Quasi-in-situ EBSD Study on the Microstructure and Tensile Properties of Selective Laser Melted Inconel 718 Alloy Processed by Different Heat Treatments

doi:10.1007/s40195-025-01887-1

Abstract

Abstract:

The effects of various heat treatments on the microstructures and mechanical properties of as-built selective laser melted Inconel 718 alloy were investigated through conventional and quasi-in-situ tensile tests. The corresponding heat treatment processes include direct aging (DA), solution + aging (SA), and homogenization + aging (HA). The DA and SA samples preserve the melt pool configuration and grain size stability, while the precipitated phase characteristics reveal the refinement of the long-strip Laves phase and the appearance of the δ phase, respectively. The HA process induces recrystallization and grain coarsening. The specimens exhibit enhanced strength concomitant with diminished elongation, which is likely attributed to the reduction of the geometrically necessary dislocation density and the intensified precipitation of the γ′′ phase after heat treatment. Tensile plastic deformation displays notable strain concentration along grain boundaries. The dimensional alterations in precipitated phases were measured to quantitatively determine the impact of grain boundary, dislocation and precipitation strengthening on the yield strength after heat treatment. Precipitation strengthening encompasses coherent, order, and Orowan strengthening. A remarkable agreement is revealed between theoretical predictions and experimental results. Insights are offered for optimizing heat treatment processes to comprehend microstructural evolution effect on the mechanical properties of additive-manufactured metals.

Key words: IN718 alloy, Selective laser melting, Heat treatment, Quasi-in-situ EBSD, Mechanical properties, Deformation mechanism

Yuanyuan Feng, Jianchao Pang, Xiaoyuan Teng, Chenglu Zou, Jingjing Liang, Yuping Zhu, Shouxin Li, Jinguo Li, Zhefeng Zhang. Quasi-in-situ EBSD Study on the Microstructure and Tensile Properties of Selective Laser Melted Inconel 718 Alloy Processed by Different Heat Treatments[J]. Acta Metallurgica Sinica (English Letters), 2025, 38(9): 1499-1512.

Figures/Tables 19

Fig. 1 a Linear arrangement along the block’s XY plane and b dimensions of tensile specimen

Table 1 Chemical composition of Inconel 718 powder (wt%)

Ni	Cr	Nb	Mo	Ti	Al	Co	C	Fe
53.43	19.42	4.11	2.97	1.02	0.48	0.28	0.02	18.27

Table 2 Heat treatment regimes

Samples	Solution treatment	Aging treatment
AB	-	-
DA	-	720 °C × 8 h/FC to 620 °C × 8 h/AC
SA	950 °C × 1 h/AC	720 °C × 8 h/FC to 620 °C × 8 h/AC
HA	1100 °C × 1 h/AC	720 °C × 8 h/FC to 620 °C × 8 h/AC

Fig. 2 SEM and EBSD images of SLM IN718 specimens after different heat treatments: a-c AB; d-f DA; g-i SA; j-l HA

Fig. 3 a Grain size and b aspect ratio of grain in the SLM IN718 specimens after different heat treatments

Fig. 4 Tensile properties of IN718 alloy after different post-heat treatments: a engineering stress-strain curves, b true stress-strain and work-hardening rate curves, c statistics of the yield strength, ultimate tensile strength and elongation d comparison data

Fig. 5 SEM images of tensile fracture surfaces of SLM 718 alloy with different heat treatments: a1-a3 AB; b1-b3 DA; c1-c3 SA; d1-d3 HA

Fig. 6 AB samples with engineering strains of 0%, 1.6%, 3.5% and 7%: a1-a4 SEM images; b1-b4 IPF maps; c1-c4 KAM maps

Fig. 7 DA samples with engineering strains of 0%, 1.6%, 3.5% and 7%: a1-a4 SEM images; b1-b4 IPF maps; c1-c4 KAM maps

Fig. 8 SA samples with engineering strains of 0%, 1.6%, 3.5% and 7%: a1-a4 SEM images; b1-b4 IPF maps; c1-c4 KAM maps

Fig. 9 HA samples with engineering strains of 0%, 1.6%, 3.5% and 7%: a1-a4 SEM images; b1-b4 IPF maps; c1-c4 KAM maps

Fig. 10 Average GND density of SLM 718 alloy with different heat treatments during quasi-in-situ tensile testing

Fig. 11 XRD patterns of a different SLM IN718 specimens and the fitted peaks of γ, γ′, and γ′′ phases of with varied heat treatments: b DA, c SA, d HA

Table 3 Volume fraction (f), lattice parameter (a and c) and lattice misfits (δ)

Sample	f_γ_′′(%)	f_γ_′ (%)	a_γ (nm)	a_γ_′ (nm)	a_γ_′′ (nm)	c_γ_′′ (nm)	δ_γ_/_γ_′	δ_γ_/_γ_′′
DA	10.07	3.16	0.3575	0.3585	0.3596	0.7638	0.0028	0.0239
SA	11.14	4.50	0.3574	0.3585	0.3599	0.7644	0.0030	0.0243
HA	16.96	2.81	0.3577	0.3583	0.3600	0.7647	0.0019	0.0235

Fig. 12 TEM bright field image of a DA, b SA and c HA samples

Fig. 13 Schematic of microstructure evolution of SLM IN718 after different heat treatments

Table 4 Taylor factor of different heat treatment of SLM IN718 specimens

Sample	DA	SA	HA
Taylor factor	2.26	2.40	2.34

Table 5 Yield strength increments of strengthening mechanisms (MPa)

Sample	Grain boundary	Dislocation	Precipitation			Total
Sample	$\Delta \sigma_{D}$	$\Delta \sigma_{\rho }$	$\Delta \tau_{{\text{coherency,}\gamma^{^{\prime\prime}} }}$	$\Delta \tau_{{\text{order,}\gamma^{^{\prime\prime}} }}$	$\Delta \tau_{\text{Orowan,Laves}}$	$\Delta \sigma_{\text{y}}$
DA	172	119	384	447	76	1198
SA	180	113	388	563	0	1244
HA	135	84	402	641	0	1262

Fig. 14 Comparative evaluation of the yield strength derived from experimental data versus theoretical predictions

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