatures and strain-rates.The strain rate sensitivity exponent and apparent viscosity demonstrate that the optimum superplastic forming temperature and strain rate are 560??belongs to T>0.5T_m and?T_x?and 5×10^-4s^-1,respectively.