Microstructure And Mechanical Properties Of TiNi Based Bulk Amorphous Composites

In this paper we choose(Ti_0.5Ni_0.5)100-x Cux alloy with shape memory effect and strong amorphous forming ability to have reaserch on influence on organization and mechanical properties of(Ti_0.5Ni_0.5)100-x Cux alloy system of the copper content（x = 0, 10, 15, 20, 25, 30,35, 40）.We have Optimized(Ti_0.5Ni_0.5)80 Cu20 alloy with the comprehensive mechanical properties.The fracture strength is 2246 MPa and the compression ductility is 12.2%.There are only the amorphous matrix,the over- cold austenite phase B2-Ti（Ni, Cu） and thermally induced martensitic phase B19’-Ti（Ni, Cu） no other intermetallic compound phase.It makes the amorphous matrix toughen by shape memory crystal phase TRIP effect in the process which we applied so much stress that the alloy samples happened deformation. It showed a strong hardening behavior.On this basis we have the study of the（Ti50Ni50-y My）₈₀Cu₂₀ alloy system, M=Zr or Co. It can improve the martensitic transformation temperature by adding zirconium.It can promote phase transformation of martensite in solidification microstructure.But it can obviously improve the amorphous forming ability. It can reduce the martensitic transformation temperature by adding cobalt.It can stabilize austenite phase separation in solidification microstructure. The composite compressive break（ing） strength of the amorphous is 2582 MPa when M=Co, y=0.02,plastic strain is 15%.On the basis of the above mentioned composition optimization semi-solid and pre-deformation annealing process were designed respectively to further optimize the organizational structure and improve the mechanical properties.This paper draws the conclusion as follows:（1）The amorphous forming ability of(Ti_0.5Ni_0.5)100-x Cux alloy presents a waveform change from rising and decreasing to rising with the increase of copper content,but the overall trend is decreasing.The Cu element in the TiNi composites in addition（about x=25） can improve the plasticity of Ti based amorphous materials.But adding more（x>30）, neither can improve the GFA nor can improve the strength of the alloy.In x=15, the there is the highest fracture strength 2440 MPa and the 17.15% plastic strain. It reached the higher yield strength value of1471 MPa.In X=25, the plastic strain of the alloy has been improved, and the plastic deformation has reached 21.35%.（2）GFA of the amorphous alloys increases and then decreases with the increasing of Zr.The temperature gradient of the solidification process determines the gradient of the composite material.It exisits mainly for the amorphous phase and martensite and austenite dendrite from the outside to the inside.With the increase of amorphous forming ability, the volume fraction of amorphous phase increases.In the whole research component system,the content of austenite decreased and the phase transformation induced plasticity decreased.Theplasticity decreased gradually and the strength increased at first and then decreased with the addition of Zr. The fracture strength of（Ti0.5Ni0.48Zr0.02）₈₀Cu₂₀ reached 2345 MPa, and the plastic strain reached 8%.With the increasing of cobalt content, the content of austenite increased, the transformation induced plasticity enhancing, and the plasticity of austenite decreased, and the strength increases at first and then decreases.（Ti0.5Ni0.48Co0.02）₈₀Cu₂₀’s comprehensive performance is the best. The breaking strength reaches 2582 MPa, and the plastic strain is 15%.It induced phase transformation during loading by deformation so that simultaneous enhancement and toughening to amorphous matrix.The comprehensive mechanical properties of the composite are excellent, and the main features are continuous yield and strong work hardening.（3）Microstructure of(Ti_0.5Ni_0.5)₈₀Cu₂₀ and（Ti0.5Ni0.48Co0.02）80 Cu20 are a composite structure of crystalline phase and amorphous phase.The crystal phase is a structure of over-cold austenite and heat-induced martensite.Stress loading induced transformation of austenite phase to martensite and preferential orientation, so that the strength and plasticity of the composite can be improved and realized.The heart of as cast specimen is large dendrite,and the growth is not uniform.After semi-solid treatment, the microstructure of the composite was optimized to obtain the fine grain size, high roundness and compact structure.With the increase of pre-deformation degree, martensite and austenite phases increased, while the martensite phase increased faster than austenite. Composite’s yield strength increase,plasticity decreases, and pre-deformation of the shaping stage can achieve yield strength controllable.