Homogenization & Solution Treatment

After IN718 is cast or solidified, Niobium is one of the most segregated alloying elements.

Homogenization is typically the first step in post-processing of castings, seeking to reset Nb segregation by thermally activated diffusion. Homogenization of ingots has shown that Laves can be solutionized and Nb homogenized. [221] The presence of Laves phase and NbC may complicate post-processing, resulting in incipient melting at the GBs and aiding

intergranular liquation cracking. [222]

Homogenization of deformed cast ingots has been observed to lead to RX. [223] It was observed that delta phase limited growth below 1010°C, but also that NbC acted as a nucleation site for RX (Figure 45). Delta phase and new nucleation of grains were related to prior GBs, which were Nb rich regions. It has also been reported that 𝛿-phase needles can contribute to particle stimulate nucleation (PSN) during dynamic RX of high temperature tensile tests of wrought IN718. [224, 225] Recent work noted unexplainable recrystallization in IN718, [226] possibly due to high solution treatment temperature and a “higher amount of stored energy observed in the microstructure”. Ragged edges, large amounts of

twinning, and a “ghost” microstructure of delta characterize the recrystallized regions of the sample processed via Gleeble. High angle GBs are greatly reduced in the recrystallized region following heat treatment. Detailed, etched SEM images of precipitate structure in the deformed state, prior to HT, are not presented, but could provide clues as to the variance in resulting recrystallization regions. For cold-rolled material, 𝛿-phase, 𝛾^′, and 𝛾^′′

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are all found to inhibit recovery and RX. [227] Newly RX grains show persistence of 𝛿-phase on GBs and intragranularly (anneals were carried out below 900°C). There must be a minimum dislocation density present to initiate RX, where dislocation density has also been modeled for forgings to predict yield stress. [228]

Figure 45. Nucleation sites shown to occur near carbides in deformed IN718. [223]

Solution treatment relies on annealing above the solvus temperature of any precipitates that are to be solutionized. For IN718, the solvus temperatures of various phases are given in Table 9. Solution treatment to reset 𝛾^′′ and 𝛾^′ is common as a standard aging procedure can then be followed to obtain peak aged material, without overaging. As NbC solutionizes at 1260-1297°C in as-cast material and 1267-1305°C in homogenized, wrought material, [181] solution treatment to reset carbide formations from solidification is likely to result in significant grain growth. Banding of fine grains in IN718 forgings was eliminated using the multi-step homogenization process. [229] With higher homogenization temperature, carbides in the forgings grew in size due to Ostwald Ripening and also became more dispersed than clustered. Homogenization and solution treatments must therefore be balanced with the effect on carbide or grain growth.

100 III.5.2 Hot Isostatic Pressing (HIP)

Hot isostatic pressing (HIP) may be performed to remove internal porosity from material.

This step subjects material to high temperatures and high pressures, which causes voids to collapse. HIP of cast material [230, 231] has explored the impact of pressure and

temperature on microstructure and mechanical properties. Porosity was observed to be reduced 64% by HIP in the castings studied. The optimal HIP parameters were found to be 100MPa, 1180°C, and a 2-hour hold time. NbC phases are still present after HIP.

III.5.3 Aging

After a material is homogenized, it may be HIP’d or aged according to standard procedures.

“Direct aged” material is aged directly following receipt or a processing step. “Peak aged”

material has an optimal amount of aging defined by achieving a peak in hardness, as shown in Figure 46. Solution treatment can be used to “reset” any precipitate phases that have formed and typically precedes aging. The term “STA” may refer to either “solution treated and aged” or “solution treatment anneal”.

Aging in IN718 is a specific heat treatment designed to promote the formation 𝛾^′′ and 𝛾^′ strengthening phases. The standard aging process is to hold at 720°C for 8 hours, furnace cool to 620°C, hold for total furnace time to reach 18 hours, and then air cool. [94] This aging treatment starts in the 𝛾^′′/𝛾^′ precipitation region (720°C) to precipitate hardening phases, then holds below the precipitation range (620°C) to coarsen the hardening phases.

Cooling rate from solution treatment can have a significant impact on mechanical properties, as aging can occur during slow cool downs. [232]

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Figure 46. Hardness of IN718 as a function of temperature. Note that peak aged material is 466HV. [198]

III.5.4 Coarsening

As discussed earlier in §III.5.1, carbides may grow by Ostwald Ripening. Lifshitz-Slyozov-Wagner (LSW) Theory is a method for quantifying coarsening by Ostwald Ripening. [233]

Original formulations point to a particle size proportionality, 𝛼 𝑡

1

3 , for particles forming directly from super saturated solution. [234] The theory for calculating growth of existing particles is more complex, and is still the subject of ongoing research. The time law for LSW theory was found to apply to the coarsening kinetics of 𝛾^′′ and 𝛾^′ in IN718, but not to the particle size distribution of the phases. [235] Interactions between growing particles or strains from matrix misfit were suggested in this work as reasons for the difference in size distribution. The same work on application of LSW theory calculated the activation energy for coarsening of 𝛾^′′ to be 298±41 kJ/mol and that for 𝛾^′ to be 271±49 kJ/mol.

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Since the 𝛿-phase is incoherent with the matrix, the surface energy of the interface is expectedly higher than for the coherent 𝛾^′′/𝛾^′ interfaces. Additionally, the incoherent interface is expected to produce the most growth for a set diffusion coefficient. Since the diffusion of Nb is key to both 𝛿 and 𝛾^′′ precipitation and coarsening, the role of prior grain boundaries in non-uniform precipitation, as shown in Figure 47, is important to consider; Nb rich zones left behind by prior grain boundaries or precipitates may create preferred

precipitation sites. The kinetics of 𝛿-phase and 𝛾^′′ growth are not typically compared directly, however, due to the differences in precipitation kinetics.

Figure 47. Mechanisms for non-uniform precipitation in IN718. [223]

III.5.5 Overaging

Overaging results from the growth of 𝛾^′′ to sizes that are larger than is optimal for

strengthening. Additionally, overaging can result in the transformation of 𝛾^′′ to 𝛿-phase.

This is associated with a loss of strength, as the growth of intragranular 𝛿-phase consumes 𝛾^′′ that would otherwise be contributing to the high temperature strength of the material.

In the case of extremely long hold times (~50,000 hrs at 538-704°C), the formation of 𝛼-Cr and 𝜎-phase was found to occur near large delta phase particles. [178] This occurs because the growth of the 𝛿-phase rejects Cr and Mo to the matrix (which is needed for formation).

103 III.5.6 Alternative Post-Processing Methods

Alternative methods for post processing IN718 exist, but are mostly held as trade secrets or proprietary standards. Patents offer clues to some methods; a patented post-processing procedure claims the ability to produce IN718 with 𝛾′ as the primary strengthening phase.

[236] While it is unclear that the claims of this patented alternative method could be

produced (given the typical co-precipitation of 𝛾^′′ with 𝛾^′), it does show that industrial work has attempted to explore and optimize processing techniques.

For powder metallurgy (PM) of IN718, powders can be compacted into dense material by HIP. Oxides and carbides may form on prior particle boundaries (PPBs), reducing ductility of HIP material. The standard HT (980°C 1hr / 720°C 8hr / 620°C 8 hr) was found to not be suitable for PM IN718. [237] Modification of the post-processing of PM IN718 showed improvement in tensile and stress rupture properties. [238] This was accomplished by a high temperature initial solution treatment (1270°C 1hr) preceding HIP to breakup PPBs and dissolution of metal carbides MCs. The quantity of PPBs was found to be influenced by the oxygen content of the powder feedstock, but the oxygen content did not influence

formation of 𝛾′,𝛾′′, or 𝛿-phase. [239]

III.6 Mechanical Properties

IN718 must maintain mechanical properties under high-temperature operating conditions, for long life-times. To meet performance requirements, mechanical testing of test batched of material is standard. This section will explore the tensile, fatigue, hardness, and other mechanical properties that are relevant. Mechanical properties are determined by the microstructure of the material, which impacts the deformation mechanism of choice. The impacts of grain size, grain morphology, segregation, porosity, phase fraction, and phase morphology are all discussed.

104 III.6.1 Grain Size & Grain Morphology

In solution treated forged bar, the difference between 40um (ASTM 6) and 59um (ASTM 5) grain size showed no notable difference in YS, UTS, or elongation, but did have an observed variation in hardness of 261.7±7.2HV (ASTM 6) compared to 222.8±10.8HV (ASTM 5). [240]

Upon aging, there was no notable difference in YS, UTS, elongation, or hardness. This data supports the claim that IN718 is relatively insensitive to grain size, though the limits of this claim are not well defined for very large or small grains.

The relationship of grain size to hardness is not very strong, according to data from Schirra et al. (though there is some loss of hardness associated with very large grains as seen in Figure 48) [241] This points towards precipitate hardening, the main strengthening mechanism, as the most important consideration. The shape of grains (columnar vs.

equiaxed) and the orientation are important in materials that display anisotropic material properties. [242] For example, elastic modulus is known to vary with crystallographic orientation in nickel based superalloys.

In FCC Cu, it has been shown that grain boundary strengthening has a greater effect in columnar grains than in equiaxed grains due to relatively more grain boundary in a unit volume. [243] Work hardening is discussed to be of more impact in larger grains, whereas grain boundary strengthening is more important to smaller grains. However, this analysis was done with dissimilar grain volumes (columnar grain smaller in volume than cubic grain).

This of course implies that more GBs will be present within a unit volume. So, this may not be universally true. Also, the “grain size” was measured by a parameter “a”, which does not take into account variation in height of columnar grains vs. equiaxed. If the premise of this work is extrapolated to AM material, it may be useful to measure the ratio of GB length to grain area for both XY and XZ faces. This may give a method for quantifying variance in columnar grain size.

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Figure 48. Relationship of machinability, grain size, and hardness. Larger grains are less machinable, due to increases in work hardening from the machining process. [241]

III.6.2 Segregation

The effects of segregation of refractory elements to the inderdendritic regions of SX and 𝛾^′ -strengthened alloys is well studied. IN718 homogenizes more readily than many other nickel-based superalloys, as it is not dependent on the sluggish diffusion of Mo, W, or Rh.

[244] Due to this property of IN718, homogenization treatments are typically used to eliminate (or greatly reduce) the impact of segregation on mechanical properties. As previously discussed with post-processing, the main impact of segregation is during homogenization treatments, and the possible introduction of GB liquation cracking from NbC or Laves phase formations.

106 III.6.3 Porosity

The influence of porosity on mechanical properties has been generally noted to negatively impact properties from a reduction in area at the failure site. [129] This can impact tensile properties and fatigue life. If pores align or are otherwise non-random, the site can become a plane of weakness. The size distribution of pores is also noted to be unimportant; only the summation of the area reduction determines the impact. For pores with sharp corners (non-spherical), these statements may no longer hold.