Investigation of High-Temperature Tensile Properties and Fracture Mechanisms of GH3536 Alloy Fabricated by Selective Laser Melting

Abstract

This work revealed the mechanism of microstructure evolution on tensile properties in selective laser melted (SLM) GH3536 superalloys in the range of room temperature to 1000 °C. The results showed that SLM-GH3536 alloy exhibited isotropic tensile properties along X, Z, and 45° directions. The carbide at the grain boundary changed from banded to granular, resulting in a significant decrease in tensile strength at elevated temperatures, and the fracture mode transitioned from a mixed fracture to transgranular fracture.

1. Introduction

GH3536 is a nickel-based superalloy strengthened by solid solution elements such as Cr and Mo. It is widely used in critical components such as turbine blades, tail cones, and afterburners due to its excellent high-temperature properties and corrosion resistance, with a short-term operating temperature reaching up to 1090 °C [1,2,3]. Selective Laser Melting (SLM) is a key additive manufacturing technique that utilizes a laser beam to melt metal powder layer by layer, enabling the rapid fabrication of complex structural components [4,5,6]. However, the mechanical properties of the SLM-fabricated GH3536 alloy, especially at a service temperature range of 900 °C to 1000 °C, are crucial for determining whether it meets usage requirements. The performance gap between it and traditional forged materials also needs to be explored.

Extensive research has been conducted on the mechanical properties and fracture mechanisms of SLM GH3536 alloy at room and elevated temperatures [7,8,9,10,11,12]. Sanchez-Mata et al. [13] reported the anisotropy of tensile properties at room temperature and found that the (001) texture along the build direction of SLM GH3536 alloy was an important factor causing anisotropy. Zhao et al. [14] identified that this anisotropy could be attributed to the weakest link interface due to the laser-melted pool geometry, the grain boundary, or the dendrite direction. High-temperature environments can alter tensile properties by affecting microstructure characteristics, such as grain size, elemental segregation, precipitation phase types, and micro-texture [15,16,17,18,19]. Zheng et al. [20] found that, unlike room-temperature fracture along the 45° shear direction, intergranular cracking was intensified at high temperature, resulting in the attenuation of plastic deformation ability. By analyzing the evolution of grain boundaries with temperature, Ma et al. [21] found that the segregation of Mo and Cr at grain boundaries might be an important factor in intergranular fracture. However, in the whole temperature range from room temperature to high temperature, the evolution of tensile properties and its relationship with microstructure are still unclear, and the corresponding fracture failure modes need to be further explored.

This work utilized SLM technology to prepare smooth and notched GH3536 alloy tensile specimens in different directions. It systematically investigated the evolution of tensile properties from room temperature to 1000 °C, revealing the mechanisms by which microstructure evolution affects tensile properties and fracture mode.

2. Experimental Section

The GH3536 spherical powder was prepared by the Beijing Aeronautical Materials Research Institute of China Aviation Engine Corporation (Beijing, China) using the vacuum induction gas atomization (VIGA) method. The powder size ranged from 15 to 53 μm, with the following chemical composition (wt.%): C: 0.05–0.15, Cr: 21.61, Mo: 8.9, Fe: 18.78, Co: 1.73, W: 0.64, Si: 0.11, Mn: 0.015, Ni: Bal.

The GH3536 alloy was fabricated using the EOS-M290 Selective Laser Melting (SLM) system (EOS Gmbh, Shanghai, China), which is equipped with a solid-state fiber laser with a maximum power of 400 W and a build volume of 250 mm × 250 mm × 325 mm. The process parameters for fabricating the GH3536 alloy were as follows: laser power of 280 W, scanning speed of 1250 mm/s, scanning distance of 110 μm, and powder layer thickness of 40 μm. The detailed SLM process was described in previous studies [22]. The forming process was conducted in a high-purity argon atmosphere (99.999%) to prevent oxidation and nitridation of the alloy. The building directions of the SLM-GH3536 alloy, designated as the X, Z, and 45° directions, are illustrated in Figure 1a.

According to GB/T145-2001 stand, two specimen types were designed for high-temperature tensile testing: the first type comprised smooth tensile specimens prepared along the X, Z, and 45° directions, while the second type consisted of notched tensile specimens prepared along the X and Z directions, with the specific dimensions illustrated in Figure 1b,c. Tests were conducted on these specimens using an Instron 8801 universal testing machine (Instron, Norwood, MA, USA), with thermocouples attached to the specimens to continuously monitor temperature variations. The equipped high-temperature furnace was used to heat up to the preset temperature (300 °C, 400 °C, 800 °C, 900 °C, and 1000 °C) and hold the temperature for 20min before tensile testing. During the tensile tests, a constant strain rate of 0.001 s⁻¹ was applied. Subsequent to the fracture of the specimens, they were meticulously cut from the gripping ends, ground, polished, and subjected to a chemical etching process (etching in a solution of HNO₃:HCl = 1:3 for 2 s) for the purpose of microstructural observation. The microstructural and fracture surface morphology analyses were conducted using a JEOL 7001 F scanning electron microscope (JEOL, Tokyo, Japan). EBSD samples were prepared using a vibratory polishing technique, and texture and crystallographic orientation analyses were conducted using a Hikari XP EBSD detector.

3. Results and Discussion

3.1. Thermophysical Properties

The variation curves of specific heat capacity (C_p) and linear expansion coefficient (α) of the SLM-GH3536 alloy with temperature are shown in Figure 2a,b, with detailed values provided in

Table 1. Specific heat capacity of SLM-GH3536 alloy at different temperatures.

Direction	θ/°C	25	300	400	500	600	650	700	800	900
X	Cp/J·(g·K)−1	0.412	0.459	0.475	0.487	0.551	0.540	0.551	0.568	0.578
Z		0.524	0.603	0.616	0.630	0.701	0.688	0.698	0.724	0.736

and

Table 2. Linear expansion coefficient of SLM-GH3536 alloy at different temperatures.

Direction	Heating Range θ/°C	20~300	20~400	20~500	20~600	20~650	20~700	20~800	20~900
X	α/10−6 K−1	13.8	14.3	14.7	15.2	15.5	15.8	16.2	16.5
Z		13.8	14.4	14.8	15.2	15.5	15.8	16.2	16.5

. As shown in Figure 2a, the specific heat capacity of the SLM-GH3536 alloy exhibits a positive correlation with temperature, with the specimens in the X-direction substantially outperforming the Z-direction specimens. Regarding the linear expansion coefficient, Figure 2b demonstrates a linear increase with temperature, accompanied by a marginal discrepancy between the X-direction and Z-direction specimens.

3.2. Microstructure Characterization

The microstructure of the SLM-GH3536 alloy is examined under various temperatures, shown in Figure 3. To accurately reflect the microstructure of tensile test specimens, all metallographic specimens were prepared from the gripping ends of the X-direction tensile samples following tensile fracture. As illustrated in the figure, all samples consist of equiaxed grains, indicating that the short-term heat treatment during high-temperature tensile testing does not significantly alter the inherent microstructural features of the SLM-GH3536. However, at temperatures exceeding 900 °C, the high-temperature environment facilitates grain coarsening, with the average grain size increasing from 53 μm to 61 μm. Furthermore, at 1000 °C, a reduction in grain boundary continuity is observed, accompanied by the precipitation of numerous discrete precipitates within the γ-phase matrix.

A comparison of the precipitate phase between specimens at room temperature and at 1000 °C was conducted, as illustrated in Figure 4. In the room temperature specimen, the banded second phase is continuously embedded along the grain boundaries. EDS analysis reveals that this phase is enriched in Cr, which is identified as M₂₃C₆-type carbide based on literature analysis. The composition of the γ-matrix after RT and 1000 °C shows no significant difference (comparison of points 1 and 3). However, the continuous carbides gradually disappear, and numerous granular carbides form at the grain boundaries and within the grains, with diameters reaching up to 2 μm. EDS results at point 4 demonstrate that, compared to the continuous carbides, the granular carbides exhibit considerably higher concentrations of Cr and Mo, consequently leading to a reduction in the concentration of the primary solid-solution elements, Cr and Mo, near the grain boundaries. This results in a decrease in solid solution strengthening in the grain boundary region.

As illustrated in Figure 5, a range of samples are presented in the form of EBSD images. The IPF map demonstrates that, following tensile testing at varying temperatures, the crystallographic orientation of the equiaxed grains remains consistent, exhibiting a random distribution with no discernible preferred growth direction. Furthermore, a substantial number of annealing twins are observed in all specimens. The twin boundaries are highlighted on the IQ map, with (111) twin boundaries marked in blue and (110) twin boundaries marked in red. Statistical analysis reveals that the length percentage of (111) twin boundaries exceeds 90%, confirming it as the predominant twin type in the SLM-GH3536 alloy.

3.3. Tensile Properties

3.3.1. Tensile Properties of Smooth Specimens

High-temperature tensile tests of smooth samples were conducted along the X, Z, and 45° directions. The curves of yield strength (σ_0.2), tensile strength (σ_b), elongation (δ₅), and reduction in area (ψ) with temperature are summarized in Figure 6. As shown in Figure 6a, the equiaxed grains in the SLM-GH3536 alloy result in similar strength in the X, Z, and 45° directions, indicating isotropic mechanical properties. As the testing temperature increases from room temperature to 1000 °C, the yield strength of the SLM-GH3536 alloy decreases from 315 MPa to 98 MPa, leading to a reduction of approximately 69%. The tensile strength decreases from 732 MPa to 128 MPa, yielding a reduction of 83%. Regarding plasticity, the dendrites’ plastic deformation capacity along the axial direction is superior to that in the transverse direction, leading to the following sequence of elongation: Z-direction > 45° direction > X-direction. Compared to the forged GH3536 alloy, the evolution of plasticity can be divided into two stages. Below 800 °C, the SLM-GH3536 alloy exhibits similar plasticity to the forged GH3536 alloy. However, the tensile plasticity of the forged GH3536 alloy is notably higher than that of the SLM-GH3536 alloy above 900 °C. In the high-temperature region, the rapid loss of plastic deformation ability suggests insufficient work hardening in the SLM-GH3536 alloy, which is likely a key factor for the reduction in tensile strength to the yield strength level.

The fracture morphology of smooth tensile specimens was examined, as shown in Figure 7. At all temperatures, the fracture morphology of the X, Z, and 45° direction specimens remains consistent, confirming the hypothesis that the SLM-GH3536 alloy exhibits good isotropy. However, at temperatures below 400 °C, the fracture surface exhibits a significant number of secondary cracks and cleavage steps (see Figure 7(a1–c3)). It is hypothesized that under tensile loading, the continuous carbides along the grain boundaries cause separation of adjacent crystals, leading to the formation of secondary cracks. Consequently, at low temperatures, the SLM-GH3536 alloy exhibits a mixed fracture mode, characterized by both intergranular and transgranular fracture features. In contrast, at temperatures above 815 °C, the fracture surface shows a sugar-like pattern in Figure 7(d1–d3,e1–e3), indicating intergranular fracture. During high-temperature tensile testing, the growth and coarsening of carbides significantly influence the fracture behavior. Carbides at the grain boundaries, as hard phases, exhibit higher yield strength and lower fracture toughness compared to the γ-matrix [23,24]. The coarsened carbide particles exacerbate stress concentration near the grain boundaries, leading to localized plastic deformation instability, which directly triggers cracking and severely impairs the plastic deformation capability at high temperatures.

3.3.2. Tensile Properties of Notched Specimens

A comparison of the tensile strength of notched specimens between SLM-GH3536 alloy and forged alloy is presented in Figure 8. The notched specimens in both the X and Z directions demonstrate high isotropy. As the testing temperature increases from 650 °C to 1000 °C, the tensile strength decreases from 600 MPa to 180 MPa. The figure clearly shows that below 800 °C, the tensile strength of the SLM-GH3536 alloy is lower than that of the forged alloy, while above 800 °C, the difference between the two alloys becomes negligible. As illustrated in Figure 9, the notching effect restricts substantial plastic deformation across the entire specimen. Under high-temperature conditions, the fracture surface displays an intergranular fracture pattern with a sugar-like appearance, and the coarsening of carbide particles further impairs the notched tensile performance.

4. Conclusions

In this work, the tensile properties of smooth and notched specimens were tested in the range of room temperature to 1000 °C, and the corresponding microstructure analysis was carried out to reveal the mechanism of microstructure evolution on tensile properties and fracture modes. The main conclusions were as follows:

• The tensile strength of the two types of specimens and the elongation of the smooth specimens decreased gradually with the increase in ambient temperature. Among them, the smooth specimens exhibited isotropy along X, Z, and 45° directions.
• Compared to conventional forged materials, SLM GH3536 alloy exhibited a similar tensile strength, but the elongation gap increased at temperatures above 900 °C.
• At elevated temperatures, the continuous carbides gradually disappeared, while a significant number of granular carbides precipitated at the grain boundaries and within the grains.
• Above 815 °C, the tensile fracture mode changed from mixed fracture to intergranular fracture, and the coarse carbide particles at the grain boundary promoted intergranular cracking.