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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.

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. • A novel fiber-diode laser hybrid welding process was successfully developed to join Fe–36Ni Invar alloy. • The diode laser widened the keyhole opening and improved the molten pool stability. • The incorporation of diode laser decreased the temperature gradient and facilitated dynamic recrystallization. • Superior tensile strength and elongation were obtained by controlling diode laser power. [ABSTRACT FROM AUTHOR]

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 + Ce 2 O 3 (0.0050% Ce) → Ce 2 O 3 + Ce 2 O 2 S (0.0170% Ce) → Ce 2 O 3 + Ce 2 O 2 S + Ni 5 Ce (0.0360% Ce). The results indicated that the grain size and the fraction of annealing twins (f AT) were fairly sensitive to the Ce content. The smallest average grain size (d ¯) and the highest f AT 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 Ni 5 Ce 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. • An approach for Fe-36Ni Invar alloy by Ce treatment was proposed. • The grain size and fraction of annealing twins were sensitive to the Ce content. • Improved mechanical properties were obtained by proper Ce addition. • The proper addition of Ce decreased the coefficient of thermal expansion. • The optimum Ce content of Fe-36Ni Invar alloy was 0.0170%. [ABSTRACT FROM AUTHOR]

The hot ductility of Fe-36Ni invar alloy doped with and without yttrium was investigated using a Gleeble-3800 thermal-mechanical simulator over the temperature range 850–1050 °C and the improvement mechanism of the hot ductility was analysed with a combination of SEM, EDS and OM. The results showed that Fe-36Ni invar alloy had a poor hot ductility below 1050 °C, which was mainly attributed to the presence of the grain boundary sliding and weak grain boundaries. The addition of 0.048% yttrium had a substantial improvement in the hot ductility of Fe-36Ni invar alloy over the whole testing temperature range especially at 950–1000 °C. At 850–900 °C, the improvement of the hot ductility was mainly associated with the grain boundary strengthening and the restriction of the grain boundary sliding because the addition of yttrium could reduce the segregation of sulfur at grain boundaries and refine the grain structure. At 950–1000 °C, the hot ductility was highly improved, which was owed to the acceleration and occurrence of dynamic recrystallization as a result of the refinement of the grain structure by addition of yttrium.

The oxidation of Fe-36Ni invar alloy at 1273–1473 K in 15%CO2-(0, 3, 5, 6)%O2-N2 atmosphere and the effect of the grain structure on the compressed cracks were investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and a microcomputer control universal material testing machine. The results showed that the oxygen content and the oxidation temperature had an obvious influence on the morphology of the external scales, which changed into only two-layer structures from three stratified structures when the oxygen content was 0% or the oxidation temperature was below or equal 1423 K. However, the subscales still consisted of a typical structure of the intergranular subscale and the intragranular subscale with different oxygen contents or oxidation temperatures. The growth rate of the scales did not depend on the partial pressure of oxygen when oxygen existed. The oxidation temperature had an obvious influence on the growth rate of the scales, the growth rate of the scales was slower below or equal 1423 K and faster over 1423 K. A linear relationship existed between the thickness of the scales and the square root of oxidation time, moreover, the estimated activation energy for the growth of the external scale, the intragranular scale and the intergranular scale were 165, 159 and 131 kJ/mol, respectively. The higher apparent activation energy corresponded to the thinner scale at each oxidation temperature. In addition, the compressed cracks were hard to propagate and grow in the specimen with equiaxed grain structures, which would result in the narrower and shallower compressed cracks and the better surface quality during the compressed process after oxidation.

The effect of Y and Mg on the solidification structure of Fe-36Ni invar alloy melted in a carbon tube furnace was investigated using digital camera, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results showed that Y or Mg addition could remarkablly refine the solidification structure and increase the fraction of equiaxed grains. Great amount of high-melting point compounds (Y2O2S, MgS and MgS-MgO) formed in the alloy with the addition of Y and Mg respectively. Based on the theory of lattice misfit, the effectiveness of high-melting point compounds as heterogeneous nuclei of γ-matrix was calculated. The calculation of the lattice misfit between Y2O2S, MgS and γ-matrix, which were far less than 12%, showed that Y2O2S and MgS acted as heterogeneous nuclei were highly effective. However, MgO could not serve as the heterogeneous nuclei because the lattice misfit between it and γ-matrix exceeded 12%.