Phase Transformation Behavior And Properties Of NiTi Alloy Deposited By Electron Beam Freeform Fabrication

NiTi shape memory alloys are widely used in aerospace,biomedical and electromechanical drive fields due to their unique shape memory effect and superelasticity.But the problem of loss of functional properties caused by multiple processes and cutting difficulties in machining when preparing complex components greatly limit the rapid preparation of NiTi alloy components.Therefore,exploring a short-process processing technology suitable for complex components of NiTi alloys is a critical problem that needs to be addressed urgently.The aim of this thesis is to obtain NiTi alloys with uniform microstructure,stable mechanical properties and functional performances through the technical route of electron beam freeform fabrication（EBF³）and post-heat treatment,and to systematically investigate the basic scientific issues involved in this process,such as phase transformation characteristics,precipitation behavior,microstructure evolution,shape memory effect and superelastic response,to explore a feasible processing route for the rapid preparation of NiTi shape memory alloy complex parts.In this thesis,the effects of different process parameters on the forming quality,chemical composition,microstructure evolution,phase transformation behavior,mechanical properties,shape memory effect and superelastic response of EBF³-fabricated NiTi alloy were investigated using NiTi shape memory alloy wire as the raw material.And the EBF³-fabrication parameters applicable to single-pass multilayer NiTi alloy components was determined,i.e.,beam current of 45mA,traveling speed of 600mm/min and feeding speed of 3m/min.The NiTi alloy prepared under this deposition process has a relative density of 99.6%,and consists mainly of columnar B2 austenite and a small amount of B19’martensite at room temperature,while submicron-scale Ti₄Ni₂O_xprecipitates within and between the columnar grains.It presented a 98.95%shape memory recovery ratio and 55.78%compression superelastic recovery ability.And its martensite phase transformation is a one-step reversible transformation of B2（?）B19’during both heating and cooling,and the austensite finish temperature is close to room temperature.But the martensite transformation characteristics of the EBF³-fabricated NiTi alloy are strongly influenced by the thermal-force environment.Moreover,the microstructural differences in the building direction and horizontal direction of the EBF³-fabricated NiTi alloy make the mechanical properties and superelastic response display an obvious directional effects.And the as-deposited NiTi alloy always maintains a higher comprehensive properties along the horizontal direction.In order to further improve the mechanical properties and superelastic recovery ability of EBF³-fabricated NiTi shape memory alloy,an optimal remelting process assisted by a remelting beam current of 20mA was determined by introducing an interlayer remelting strategy with a small beam current based on a process with a beam current of 45 mA,a traveling speed of 600 mm/min and a feeding speed of 3m/min.Specifically,the NiTi alloys prepared under this process are still mainly composed of B2 austenite in cubic texture{001}<100>at room temperature,and the Ti₄Ni₂O_xprecipitates are distributed between the epitaxially grown columnar crystals and inside the grains.Its ultimate tensile strength and elongation are553.8MPa and 6.03%,respectively.Meanwhile,the NiTi alloy prepared by interlayer remelting process presents a single reversible martensite transformation between B2 and B19’during both heating and cooling.But the introduction of interlayer remelting promotes the volatilization of Ni elements,resulting in an increase of the martensite start temperature.And there is a dependence of-3.8MPa/°C between the critical stressσ^Msand M_sfor the stress-induced martensite transformation during tensile process.In addition,with the gradual increase of tensile stress,the stress-induced martensite（SIM）transformation in the NiTi deposit is discontinuous and intermittent.There is often a large strain at the SIM reorientation,which is easy to form local stress concentration and plastic deformation.The submicron-scale Ti₄Ni₂O_xprecipitates cause the austenite matrix to be tensile-strained,which promotes the formation of local dislocations at lower stress states.B19’martensite can use these dislocations as“nucleation sites”to take the lead in cubic texture{001}<100>austenite with a specific orientation relationship to shear into different martensite variants.When the tensile stress continues to increase,the stress-induced martensite variants exist in the form of（100）compound twins,while the austenite is characterized by the activation of{011}<001>and{011}<1-11>slip dislocations as a typical plastic deformation feature.A significant detwinning phenomenon occurs in the late tensile deformation,with SIM transforming to a single lamellar martensite.But a small amount of discontinuous{-1-11}typeⅠtwins also remain,and accompanied by obvious faults and local amorphous bands in severely deformed austensite.In this thesis,a heat treatment regime including the synergistic treatment of solution and aging is specified based on the interlayer remelting process,i.e.,10 h of solution treatment at 1000°C followed by 4 h of aging treatment at 500°C.The Ti₄Ni₂O_xin obtained NiTi alloy transformed from the original elongated rod-like to granular shape,and nano-scale lenticular Ni₄Ti₃precipitated in a orientation relationship of{123}<111>_B2//{11-20}<0001>_Ni4Ti3,which promoted the strength-plasticity coordination of the as-deposited NiTi alloy.Its tensile strength and elongation increased to 680MPa and 7.14%.Meanwhile,the heat-treatment-modulated NiTi alloy exhibited a two-step phase transformation of B2→R→B19’during cooling process,and exhibited a good shape memory effect and superelastic recovery ability.But its superelastic response behavior was significantly dependent on temperature and strain rate.Specifically,at the same strain rate,the superelastic response gradually changed to a nearly linear mode with increasing ambient temperature,and the superelastic recovery ratio was characterized by increasing and then decreasing.However,at the same test temperature,the response hysteresis during superelastic cycling of NiTi alloys decreases with increasing loading rate,and the superelastic recovery ratio decreases overall with increasing strain rate.In fact,dislocation plugging caused by Ti₄Ni₂O_xand Ni₄Ti₃precipitates and plastic deformation caused by dislocation slip in the（011）[001]slip system of austensite are mechanism for the decline of superelastic recovery ability of NiTi alloys.