Grain coarsening behaviour of solution annealed Alloy 625 between 600–800 °C

Abstract

As with all alloys, the grain structure of the nickel-base superalloy 625 has a significant impact on its mechanical properties. Predictability of the grain structure evolution in this material is particularly pertinent because it is prone to inter-metallic precipitate formation both during manufacture and long term or high temperature service. To this end, analysis has been performed on the grain structure of Alloy 625 aged isothermally at temperatures between 600-800 • C for times up to 3000 hours. Fits made according to the classical Arrhenius equation describing normal grain growth yield an average value for the activation energy of a somewhat inhomogeneous grain structure above 700 • C of 108.3±6.6 kJ mol −1 and 46.6±12.2 kJ mol −1 below 650 • C. Linear extrapolation between 650-700 • C produces a significantly higher value of 527.7 ± 23.1 kJ mol −1 . This result is ultimately a consequence of a high driving force, solute-impeded grain boundary migration process operating within the alloy.Comparison of the high and low temperature values with the activation energy for volume self-diffusion and grain boundary diffusion identifies the latter as the principle governing mechanism for grain growth in both instances. A decrease in the value of the time exponent (n) at higher temperatures despite a reduction in solute drag is attributable to the Zener pinning imposed by grain boundary M 6 C and M 23 C 6 particles identified from Transmission Electron Microscopy (TEM) and Energy Dispersive X-ray Spectroscopy (EDXS) analysis.Vickers hardness results show the dominance of intermetallic intragranular precipitates in 1 the governance of the mechanical properties of the material with grain coarsening being accompanied by a significant increase in hardness. Furthermore, the lack of any correlation with grain growth behaviour indicates these phases have no significant effect on the grain evolution of the material.

Effect of Nb on δ Phase Precipitation and the Tensile Properties in Cast Alloy IN625

“…Although there are many studies in the literature on alloy IN625, most of the reports are about wrought alloy IN625 [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. So, studies on cast alloy IN625, such as the one presented in this paper, is needed to investigate its potential application in A-USC power plants.…”

“…According the studies on wrought alloy IN625, the δ phase is observed to precipitate easily after long-term aging at 700-760 • C (the service temperature of A-USC power plants) [9][10][11][12][13], although it is initially designed as a solid solution hardening alloy [14]. The δ phase is an equilibrium phase corresponding to the γ phase with a composition of Ni 3 Nb and an orthorhombic (D0 a ) crystal structure [15][16][17][18].…”

Mechanical Behavior of Inconel 625 at Elevated Temperatures

“…The value of core diffusion activation energy (Q(core)) for Nickel reported from Frost and Ashby [39] was 170 kJ/mol. Moore et al [40] found different values of self-diffusion activation energy for where B0 is a structure-dependent constant,  is the applied stress, R is the ideal gas constant, and T is the test temperature. The value of core diffusion activation energy (Q(core)) for Nickel reported from Frost and Ashby [39] was 170 kJ/mol.…”

“…The value of core diffusion activation energy (Q(core)) for Nickel reported from Frost and Ashby [39] was 170 kJ/mol. Moore et al [40] found different values of self-diffusion activation energy for The value of core diffusion activation energy (Q (core) ) for Nickel reported from Frost and Ashby [39] was 170 kJ/mol. Moore et al [40] found different values of self-diffusion activation energy for the Inconel 625 alloy according to the temperature.…”

Long Term Aging Treatment Impact on Ultralow Iron Alloy 625 Intergranular Corrosion Property

“…Because Nb element precipitate δ phase intergranular besides grain boundary carbides precipitation and it forms many peaks in scanning result. At750°C aging process, compared with ferrous Alloy 625, γ'' 'phase precipitation and γ'' →δ phase is delayed, which have good organizational stability [8,19,20]. In the process of aging, the generation and evolution of γ'' phase, δ phase, precipitation in the grain boundary and alloy element in the grain boundary affects the performance of the alloy [10,21,22].…”