Formation And Toughening Mechanism Of Direct Metal Deposited NiTi-based Alloys With Rare Earth Modified

With the continuous expansion of the application scope of NiTi alloy,higher requirements have been put forward for its comprehensive performance.However,the conventional casting and powder metallurgy method are difficult to manufacture NiTi alloy parts with large complex structure.And these NiTi alloy parts has some limitations in poor cold working performance,mechanical properties and wear resistance deficiencies,which have been unable to meet the demand of increasingly stringent engineering application.Additive manufacturing technologies such as direct metal deposition(DMD)have provided a new way to prepare NiTi alloy components with complex shape,controllable structure,large size and high performance.The research on additive manufacturing of NiTi alloy gradually become one of the focuses in the field of intelligent materials.However,the research of NiTi alloy manufactured by DMD is still in its infancy,and there are still some technical problems to be studied and solved,such as poor microstructure uniformity,insufficient mechanical properties and wear resistance.Therefore,in this paper,firstly,the forming mechanism of DMDed NiTi alloy is studied.Then,adopts La2O3 and LaB6 rare earths were adopted to regulate the solidification process,achieving effective regulation of tissue uniformity and significantly improving of mechanical properties In addition,the DMDed NiTi alloy was modified by Zr/N double ion implantation.Furthermore,the forming mechanism and related regulation mechanism of DMDed NiTi alloy before and after the regulation were systematically studied.And the influence microstructure characteristics before and after the regulation on the phase transformation characteristics,mechanical properties,tribological properties and superelasticity was also investigatedLMDed NiTi and PADed NiTi alloys are mainly composed of NiTi(B2)dendrites and NiTi2 phase at the intercrystalline or transgranular of NiTi phase,but the morphology and quantity of NiTi(B2)dentrite and NiTi2 phase are different.Among the three DMDed NiTi alloy,LMDed NiTi alloy possessed the best microstructure,phase transformation enthalpy,plasticity and elasticity strain,but the microstructure uniformity,intensity,hardness and cycle stability needs to be promoted.The PADed NiTi alloy was characterized by two-step phase transformation(B2 →R→B19’)in the cooling process and quasi-linear superelasticity due to the existence of a large number of NiTi2 phases closely related to the NiTi phase,which presenting a certain directional relationship with NiTi phaseThe solidification process of the single-channel DMDed NiTi alloy is as follows:(L→NiTi)→(L+NiTi→NiTi2)→(NiTi→NiTi2).The NiTi phases mainly presented slender columnar dendrites by epitaxial growth,and partly NiTi equiaxed dendrites were formed at the top of the melting pool by heterogeneous nucleation.The NiTi2 phase was mainly formed in the interdendritic of NiTi phase by peritectic transformation,and the other part was precipitated in the NiTi crystal.During the formation of multilayer multichannel DMDed NiTi alloy,the remelting action and heat conduction cycle heat affected directly the composition of molten pool,thus influencing the epitaxial growth of primary NiTi phase and peritectic transformation of NiTi2 phase.And the number of heat affect of LMDed NiTi alloy is less than that of PADed NiTi alloy,thus LMDed NiTi alloy possessed a better microstructureBased on the first principle,single rare earth element doping can reduce the C44 of NiTi(B2),namely can increase the Ms,but only La doping can simultaneously improve its rigidity and ductility,which is due to the fact that La doping can simultaneously strengthen the covalency of Ni-Ti bond and the ionic property of La-Ni/Ti bond.In the co-doping of La and non-rare earth elements,only the co-doping of B and La can reduce its C44 and improve its ductility and rigidity.This is because the co-doping of B and La can simultaneously enhance the covalency of Ni-Ti bond and the ionic bond of La-Ni/Ti,and form the covalent B-Ni/Ti bond.Overall,single La doping and its co-doping with B showed the best enhancement effectLaser direct deposition technique was used to prepare NiTi alloy with nano La2O3 added.La existed in the form of fine ellipsoidal La2O3 and LaNi precipitated phases.La2O3 is the effective heterogeneous nucleation particles of NiTi(B2)phase,significantly improving the heterogeneous nucleation of primary NiTi phase formation.In addition,the precipitates have nail binding effect relative to the NiTi and NiTi2.Thereby,the NiTi phases with large size of columnar crystals were transformed into into a tiny state of columnar crystal and the equiaxial crystal,and the grain of NiTi and NiTi2 phase were effectively refineAfter the addition of nano La2O3,the phase transformation temperature and enthalpy of the LMDed NiTi alloy were significantly increased,which was caused by the reduction of Ni element in the NiTi matrix phase,the reduction of C44 and the purification of grain boundary.The hardness,strength and fracture toughness of LMDed NiTi alloy were also significantly improved,which was related to the fine crystal strengthening effect of NiTi matrix phase and the dispersion strengthening effect of rare earth precipitated phase and NiTi2 phase.In addition,because of the enhancement effect of La2O3 on the phase uniformity and yield phase strength of NiTi matrix,the superelastic recoverable strain and its cyclic stability of the LMDed NiTi alloy were alos effectively improved.Under the effect of laser direct deposition technology,the NiTi alloy with quasi-continuous reticular structure was in-situ synthesized,and in-situ strengthening phases such as NiTi2,TiB,LaNi and La2O3 uniformly distributed around the matrix phase of NiTi.The phase transformation temperature and enthalpy of LMDed NiTi alloy were improved to some extent.On the basis of not affecting the plasticity of LMDed NiTi alloy,its hardness and strength were significantly improved,and its yield and tensile strength were increased by 73.5%and 31.1%,respectively.This was due to the combined effect of fine grain strengthening of NiTi matrix phase,load transfer strengthening of TiB phase,dispersion strengthening of rare earth precipitated phase and NiTi2 particle phase.The quasi-continuous network distribution of the strengthening phase guaranteed the connectivity of the matrix phase and inhibited the crack propagation and the bridge grounding.In addition,the super-elastic recoverable strain and its cyclic stability of the LMDed NiTi alloy were significantly improved by LaB6 added.In addition,a nanometer polycrystalline layer composed of TiO2,ZrO2,TiN and ZrN was formed on the surface of LMDed NiTi alloy with LaB6 added after Zr and N double ion implantation,which significantly improved the wear resistance of LMDed NiTi alloy with LaB6 added and reduced its friction coefficient from 0.45 to 0.15.