Invar Effect Research Articles

A novel method to relieve residual stress in dissimilar brazed joint via micro-scale additive manufacturing

To alleviate residual stress of ZrO2/GH536 brazed joints, the micro-scale structures were fabricated on ZrO2 via selective laser melting technology before its vacuum brazing with GH536 superalloy. The effects of micro-scale structures on the interfacial microstructure and residual stress of brazed joints were studied. The characterization results indicated that due to its reaction with excessive Ti elements from filler, the micro-scale structures prevented the detrimental effect of over-thick brittle reaction layer on the mechanical properties of joints. Furthermore, the Invar effect of the micro-scale structures facilitated the transfer of residual stress from the ceramic side to the micro-scale structures. According to the numerical simulation results, the maximum residual stress in the brazed joint with micro-scale structures decreased from 622 MPa to 310 MPa. As a result, the fracture path was changed from the flat shape to zigzag. This work reveals the stress-relaxed mechanism of micro-scale structures and provides a new method for fabricating the ceramic/metal joints with low residual stress.

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.

Study on the microstructure and properties of a laser cladding Fe–Ni–Al coating based on the invar effect

The purpose of this study was to investigate the effect of Al content on Fe–Ni–Al coatings. A Fe–Ni–Al coating was prepared using a semiconductor laser, and the influence of the Al content on the microstructure and properties of the coating was examined. The microstructure of the coating was characterized using scanning electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. The coefficient of thermal expansion of the coating was measured using a static thermomechanical analyzer. The microhardness and wear performance of the coating were analyzed using a microhardness tester and a wear testing machine. The results were as follows. The addition of Al to the Fe–Ni ferroalloy powder resulted in the in situ formation of an AlNi/Fe–Ni laser cladding layer. When the Al content was low, the coating mainly consisted of γ-[Fe,Ni] austenite. As the Al content increased, the matrix phase structure of the cladding layer transformed into the α phase. Consequently, the Invar effect was gradually compromised, leading to the generation of defects in the coating. When the Al content was 4%, the coating performance improved while maintaining a low coefficient of thermal expansion. At this point, there were relatively few cracks in the cladding layer, and it exhibited the best wear resistance.

Fabrication of low thermal expansion coefficient electrodeposited Invar alloy films by hydrogen annealing for OLED fine metal masks

This study unveils the dependence of the Invar effect on grain size. An electrodeposited Invar alloy film was prepared with near-zero inclusions and near-zero thermal expansion, which will advance its application in OLED fine metal masks.

Predictive Theory of Anomalous Volume Magnetostriction in Fe–Ni Alloys: Bond Repopulation Mechanism of the Invar Effect

Predictive Theory of Anomalous Volume Magnetostriction in Fe–Ni Alloys: Bond Repopulation Mechanism of the Invar Effect

Exploration of novel structures and related physical properties of Fe2Pt ordered alloys

The Fe–Pt phase diagram suggests the possible existence of ordered Fe2Pt structures, however, the ordered structures of Fe2Pt and related physical properties are unclear until now. In this work, four new low-energy ferromagnetic Fe2Pt ordered structures are predicted based on the particle swarm optimization algorithm. Their structural stability is verified by the formation enthalpy, mechanical stability and lattice dynamic stability. The order of the thermodynamic stability is Cmcm > Immm > I4/mmm > P3¯m1. All four structures are dynamically (except tetragonal I4/mmm structure) and mechanically stable. The fundamental physical properties including magnetic, elastic, phonon and thermal properties are investigated in detail. Their total magnetic moments, as well as the bulk modulus are almost the same. The shear modulus and Young's modulus of Cmcm structure are close to the P3¯m1, and larger than that of other two structures. The order of the elastic anisotropy is Immm > P3¯m1 > I4/mmm > Cmcm. The orthorhombic Immm structure can be regarded as a distorted tetragonal I4/mmm structure, and the distortion effectively improves the physical properties of the I4/mmm structure. The thermal properties at finite temperatures show that the trigonal P3¯m1 structure is more suitable for high-temperature applications compared to other structures. The results of linear thermal expansion coefficient indicate that anisotropy order of thermal expansion is Immm > I4/mmm > Cmcm > P3¯m1. More interesting is the discovery of the Invar effect along the y-axis direction in orthorhombic Immm structure.

Invar Effect in the Wide and Higher Temperature Range by Coherent Coupling in Fe‐Based Alloy

AbstractInvar alloys with zero thermal expansion (ZTE), isotropy, high operation temperature, and wide temperature range are of great significance in fields such as aerospace and modern industry. However, it is difficult to simultaneously meet the above characteristics in one material. The traditional methods like element or vacancies doping would increase the operation temperature but at the expense of ZTE behavior. Here, by the coherent coupling between two cubic phases, the isotropic ZTE metallic material with the higher cut‐off temperature (560 K) and a wide operation temperature range (ΔT = 200 K) is obtained in Fe2.75Co0.25PtB0.25. The operation temperature range is much higher than that of commercial Fe0.65Ni0.35 Invar alloy and meets the applications at elevated temperatures. High‐angle annular dark field images and synchrotron X‐ray diffraction prove that by the coherent interfacial coupling between the A1 and its coherent L12 nanodomains, the original negative thermal expansion or positive thermal expansion performances for them can be changed into the same ZTE performances. This thermal expansion regulation strategy realized by coherent coupling will lead to an innovative and significant revolution for ZTE design and promote its development and application.

Emergence of Invar effect with excellent mechanical property by electronic structure modulation in LaFe11.6-xCoxSi1.4 magnetocaloric materials

Materials with Invar effect, i.e. zero thermal expansion (ZTE), have important applications in precision manufacturing. Here we report the emergence of Invar effect with excellent mechanical property by modulating electronic structure in LaFe11.6-xCoxSi1.4, a family known for magnetocaloric effect. The observed linear thermal expansion coefficient (αl ∼ 1.5 × 10−8 K−1, 5–250 K, by XRD; αl ∼ 2.7 × 10−7 K−1, 109–250 K, by dilatometer) is superior to most other ZTE materials including the famous Invar alloy Fe0.65Ni0.35. By taking advantage of distinct resolution ability of neutrons and X-rays to Fe/Co adjacent elements, the site occupancy of Co atoms was determined by joint refinements of neutron powder diffraction (NPD) and X-ray diffraction (XRD). On the basis, ab initio calculations were performed on the atomically resolved electronic band structure. The incorporation of Co atoms alters the electron transfer properties by increasing 3d bonding electrons at Co sub-lattice, as a result, spontaneous magnetostriction is inhibited and Invar effect prevails in the ferromagnetic region of LaFe10.6Co1.0Si1.4. The electron local function (ELF) evidences the enhanced bonding strength and origin of excellent mechanical properties. The measured compressive strength can be up to ∼340 MPa, which endows the LaFe10.6Co1.0Si1.4 good machining ability. Further, the mode-mode coupling coefficient b calculated from density of states (DOS) explains the evolution of phase transition order with Co doping, which gives LaFe10.6Co1.0Si1.4 a significant advantage in its effective cooling capacity under low magnetic fields. The present study provides a solid foundation for exploiting multifunctional application of La(Fe,Si)13 alloys with Co incorporation from the perspective of electronic structure modulation.

The nature of the atomic-scale Invar effect in Fe-based bulk metallic glasses

Fe-based bulk metallic glasses (BMGs) universally show an anomalously low coefficient of thermal expansion below their Curie temperature. This effect is known as the Invar effect, is rarely seen in crystalline materials and also vanishes after the crystallization of BMGs. While it is known that Co and Ni reduce the strength of the Invar effect, the influence of other elements is unknown. Moreover, it is unclear to what extent structural modifications and magnetic interactions of minor alloying elements contribute to the Invar effect. Using synchrotron-based in-situ X-ray diffraction, we show that (Fe71B24RE5)96Nb4 BMGs with the rare earth elements (RE) = Tm, Er, Ho also reveal the Invar effect. This can be seen in the diffraction peaks that shift in accordance with the macroscopically measured thermal expansion. In comparison to a similar FeBYNb BMG, the Invar effect is not influenced by the substitution of Y with RE. This suggests that the minor alloying elements do not contribute to the Invar effect. Neither their influence on the atomic arrangement, the (anti)ferromagnetic interactions of the 4f electrons of the RE elements nor paramagnetic interactions such as those from Y and Nb have an influence on the Invar effect. These elements only serve to increase the glass-forming ability and the formation of a disordered Fe network but do not contribute themselves to the Invar effect. Additionally, their (anti-)ferromagnetic interaction with Fe does not influence the magneto-structural correlations of the Fe network. Therefore, the Invar effect in these materials not only originates at the atomic scale but can be attributed solely to the magnetic interaction in the disordered Fe network.

A novel stress-relaxing approach based on forming Fe–Ni invar effect interlayer for joining C/C composites

Taking advantage of the invar effect of Fe–Ni alloy at low temperatures, a novel stress-relaxing approach based on forming Fe–Ni invar interlayer was proposed for joining carbon fiber reinforced carbon (C/C) composites in this paper. Mixed Fe + Ni62Ti38 (at %) powders were used as the filler materials. Microstructure, mechanical property, and formation mechanism of the bonded joints were investigated. The results indicate that, based on the eutectic reaction of (Fe,Ni)–C at the C/C/interlayer interface and the in-situ reaction of Ti–C in the interlayer, a (Fe,Ni)–C eutectic reaction layer was formed at the C/C side interface and the Fe–Ni invar effect interlayer reinforced by TiC particles was obtained. Owing to the low thermal expansion coefficient and good plasticity of the Fe–Ni alloy matrix at low temperatures, the high residual thermal stress of the joint was effectively reduced and the shear strength of the joint was improved. With the increase of Ni content in the Fe–Ni alloy matrix of the interlayer, the shear strength of the joint decreases gradually. When the composition of the Fe–Ni matrix was designed to be Fe–36Ni, i.e., the invar alloy with the lowest thermal expansion coefficient at 100 °C, the average shear strength of the bonded joint reached the highest value of 36 MPa. The fracture of the joint occurred in the (Fe,Ni)–C eutectic reaction layer close to the C/C composite.

The mechanism of multi-directional Invar effect in a cross-rolling TiNb alloy

In this work, a binary metastable β-type Ti-41 Nb (wt. %) with a multi-directional Invar effect (i.e., near-zero thermal expansion) was fabricated through the cross-rolling (Cro-R), with an aim to elucidate the relationship between martensitic phase transformation and multi-directional Invar effect. It was found that the Cro-R TiNb alloy shows the Invar effect along the transverse direction (TD) and rolling direction (RD) across a wide temperature range (∼180 K), and the coefficient of thermal expansion (CTE) along the TD and RD are αTD = +0.64 × 10−6 K−1 and αRD = −0.016 × 10−6 K−1, respectively. Further experimental results show that the randomly oriented β parent phase and α″ martensitic phase are distributed in the tiny grains with a high density of dislocations. This high density of dislocations can effectively impede martensitic phase transformation from β to α″ that appears during the subsequent cooling, resulting in the martensitic phase transformation occurring over a wide temperature range. In such cases, the multi-directional Invar effect was achieved via the compensation between the negative thermal expansion (NTE) caused by martensitic phase transformation and the positive thermal expansion (PTE) due to the TiNb matrix lattice vibration during cooling. These experimental results act as a foundation for the design and development of the metastable β-type Ti alloy with a multi-directional Invar effect.

First-principles theory of the pressure-induced invar effect in FeNi alloys

Beside the classic Fe${}_{64}$Ni${}_{36}$ Invar alloy, which shows an anomalously low thermal expansion at ambient pressure, Fe-Ni alloys with a higher nickel composition can be driven to Invar behavior by pressure. The authors investigate the pressure induced Invar effect theoretically and demonstrate that it can be explained by a magnetic transition from a ferromagnetic state at high volume to a complex noncollinear state upon compression. They relate the Invar effect to the increasing contribution of magnetic entropy with pressure.

Alloy modification for additive manufactured Ni alloy components—part I: effect on microstructure and hardness of Invar alloy

Alloy 36 (1.3912), also known as “Invar,” is an alloy with 36% nickel. The alloy has a remarkably low thermal expansion coefficient in certain temperature ranges. This peculiarity is called the invar effect, which was discovered in 1896 by the Swiss physicist Charles Édouard Guillaume Sahoo and Medicherla Mater today Proc 43:2242-2244, (2021). Therefore, it is used in applications in which dimensional stability is critical, such as molding tools for composite materials in aerospace, automotive applications, or liquified natural gas (LNG) cargo tanks. Moreover, increasingly complex structures and the optimization of resource efficiency also require additive manufacturing steps for the production or repair of components Frazier J Mater Eng Perform 23:1917-1928, (2014); Treutler and Westling, (2021). Additively manufactured components have a heterogeneous microstructure and anisotropic mechanical properties Guévenoux et al. (2020). In addition, the manufactured components require subsequent machining surface finishing, like finish milling, to achieve their final contour. Nickel iron alloys are difficult to machine Zheng et al. Adv Mater Res 988:296–299, (2014). Additionally, inhomogeneous microstructure may lead to unstable cutting forces and conditions. In part I of this investigation, the initial alloy 36 is modified with the elements Ti, Zr, and Hf up to a maximum of 0.33 wt.-%. The influence of the modification elements on the microstructure as well as on the hardness of the AM components is examined. Furthermore, one modification is applied to metal arc welding process and investigated. Part II focuses on the effect of the alloy modifications on machinability as well as on the surface integrity of plasma-transferred-arc-welded (PTA) and finish milled invar components.

An in-situ study of low thermal expansion and internal stress evolution in FeMn alloys

The mechanism of low thermal expansion phenomenon in antiferromagnetic FeMn alloys needs to be revealed. In this work, the evolution of lattice thermal expansion and internal stress in FeMn alloys was investigated by in-situ synchrotron X-ray diffraction during thermal cycling. Preceding the martensitic transformation, the coefficient of lattice thermal expansion (CLTE) decreased and the full width at half maximum (FWHM) of the diffraction peaks broadened abnormally. The evolution of the lattice thermal expansion and the FWHM were both reversible and isotropic. The additional reflection spots around the fundamental diffraction spots in the electron diffraction patterns of austenite and the dispersed nano-scale martensite embryos in austenite observed in the dark field TEM images proves the existence of premartensitic transformation. Similar phenomena were found in FeMn Invar alloy, which indicate that the Invar effect is closely related to the internal stress evolution caused by premartensitic transformation. • In this paper, many abnormal phenomena were observed in the Fe Mn alloys. The lattice thermal expansion behaviors and internal stress evolution in FeMn alloys was investigated by in situ synchrotron X-ray diffraction. The correlation between low thermal expansion property and premartensitic transformation were revealed by comparing the similar abnormal phenomena during Invar effect and the premartensitic transformation. The highlights are as follows: • During the Invar effect of Fe 27Mn alloy and preceding the martensitic transformation of Fe 24Mn alloy, the coefficient of thermal expansion decreased and the FWHM of diffraction peaks of austenite increased abnormally. The evolution of thermal lattice strain and FWHM was reversible and isotropic. • Transmission electron microscopy observations confirmed that there are massive martensite embryos embedded in austenite phase during Invar effect and premartensitic transformation. The existence of martensite embryos caused the abnormal increase of FWHM and may be related to the low thermal expansion behavior of FeMn alloy. • These similar phenomena imply a close relationship between the mechanism of Invar effect and the premartensitic transformation in FeMn alloy. • Our results may help to reveal the relationship between Invar effect and premartensitic transformation in antiferromagnetic Invar alloys, and lead to an in-depth understanding of the mechanism of low expansion property.

Electrodeposition of Ni-Fe Alloy in Presence of Complexing Agent

The nickel-iron alloys have been widely used for versatile soft magnetic applications and electro-magnetic applications due to their magnetic properties for a long time. Also, it has been discovered that Ni-Fe alloy near 36 wt% Ni shows almost zero thermal expansion composition(CTE) in a wide temperature range (~150 ℃). The phenomenon showing low CTE in a specific composition is called the Invar effect. Due to the Invar effect, Invar alloys are used for temperature-sensitive applications such as oscillators and fiber-optic package parts. Invar alloy films are used for fine metal masks, an essential part of OLED fabrication. The invar film currently in use is fabricated using the pyrometallurgy method, more than 20um thick. However, miniaturization and high performance of the products require the thinner thickness of the Invar film, around 10um thick which shows the need for a bottom-up fabrication process of invar film. There are various methodologies to fabricate thin films, such as thermal evaporation, sputtering, CVD, and electrodeposition. Most fabricating processes are the vacuum process, a high-cost and non-continuous process. On the other hand, electrodeposition is a non-vacuum process, relatively low-cost, large-scale, and continuous process.Despite these advantages, there are some obstacles to gaining the Invar thin film by electrodeposition, such as anomalous deposition, composition uniformity, and higher CTE of electrodeposited Invar film. There are researches[1-3] about electrodeposited Ni-Fe thin films in the aspect of CTE, magnetic properties, crystal structures, etc. However, very few researchers have seen the effect of complexing agents other than specific complexing agents such as boric acid and ascorbic acid. In addition, Ni-Fe electrodeposition shows very different results depending on temperature and agitation, but no research confirms the effect on various complexing agents under the same condition.In this study, we synthesized Ni-Fe alloy thin film by electrodeposition in the presence of various complexing agents with different functional groups (amine, carboxyl, alcohol, etc.) such as glycolic acid, malonic acid, ethylenediamine, etc. on 20um stainless 304 foil substrates. The electrodeposition was conducted under the aqueous acidic bath(pH 2) at 55℃. Electrochemical studies were conducted to set appropriate potential and current density for electrodeposition. The thin film was characterized by SEM, EDS, XRD, and copper strip tests to observe the difference in composition, surface morphology, phase, and internal stress.

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.

Abnormal strain evolution of austenite upon martensitic transformation in FeNiCo alloy with invar effect: An in situ study

The lattice strain evolution is essential for the understanding of the invar effect. In this letter, the transformation behavior and lattice strain of austenite in a Fe66Ni29Co5 alloy is studied by in situ synchrotron X-ray diffraction during cooling. An abnormal increase of the lattice strain is observed upon martensitic transformation. The strain inhomogeneity is found to increase monotonically before transformation during cooling. The anomalies are suggested to be related to the restricted nature of the transformation, which is further evidenced by the abnormal lattice strain evolution in a laminated Cu/steel nanocomposite.

Invar Effect of the Fe13 Cluster.

The Invar effect, which refers to magnetic alloys having an anomalously small thermal expansion, has been under extensive investigations over the past century. However, study on the thermal expansion of clusters and their Invar effect is still lacking. In this letter, we calculate the thermal expansion properties of Fe13-nNin (n = 0, 3, 6, 9, 12, 13) clusters and find that the Fe13 cluster has an abnormally small thermal expansion, which is due to its remarkable magneto-volume effect. It is also revealed that the strong magneto-volume effect of the Fe13 cluster is highly related with the two magnetic states, the same as the interpretation for the Invar effect of a bulk alloy. The anomalously small thermal expansion of the Fe13 cluster is the manifestation of the Invar effect at the cluster scale.

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.

Pathways to Titanium Martensite

The structural relationship between the parent and product phases in the martensitic transformation from the parent β phase is described. The atomic movements leading to the martensite are accomplished by a long-range shear {112} < 111 > that transforms the parent to the product lattice and a short-wavelength displacement or shuffle {110} < 110 > that induces the correct stacking. The microstructures arising out of these paths depend on which of these modes initiates the transformation. When the shear precedes the shuffle, conventional martensite forms as dislocated laths or internally twinned plates. A signature of the {110} < 110 > shuffle that follows or accompanies the shear is present in both cases as stacking fault-related domains. The shuffle displacement precedes the shear with increasing β stabilizer addition, resulting in a nanodispersion of a structure with orthorhombic symmetry, which we have designated as O′. Al, Sn, Zr and O additions promote the shuffle. The O′ dispersion acts as embryos for the formation of nanomartensite on cooling or with the application of stress. The resulting continuous and controlled strain incorporation into the lattice within the constrained nanoembryos results in nonlinear superelasticity or the invar and elinvar effects. The stability of the bcc parent is discussed in terms of phonon mode or related elastic constant softening.