3D printing of high performance nano-sheet layer eutectic high entropy alloys

At 23:00 Beijing time on August 3, 2022, Professor Chen Wen of the University of Massachusetts (UMass-Amherst) and the team of Professor Zhu Ting of the Georgia Institute of Technology published an article in the journal Nature entitled “Strong yet ductile nanolamellar high-entropy alloys by additive.” manufacturing” research results.

This achievement reports for the first time that a duplex nano-sheet eutectic high-entropy alloy with high strength and toughness and isotropic characteristics is prepared by laser 3D printing technology, and the toughness mechanism of the alloy is revealed by three-dimensional atomic probe, in situ neutron diffraction, crystal plastic finite element simulation and other characterization methods.

The corresponding authors of the paper are Chen Wen and Zhu Ting; The first authors are Ren Jie and Zhang Yin. Other partners include Texas A&M University, Oak Ridge National Laboratory, Rice University, and Lawrence Livermore National Laboratory) and the University of California, Los Angeles.

The extremely high temperature gradient and ultra-fast cooling rate in Laser powder bed fusion (L-PBF) can effectively refine the grain to achieve high strength of the material. At present, the nanoalloys prepared by L-PBF technology have high strength but low tensile plasticity. The strength-ductility tradeoff between material strength and plasticity is a common challenge in materials science. By shifting the focus of alloy design from the corners of the phase diagram to the center, thus enabling a wide range of composition and phase space, the emergence of high-entropy alloys provides a new paradigm for alloy design and material development. In particular, as a potential high-entropy alloy, eutectic high-entropy alloys have a biphase layered isomeric structure that exhibits better mechanical properties than conventional alloys. The eutectic layers prepared by the traditional casting method are organized at the micron or sub-micron scale, which severely limits the strength of the material. In contrast, the nano-sheet layer of tissue has high strength but low plasticity. In addition, the nanosheet layer structure is mainly prepared by thin film deposition and large plastic deformation, and the strong texture will lead to the mechanical behavior of the material with anisotropy, which limits the application of high entropy alloys in actual production.

Therefore, Chen Wen’s team at the University of Massachusetts used L-PBF technology to prepare a high-performance duplex nano-sheet AlCoCrFeNi2.1 eutectic high-entropy alloy. The material exhibits excellent strong plasticity matching ability (yield strength > 1.3 GPa, and uniform elongation greater than 14%), and the excellent strong plasticity matching ability is significantly better than other alloys prepared by publicly reported 3D printing technology. At the same time, in situ neutron diffraction reveals the real-time distribution of stress in different crystal planes and FCC and BCC phases and the evolution of dislocation density of two phases. Ting Zhu’s team at Georgia Tech has developed a finite element model of crystal plasticity of biphasic materials, revealing for the first time the rarely significant process hardening behavior of BCC nanosheets.

Multi-scale non-equilibrium nano-sheet microstructure achieves strength plastic synergy effect. The extremely high temperature gradient and cooling rate during laser selection melting printing allowed AlCoCrFeNi2.1 eutectic high-entropy alloys to form multi-scale non-equilibrium structures: BCC+FCC nanosheet structures (average sheet spacing: ~215 nm) were distributed in micron-scale eutectic colony with random textures, and amplitude modulation decomposition in BCC sheets further led to nanoscale chemical isomerization. The random crystallographic orientation and growth direction of eutectic groups contribute to the isotropic mechanical properties of materials.

Figure 1: Multiscale non-equilibrium tissue characterization of AM AlCoCrFeNi2.1. Optical microstructure and EBSD results show that eutectic colony has a random texture. HaADF and APT characterization confirm chemical modulation in BCC nanosheet layers.

Figure 2: (a) Tensile mechanical properties of AM AlCoCrFeNi2.1. (b) The red five-pointed star in the figure represents the results of this study, the solid sign represents the printing state performance of the material, and the hollow mark represents the heat treatment state performance of the material.

The rare significant process hardening behavior of BCC nanosheets helps to improve the plasticity of the material. Conventional BCC nanometals exhibit limited plasticity due to the lack of strain hardening behavior. In this study, in situ neutron diffraction, duplex crystal plastic finite element simulation and transmission electron microscopy were used to prove that BCC nanodisks have a higher dislocation density proliferation rate and process hardening rate than FCC nanodisks. The constraints of the interfaces of the FCC sheet layer and the semi-common lattice sheet layer during the deformation process, the constraints of adjacent eutectic and eutectic interfaces with different orientations, and the nanoscale chemical isomerization in the BCC sheet layer all help to improve the strain hardening ability of the BCC nanosheet layer, thereby improving the plasticity of the material.

Figure 3: Study the deformation mechanism of AM AlCoCrFeNi2.1 by in situ neutron diffraction. (a) The lattice strain of the characteristic crystal faces of the tensile direction FCC and BCC evolves with true stress, and the markers and solid lines in the figure represent the neutron diffraction experiments and the crystal plasticity finite element simulation results, respectively. (b) Real-time distribution of stresses in the FCC and BCC phases during tensioning. (c) Neutron diffraction patterns of alloys under different strains. (d) Changes in FCC and BCC phase fault densities as a result of strain calculated by improving the Williamson-Hall method.

Figure 4: Evolution of the deformed microstructure of AM AlCoCrFeNi2.1. (a-c) A diagram of the virtual brightfield screw-in electron diffraction of the alloy under different strains, in which the red flag represents the BCC nanosheet layer and the green flag represents the FCC nanodisk layer. (d-f) High-power brightfield TEM diagram of alloys under different strains, the yellow arrow indicates the deformation-induced stacking faults in the FCC nanosheet layer under 5% of the variable, and the yellow dotted line indicates the FCC-BCC phase interface. (g-i) High resolution TEM plot showing atomic-scale FCC-BCC phase interface features. (j-l) Corresponding inverse fast Fourier transform (IFFT) plot under different response variables, the yellow circle indicates edge dislocations.

This study reveals the idea of designing high-performance biphasic/heterogeneous nanostructures by using the unique thermophysics characteristics of laser 3D printing and the multi-principal original characteristics of high-entropy alloys. The unique toughening mechanism of nano-sheet layer structure can effectively guide the design of multi-phase layer structure of high-performance aluminum alloy and titanium alloy. (Source: Science Network)

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