基于激光直接能量沉积制备的 Invar/NiTi 双金属结构的界面组织及性能（特邀）

Objective Bimetallic structures, integrating the unique properties of two distinct materials, offer remarkable advantages for advanced engineering applications. This study focuses on an innovative Invar/NiTi bimetallic structure designed to combine the exceptional thermal stability of Invar alloy with the prominent shape memory effect and superelasticity of NiTi alloy. The integration aims to enable components to maintain overall dimensional stability under temperature fluctuations while leveraging the reversible deformation of NiTi alloy to counteract thermal expansion. However, existing multi-material systems involving Invar or NiTi frequently encounter critical challenges, including the formation of brittle intermetallic phases, accumulation of thermal stress, and interfacial defects. Furthermore, no prior studies have reported the direct bonding of Invar and NiTi alloys, leaving the issues associated with such a connection largely unexplored. Therefore, investigating the feasibility of Invar/NiTi bimetallic structures and comprehensively understanding their interfacial characteristics, phase evolution mechanisms, and mechanical properties are crucial for bridging this critical knowledge gap and providing an experimental foundation for the development of high-precision devices and multi-functional components. Methods Spherical Invar and NiTi alloy powders, prepared via gas atomization, are selected as raw materials in this study. Invar/ NiTi bimetallic structure samples are fabricated using laser directed energy deposition (LDED) technology. Industrial computed tomography (CT) is then employed to detect defects within the samples. On this basis, a scanning electron microscope (SEM) equipped with an energy dispersive spectroscopy (EDS) probe is used for precise analysis of the microstructure, elemental distribution, and elemental content, with a particular emphasis on the interfacial transition zone. Electron backscattered diffraction (EBSD) is applied to characterize grain morphology, crystal orientation, and phase composition from the Invar region, across the interface, and to the NiTi region. Additionally, thermodynamic calculations using the Scheil non-equilibrium solidification model in Thermo-Calc software assist in interpreting and validating the experimentally observed phase evolution paths. Finally, microhardness testing and room-temperature tensile testing are conducted to evaluate the mechanical properties of the fabricated Invar/NiTi bimetallic structure. Results and Discussions The Invar/NiTi bimetallic structure fabricated via LDED exhibits high metallurgical quality with minimal defects. Industrial CT analysis reveals an overall porosity of merely 0.0279% and a maximum pore diameter of 0.1153 mm, with no continuous defects such as cracks. A unique interfacial transition zone, approximately 30‒40 μm wide, forms as a result of elemental interdiffusion driven by Marangoni convection during the melting process. This zone displays significant microstructural heterogeneity, characterized by complex evolution of grain morphologies and phases. EBSD and SEM analyses reveal a distinct transformation in grain morphology: columnar grains in the Invar region evolve into a mixture of equiaxed and columnar grains in the fine-grained interface region, and further become columnar grains in the pure NiTi region. The fine-grained interface zone exhibits irregular eutectic structures. The phase evolution from the Invar region to the NiTi region is identified as follows: γ-(Fe, Ni) → γ-(Fe, Ni) + Ni₃Ti + Fe₂Ti → Ni₃Ti + Fe₂Ti → Ni₃Ti + Fe₂Ti + NiTi → Ni₃Ti + NiTi → NiTi. Thermodynamic calculations using the Scheil model show high consistency with experimental results, confirming that Ni₃Ti precipitates prior to Fe₂Ti, and that Fe-containing phases cease to form as Fe content decreases due to diffusion. This finding highlights the direct influence of Fe content on phase composition and precipitation sequence. The presence of brittle intermetallic compounds (Fe₂Ti and Ni₃Ti) in the fine-grained interfacial region significantly affects mechanical properties. The average hardness in the fine-grained interface zone exceeds 700 HV, reaching a maximum of 837.2 HV—substantially higher than the 130.6 HV in the Invar zone and 445.3 HV in the NiTi zone. This significant hardness increase is attributed to the combined effect of intermetallic precipitation and grain refinement. However, tensile tests reveal brittle fracture behavior, with a low average tensile strength of (182±12)MPa and elongation at break of (0.39±0.11)%. Despite the inherent brittleness of the interface, crack-free bimetallic structures are successfully fabricated, and this success is attributed to three key factors: first, the relatively small sample size (30 mm×15 mm×70 mm) facilitates residual stress relaxation; second, higher thermal input (high laser power, low scanning speed) enhances molten pool fluidity, enables gas escape, and reduces steep temperature gradients, thereby mitigating thermal stress accumulation; third, reduced interlayer dwell time maintains a higher overall deposition temperature, promotes continuous interlayer heat transfer, and allows gradual release of thermal stresses—ultimately improving metallurgical bonding quality and interface stability. Conclusions This study first successfully fabricates crack-free Invar/NiTi bimetallic structures using LDED technology. Industrial CT results indicate that the samples are generally crack-free, which verifies the feasibility of combining Invar and NiTi. The interfacial transition zone exhibits significant non-uniformity, characterized by the coexistence of irregular eutectic microstructures (e.g., lamellar and lath structures) and a complex grain transformation process that evolves from columnar grains to equiaxed grains and then back to columnar grains. The presence of intermetallic compounds such as Ni₃Ti and Fe₂Ti is also confirmed. The precipitation of Fe₂Ti and Ni₃Ti results in a significantly higher hardness at the Invar/NiTi interface than that in the Invar and NiTi base regions. Owing to the inherent rigidity and low plasticity of these intermetallic compounds, tensile samples exhibit brittle fracture, accompanied by low average tensile strength and elongation.