PSI - Issue 58

Juraj Belan et al. / Procedia Structural Integrity 58 (2024) 109–114 Juraj Belan et al. / Structural Integrity Procedia 00 (2023) 000–000

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The influence of precipitates on hardness can be felt only if they reach a critical size to affect the dislocation movement. Which phases are generated and in which amount, the shape and size of these structural components have a significant influence on final the mechanical properties of alloys and mainly on creep rupture life (Goodfellow 2018, He et al. 2005, Kim et al. 2021, Gonzalo et al. 2020). IN 718 must be annealed first to ensure the ageing constituents (Al, Ti and Nb) are dissolved in the matrix. If already combined, they will not properly precipitate and realize the optimal hardness of the alloy. When stress and creep resistance is expected, applications are restricted below 650 °C because γ  , meta-stable, rapidly overages under prolonged exposure at or above this temperature. A rapid coarsening of γ  - phase, solutions of both γ  and γ  and microstructural shift, from the coherent disk-like γ  phase to the stable, plate like δ phase of Ni 3 Nb, followed by a loss of strength and especially creep life (Kim et al. 2021, Gonzalo et al. 2022). The article is focused on the quantitative analysis of the IN 718 microstructure. The emphasis is on the influence of the grain size of the  solid solution, the coarsening of the  -phase and the precipitation of the δ-phase on dynamic properties (Charpy impact strength and fatigue characteristics). For this purpose, the techniques of quantitative metallography, observation and evaluation using a light microscope using DIC and SEM were used. 2. Experimental material and methods The experimental material, wrought superalloy IN 718, was supplied by BIBUS METALS s.r.o., Brno, Czech Republic in the form of a cut strip with dimensions of 10x10x55 mm. Melting No. HT3241 (VIM + ESR-electro slag remelted), chemical composition, selected mechanical properties and grain size are listed in Table 1. Mechanical properties are taken from the relevant material sheet unless otherwise stated. Heat treated by solution annealing 954°C/1h cooled in air, precipitation hardening 718°C/8.25h cooled at 100°C/1h in a furnace at temperature 621°C, duration 8.25h and subsequent cooling in air were applied. The grain size of IN 718 superalloy was evaluated and determined using ASTM E112-12 - Standard Test Methods for Determination of Average Grain Size (ASTM E112-12). This standard describes methods for determining the average grain size in metallic materials. Samples were taken from each melt in the transverse but also the longitudinal direction with respect to the direction of rolling. On all samples, the initial grain size was evaluated by the Saltyk method in accordance with the ASTM E112-12 standard. Grain size measurements were performed on etched samples using a Neophot 32 light microscope with NIS-Elements AR 5.20 metallographic software on cross-sections at ten different locations. The current magnification has been recalculated according to the inserted scale. In order to quantify the δ-phase in the initial state, in the state after annealing 800°C/72h and after high-temperature fatigue, quantitative metallography procedures, specifically coherent test grids, were used (Belan 2012). The dynamic mechanical properties of alloy IN 718 were obtained using a bending impact test (STN EN ISO 148 1 - Charpy's principle, blocky shape specimens with 10 mm x 10 mm x 55 mm dimensions without notch) and fatigue tests in the initial state and after annealing. This alloy was used for high-frequency high-cycle three-point bending fatigue tests with cycle asymmetry parameter R  1 and ambient temperature. In the case of this set of samples (the same dimensions as for Charpy's principle test), additional annealing at a temperature of 800°C ± 5°C/72h + cooling in the furnace was also used, in order to increase the volume fraction of the δ-phase.

Table 1. Chemical composition (wt. %) and selected mechanical properties of experimental material

IN 718

Ni

C

Cr

Mo

Co

Mn

Al

Ti

Nb

B

Ta

53.62

0.03

18.44

2.89

0.18

0.07

0.58

0.93

5.01

0.002

0.05

HBW 2,5/187,5

Grain size [G]

Grain diameter d [µm]

UTS [MPa] YS [MPa] A [%]

 TPt at 649 °C [MPa] Elongation at 649 °C [%]

1306 1084*

1062.5 893.6*

26.5 22.1*

399

689

6.3

8.5

18.9

* values at testing temperature 650°C

All sample sets for metallographic analysis were prepared by classic metallographic procedures optimized for nickel superalloys. The prepared samples were chemically etched – classically (black and white contrast - Marble and Kallings 2 etchers) or in color (Beraha III etcher). Marble etchant (4g CuSO4 + 20ml HCL + 20ml dist. H2O) was used to etch the initial