Mechanical Properties,Strengthening And Toughening Mechanisms Of Ti-Ni Based Bulk Amorphous Composites

In this paper,the Ti-Ni alloy with shape memory effect and strong amorphous forming ability were prepared by the levitation suspend melting-water cooled Cu mold process.The microstructure,mechanical properties and toughening mechanism of the shape memory metallic glass composites,which obtained by component control and micro alloying were studied.(Ti_0.5Ni_0.5)₈₀Cu₂₀ alloy was optimized by adjusting the composition of Ti-Ni-Cu ternary alloy.And the microstructure is consist of glassy matrix,supercooled austenite phase B2-Ti?Ni,Cu?,thermally induced martensitic phase B19'-Ti?Ni,Cu?without intermetallic phases,presenting gradient microstructure in casting process of temperature gradient.Moreover,its excellent mechanical properties includes fracture strength 2246 MPa and compressive plasticity 12.2%.The deformation-induced phase transition at loading enhances the toughening of the amorphous matrix,so that the comprehensive mechanical properties of composite materials is excellent.And the mechanical behavior in room temperature is continuous yield and strong work hardening.The fracture surface has a large number of dense uniform vends,multiple shear bands formed around the precipitation phase and the shear zone on the glass substrate which exists direction 90°and 45°respectively with the loading direction.The amorphous composite material from the outside to the inside is showing the creep quantity and the plastic deformation gradually increases and the nanometer hardness gradually decreases.Ti₄₅Zr₅Cu_XNi_50-X?x = 5,10,15,20,25,30,35,40?and Ti₄₀Cu₂₀Ni_40-XZr_X?x = 2,4,6,8,10,12?alloys was prepared by the substitution of Cu instead of Ni or Zr instead of Ni in the Ti-Ni-Cu-Zr quaternary alloy.It was found that in Ti₄₅Zr₅Cu_xNi_50-X alloy system,the volume fraction of the shape memory crystal phase,strength and plasticity of the composites decreased first and then increased with the increasing of Cu content.And X = 5 alloy was optimized as the component with best comprehensive mechanical properties.The fracture strength and the plastic strain are 2485 MPa and 16%,respectively.In Ti₄₀Cu₂₀Ni_40-XZr_X alloy system,the X=10alloy was optimized as the component with best comprehensive mechanical properties.The yield strength,fracture strength and the plastic strain are 1180 MPa,2300 MPa and11.6%,respectively.For this alloy,the fracture strength is much higher than the yield strength,and no stress-induced phase transformation platform is found.The deformation process can be described as elastic deformation,continuous yielding and strong work hardening.Ti_40-xCe_xNi₄₀Cu₂₀?X=0.2,0.4,0.6,0.8,1.0?was obtained by adding rare earth element Ce.Addition of rare earth elements significantly improved the mechanical properties of the alloy.The alloy?x=0.2?had the best comprehensive mechanical properties,the fracture strength was2500 MPa,plastic strain was 12.0%;X=0.4 alloy plastic?14.5%?is the highest and strength reach to 2400 MPa.The influence of microalloying on the transformation and work hardening behavior of alloy was studied in(Ti_0.5Ni_0.48M_0.02)₈0Cu₂₀?M=Fe,Ce,Zr?alloy system.The micorstructure of as-cast alloy embrace B2-Ti?Ni,Cu?undercooled austenite phase,B19'-Ti?Ni,Cu?thermally induced martensite phase and amorphous matrix,proving the material is amorphous composite.The martensite diffraction peak of each sample after compressive stress loading is stronger than the cast alloys,and the amount of austenite transformed to martensite after stress loading,so that the amount of martensite precipitation increases.The work hardening behavior of three alloys was analyzed by the true stress-true strain curve.It was found that the work hardening behavior of the composites can be divided into three stages in the processing hardening rate-true strain curve and the instantaneous hardening rate-true strain curve.The mechanical hardening rate,the average work hardening index and the instantaneous hardening index of the alloy?M=Fe?are the highest in the alloy system,the yield strength of the composites is the lowest?920MPa?,the plasticity is best?13.1%?,The average work hardening index is the largest?n = 0.31?.The work hardening ability of M = Ce is slightly lower than of M = Fe alloy.However,in the work hardening rate-true strain curve and n *-true strain curve,the process of hardening is the longest and the comprehensive mechanical properties of the alloy are the most excellent,breaking strength?2645MPa?,plastic deformation of 12.2%.Adding Zr element will promote the phase transformation of martensite in the solidified structure and inhibit the formation of the austenite phase in the alloy structure,so that the work hardening ability of the alloy is the smallest.Therefore,alloy yield strength of M = Zr is the largest?1300MPa?Minimum?7.5%?.