Thermal analysis and microhardness of nanostructured alloy Invar 36

Sahoo A., Medicherla V.R.

• 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.

Wegener T., Brenne F., Fischer A., Möller T., Hauck C., Auernhammer S., Niendorf T.

Specimens made from pre-alloyed Invar (Fe-36Ni) powder were fabricated by selective laser melting and stress-relief heat treated afterwards. A relative density of the fabricated parts of 99.6 % was determined by computed tomography. The microstructure and mechanical behavior under monotonic and particularly cyclic loading at ambient temperature were investigated. Results reveal a bimodal microstructure containing columnar and equiaxed grains with an average grain size of 75 μm and pronounced texture 〈001〉 || BD. The selective laser melted Invar is characterized by a homogeneous hardness distribution revealing no gradient with respect to the height of the fabricated parts. The mechanical response under monotonic tensile loading is characterized by a fairly ductile behavior. Total strain controlled low-cycle fatigue tests with varying strain amplitudes were performed. The cyclic deformation response is characterized by highly reproducible behavior in terms of stress amplitudes and number of cycles to failure. The corresponding half-life hysteresis loops reveal perfect Masing behavior. Post-mortem fractography revealed crack initiation in direct vicinity of the surfaces. The damage tolerance of Invar is found to be promoted by its high ductility. Finally, a low coefficient of thermal expansion, very similar to conventionally processed Invar, was shown by dilatometer tests in the temperature range of 0 °C–180 °C.

Krylova K.A., Bitkulov I.K., Mulyukov R.R.

Abstract Nanostructured samples of Fe - 36% Ni (Invar alloy) and Fe - 50% Ni alloys with a fragment size of about 100 nm were obtained by high pressure torsion using Bridgman anvils. The formation of the nanostructure leads to a decrease in the coefficient of thermal expansion of the Fe - 36% Ni and Fe - 50% Ni alloys by 2.2 and 1.2 times, respectively. Annealing of the Fe - 36% Ni alloy after nanostructuring leads to the formation of a dispersed bcc phase, which is not observed in the coarse-grained Invar alloy. The release of this phase affects the anomalous growth of thermal expansion (above the values typical for a coarse-grained Invar alloy) and the retention of saturation magnetization at temperatures above the Curie temperature of 260 °C. It is found that heating the nanostructured Fe - 50% Ni alloy above the Curie temperature (460 °C) saves the saturation magnetization. By analogy with the results for the Fe - 36% Ni, it can be assumed that this is due to the appearance of a dispersed bcc ferromagnetic phase. Detailed structural analysis will be carried out in future works.

Asgari H., Salarian M., Ma H., Olubamiji A., Vlasea M.

In the present research, structural analyses are conducted on Invar (64% Fe-36% Ni) samples manufactured by modulated laser powder bed fusion (LPBF) under different laser power regimes. Within the selected process window, the results obtained from X-ray computed tomography analysis indicate that the relative density of as-built samples increases with an increase in laser power. In addition, it is observed that irregular-shaped pores with the longest axis normal to the building direction are significantly reduced with increasing the laser power. Microstructural investigation suggests that with increasing the laser power, a transformation from conduction to transition and keyhole modes occurs. In X-ray diffraction patterns of the as-built samples, only fcc γ-phase is detected and no reflection of bcc α-phase is found. Texture measurements show that the intensity of 〈110〉 crystallographic orientation along the building direction of the as-built samples increases with increasing laser power. Coefficient of thermal expansion of the as-built samples is very low and comparable to that of conventionally manufactured Invar alloy. Furthermore, as-built samples fabricated at lowest (250 W) and highest (400 W) laser powers exhibit the lowest and highest thermal expansion displacement, respectively. Finally, the thermal expansion behavior and its correlation with structural integrity and texture are discussed.

Krylova K.A., Bitkulov I.K., Mulyukov R.R.

The influence of deformation nanostructuring and subsequent annealing on the temperature dependence of the thermal expansion coefficient of the Fe-36% Ni Invar alloy is studied. The nanostructured state was obtained by severe plastic deformation via high pressure torsion in Bridgman anvils. An anomalous contraction of the nanostructured Fe-36% Ni alloy was detected during heating in certain temperature ranges. The phase composition of the nanostructured Invar was studied to explain the behavior of the thermal expansion coefficient. The revealed anomalies of the coefficients are explained by the formation of a bcc phase not observed in the Fe-36% Ni Invar alloy in the coarse-grained state. The formation of this phase becomes possible due to a significant increase in the diffusion coefficient as a result of nanostructuring.

Nazarov A. .

Nagayama T., Yamamoto T., Nakamura T.

Electrodeposited Invar Fe–Ni alloys with 36 to 40 mass% Ni were prepared from plating baths containing saccharin as a stress reducer and containing various Fe2+ concentrations. The Invar Fe–Ni alloys contained of small amount of S (∼0.02 mass%). The coefficients of thermal expansion (CTEs) of the as-deposited Invar Fe–Ni alloys were approximately 9 to 11 ppm/°C and were larger than those of pyrometallurgically produced Invar alloys. When the alloys were heat-treated at 400 to 500 °C, their CTEs drastically decreased to approximately 5 ppm/°C. Furthermore, upon heat treatment at 600 °C, the CTEs reached approximately 2 to 4 ppm/°C depending on alloy composition; these CTEs are comparable with those of pyrometallurgically produced alloys. The as-deposited Invar Fe–Ni alloys were mainly composed of metastable bcc phases, resulting in larger CTEs. When the alloys were annealed at 400 °C or above, the equilibrium fcc phases became the predominant phases, accompanied by a drastic decrease of the CTEs. The bcc-to-fcc transformation led to a decrease of the CTEs and to thermal contractions. Upon the heat treatment, an S (sulfide) at bcc grain boundaries segregated not as a thin film but as a granular sulfide form at primary bcc grain boundaries in the electrodeposited Invar alloys. Upon heat treatment at 500 °C or above, bcc grain eliminated accompanied by fcc grain growth and the granular sulfide agglutinated further. Consequently, the agglutinating granular sulfide was entrapped in the matrix grains or grain-boundary triple-points of transformed fcc grains. In addition, it is considered that these two-type morphologies of the sulfide, as a thin film at grain boundaries or as a precipitate, will be determined by grain growth form during an annealing. Ductile behavior of the electrodeposited Invar Fe–Ni alloys was confirmed, irrespective of whether the alloys were heat-treated. Upon heat treatment at 400 to 500 °C, the Invar Fe–Ni alloys exhibited high strength with good ductility, consistent with their low CTE. After the heat treatment, no severe embrittlement of the electrodeposited Invar alloys was observed despite the codeposition of S because the S existed as a granular sulfide, thereby preventing grain-boundary embrittlement.

Qiu C., Adkins N.J., Attallah M.M.

Invar 36 samples have been fabricated by selective laser melting at a constant laser power but with varied laser scanning speeds. Some samples were further heat treated or hot isostatically pressed (HIPed). The obtained microstructures were studied using optical and electron microscopes, X-ray diffraction and electron backscatter diffraction techniques and the properties evaluated through both tensile testing and thermal expansion measurement. It was found that the as-fabricated samples show very low porosity (

Muluykov R.R., Pshenichnuk A.I., Baimova J.A.

Vicenzo A.

Nickel-iron alloy coatings were produced by electrodeposition from an additive free electrolyte, at room temperature and current density in the range of 1 to 5 A dm −2 , with Fe content up to 75 wt%. The structure and mechanical properties of the electrodeposited alloys are reported in the present work and analyzed focusing on structure-property relationships. In particular, the influence of the hydrogen evolution reaction is highlighted as a process factor affecting alloy phase structure, notably the composition limit of the γ-phase field. The variations of the mechanical properties with alloy composition are analyzed in the light of the concurrent modifications in phase structure and crystal size of the alloys. In particular, an assessment of the different factors influencing the hardness of γ phase alloys is proposed. Solid solution effects contribute significantly to the strength of γ phase alloys over a wide composition range, approximately from 5 to 25%, though a complex interplay between solid solution and Hall-Petch strengthening needs to be envisaged to account for the variations in hardness with composition over this range. Moreover, it is emphasized that with decreasing grain size, the increasing level of internal stresses and decreasing stiffness engender significant softening in nanocrystalline γ phase alloys with Fe content exceeding about 25%.

Wang C., Yuan S.Q., Yao C.G., Feng Z.P.

The element W, Mo with paremagnetism and grain refinement is selected according to the theory of magnetostrictive and fine grain strengthening method. The sample with low coefficient of thermal expansion (CTE) and high strength are made by W and Mo alloying of Fe-Ni36, forging and heat treatment. The mechanism of low coefficient of thermal expansion and high strength is analyzed by means of the results of Chemical analysis, metallographic and scanning electron microscopy, grain size, micro-hardness and CTE testing. The result shows: the strength of matrix is improved by W and Mo alloying, the CTE is lower at the same time.

Yokoyama T., Eguchi K.

We have investigated the anharmonicity and quantum effects in the Invar alloy Fe(64.6)Ni(35.4) that shows anomalously small thermal expansion. We have performed Fe and Ni K-edge extended x-ray-absorption fine-structure spectroscopic measurements and the computational simulations based on the path-integral effective-classical-potential theory. The first nearest-neighbor (NN) shells around Fe show almost no thermal expansion, while those around Ni exhibit meaningful but smaller expansion than that of fcc Ni. At low temperature, the quantum effect is found to play an essentially important role, which is confirmed by comparing the quantum-mechanical simulations to the classical ones. The anharmonicity (asymmetric distribution) clearly exists for all the first NN shells as in normal thermal expansion systems, implying the breakdown of the direct correspondence between thermal expansion and anharmonicity.

Park W.S., Chun M.S., Han M.S., Kim M.H., Lee J.M.

Austenitic stainless steels (ASSs), aluminum alloys, and nickel alloys are potential candidate materials for cryogenic applications owing to their superior mechanical properties under low temperatures. In the liquefied natural gas (LNG) industry, these materials are widely used in the construction of thermal barriers for the insulation systems of storage tanks and LNG equipment. Among the typical material nonlinearities of ASSs, phase transformation induced plasticity (TRIP)-related nonlinear hardening characteristics have been experimentally and numerically reported in a number of detailed studies [1] , [2] ; however, to the best of our knowledge, quantitative studies on aluminum and nickel alloys are not available for reference. Moreover, although these materials are used under various temperatures and strain rates, their temperature- and strain rate-dependent properties have not been determined thus far. In this study, a series of tensile tests is carried out under various temperatures (110–293 K) and strain rate (0.00016–0.01 s−1) ranges as a preliminary step in the overall process for understanding the material characteristics of ASSs, aluminum alloys, and nickel alloys. On the basis of the experimental results, the essential mechanical properties are summarized in a quantitative manner in terms of the temperature and strain rate. The strain-hardening rate and strain sensitivity, which can be used to describe cryogenic temperature dependent material nonlinearities, are also proposed for the selected materials.

Mulyukov R.R., Bitkulov I.K., Bukreeva K.A.

Tabakovic I., Inturi V., Thurn J., Kief M.

Electrodeposition of Ni 1− x Fe x ( x  = 0.1–0.9) films was carried out from a chloride plating solution containing saccharin as an organic additive at a constant current density (5 mA/cm 2 ) and a controlled pH of 2.5. X-ray diffraction studies revealed the existence of an fcc, or γ phase, in the range of 10–58 wt.% Fe, a mixed fcc/bcc phase in the range of 59–60 wt.% Fe, and a bcc, or α phase in the range of 64–90 wt.% Fe. The saturation magnetization, B s , of electrodeposited Ni 1− x Fe x alloys at the room temperature was found to increase with the increase of Fe-content and follows the Slater–Pauling curve, but deviates from as-cast bulk NiFe alloys. The coefficient of thermal expansion, CTE, of electrodeposited alloys at room temperature also deviates from as-cast bulk NiFe alloys. Annealing of α-Ni 36 Fe 64 alloy results in a martensitic α → γ phase transformation, which takes place between 300 and 400 °C. It was demonstrated that thermal treatment above 400 °C was necessary to obtain magnetic and mechanical properties similar to those to conventional Invar alloy. Annealing of α-Ni 36 Fe 64 alloy at 700 °C brings about a decrease of B s from 1.75 to 0.45 T. By controlling the annealing conditions of α → γ martensitic transformation, it is possible to adjust the CTE of Ni 36 Fe 64 alloy over the broad limits from 2.7 to 8.7 × 10 −6 /°C.

Sahni S.K., Bhowmick S., Upadhyaya A.

In this work, the molecular dynamics simulation method is employed to understand the sintering behaviour and mechanical properties of the Invar alloy. The densification behaviour of Invar alloy nanoparticles with different sizes at a fixed sintering temperature is investigated. The influence of external pressure is also simulated. Finally, the uniaxial tensile test is employed to study the mechanical response of the sintered product. The results show a qualitative relationship between particle size, external pressure, densification, and mechanical properties. Smaller particle sizes and higher external pressure promote densification. The uniaxial tensile results show that the sintered structure has a lower Young’s modulus than the bulk crystal because of the porosity, and the sample with high porosity has a low value of mechanical strength.

Khisamov R.K., Andrianova N.N., Borisov A.M., Ovchinnikov M.A., Timiryaev R.R., Musabirov I.I., Mulyukov R.R.

The results of the study of the effect of deformation nanostructuring on the formation of a cone-shaped relief on the surface of ultrafine-grained tungsten with an average grain size of 300 nm under high-fluence irradiation with argon ions with an energy of 30 keV are presented. The thermal stability of the resulting cone-shaped relief on the surface and the ultrafine-grained structure in the volume of tungsten under heating up to 1400°C has been studied. Changes in microhardness have been measured.