Nano-scale solute heterogeneities in the ultrastrong selectively laser melted carbon-doped CoCrFeMnNi alloy

Abstract

Rapid melting and solidification cycle during additive manufacturing provides a non-equilibrium environment that generates a large amount of internal defects, including dislocations, precipitations, and solute heterogeneity. These internal defects not only enhance the strength of materials by interacting with mobile dislocations but also reduce ductility due to coherency loss. To minimize the coherency loss from internal defects, defect size control in the additively manufactured products becomes an important issue. In this work, the high strength-ductility combination of additively manufactured carbon-doped CoCrFeMnNi is achieved by designing nano-scale solute heterogeneities in the matrix. The CoCrFe–MnNi solid-liquid two-phase region and interstitial carbon promote Mn and Ni segregation at cell networks and nano-sized precipitations, respectively. Laser scan speed during additive manufacturing determines the solidification rate that controls the solute cell network size. The MnNi co-segregated solute network not only interacts with dislocations but also induces strong back-stress hardening that contributes to achieving ~900 MPa yield strength with ~30% elongation which combination is significantly larger than the recent additively manufactured high-entropy alloys. This work demonstrates the importance of heterogeneity control in the additively manufactured materials to gain outstanding mechanical properties.

Introduction

Since the effective, accurate, and rapid processing are important to produce the on-demand parts, additive manufacturing (AM) becomes an attractive process to produce accurate products which can be used in an as-built state without additional post-treatment [1,2]. By combining with the computer-aided design, the degree of freedom of AM is getting wider from simple structures to complex parts. In general, AM products are used without any post-processing (i.e., rolling, machining, and forging), which means sufficient mechanical property should be ensured in the as-built products. Because yield strength of the as-built product is essential to sustain from external stress, designing structural materials with a yield strength-ductility improvement becomes a challengeable issue. Repetitive high energy laser scan in AM induces rapid melting-solidification cycle that generates a non-equilibrium state which induces severe lattice distortion, a large amount of defects, and solute heterogeneity into materials [[3], [4], [5]]. The solute heterogeneities of the AM-processed materials generate boundary segregation and precipitation which impede dislocation movement during plastic deformation. Because of these excessive defects from AM, we can expect strength enhancement of the AM-processed materials, on the other hand, we need to keep in mind ductility reduction due to the incoherent interfaces [6]. To avoid incoherent interfaces during AM, designing coherent segregation and precipitation with their size reduction into nano-scale needs to be considered at the same time [7,8].

High-entropy alloys (HEAs) having compositional complexity and structural simplicity have attracted substantial attention for the last decade as advanced structural materials leading Metallurgy Renaissance [[9], [10], [11]]. This new alloy-design strategy allows designing an unknown and huge compositional space in the multi-component systems with high configurational entropy. Interestingly, multi-composition in HEAs allows designing various phases by varying thermomechanical treatment conditions even they can generate solute segregation and clusters with a non-equilibrium environment [12,13]. These results imply HEAs have a strong potential to create excessive lattice distortion, a significant amount of defects, segregation, and coherent nano-scale precipitations by applying AM. From this point of view, some researchers prepared bulk HEAs using various AM techniques and investigated their mechanical and microstructural characteristics.

Many results have been obtained from the CoCrFeMnNi equi-atomic system, so-called Cantor alloy, the first developed HEA, achieving 500–600 MPa yield strength with a heterogeneous microstructure [[14], [15], [16]]. One of the important microstructural features in the AM-processed equiatomic CoCrFeMnNi alloy is the MnNi segregation in the nano-scale interdendritic region, which was also reported in the conventional welded or cast HEAs [17,18]. This segregation structure was only reported in the results from the slow laser scan speed with a sufficient laser power while the homogeneous compositional distribution was reported in fast speed AM. This means the MnNi segregation in equiatomic CoCrFeMnNi alloy is sensitive to the heat infiltration and solidification rate due to their narrow solid-liquid two-phase region in the CoCrFe–MnNi system. Although this segregation structure can increase the strength by interacting with dislocations [19], micro- and macro-scale segregation structures induce a harmful effect due to the coherency loss [20]. However, if these solute heterogeneities occur at a fine-grained structure with a nano-scale, these boundaries can be less susceptible to inhomogeneous slip and they can overcome a drawback from the segregation structure [21,22]. Therefore, to achieve the high strength and ductility combination in the AM-processed HEAs, designing the segregation and coherent nano-scale precipitations into the fine-grained matrix becomes an important issue.

Based on this purpose of simultaneous high yield strength and ductility, in this study, the role of nano-scale solute heterogeneities on the mechanical properties of the AM-processed C–CoCrFeMnNi alloy was investigated. To design nano-sized segregation boundaries and precipitations into the matrix, slow laser scan speed (200 mm/s) AM was applied to adjust an intermediate cooling rate. The intermediate cooling rate allows time to stay in the solid-liquid two-phase region of the CoCrFe–MnNi system that creates a solute partitioning during solidification [23]. Because the excessive heat from high energy laser scan speed can induce abnormal grain growth in HEAs, we added 0.2 wt% interstitial C-dopants into the CoCrFeMnNi equiatomic HEA to delay the grain growth. The added C-dopants not only interrupt the grain growth but also generate nano-Cr carbide that can contribute to the additional strength enhancement [24]. Following these microstructural design strategies, we discuss the role of back-stress from the various heterogeneities (i.e., solute segregation, nano-precipitation, and bimodal microstructure) on the mechanical property of the AM-processed HEA.