Invar Properties Research Articles

Effect of subgrain microstructure on the mechanical properties of Invar 36 specimens prepared by laser powder bed fusion

Invar 36 samples were fabricated via laser powder bed fusion (LPBF) at various energy densities. In this work, a parameter range for the preparation of full-density Invar 36 samples was determined. The microstructural evolution and tensile properties of the materials were studied in the as-LPBFed state, and the role of subgrain structures on the fracture mechanism was investigated. According to the results of this analysis, as the energy density increased, the density first increased and then decreased. The maximum density (99.94 %) reached 65 J/mm3, and the simple density reached full density at 40–90 J/mm3. Due to constitutional supercooling, the microstructure of the alloy is composed of cellular subgrains and columnar subgrains. Columnar subgrains grow perpendicular to the boundary of the molten pool to the center of the molten pool, while cellular subgrains primarily exist in the remelting area of the molten pool. In the full density range, the ultimate tensile strength of the samples reaches 437–484 MPa, and differences in the morphology, distribution, and micronicomechanical properties of the subgrains are responsible for the differences in macroscopic properties. At 40, 65, and 90 J/mm3, the elastic moduli of the columnar subgrains are approximately 155.1 GPa, 161 GPa, and 162.4 GPa, respectively; the elastic moduli of the cellular subgrains are approximately 136.2 GPa, 140.2 GPa, and 139.9 GPa, respectively, which means that in the elastic deformation stage, the columnar subgrains are more resistant to elastic deformation; in the plastic deformation stage, different subgrains have different effects on crack propagation; the columnar subgrains grow perpendicularly to the melt pool boundary, which has a hindering effect on the extension of microcracks; and the cellular subgrains have a homogeneous grain size so that the microcracks can be easily extended along the cellular subgrain boundaries.

Effect of processing parameters on the microstructure, mechanical properties and thermal expansion behavior of Invar 36 alloy manufactured by laser powder bed fusion

Invar 36 alloy manufactured via the laser powder bed fusion (LPBF) process have attracted interest for their excellent low expansion properties. The properties of Invar 36 alloy, particularly thermal expansion, are highly sensitive to variations in process parameters. However, the effect of process parameters on microstructure and properties has not been systematically studied. This study examines the microstructure, mechanical properties, and coefficient of thermal expansion (CTE) of Invar 36 alloy under various processing parameters with different volumetric energy densities (VED). The results demonstrate that VED variations significantly impact the microstructure, mechanical properties, and CTE. Increasing VED leads to larger grain size and higher high angle grain boundaries, while geometric dislocation density and Taylor factor do not exhibit a linear relationship. At 89.29 J/mm3 VED, the Invar 36 alloy displays the optimal yield strength (405.4 MPa), tensile strength (489.3 MPa), and elongation (43.3%). The CTE meets the ASTM F1684 standard for all specimens. Lower relative density corresponds to smaller CTE, with the lowest CTE achieved at VED of 41.67 J/mm3. Furthermore, the influence of porosity on CTE diminishes with rising temperature. This research opens up the possibility of process parameters optimization for improved CTE and mechanical properties in as-printed Invar parts.

Design and analysis of an optical planar waveguide using Invar as core and silicon nitride as cladding layer to control birefringence

Abstract The main problem with optical planar waveguide is thermal stress which arises when layers inside the planar waveguide get fused together with different coefficient of thermal expansion (CTE) and due to variation in thermal properties, the problem of thermally induced strain and birefringence occurs inside the waveguide glass layer. In proposed work, the mathematical analysis of thermal induced stress and the thermal strain in waveguide glass layer is calculated. ‘C’ software is used for analysis of an optical planar waveguide to calculate the thermal stress. By using material properties of Invar (64 % iron-36 % nickel alloy) and silicon nitride (Si3N4) an optical planar waveguide is designed. Here when silicon nitride (Si3N4) is used as a cladding layer and Invar (64 % iron-36 % nickel alloy) as a core layer in an optical planar waveguide, the problem of birefringence get eradicated as zero thermal stress was achieved.

Magnetic properties of the FCC and BCC phases of (MnFeCoNi)80Cu[formula omitted] ([formula omitted]: Al, Ga) high-entropy alloys

In this work, the FCC-BCC phase transition in (MnFeCoNi)80Cu20−xZx (Z: Al, Ga) high entropy alloys were investigated on the basis of structural, magnetic, hardness and thermal expansion properties. It is found that the equiatomic MnFeCoNiCu alloy is FCC and exhibits mixed ferromagnetic/anti-ferromagnetic interactions with a Curie temperature, Tc, close to room temperature. However, the BCC structure emerges with increasing amount of Al (10 at%) and Ga (15 at%) with additional ferromagnetic interactions and associated Tcs. It is evidenced that in terms of the valence electron concentration and the average magnetic moment values, the investigated HEAs obey the Slater-Pauling rule in a region close to the Heusler alloys. Additionally, they may exhibit similar physical properties like Invar property with low thermal expansion coefficient with the 3d-transition metal-alloys having same valence electron concentration.

Directed energy deposition of Invar 36 alloy using cold wire pulsed gas tungsten arc welding: Effect of heat input on the microstructure and functional behaviour

Invar alloys exhibit low thermal expansion and are useful in applications requiring dimensional stability when subject to temperature changes. Conventional production of Invar faces certain challenges that can be offset by exploiting additive manufacturing processes. This study employed pulsed gas tungsten arc welding (GTAW) to deposit Invar 36 alloy blocks at five heat inputs (HI) ranging from 200 to 550 J mm−1. The results show that the microstructure comprised of columnar grains and remained in the austenitic phase regardless of the HI. Ductility dip cracking was found to prevail in all the blocks except the block deposited at the lowest HI. The decreased susceptibility to cracking with a reduction in the HI was due to the preservation of the grain boundary area, consequently leading to an improved partitioning of strain among the grain boundaries. On lowering the HI from 550 to 200 J mm−1 the average yield strength, tensile strength and elongation improved by 16%, 23% and 38%, respectively. The HI had a negligible effect on the mean linear coefficient of thermal expansion (CTE) in different temperature ranges as the CTE values were nearly identical between the blocks deposited at 200 and 550 J mm−1. In general, the CTE in the building direction was slightly higher than the travel direction, with a maximum difference between the CTE of the two directions being 15%. In summary, this work demonstrates the application of the cold wire GTAW process as an alternative to conventional/laser based methods for realizing the functional properties of Invar.

Investigation of cryogenic friction and wear properties of Invar 36 alloy against Si3N4 ceramic balls

The existing materials for liquefied natural gas (LNG) cryogenic submersible pump bearings are very susceptible to wear and failure in LNG environments where oil lubricants and greases are ineffective, posing a major hazard to the safety of LNG storage and transportation. Therefore, searching for a type of material with excellent cryogenic tribological properties for improving the life of LNG bearings has become a huge challenge for researchers. Invar 36 alloy, as a low expansion nickel-titanium alloy, is used in control instruments, moulds, laser control equipment, and materials of LNG transport hull at temperatures below −162 °C. However, there is a lack of research data in the field of bearings. Currently, the Si3N4 ceramic balls are widely used in ceramic hybrid ball bearings and exhibit excellent tribological properties. In this work, the tribological performance of Invar 36 alloy is mainly investigated against Si3N4 ceramic ball under extremely cryogenic and dry conditions using the modified ball-on-disk tribometer with a cryogenic module, with temperatures as low as −196 °C. All experiments are performed at loads of 0.5 N, 1 N, 1.5 N, and 2 N, and at temperatures of 20 °C, 0 °C, −78 °C, and −196 °C, respectively. The experimental results show that the coefficient of friction (COF) and wear rate decrease with temperature and load, with a minimum value of −196 °C. The Invar 36 alloy exhibits excellent tribological properties, whose COF decreases gradually from 20 °C to −78 °C. However, as the temperature drops to −196 °C, the COF decreases abruptly, reaching a minimum value at −196 °C. On the contrary, the COF gradually increases with load. In addition, a commercial bearing grade material (G95Cr18 steel) is tested and compared under the same conditions, proving that Invar 36 alloy exhibits better tribological properties than G95Cr18 steel, with a wear rate 55.43% lower than G95Cr18 steel at −196 °C. Further simulation comparisons demonstrate that, due to its extremely low coefficient of thermal expansion, Invar 36 alloy exhibits higher toughness and strength than G95Cr18 steel under cryogenic conditions. This study complements the tribological data of Invar 36 alloy and G95Cr18 steel against Si3N4 ceramic balls under different temperatures and loads and provides important theoretical guidance and technical support for the bearings’ future development of LNG cryogenic submersible pump.

Improving Tribological Behaviors of Invar 36 Alloy under Extremely Cryogenic and Dry Conditions Through Surface Micro-Texturing

Invar 36 alloy is an iron-nickel alloy with an ultra-low coefficient of thermal expansion, which is widely used in cryogenic fields of liquefied natural gas (LNG) and aerospace. However, the wear under extremely cryogenic and dry conditions is a key problem for Invar 36 alloy applied in the bearing of cryogenic submersible pump. In this work, we are mainly for improving the cryogenic tribological performance of Invar 36 alloy through laser surface texturing. Three different types of surface micro-textures were prepared on Invar 36 alloy specimens. And the effects of these micro-textures on the friction and wear properties of Invar 36 alloy were investigated. The results show that groove-channel micro-texture has the lowest coefficient of friction and wear rate under normal and -196 °C. Further research shows that this is because the channel structure of groove-channel micro-texture is more conducive to the transfer of most debris generated caused by wear, which successfully reduces the occurrence of abrasive wear. Scanning Electron Microscopy/Energy-dispersive X-ray spectroscopy was utilized to explain the cryogenic tribological performance of the micro-textures.

Multifunctional nanostructured NiTi alloy with Invar, Elinvar and Rinvar properties

A multifunctional NiTi alloy with Invar (dimension-invariant), Elinvar (elastic modulus-invariant) and Rinvar (resistivity-invariant) properties over the temperature range from 125 K to 375 K is fabricated through cold rolling of 60% thickness reduction. The underlying microstructure of such multifunctional NiTi alloy mainly consists of B2 and B19′ nanocrystals which can continuously undergo a small fraction of thermally induced B2↔B19′ phase transition over a wide temperature range. Such phase transition can induce a negative thermal expansion to compensate for the positive expansions of the B2 and B19′ phases and thus yield the Invar property. Meanwhile, the opposite temperature dependences of elastic modulus of the B2 and B19′ phases can compensate each other and cause the macroscopic overall Elinvar property. In addition, the presence of nanocrystals enhances the negative temperature dependence of resistivity (TDR) caused by boundary scattering, which compensates for the positive TDR due to background scattering and thus leads to the Rinvar behavior. The three compensation mechanisms in the dual-phase nanostructure can also be exploited in other shape memory alloys to obtain the Invar, Elinvar and Rinvar properties so as to broaden their applications. • With cold rolling of 50% thickness reduction, NiTi alloy can be tuned to possess both Invar and Rinvar properties. At 60% thickness reduction, it can possess Invar , Elinvar and Rinvar properties simultaneously over the temperature range from 125 K to 375 K. • The above multifunctional NiTi alloys have dual-phase nanostructures consisting of B2 and B19′ nanocrystals and can continuously undergo a small fraction of thermally induced B2↔B19′ phase transition over a wide temperature range. • Such phase transition induces a negative thermal expansion to compensate for the positive those of the B2 and B19′ phases, resulting in the Invar property. • Meanwhile, the opposite temperature dependences of elastic modulus of the B2 and B19′ phases compensate each other to cause the macroscopic Elinvar property. • In addition, the strongly negative temperature dependence of resistivity (TDR) caused by the boundary scattering of nanocrystals compensates for the positive TDR due to background scattering , thereby leading to the Rinvar behavior.

Thermal expansion behaviour of Invar 36 alloy parts fabricated by wire-arc additive manufacturing

Invar 36 alloy is of high interest in various industrial sectors, due to its reduced thermal expansion properties. This study aims to validate Wire-Arc Additive Manufacturing (WAAM) technology as a valid method for manufacturing aerospace tooling in Invar 36. The main novelty and the objective of this work is to study the properties of Invar deposited by WAAM technology and to provide guidelines for the manufacture of parts using this technology. To do so, the thermal expansion behaviour of Invar specimens manufactured using Gas Metal Arc Welding (GMAW)-based WAAM technology and Plasma Arc Welding (PAW)-based WAAM technology is analyzed for subsequent comparison with the values obtained from the laminated Invar sample used as the reference specimen. A wall is manufactured with each technology, for comparative purposes, from which specimens were extracted for the dilatometry test and metallographic analysis. The results of these analyses show the advantages of GMAW technology for the manufacture of Invar alloy parts, as it presents the same thermal expansion behaviour as the laminated reference material with less presence of precipitates and no macrostructural failures such as pores, cracks and lacks of fusion. Furthermore, to conclude, an aeronautical tooling that has been manufactured within this work demonstrated the potential of this technology to manufacture specialized aeronautical parts.

Impact of corrosion on mechanical properties of additively manufactured Invar 36

AbstractInvar alloys are used for high‐precision applications due to their invariance of the coefficient of thermal expansion over a temperature range up to about 250 °C. The present study determines the mechanical and corrosive properties of Invar 36 (64 % Iron – 36 % Nickel) specimens produced by laser powder bed fusion. Tensile specimens were fabricated to investigate the mechanical properties. The influence of a horizontal and vertical building orientation is considered. An apparent mechanical anisotropy depending on the respective building is observed. Based on this, further tensile specimens were stored in a hydrochloric acid pickling solution, a sulphate buffer solution and a sodium chloride solution to analyse the corrosion behaviour. The findings after eight weeks in the individual media show that outsourcing the Invar 36 specimens in a sulphate buffer solution results in significant changes in the mechanical properties, especially fracture elongation.

Metal matrix composites based on valence-unstable systems

The work is focused on the thermal expansion of composites based on valence-unstable systems. The main motivation was to create invar-like materials of a new type and to develop approaches to the quantitative description of coefficients of thermal expansion (CTE) of both the composite itself and its key component - a material with a negative contribution to thermal expansion due to valence fluctuations. The experimental data on the linear CTE of Al/SmB6 composite was performed by capacitive dilatometry in the temperature range 5-220K. It turned out that this composite shows invar properties up to 60K. Several models were used to describe thermal expansion in Al/SmB6. A quantitative approach is proposed for describing the negative magnetic contribution and the lattice contribution to the CTE of systems based on intermediate-valence samarium hexaboride.

Fe-Ni Invar alloys: A review

• Historical development of Invar alloys. • Explanation of Invar anomaly. • Review of magnetic properties of Invars. • Mixed ground state of Invars. Fe-Ni alloys with face centred cubic structure having Ni concentration around 36% exhibit extremely low or no coefficient of thermal expansion over a wide range of temperatures around room temperature which is called Invar behaviour. The Invar behaviour was first observed in Fe-Ni alloys by Charles Édouard Guillaume in the year 1896 and was awarded Nobel prize in physics in the year 1920. The observed Invar behaviour was puzzling and elusive for physicists. The origin of Invar phenomena has been the subject of serious concern for physicists since the observation of the phenomenon. The first model explained the Invar behaviour is called two gamma state model suggested by Weiss, considers two magnetic states of Fe in Invar alloys one with high spin high volume and another with low spin low volume states. When the temperature is increased low spin low volume states get populated at the cost of high spin high volume states and thus compensate the expected thermal expansion. However, this was shown to be incorrect and it seems that high moment to low moment transition is preceded by a frustrated ferromagnetic state. The moment volume instabilities in Invar alloys also lead to anomalous elastic properties. The Invar alloys find applications in the fabrication of watches, cryogenic storage dewars and aerospace engineering parts.

Mechanical properties of Invar 36 alloy additively manufactured by selective laser melting

Invar 36 alloy is widely used in aerospace engineering, owing to its extremely low coefficient of thermal expansion. This work aims to explore the underlying influence of process parameters on the microstructures and mechanical properties of Invar 36 additively manufactured by selective laser melting (SLM) with island scanning strategy. A full factorial design for a wide range of SLM process parameter sequence was established. Density and the mechanical properties including hardness, tensile strength as well as microstructures, fractography were characterized. The results confirm that the increment of exposure time and the reduction of point distance remarkably induce stable melting and achieve superior density and hardness. The laser energy density shows a pronounced effect on the microstructures, as extremely low laser energy density induces considerable lack-of fusion pores, unmelted powder particles and blurry boundaries of islands. While, very high laser energy density induces a few keyhole pores, and makes the laser scanning tracks and island boundaries transfer to be irregular. The optimal laser energy density is recommended to be 99.2 J/mm3, and the as-fabricated Invar 36 shows the relative density of 99.5%, Vickers hardness of 1.8 GPa and ultimate tensile strength of 480 MPa those are very comparable to conventionally fabricated one. The testing temperatures of 200 °C and 600 °C induce the coarsening of γ phase, and result in a significant degeneration of tensile properties. The correlations between the SLM process and mechanical properties provide experimental basis of the additive manufacturing Invar 36 alloy.

Structure and properties of invar alloys for MIM technologies

The paper studied Fe-Ni alloys obtained by sintering from mixtures of powders. Powders with 30-40 wt. Ni were mixed with paraffin and wax binder and then pressed and sintered at 1300 °C during 1 hour. It has been shown that the face centered cubic lattice (γ-phase) and body centered cubic lattice (α-phase) are formed after sintering of powders mixture. The content of the α-phase decreases from 37 % to 2 % with increase of Ni concentration from 30 to 35 wt. % Ni in powders mixture. The average grain size of the γ-phase decreases from 26.5±12.3 μm to 18.3±9.1 μm, and porosity of samples increases from 13±1.4 % to 18.4±1.1 %.The lattice parameter of the γ-phase increases from 3.5849·10−10 m to 3.597·10−10 m and it corresponds to well-known invar alloys. The maximum value of the microhardness of sintered samples is 1.04±0.07 GPa. Thus, alloys obtained by sintering of Fe and Ni powder mixtures has an invar properties typical of cast alloys.

Invar Properties in Ti-Alloys Achieved Through Alloy Design and Thermomechanical Treatments

Invar non-ferromagnetic Ti-alloys with zero thermal expansion coefficients are designed by optimizing both the alloy composition and the thermomechanical treatment. The thermal expansion coefficient is controlled in those alloys which exhibit high elastic softening, since unique microstructures tend to be formed in them by cold deformation. The strains and stresses stored in the alloys by cold deformation are released to some extent in the course of the subsequent heating treatment, which leads to either a negative, positive or zero thermal expansion coefficient, depending on the conditions of cold deformation and heat treatment. Such unique thermal expansion behavior is fully reversible in a certain temperature range.

Bonding strength of Invar/Cu clad strips fabricated by twin-roll casting process

Abstract Based on traditional twin-roll casting process, Invar/Cu clad strips were successfully fabricated by using solid Invar alloy strip and molten Cu under conditions of high temperature, high pressure and plastic deformation. A series of tests including tensile test, bending test, T-type peeling test and scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) measurements were carried out to analyze the mechanical properties of Invar/Cu clad strips and the micro-morphology of tensile fracture surfaces and bonding interfaces. The results indicate that no delamination phenomenon occurs during the compatible deformation of Invar/Cu in bending test and only one stress platform exists in the tensile stress−strain curve when the bonding strength is large. On the contrary, different mechanical properties of Invar and Cu lead to delamination phenomenon during the uniaxial tensile test, which determines that two stress platforms occur on the stress−strain curve of Invar/Cu clad strips when two elements experience necking. The average peeling strength can be increased from 13.85 to 42.31 N/mm after heat treatment at 800 °C for 1 h, and the observation of the Cu side at peeling interface shows that more Fe is adhered on the Cu side after the heat treatment. All above illustrate that heat treatment can improve the strength of the bonding interface of Invar/Cu clad strips.

Thermal expansion coefficients in Invar processed by selective laser melting

This work investigates whether the unique low thermal expansion property of Invar (64Fe–36Ni) is retained after processing using the additive manufacturing process selective laser melting (SLM). Using this process, near-full-density components (99.96%) were formed by melting thin (20 μm) layers of powdered Invar (15–45 μm particle size). The mechanical properties of SLM Invar were comparable to that of cold-drawn Invar36®; however, the thermal coefficient of expansion was observed to be a lower value and negative up until 100 °C. This negative value was attributed to residual stress in the as-deposited parts. The low thermal expansion property of Invar was still maintained when processed using a non-conventional layer-based additive manufacturing technique.

IMPACT OF COOLING CHARACTER ON STRUCTURE AND CTE OF INVARALLOY WITH 0,6% C

The qualitative and quantitative analysis of distribution of structural components (-phase + graphite) in the carbon-containing invar alloy with 0,6% C was conducted. It is demonstrated that velocity of crystallization produces a strong influence on the dispersion of graphite and on the volume fraction of graphite components in the alloy, i.e. at faster cooling the graphite volume fraction and its dispersion are noticeably higher than at slower cooling. The linear thermal expansion coefficient of both slowly cooled and fast cooled alloys maintains its values constant at heating temperatures lower 200 oC. However, in the alloy received as a result of fast cooling this coefficient values are lower than at slow cooling which can be explained by a lower content of carbon in the -phase, being a carrier of invar properties, of the slowly cooled alloy.

Structure and Properties of Nanostructured Vacuum-Deposited Foils of Invar Fe–(35–38 wt%)Ni Alloys

The process of electron beam vacuum deposition of the Fe–(35–38 wt%)Ni alloys at substrate temperatures Ts from 300 to700 °C were used to produce vacuum-deposited foils with the FCC structure, differing by the size of characteristic microstructural elements (grains and subgrains). It was shown that refinement of foil microstructural elements to nanoscale is accompanied by their microhardness increase up to 4–5 GPa. The change of the thermal expansion coefficient (TEC) of the nanostructured (NS) foil of the Fe–35.1Ni alloy within the temperature range from −50 to 150 °C has some deviation from that observed for cast Invar alloy of the same composition. It has been found that the main factors affecting the peculiarities of thermal expansion of the NS foil can be related to the presence of small fraction of BCC-phase in them, high level of crystalline lattice microstrains and inhomogeneous magnetic order in FCC-phase. It was shown that as a result of additional thermal treatment of NS foils their invar properties become similar to that observed for cast Invar alloy but mechanical properties remain on the same level.

Isotope effects on the vibrational, Invar, and Elinvar properties of pristine and hydrogenated graphene

Materials with Invar (Elinvar) property have a negligible temperature dependence in lattice constant (elasticity), and both pristine and hydrogenated graphene possess these two superior properties. The isotope effects on the vibrational, Invar, and Elinvar properties of them are studied using density-functional simulation. The increase in mass not only lowers the vibrational frequencies of (hydrogenated) graphene, but also influences the inter-atom and inter-branch hybridizations in hydrogenated graphene. However, the Invar and Elinvar properties have a negligible isotopic dependence, and the underlying mechanism is revealed by analyzing the isotope effects on phonon dispersions, anharmonicity, and thermodynamics. When carbon/hydrogen isotope is used to selectively engineer some properties (e.g., thermal transport and kinetic stability), such isotopic independence of the Invar and Elinvar properties can guarantee the minimized side effects on structure and stiffness.