Effect of High Temperature on the Mechanical Performance of Additively Manufactured CoCrNi Medium-Entropy Alloy Octet-Truss Lattice Materials

Abstract

In this study, the effect of high temperature on the mechanical performance of CoCrNi medium-entropy alloy octet-truss lattice material fabricated via laser powder bed fusion (LPBF) is investigated by compressive test and numerical simulation method. The results reveal that the strength and energy absorption performance of CoCrNi octet-truss lattice material with a hollow truss are higher than those of ones with a solid truss; however, they diminish by 30% and 50%, respectively, as temperature rises from 25 °C to 600 °C. As the temperature rises, the potential barrier for dislocation slip decreases, making it easier for dislocations to move at high temperatures and thus reducing the strength. CoCrNi octet-truss lattice materials present the failure mechanism of progressive collapse at varied temperatures. Meanwhile, the mechanical performance of the experimental testing agreed well with numerical simulation results. The numerical results show that the strength and energy absorption properties of the CoCrNi lattice materials increase as the relative density, however, decreases with increasing temperature. Additionally, CoCrNi octet-truss lattice materials maintain exceptional energy absorption performance at varied temperatures.

1. Introduction

High/medium entropy alloys (MEA/HEA) have indeed garnered considerable attention due to their exceptional comprehensive properties, such as superior mechanical performance at both ambient and cryogenic temperatures, remarkable fracture toughness, and excellent corrosion resistance [1,2]. High-entropy alloy was defined as one with at least five major elements whose individual atomic concentrations are between 5 and 35%. Alloys based on one principal metallic element were classified as low-entropy alloys and those composed of two to four principal elements as medium-entropy alloys [3,4]. Notably, the equiatomic CoCrNi transcends the traditional “trade-off” between strength and ductility through its pronounced lattice distortion, multi-component solid solution strengthening, and deformation-induced twinning [5,6]. This high-performance material is promising for extreme conditions and has wide application prospects and important academic value to meet extreme temperature conditions such as outer space exploration, the nuclear industry, and aviation.

To date, the primary manufacturing methods for medium entropy alloys encompass powder metallurgy, arc melting, casting, and rolling [7]. For instance, as-cast CoCrNi alloys with grain sizes ranging from 15 to 25 µm exhibit a yield strength of less than 400 MPa at room temperature, which restricts their broader application as structural materials [8]. To address these limitations, additive manufacturing (AM) provides innovative approaches for fabricating high-performance metallic components.

Laser powder bed fusion (LPBF) stands as one of the most revolutionary additive manufacturing technologies, garnering significant attention over the past few decades. This cutting-edge technique enables the creation of metallic components through the meticulous layer-by-layer deposition of raw metal powders, guided by a high-power laser source. A paramount advantage of this advanced methodology lies in its unparalleled design flexibility, permitting the fabrication of metallic parts with intricate geometries that were previously unattainable. Moreover, the resulting components exhibit near-perfect density and superior performance characteristics [9]. It has been demonstrated that the cellular substructure with high-density dislocation by adjusting the dislocation motion can be formed during the deformation process, which significantly contributes to enhancing the ductility of additively manufactured medium entropy alloys. Concurrently, the synergistic effect between slow diffusion and the inherently rapid non-equilibrium solidification characteristic of additive manufacturing promotes the formation of nanoscale precipitates and inhibits grain growth. These microstructural features ultimately contribute to an increase in the yield strength of MEAs [10,11,12].

In recent years, three-dimensional mechanical metamaterials have emerged as an effective strategy for achieving high specific strength and specific energy absorption by leveraging the synergistic combination of rationally designed topologies and the intrinsic microstructural features of matrix materials [13,14,15]. Medium entropy alloy mechanical metamaterials, as a unique class of advanced lightweight structural materials, have garnered increasing attention due to their tunable composition and their mechanical and functional properties. For instance, Feng et al. [16] deposited CoCrNiTi medium entropy alloy-coated nanolattices with ultra-low dislocation energy, which exhibited high energy absorption and specific strength. Zhaoyi Wang et al. [17] fabricated CoCrNi TPMS cellular materials via selective laser melting. TPMS materials with low density and high energy absorption capacity were obtained by optimizing the design of the unit. James Utama Surjadi et al. [18] reported the ultralight, damage-resistant CoCrNi entropy alloy metamaterials, which exhibit outstanding specific energy absorption (~25 J/g) and elasticity (~90% recoverability) by utilizing the size-induced ductility and well-designed MEA microstructure defects.

Currently, the mechanical properties of metamaterials have primarily been investigated at room temperature. However, the mechanical performance of metal-based metamaterials exhibits significant variation over a wide range of temperatures. Zhang et al. [19,20,21] studied the temperature-dependent compression behaviors of metal alloy lattice structures. The results revealed that, as the temperature increased from 25 °C to 450 °C, the compressive modulus, strength, and energy absorption performance of the lattice structures decreased by 46.2%, 42.3%, and 46.3%, respectively. Lijun Xiao et al. [22] investigated the compression behavior of Ti–6Al–4V lattice structures with a rhombic dodecahedron cell shape. The findings are that higher temperatures resulted in lower strengths, modulus, densification strains and plateau stresses. Currently, most metal lattices are particularly sensitive to temperature variations, with their strength and plasticity showing varying degrees of degradation over a wide temperature range, which becomes a limit to the high temperature application of the metal lattice.

Numerous studies have demonstrated that medium-entropy alloys (MEAs) exhibit excellent strength and toughness over a wide temperature range [23,24,25,26]. Therefore, it is crucial to develop a rational architectural design for lattice materials to achieve superior mechanical and energy absorption performance by integrating MEAs with lattice configurations. However, research on the cryogenic and room-temperature mechanical properties of MEA-based lattice materials remains limited. To our knowledge, the high-temperature mechanical properties of such lattice materials have been scarcely reported. In this study, CoCrNi medium-entropy alloy octet-truss lattice materials were manufactured using laser powder bed fusion (LPBF). The mechanical performance and energy absorption capacity of these materials were evaluated through experimental testing and numerical simulation.

2. Materials and Methods

2.1. Materials and Fabrication

In this work, octet-truss lattice materials with a hollow truss (HT) and solid truss (ST) are designed as shown in Figure 1, whose unit cells are characterized by l × w × h = 6 mm × 6 mm × 6 mm, d₁ = 1.4 mm or 1.3 mm, and d₂ = 0.4 mm. A first-order approximation of the relative density can be given by $\overline{\mathrm{\rho }}=3\sqrt{2\mathrm{\pi }}{({\displaystyle \frac{\mathrm{d}}{\mathrm{L}}})}^2$ for the ST and $\overline{\mathrm{\rho }}=6\sqrt{2\mathrm{\pi }}{({\displaystyle \frac{{\mathrm{d}}_1}{\mathrm{L}}})}^2(1-{\displaystyle \frac{{\mathrm{d}}_2}{{\mathrm{d}}_1}})$ for the HT. The designed relative densities are 34% (ST) and 43% (HT), respectively. The equiatomic CoCrNi pre-alloyed powders (Xi’an Bright Additive Technologies Co., Ltd., Xi’an, China) with a spherical shape and an average size of 35 μm are employed in this work. All designed samples were manufactured by laser powder bed fusion using the BLT s200 laser printing machine (Xi’an Bright Additive Technologies Co., Ltd., Xi’an, China). This process is characterized by several key features [27]: the laser power of 180 W, the applied laser wavelengths of 1.07 μm, a laser spot size of 60 μm, a scan speed of 1000 mm/s, an overlapping rate of 50%, and the printed layers with a thickness of 40 µm. The as-fabricated octet-truss lattice materials exhibit an average relative density of 35% with a standard deviation of 0.4% for the solid truss (ST) and 44.6% with a standard deviation of 0.5% for the hollow truss (HT). The measured dimensions deviate by less than 5% from the designed values, which may be attributed to high surface roughness. Additionally, dog-bone-shaped specimens with a gauge length of 15 mm and a diameter of 3 mm were fabricated using the same manufacturing process parameters. Figure 2 shows the macro- and micro-morphologies of the laser powder bed fusion (LPBF)-manufactured octet-truss lattice materials with hollow and solid trusses. It can be observed that the staircase effect and adhered powder are found in the manufactured metallic octet-truss lattice material.

2.2. Experimental Tests

To investigate the mechanical performance of the as-designed octet-truss lattice material, quasi-static compression tests were conducted using an LE5105 electronic universal testing machine equipped with an electrical furnace (Lishi Scientific Instruments Co., Ltd., Shanghai, China). The tests were performed at a strain rate of 1 mm/min at temperatures of 25 °C, 400 °C, and 600 °C. Prior to testing, all samples were annealed in the furnace for 20 min to ensure uniform temperature distribution. During the compression tests, the load–displacement curves were automatically recorded. Engineering stress and strain were calculated by dividing the measured load and compressive displacement by the cross-sectional area and initial height of the tested samples, respectively. Simultaneously, a video camera was employed to capture the deformation process. Three as-fabricated specimens were tested to ensure repeatability. Furthermore, tensile tests of the dog-bone-shaped samples were conducted. Figure 3 illustrates the tensile stress–strain responses of the CoCrNi alloy at various temperatures, which serve as base material parameters for the simulation model. As temperature increases, the potential barrier for dislocation slips decreases, facilitating easier dislocation movement and consequently reducing the material strength. The notable decline in high-temperature plasticity can be ascribed to the inhibition of deformation-induced twinning during tensile loading, coupled with the pervasive occurrence of transgranular fracture at elevated temperatures. Notably, the plasticity of the CoCrNi printed sample significantly decreased at 600 °C, indicating the onset of high-temperature brittleness, a phenomenon commonly observed in many high-temperature alloys [28,29]. This is generally attributed to the intensified diffusion of oxygen (O) at grain boundaries at this temperature, leading to reduced grain boundary cohesion and transgranular fracture, thereby diminishing plasticity.

3. Numerical Modeling

The mechanical response and failure mechanism of lattice materials subjected to compressive loading at various temperatures were studied using ABAQUS 2020. As shown in Figure 4, octet-truss lattice models were meshed with hexahedral C3D8 elements (type C3D8 in the Abaqus library). To avoid convergence issues and ensure numerical accuracy, an average element size of 0.5 mm was used, which provides a convergent load–displacement curve while maintaining efficient computational cost and simulation accuracy. All degrees of freedom of the lower plate were constrained, while all degrees of freedom except the three directions of the top plate were fixed. The top platen of the 1–2 plane was displaced in the negative 3 direction at a speed of 1 mm/min to simulate quasistatic compression. After the convergence analysis, the element size of 0.2 mm is employed to meet both computing precision and efficiency. A 3 × 5 unit cell for 25 °C, 3 × 4 unit cell for 600 °C, 2 × 2 unit cell for 800 °C in the x-y plane, and 3 × 1 unit cells in the z-y plane are employed in the simulation model to be consistent with the experimental samples. Due to the deformation symmetry of the specimen, the symmetric boundaries (PB) are imposed on the front and back of the representative model in the 1–2 plane. The friction coefficient is 0.2 in the numerical model. The basic properties of the CoCrNi alloy at different temperatures are shown in Figure 5. As the temperature rises, both the tensile strength and elongation decrease. In order to ensure quasi-static compression, the kinetic energy during the finite element simulation process is always less than the internal energy (less than 5%) throughout the compression process. In the FEM, the material was modeled as elastic-perfectly plastic, as described by its elastic modulus and yield point.

4. Results and Discussion

4.1. Mechanical Response of Octet Metamaterials

Figure 6a,b illustrate the compressive stress–strain responses of two as-fabricated octet-truss lattice materials obtained by experimental testing and numerical simulation at various temperatures. It is evident that the stress–strain curves exhibit three distinct regions, characteristic of typical cellular materials: the elastic region, a wide stress plateau region, and the densification region. In the elastic region, all octet-truss lattice materials undergo recoverable deformation, and the compressive modulus is determined by calculating the slope of this portion of the curve. Subsequently, the stress gradually increases and reaches a long and stable plateau region with slight strain hardening at 25 °C and 400 °C, indicating ductile behavior under static compressive loading. However, a minor stress drop is observed in this region for samples tested at 600 °C, attributed to strut fracture. The reason is that the plasticity of lattice material deteriorates under a high-temperature environment, which is consistent with the matrix material as shown in Figure 4. Notably, the plateau stress remains relatively smooth, making it suitable for energy absorption. As compression continues, the struts begin to come into contact with each other, leading to a sharp increase in stress, marking the onset of the densification stage. In addition, the FEM-predicted curves present reasonable agreement with the experimental curves. The comparison results further validated the accuracy of the finite element model.

It is obvious that the energy per unit volume of octet metamaterials increases as compressive strain increases, as shown in Figure 6c. The energy per unit volume of HT metamaterials consistently surpasses that of ST metamaterials. It is worth noting that the strength of octet metamaterials decreases by 27% (HT) and 33% (ST), respectively; however, the energy per unit volume of samples decreases by 17% (HT) and 58% (ST), respectively, as temperature increases from 25 °C to 600 °C as shown in Figure 6d. Obviously, the amount of SEA of HT metamaterials is 50% higher than that of ST metamaterials at a strain of 0.5.

4.2. Deformation Patterns

To reveal the underlying mechanism for such superior mechanical performance of the ST and HT metamaterials, the large-strain deformation characteristics are presented in Figure 7 and Figure 8 experimentally and numerically. As shown, the images captured by numerical simulation are consistent with those observed in experimental tests. Specifically, when ε = 0.2, it can be seen that both types of specimens undergo progressive failure, accompanied by a significant bending deformation of struts at different temperatures. This bending behavior is clear for the hollow struts. Compared to the hollow truss, the solid truss emerges with a small number of fractures at the conjunction nodes. Then, it entered a stable folding mode. Through continuous compressive loading, the lattice material exhibits an ideal layer-by-layer collapse pattern until it becomes dense. For the first collapse layer, the special perforation design at the nodes causes the pores to transition from open to closed. Subsequently, as bending deformation progresses, cracks initiate and propagate near the perforations. Notably, these cracks form at the neutral axis of the hollow struts rather than at the lattice nodes. Summarily, both octet-truss lattice materials show a hierarchical deformation mode characterized by being stable macroscopically but unstable microscopically. To further demonstrate the underlying reasons for the mechanical properties of the ST and HT metamaterials, Von Mises stress distributions of ST and HT are compared in Figure 9 and Figure 10 via unit cell representations with a compression strain of 0.2 at 25 °C (the Von Mises stress distributions of the ST and HT materials at 25 °C, 400 °C, and 600 °C are similar. To simplify the paper, the corresponding results are not presented here).

To investigate the underlying reasons for the exceptional mechanical properties of the presented metamaterial, the distributions of Von Mises stress for the ST and HT materials are compared by using unit cell representations under a compression strain of 20% in Figure 9a. As illustrated, as a typical stretch-dominated truss structure, significant and highly localized stress concentrations are observed on the struts. The high stresses are primarily localized at the inclined struts, where the largest deformations are anticipated. This can be attributed to the relatively high strut-to-node ratio. The regions surrounding the nodes are typically constrained, leading to deformation primarily occurring through the bending or twisting of the strut members. This uneven stress distribution results in a lower strain energy required for deformation, leading to severe fractures at the nodes under compression. Similarly, although the incorporation of the hollow design in HT alleviates the stress in struts to some extent, the overall stress distributions remain similar to those in ST, as the architectural configuration and the number of struts are unchanged. In addition, to further analyze the deformation behavior and stress distribution within the cross-sections of the struts, the struts on one side of the unit cell were employed. Figure 9b presents a schematic illustration of the stress distribution and deformed patterns under a compressive strain of 0.2. The surfaces of the intersecting struts on one side of the ST unit cell had eight compressive stress and four tensile stress regions, respectively. While the surfaces of the intersecting struts on one side of the HT unit cell had sixteen compressive stress and tensile stress regions, respectively, which are distributed on both sides of the hollow struts. The diagonal struts of both the ST and HT materials show bending deformation during compression, accompanied by an X-shaped deformation mode.

4.3. Parameters Affecting Deformation Modes

Based on the aforementioned results, octet lattice metamaterials exhibit superior energy absorption performance. To further enhance this property, a parametric study was conducted to investigate potential improvements in the energy absorption characteristics of octet lattice metamaterials. Relative density, being one of the most critical factors influencing both mechanical properties and energy absorption, is discussed in detail. Figure 10 illustrates the influence of temperature and the relative density on the mechanical performance of the ST and HT lattice materials at different temperatures by numerical simulation. It is evident that the strength and energy absorption of CoCrNi octet-truss lattice materials slightly decrease with increasing temperatures, while they increase with higher relative density. In octet lattices with small relative density, slender struts are prone to buckling or bending, and plastic hinges tend to form at the midpoints of these struts. Prior to strut contact, these slender struts undergo local plastic deformation and rigid body rotation, resulting in lower peak strength, platform stress, and specific energy absorption. As the diameters of the struts increase, strut contact occurs earlier, and the mutual interaction between struts prevents post-yield stress drop and subsequent fluctuations, thereby significantly increasing the overall compressive stress and post-yield stress. It is more suitable for energy absorption applications compared to one with a lower relative density. In addition, at the same relative density, HT has higher strength and specific absorption energy than ST.

4.4. Ashby Plot for Energy Absorption

Figure 11 highlights the energy absorption capability of the proposed CoCrNi alloy octet-truss lattice material with other previously reported architecture configurations materials such as Ti-6Al-4V lattice material [22], metal foam [30,31], and high entropy alloy metamaterials [32,33] at varied temperatures. It is evident that the CoCrNi octet-truss lattice material achieves unprecedented specific energy absorption with unit mass, E_m, and the specific energy absorption with unit volume, E_v, with similar service temperature, which results from the combinations of structures and properties of medium entropy alloy. Therefore, the as-designed CoCrNi alloy octet-truss lattice material can broaden the limit of energy absorption capability of architectural materials at varied temperatures, and offer interesting possibilities for high-temperature materials and thermal management applications.

5. Conclusions

In this study, two types of CoCrNi alloy octet-truss lattice materials were designed and manufactured using laser powder bed fusion (LPBF). The mechanical performance and energy absorption characteristics of the proposed CoCrNi alloy octet-truss lattice materials were investigated through quasi-static compression tests, both experimentally and numerically, at various temperatures. The results indicate that the strength and energy absorption of the hollow truss (HT) configurations are superior to those of the solid truss (ST) configurations. Specifically, as the temperature increases from 25 °C to 600 °C, the compressive strength of the octet-truss lattice materials decreases by 27% for HT and 33% for ST, while the specific energy absorption per unit volume decreases by 17% for HT and 58% for ST. As the temperature rises, the potential barrier for dislocation slip decreases, making it easier for dislocations to move at high temperatures and thus reducing the strength. As the temperature increases, the deformation-induced twinning gradually decreases, the strain hardening rate becomes unstable, and plastic instability occurs. FEM and experimental results show good consistency, but there are still small differences here. The presence of particles in the manufacturing process on the surface will increase the relative density of the specimen. In addition, the presence of particles causes the surface roughness of the specimen and causes the fracture failure to occur at ε = 0.2, resulting in a decrease in the strength of the samples. Both temperature and relative density significantly influence the strength and energy absorption properties of the CoCrNi lattice materials. Notably, the specific energy absorption capacities of the CoCrNi alloy octet-truss lattice materials are slightly higher than those of previously reported architectural configurations at varying temperatures. However, defects in the printing process reduce the mechanical properties of the lattice to some extent, and it is also very important to analyze the influence of defects on the mechanical properties of the lattice.