Tailoring Mechanisms Of Functional Properties In NiTi Shape Memory Alloys Fabricated By Selective Laser Melting

NiTi shape memory alloys（SMAs）are widely used in aerospace applications,sensing drivers,surgical instruments,and medical devices due to their unique shape memory effect,superelasticity,and excellent biocompatibility.However,the processing and machining of NiTi SMAs are challenging owing to their low thermal conductivity and high reactivity,which make it very difficult to post-process/machine pre-prepared NiTi SMAs.Hence,NiTi parts fabricated by conventional processes are usually simple in geometry,such as wires,plates,bars,and pipes.As a typical metal additive manufacturing technology,selective laser melting（SLM）shows remarkable advantages and can fabricate complex metallic parts directly compared with conventional processes.Therefore,in this work,we try to establish the relationship among processing parameters,microstructure,and functional properties for SLM NiTi SMAs,and consequently obtain tailoring mechanisms of functional properties in SLM NiTi SMAs,which can provide a theoretical basis for expanding applications of SLM NiTi SMAs.During the optimization of processing parameters for SLM NiTi SMAs based near-equiatomic NiTi powder,it is found that high energy density can not guarantee high relative density.The phase transformation temperature and the evaporation of Ni element increased with the increase of laser power,and decreased with the increase of laser scanning speed during SLM.The highest tensile strength and elongation of the as-fabricated NiTi sample were 776 MPa and7.2%,respectively.In the ten-cycle bending test,the bending deformation was fully recovered when the bending angle was 180°,and the recovery rate was as high as 97%when the bending angle increased to 360°.These good mechanical and shape memory properties were attributed to the formation of high-density dislocations and dispersed nano precipitates in the matrix of NiTi by optimized processing parameter.Microstructural observation shows that the change of process parameters caused the changes in the precipitation phase and dislocation density in the matrix of SLM NiTi SMAs,which is the underlying mechanism for tailoring shape memory effect of SLM near-equiatomic NiTi SMAs.In addition,the two-way shape memory strain of0.42%-0.84%was successfully obtained in SLM NiTi SMAs after shape memory training.For SLM Ti-rich Ni_49.4Ti_50.6 SMA,the results show that Ti₂Ni precipitates in the matrix can be homogenized by heat treatment.By complete solution annealing at 1000°C,the semi-continuous acicular nano Ti₂Ni precipitates along grain boundaries transformed into uniformly distributed spherical precipitates in the matrix.The tensile strength and fracture strain of the heat-treated NiTi sample with uniformly distributed Ti₂Ni precipitates were 880 MPa and22.4%,respectively.Compared with the results reported in previous works,the strength and fracture strain obtained in this work were improved simultaneously.In addition,the complete solution annealed NiTi sample had excellent recoverable strain of 5.32%and recovery rate of98.2%.Controlling the size and distribution of Ti₂Ni in SLM Ti-rich NiTi SMAs can effectively tailor their mechanical properties and shape memory effect,and it is found that uniform distribution of spherical Ti₂Ni precipitates in the matrix can impede the movement of dislocations,enhance the strength and ductility of SLM Ti-rich NiTi SMAs,and improve their shape memory effect.For SLM and heat-treated Ni-rich Ni_50.4Ti_49.6 SMA,the multi-step phase transformation behavior after solution and aging treatment resulted from the strain/stress field caused by the different nano Ni₄Ti₃ precipitates formed in the matrix.The three-step phase transformation was ascribed to the larger strain/stress field in the inner region of the grain compared to the grain boundary region.During the cyclic tensile loading,the recovery strains of aged Ni_50.4Ti_49.6samples gradually decreased to stable values.This was attributed to that formed dislocations after a certain number of tensile cycles reached a saturated state in aged Ni_50.4Ti_49.6 samples.The spherical nano Ni₄Ti₃ precipitates caused greater stress/strain field in the matrix relative to lens-like nano Ni₄Ti₃ precipitates with a large aspect ratio,which made the NiTi sample with spherical nano Ni₄Ti₃ precipitates form smaller plastic deformation during the tensile cycle.TEM results show that nano Ni₄Ti₃ precipitates with different sizes and morphologies had different interaction mechanisms with the dislocations formed in the matrix.Superelasticity of SLM Ni-rich NiTi SMAs can be tailored by controlling the size and distribution of Ni₄Ti₃nanoprecipitates efficiently.By nano Ni particles modifiing as-atomized Ni_49.4Ti_50.6 powder,the modified Ni_50.6Ti_49.4powder had a similar particle-size distribution to that of the as-atomized powder.The uniform distribution of nano Ni particles on the surfaces of the modified powder was due to their opposite Zeta potentials.The orderly constructed function domain in SLM Ni_50.6Ti_49.4 contained three characteristic zones:heat-affected zone,transition zone,and cellular grain zone.Although the matrix in all these zones was B2 austenite,they exhibited inhomogeneous precipitation behavior,with 1–3 nm Ni₄Ti₃ precipitates in the grain interior of the heat-affected zone,8–18nm Ti₂Ni precipitates in the grain interior of the transition zone,and 30–45 nm Ti₂Ni precipitates along the grain boundaries in the cellular grain zone,which can be explained by the specific thermal histories of the three zones.The results of cyclic compression demonstrate that modified Ni_50.6Ti_49.4 SMA exhibited a more stable recovery strain and smaller hysteresis area than those of as-atomized Ni_49.6Ti_50.4 SMA.The stable recovery strain and smaller hysteresis of modified Ni_50.6Ti_49.4 SMA were attributed to the formation of Ni₄Ti₃,smaller Ti₂Ni nanoprecipitates,and consequently the orderly occurrence of stress-induced martensite transformation in the three zones during loading.It is found that the addition of Ni particles can change the phase transformation temperature and microstructure of SLM NiTi SMAs,which provides a new strategy for controlling the microstructure and functional properties of NiTi SMAs by SLM.For SLM porous NiTi scaffolds,the pore size and porosity of porous NiTi scaffolds were smaller than the designed values owing to the formation of spatter powders on the strut of the scaffolds.The change in pore size did not affect the defect volume fraction or accuracy of porous NiTi scaffolds.The elastic modulus of porous NiTi scaffolds decreased with the decrease in pore size.The elastic modulus of P900（900μm pore size）,P835（835μm pore size）,and P618（618μm pore size）scaffolds were 2.45±0.10,2.02±0.12,and 1.75±0.08 GPa,which were similar to the elastic modulus of human bones.The recovery strain of porous NiTi scaffolds decreased with the decrease in pore size because a higher stress concentration occurred in the smaller pore-size NiTi scaffolds,which resulted in increased plastic deformation during loading.The porous NiTi scaffolds exhibited good in vitro biocompatibility,and their cell adhesion rates and cell morphologies were equivalent to those of dense SLM NiTi.These results make it possible for the application of porous NiTi scaffolds in orthopaedic implants.