Additive manufacturing (AM) has now been established as a versatile near-net-shape manufacturing technology that allows for the free design and integrated molding of complex structures through layer-by-layer structures. At present, some studies using laser directed energy deposition (L-DED) AM to fabricate layered heterostructure high-entropy alloys have shown that the strength can be improved by a combination of soft and hard components. However, the interface between the two layers is prone to failure in the early stages of deformation, resulting in limited strength gain and low ductility. In addition, remelting and dilution inevitably occur at the interface during deposition, which may form some brittle intermetallic compounds, which can deteriorate the interfacial properties of the material. On the other hand, compared with as-cast and thermomorphic metal materials, metal components fabricated by AM usually have problems such as poor strength and/or ductility, relatively obvious anisotropy of mechanical properties, and insufficient volume fraction of precipitated phase due to the initial coarse columnar crystal microstructure. Traditional thermomechanical methods for performance manipulation (e.g., cold rolling or recrystallization) are shape-damaging and unsuitable for post-processing of AM components. Therefore, it is an important and challenging problem to find a suitable AM process to obtain metal components with high-strength-ductility synergy.
为了解决上述问题，哈尔滨工业大学材料学院黄永江教授课题组与英国皇家工程院院士、香港大学颜庆云教授（A.H.W.Ngan），太原理工大学张长江副教授合作，采用激光定向能量沉积技术制备了具有优异强度和延展性协同的缓梯度界面异质结构中熵合金，探究了其界面处微观组织和相形成机制，揭示了该异质结构中熵合金的塑性变形机理。 第一作者为博士生孙永刚，论文参加者还有宁志良副教授(共同通讯)以及孙剑飞教授。 相关成果以题“Additively manufactured low-gradient interfacial heterostructured medium-entropy alloy multilayers with superior strength and ductility synergy”发表于 Composites Part B: Engineering。
Paper Links:
https://doi.org/10.1016/j.compositesb.2024.111522
In this study, we demonstrate an interesting two-powder source alloy system: (i) CoCrNi medium-entropy alloy (MEA) and (ii) the addition of elemental Ti and Al and the same MEA (its nominal composition is (CoCrNi)86Al7Ti7, hereinafter referred to as Al7Ti7), the two powder sources are alternately deposited by L-DEC, followed by non-shape-destructive heat treatment of the deposited components. The obtained components exhibit a significant synergy of strength and ductility (ultimate tensile strength exceeds 1 GPa, ductility is close to 50%). Using strength multiplied by ductility as the merit value, this L-DED alloy system exhibits superior strength-ductility synergies than other reported medium/high entropy alloys. The key to the excellent mechanical properties is that the L-DED process promotes the formation of a heterostructure (HS), which consists of a layered heterostructure composed of ductile CoCrNi MEA and the same alloy with diffusely distributed intermetallic compound phase (L12), which alternately forms a microstructure with a shallow gradient interface, and the MEA with such a structure is both strong and tough. It is worth noting that due to the dilution of the components of the Al7Ti7 layer, a shallow gradient interface heterostructure is formed, and its mechanical properties gradually fluctuate on each layer, but a small plastic strain mismatch will occur between the two layers during the deformation process, so the crack will not preferentially occur at the interface. Based on the microstructure observation, the high strength of HS MEA in the heat-treated state can be attributed to solution strengthening, precipitation strengthening, dislocation interaction, and the interaction between dislocations and lamination faults (SF) and twins, which correspond to the significant back stresses in the system, and the resulting high strain hardening rate will also cause necking resistance, which is an important factor to inhibit necking to improve ductility, while the deformation of the Al7Ti7 layer requires greater external stress, which ultimately leads to the higher strength of HS MEA.
Figure 1. Laminar morphology of HS MEA printed on the BD-SD plane: (a) SEM image; (b-f) Electron probe mapping
Figure 2. Mechanical properties of MEA alloy samples: (a) Engineering stress-strain curves along SD of MEA samples in printed and heat-treated states; (b) Comparison of the mechanical properties of the printed and heat-treated HS MEA samples predicted by experiments and ROMs; (c) Comparison of engineering stress-strain curves along SD and BD of heat-treated HS MEA samples; (d) Comparison of ultimate tensile strength and elongation at break of heat-treated HS MEA and other high-performance MEAs/HEAs
Fig. 3.Interface characteristics of HS MEA in the heat-treated state: (a) SEM image of HS-MEA in the heat-treated state; (a1), (a2) SEM-EDS patterns of the interface region and line scan analysis of components along the yellow arrows in (a); (b) BSE images and electron probe maps at the layer interface; (c) Schematic diagram of the microstructure of HS MEA in the heat-treated state; Component mixing (migration of Al and Ti from the Al7Ti7 layer to the surrounding CoCrNi layer) results in (i) precipitation of the L12 phase in the CoCrNi layer, and (ii) gradient eutectic transition in the Al7Ti7 layer
Figure 4. Typical microstructures in the CoCrNi layer (a-e) and Al7Ti7 layer (f-h) in the heat-treated HS MEA: (a) Low-power brightfield TEM plot in Region II; (b) Darkfield TEM plot of L12 phase precipitation in Region II; (c) High-resolution transmission images of the L12 phase; (d) STEM-EDS element map of the L12 phase in Figure (b); (e) Diameter distribution of the L12 phase; (f) Brightfield TEM image of Region III; (g) Brightfield TEM images of L12 and σ phases in Region IV; (h) High-resolution transmission image correspondence (g) of L12 and σ phases
Figure 5. Typical deformation substructures of HS MEA in the heat-treated state when deformed to ~3% true strain: brightfield TEM and high-resolution transmission patterns of dislocation structures and SF in (a-c) CoCrNi layer; (d-f) Brightfield TEM plot of Al7Ti7 layer and corresponding selective electron diffraction pattern
Figure 6. Representative deformation substructures of HS MEA deformed to ~21% true strain (a-d) and fracture strain (e-h) in the heat-treated state: (a) Dense dislocation accumulation in the CoCrNi layer in Zone I; (b) Sharp sub-grain boundaries in the CoCrNi layer in Region II; (c) Dense dislocations and SFs in the Al7Ti7 layer in region III and the corresponding high-angle annular darkfield image; (d) Lamellar structure composed of σ phase IV., L12 phase and residual matrix in the Al7Ti7 layer; (e, e1) Dense defects in the CoCrNi layer; (f, f1) Brightfield and darkfield TEM plots show rare deformed twins in the CoCrNi layer. (g) High-density deformed twins in the Al7Ti7 layer; (h) SEM image of the Al7Ti7 layer
Source: Materials Science and Engineering, thanks to the strong support of the team of authors of the paper.