Fe-Ni Invar Alloys Research Articles

Oxidation behavior of Fe-Ni Invar alloy under high pressure: A ReaxFF molecular dynamics study

The Fe-Ni Invar alloy is often employed under extreme conditions such as high temperatures, high pressures, and corrosive environments, making it susceptible to oxidation and subsequent failure. Hence, a thorough understanding of its oxidation behavior is crucial. We performed ReaxFF-based molecular dynamics (MD) simulations to study the oxidation behaviour of Fe-Ni Invar alloy at the atomic scale under extreme conditions. The initial stage of oxidation involves the preferential adsorption and dissociation of O2, demonstrating its surface-site selectivity. The initial oxidation kinetics follows a logarithmic law, and the whole oxidation process results in dominant low-coordinated clusters configurations in the oxide film. Simultaneously, Fe atoms tend to donate more electrons to O atoms than Ni atoms. Moreover, the O2 consumption rate were found to increase with pressure and amorphous oxides formed more readily under high pressure. Our results indicate that adjusting the pressure may enhance the oxidation resistance of the Fe-Ni alloy, which is significant for the design and application of alloys in extreme conditions.

Unveiling the influence of zirconium on the corrosion behavior of Fe-36Ni Invar alloy

The influence of zirconium (Zr) on the corrosion resistance of Fe-36Ni Invar alloy in a 3.5 wt% NaCl solution was systematically investigated. Results indicated that adding Zr promoted the formation of Zr-bearing intermetallic compounds (Ni7Zr2 and Ni2Zr). The phases suppressed the grain growth and numerous dislocations were enriched around the Ni-Zr phases. Unfortunately, the incorporation of Zr into Fe-36Ni Invar alloy impaired corrosion resistance. The marked disparity in Volta potential between the Ni-Zr phases and the matrix signified the existence of micro-galvanic coupling, which caused the matrix to dissolve preferentially. Furthermore, the matrix around the Ni-Zr phases possessed high dislocation density, promoting the emergence of local galvanic corrosion. The introduction of Zr also decreased the protectiveness of the passive film, which was ascribed to the reduction of the thickness of the passive film and the beneficial oxide content in the passive film.

On the cyclic deformation behavior of wire-based directed energy deposited Fe-Ni Invar alloy

Fe-basis alloys with Ni concentrations of approximately 36 wt.-% are widely used for components with high requirements on mechanical properties and dimensional accuracy. Due to the remarkably low coefficient of thermal expansion, these so-called Invar alloys provide for dimensional accuracy over a wide temperature range. Generally, additive manufacturing technologies have already been qualified for processing of Invar alloys in terms of maintaining mechanical as well as functional properties. However, the mechanical behavior under cyclic loading still is not comprehensively understood. Therefore, the present study focuses on the fatigue properties of the Fe-Ni36 Invar alloy manufactured by wire-based directed energy deposition (DED) employing either an electric arc (DED-Arc) or a laser beam (DED-LB) as energy source. Here, a wire-based DED-LB Invar condition is comprehensively studied for the first time in open literature. For initial assessment, specimens were analyzed using optical and scanning electron microscopy as well as computed tomography and tensile testing. Here, the DED-LB material is characterized by a heterogeneous grain size distribution, less pronounced crystallographic texture as well as increased tensile strength and ductility compared to the DED-Arc counterparts. Despite increased strength and ductility, the DED-LB Invar consistently shows inferior fatigue properties. This behavior is discussed based on microstructural heterogeneity, cyclic deformation response, process-induced porosity and the contribution of plastic strain. Furthermore, investigations on the thermal expansion behavior show that the DED Invar retains its outstanding thermal properties, respectively.

Effect of Si addition on Curie temperature and thermal expansion coefficient of (Fe71.2B24Y4.8)96Nb4 Invar bulk metallic glasses

The ultra-low thermal expansion coefficient α makes the Fe-Ni Invar alloys useful in various applications. Their low strength and low Curie temperature Tc are, however, limiting factors. Interestingly, some Fe-based bulk metallic glasses (BMGs), with inherent high strength, exhibit the clear Invar effect. In particular, the (Fe71.2B24Y4.8)96Nb4 BMG has the lowest α among Fe-based BMGs, but it unfortunately also has the lowest Tc. In this work, silicon was added into this alloy with the aim to elevate Tc while maintaining a low α. It was found that when silicon partially substituted boron, Tc did not increase significantly but α did, which is not ideal. On the other hand, when silicon partially substituted yttrium and niobium and especially niobium, Tc increased significantly while α did not, which is close to the ideal scenario. When 3% of niobium was substituted by silicon, Tc reached the maximum value of 296 °C while α remained a low value of 7.4 × 10−6/°C. Comparing to the Fe-Ni Invar alloy, although this BMG has an inferior α, it has much higher Tc (+115 °C) and strength (∼9 times), presenting a potential for application as a new Invar material with moderate (low) thermal expansion, high operating temperature, and high strength.

Oxidation mechanism and microstructure evolution of invar alloy with high temperature annealing process

The high temperature oxidation behavior of Fe–36Ni Invar alloy in the forged billet stage and hot rolled rod stage in the compressed air atmosphere has been systematically studied. The results show that the oxide layer can be divided into the external oxide layer composed of Fe2O3, Fe3O4 and NiO, and the inner transition oxide layer composed of FeO, Fe2O3, Fe3O4 and NiO. In addition, hot rolled rod samples treated by solution and rolling have higher oxidation resistance. The mechanism of O diffusion during oxidation is analyzed, and it is found that the O atom adsorbed on the surface of the substrate diffuses into the grain along the grain boundary. The grain boundary is the optimal path for O diffusion to the interior of the material, which results in the degree of intergranular oxidation of the alloy being greater than the degree of intragranular oxidation. In summary, the oxidation resistance of Invar alloy can be improved by proper solution treatment and avoiding the formation of grain boundary cracks during production.

Unraveling microstructure evolution induced mechanical responses in coaxial fiber-diode laser hybrid welded Fe–36Ni Invar alloy

Invar alloys, renowned for its ultra-low coefficient of thermal expansion, are qualified for widespread use in aerospace applications. This study explores the utilization of fiber-diode laser hybrid welding in Fe–36Ni Invar alloy, specifically focusing on addressing the undesirable microstructure and properties arising from unstable molten pool and high-temperature gradient encountered during single fiber laser welding. Experimental and simulation analyses were conducted to systematically explore the influence of diode laser power on the microstructure evolution and mechanical responses of fiber-diode hybrid laser welded Invar alloy joints. Results indicate that the addition of diode laser could notably widen the keyhole opening and improve the molten pool stability. An acceleration of dynamic recrystallization is observed in the weld seam, with a significant increase in high-angle grain boundaries, which account for over 90 % of the total. Moreover, with increasing diode laser power, temperature gradient decreases, leading to the formation of numerous equiaxed grains. Electron backscatter diffraction (EBSD) results demonstrate a considerable enhancement in the intensity of the γ texture, particularly the {111}<110> texture. Comparatively, fiber-diode laser hybrid welding exhibits superior plasticity and tensile strength over single fiber laser welding, with a tensile strength reaching 493.1 MPa. The research findings presented here provide essential theoretical foundations and technical guidance for enhancing the quality of Invar alloy weld joints and facilitating the broader application of fiber-diode laser hybrid welding in the future.

Clarifying the role of Ce in the corrosion behavior of Fe–36Ni Invar alloy

The effect of Ce on the corrosion behavior of Fe–36Ni Invar alloy was investigated in a 3.5 wt% NaCl solution. The proper addition of Ce enhanced the corrosion resistance. The second-phase particles were gradually transformed in type, size, and number following the addition of Ce, which alleviated the localized corrosion behavior. The appropriate Ce content increased the fraction of low coincident site lattice boundaries. Furthermore, the enhancement of protectiveness and compactness of the passive film was achieved. However, overadded Ce led to the degradation of corrosion resistance. 0.0170 wt% Ce was advised for the sake of the enhancement of corrosion resistance.

Effects of Ce addition on mechanical properties and thermal expansion behavior of Fe-36Ni Invar alloy

To pursue superior yield strength and low thermal expansion characteristic of Fe-36Ni Invar alloy, Fe-36Ni Invar alloys containing various cerium (Ce) contents (0 wt%, 0.0050 wt%, 0.0170 wt%, and 0.0360 wt%) were prepared by vacuum induction melting, followed by the heat treatment process. With increasing Ce content, the evolution of the second-phase particles followed the sequence: MnO-MnS (0% Ce) → MnO-MnS + Ce2O3 (0.0050% Ce) → Ce2O3 + Ce2O2S (0.0170% Ce) → Ce2O3 + Ce2O2S + Ni5Ce (0.0360% Ce). The results indicated that the grain size and the fraction of annealing twins (fAT) were fairly sensitive to the Ce content. The smallest average grain size (d¯) and the highest fAT were obtained by adding 0.0170% Ce. The small addition of Ce was beneficial to the improvement of strength and low thermal expansion behavior. In particular, 0.0170% Ce alloy achieved the optimal combination properties. However, the excessive addition of Ce (0.0360%) induced a sharp decrease in mechanical performance and an undesirable increase in the coefficient of thermal expansion (CTE). The Ni5Ce phase was unfavorable to the enhancement of mechanical properties and low thermal expansion behavior. Last but not least, 0.0170 wt% Ce content was advised for Fe-36Ni Invar alloy by considering the balance of strength and thermal expansion behavior.

Elastic constants of equiatomic Fe–Ni Invar alloy single crystal

In a disordered fcc structure, the equiatomic Fe–Ni alloy is of great interest as a soft magnetic Invar alloy with a near zero or low thermal expansion coefficient. In the L10 ordered phase, it is of interest as a potential high-performance permanent magnet alloy. In this work, the first detailed measurements of the elastic constants in a [001]-oriented Fe-50 at. % Ni alloy single crystal under a no magnetic field condition were made using the resonant ultrasound spectroscopy technique. The elastic constants measured allow one to understand the nature of atomic bonding in a material and the mechanical response to applied stress. The single crystal sample having a rectangular parallelepiped shape was prepared from a single crystal grown using the vertical Bridgman technique. The elastic constants C11, C12, and C44 were obtained from the resonant frequencies observed over a frequency range of 100–900 kHz. Using these C11, C12, and C44 values, Young’s modulus, shear modulus, bulk modulus, and Poisson’s ratio were calculated to be 99.1 GPa, 73.4 GPa, 176.7 GPa, and 0.20, respectively. The large anisotropy factor of 3.29 highlights the strong directional dependence of the material’s mechanical behavior.

Effects of Trace Additions of Magnesium on Microstructure and Properties of Fe–36Ni Invar Alloy

Low‐thermal‐expansion alloys play a crucial role in high‐precision instruments and devices. Simultaneously improving mechanical performance and keeping or even decreasing low thermal expansion behavior are urgently required for their industrial application. Herein, a new attempt to treat Fe–36Ni Invar alloy by adding trace magnesium (Mg) in a concentration ranging from 0 wt% to 0.0030 wt% (similarly hereinafter) is conducted. The introduction of Mg results in grain refinement and an increase in the volume fraction of the annealing twins. Compared with the Mg‐free sample, the coefficient of thermal expansion (CTE) of 0.0030% Mg alloy is significantly decreased by more than 20%, which is mainly related to lattice distortion and matrix purification. The yield strength of 0.0030% Mg alloy improves by 10% with respect to Mg‐free alloy, because of grain boundary strengthening and solid‐solution strengthening. The study may lay the basis for a better understanding of the application of Mg in low‐thermal‐expansion alloys.

Effects of Bath Composition and Current Density on the Electrodeposition Behavior of Fe–Ni Invar Alloy

Fe–Ni invar alloy (Fe 64–Ni 36 wt%) has a very low coefficient of thermal expansion (CTE) than any other metals. For this reason, it has been used as fine metal mask (FMM) in RGB patterning of OLED manufacture process. However, as the resolution of OLED display is getting higher, the thickness of FMM is getting thinner and then the conventional extruded invar sheet cannot be used directly. The electrodeposition of invar can be the alternative for fabrication of thin FMM. In this study, the Fe–Ni alloy were electrodeposited varying bath compositions and current density. Also, the effects of ferric ion (Fe3+), produced during electrodeposition on inert anode, on the behavior of deposition were investigated. Finally, the Fe–Ni alloy with 36–40 wt% Ni were obtained at 50 mA/cm2 in 0.30 M Fe2+ bath. The back side and front side composition of the deposits were analyzed to evaluate the composition uniformity of the Fe–Ni alloys. Generally, the Fe content of back side of the deposits were higher than that of front side.Graphical

Enhancing low thermal expansion behavior and strength via induced Zr-rich intermetallic phase in Fe-36Ni Invar alloy

Nowadays, Invar alloy combining low thermal expansion behavior with improved mechanical properties is highly required. This study investigated the influence of Zr-rich intermetallic compounds (IMCs) on microstructure, thermal expansion behavior, and mechanical properties in Fe-36Ni Invar alloy. Zr-rich IMCs, namely Ni7Zr2 and Ni2Zr were introduced into Fe-36Ni Invar alloy by the addition of 0.22 wt% Zr. The microstructure of Zr-containing alloy was remarkably refined and accompanied by high dislocation density, which was associated with the resistance of recrystallization by the introduction of Zr-rich IMCs. The Invar alloy displayed a sufficiently low coefficient of thermal expansion (CTE) in the temperature interval of 20 °C ∼ 100 °C and 20 °C ∼ 200 °C after Zr treatment, with a CTE constant of 0.66 × 10-6 /°C and 2.07 × 10-6 /°C, respectively. The existence of residual stress and refinement of grain were responsible for an obvious decrease in CTE. Meanwhile, the yield strength increased from 256 MPa to 401 MPa after Zr treatment. The strengthening contribution mainly arose from dislocation strengthening and grain refinement strengthening.

Effect of Heat Treatments on Magnetic Properties and Thermal Expansion Behavior in Two Distinct Types of Fe-Ni Invar Alloys

The casting process of Fe-Ni Invar alloy could drastically affect magnetic properties and thermal expansion. We have observed analogous trends of saturation magnetization and thermal expansion coefficient in two distinct types of casted samples, as we perform post-annealing at different temperatures. X-ray diffraction measurements show that all the alloys remain in the same face-centered cubic structure after different heat treatments without phase transformation. With the help of X-ray diffraction and differential thermal analysis, we have inferred that these changes in properties might be induced by magnetic lattice transitions and analyzed the possible reason for property differences in samples. This work delivers a perspective on the relationships between thermal expansion, magnetic properties, and heat treatments, which could help to improve the industrial assembly line design.

Visualization of the disordered structure of Fe-Ni Invar alloys by reverse monte carlo calculations

Reverse Monte Carlo (RMC) calculation was performed to visualize the atomic arrangement in a disordered Fe55Ni45 alloy, which is classified as an intermediate structure between the non-crystalline glass and crystalline structures. The optimized structure of the ferromagnetic phase at low pressures revealed that Fe and Ni atoms were displaced from a perfect fcc lattice to elongate the nearest neighboring Fe-Fe atomic pairs, therefore, Fe-Fe atomic pairs have longer bond length than Fe-Ni and Ni-Ni atomic pairs. Because the elongation becomes negligible during the pressure-induced destabilization of the ferromagnetic state, the elongation of Fe-Fe pairs is the atomic scale origin of the volume expansion due to a large magnetovolume effect. Compared with the atomic arrangement in the Fe65Ni35 Invar alloy, a relationship between Fe-Fe atomic pairs, the Invar effect and elastic anomalies in the compression curve is elucidated.

Effects of energetic copper ions irradiation on the structural and physical properties of Fe–Ni invar alloy

Effects of energetic copper ions irradiation on the structural and physical properties of Fe–Ni invar alloy

Thermal expansion of FeNi Invar and zinc-blende CdTe from the view point of local structure

Thermal expansion of FeNi Invar and zinc-blende CdTe was investigated from the view point of local structure using the extended x-ray absorption fine structure (EXAFS) spectroscopic data and the path-integral effective classical potential (PIECP) Monte Carlo computational simulations. In this Review article, first the quantum statistical perturbation theory is intuitively described to see different character concerning thermal expansion between the quantum and classical theories. The diatomic Br2 molecule is employed as a simple example. Second, the PIECP theory is briefly described to note advantages and disadvantages of this simulation technique. Historical background is also discussed for the EXAFS investigation of thermal expansion based on the quantum statistical theories. The results of the FeNi Invar alloy are then summarized. The origin of zero thermal expansion in the FeNi alloy is ascribed to the so-called Invar effect that implies the variation of the electronic structure of Fe atoms depending on temperature. Zero thermal expansion at low temperature is however found to originate from the vibrational quantum effect. It is also noted that the interatomic distances of Fe-Fe, Fe-Ni, and Ni-Ni pairs are slightly but meaningfully different from each other, although the alloy exhibit a simple fcc crystal. Such a pair- dependent difference is also true for thermal expansion and we will discuss thermal expansion from the local point of view, which is interestingly different from the lattice thermal expansion significantly. Finally, the results of the zinc blende (or diamond) structure are presented. Although the origin of negative thermal expansion in these tetrahedral crystals is known as a result of classical vibrational anomaly within the Newton dynamics theory, the quantum statistical simulation is found to be essential to reproduce the negative thermal expansion of CdTe. It is emphasized that the vibrational quantum effect and classical anharmonicity are of great importance for the understanding of low-temperature thermal expansion as well as the elastic constants.

Investigation of Temperature-Dependent Magnetic Properties and Coefficient of Thermal Expansion in Invar Alloys.

Invar Fe–Ni alloy is a prominent Ni steel alloy with a low coefficient of thermal expansion around room temperature. We investigate the correlation between magnetic properties and thermal expansion in cold-drawn Fe–36Ni wires with different heat treatment conditions, where the annealing parameters with furnace cooling (cooling from the annealing temperature of 300, 400, 500, 600, 700, 800, 900, and 1000 °C) are used. The variation trend of magnetic properties is consistent with that of thermal expansion for all samples, where the maximum appears at 600 °C -treated sample and 400 °C shows the minimum. The domain size and the area of domain walls determine the total energy of the domain wall, and the total energy directly determines the size of magnetostriction, which is closely related to the coefficient of thermal expansion. Also, the differential thermal analysis (DTA) shows endothermic and exothermic reactions represent crystalline transitions, which could possibly cause the abrupt change of magnetic properties and thermal expansion coefficient of materials. The results indicate that there is a certain relation between thermal expansion and magnetic properties. Besides the fundamental significance, our work provides an Invar alloy with a low coefficient of thermal expansion for practical use.

インバー型Fe-Ni合金膜の無電解めっき法による作製

インバー型Fe-Ni合金膜の無電解めっき法による作製

Tailoring thermal expansion of shape memory alloys through designed reorientation deformation

Manipulating macroscopic linear thermal expansion (TE) through microstructure engineering is emerging as a new research topic of shape memory alloys (SMAs). Here, we perform a modelling-and-experiment combined study to design and tune the coefficient of TE (CTE) of polycrystalline martensitic SMAs based on reorientation deformation. Using Landau thermodynamic theory of first-order phase transition, we prove that the martensite lattices of most SMAs possess intrinsic negative (positive) TE along the directions of transformation elongation (contraction). As a combined result of such intrinsic lattice-level TE and the extrinsic reorientation-deformation texture, the overall linear CTEs of martensite polycrystals decrease (increase) with the amount of reorientation stretching (contraction). The deformation paths and strain magnitudes are designed to obtain zero linear TE in 17 types of bulk SMAs and are verified by the available experimental data. Furthermore, through designed cross-rolling on martensitic NiTi sheets, we obtain ultralow in-plane CTEs (+0.13 × 10−6 K−1 ∼ +1.4 × 10−6 K−1), which are smaller than the reported values of SMAs and are even smaller than the commercial FeNi Invar alloy (+2.0 × 10−6 K−1). Our work provides a theoretical base to design SMAs with desired linear CTEs for many applications.

Effect of Additives on the Deposition Behavior and Micro Structure of Invar Fe–Ni Alloys with Low Thermal Expansion Electrodeposited from Watt’s Solution

Effect of Additives on the Deposition Behavior and Micro Structure of Invar Fe–Ni Alloys with Low Thermal Expansion Electrodeposited from Watt’s Solution