Synergistic coupling of Mn-doped skeleton and Mg-toughened matrix: towards a heat-resistant Al–La–Mg–Mn alloy

Thermally stable three-dimensional (3D) skeleton coupled with the Mg-toughened matrix is employed to enhance the high-temperature strength in an Al–La–Mg–Mn alloy fabricated via laser powder bed fusion. The 3D skeleton exhibits a network and trans-granular structure with submicron cells, providing effective boundary strengthening to counter conventional grain boundary softening at elevated temperatures. Notably, Mn-doping introduces nanoscale Al6Mn precipitates into the skeleton, inducing additional microcracks that aid deformation coordination yet are buffered by the high-Mg toughened α-Al matrix during deformation. Quantitatively, the network structure contributes to over 30% and 40% yield strength increments at 200°C and 300°C, respectively. GRAPHICAL ABSTRACT IMPACT STATEMENT LPBF-fabricated Al–La–Mg–Mn alloy achieved outstanding high-temperature strength via boundary strengthening mechanism provide by the 3D skeleton.


Introduction
Aluminum alloys are widely employed as structural materials owing to their attractive attributes, including high specific strength and remarkable corrosion resistance [1].The advent of laser powder bed fusion (LPBF) as a cutting-edge manufacturing process has substantially expanded the potential applications of aluminum alloys.LPBF endows these alloys with custom-designed shapes and exceptional mechanical properties, facilitated by its distinctive layer-by-layer construction and rapid solidification [2][3][4].To further propel the utilization of aluminum alloys in LPBF, the industry calls for enhancing the value of LPBF-fabricated aluminum alloys.In response to this imperative, substantial attention has been redirected toward developing heat-resistant aluminum alloys specifically tailored for LPBF.The objective CONTACT Liejun Li liliejun@scut.edu.cn;Zhengwu Peng persistmiracle@163.comNational Engineering Research Center of Near-Net-Shape Forming for Metallic Materials, South China University of Technology, Guangzhou, 510640, People's Republic of China Supplemental data for this article can be accessed online at https://doi.org/10.1080/21663831.2023.2301132.
is to provide viable alternatives to expensive titanium alloys or bulky iron-based alloys within the temperature range of 200°C to 450°C [5].
LPBF, in particular, imparts aluminum alloys with a fine-grained structure that significantly augments their mechanical properties at room temperature [6,7].However, this fine-grained structure presents formidable challenges in high-temperature applications.At elevated temperatures, grain boundaries become susceptible to sliding, acting as weak points and thus compromising mechanical properties [8].To address this challenge, second phases are introduced to immobilize grain boundaries and facilitate precipitation strengthening [9,10].Striking the right balance here is crucial; a low volume fraction may result in insufficient pinning and limited strength enhancement [11,12], while an excessive vol-ume fraction would substantially reduce ductility [13,14] Recent research findings [15,16] intriguingly demonstrated that continuous intermetallics enhanced strength without a significant compromise in ductility.However, the coarsening of intermetallics would severely impair mechanical properties at elevated temperatures [17], underscoring the pivotal role of thermal stability.Thanks to the thermally stable Al 11 (La, Ce) 3 intermetallics, Al-(La, Ce) alloys have emerged as promising candidates for heat-resistant aluminum alloys in recent years [18,19].Several successful instances of manufacturing Al-(La, Ce) alloys using the LPBF have been documented [20][21][22].Of note, LPBF generates diverse intermetallic morphologies, including ribbonlike, lamellar, and rosette-like structures [13,23], resulting in distinct high-temperature mechanical properties.
This diversity prompted us to explore the customization of thermally stable intermetallic morphologies as an effective approach to enhancing high-temperature mechanical properties.It is well-established that the grain boundary constitutes a continuous 3D structure.In order to suppress the softening of grain boundaries, the present work designed a trans-granular 3D network intermetallics enveloped within a high-Mg aluminum matrix in an Al-11.5La-5.5Mg-0.6Mnalloy (in wt.%) via LPBF.This distinct structure mainly provides an effective 'boundary strengthening mechanism' at elevated temperatures.Consequently, the as-printed alloy exhibited outstanding mechanical properties at 200°C and 300°C.To gain insight into the underlying high-temperature strengthening mechanism, we conducted comprehensive microstructure characterizations.For details regarding the LPBF process and characterization methods, please refer to the supplementary materials.

Characterization and formation mechanism of 3D network intermetallics
In terms of phase composition, the X-ray diffraction (XRD) pattern of the LPBF-fabricated Al-La-Mg-Mn alloy (Figure 1a) reveals the presence of the primary Al 11 La 3 intermetallic phase coexisting with the α-Al phase.The volume fraction of the intermetallic phase was determined to be 19% using the conventional K value method.Moreover, compared to the diffraction angles of pure aluminum (as indicated by dashed lines), the lower angles of the α-Al peaks suggest lattice expansion, revealing a supersaturated α-Al matrix with a high-content Mg.Quantitatively, the lattice parameter was calculated to be 4.071 Å using the Nelson-Riley function [24].Meanwhile, the Mg content in the matrix was determined to be 4.2 wt.% because of the linear relationship between the solute Mg content and lattice parameter [25,26], with the calculation process detailed in the supplementary materials.Backscattered electron-scanning electron microscopy (BSE-SEM) image (Figure 1b) distinctly reveals a fishscale pattern, with the α-Al matrix appearing dark and the Al 11 La 3 intermetallics displaying bright contrast.Notably, the intermetallics exhibit morphological heterogeneity, featuring granular structures along the melt pool boundaries (MPB) (Figure 1c) and network structures within the melt pool interior (MPI) (Figure 1d).Further insights into the network intermetallic morphology are provided by bright-field transmission electron microscopy (BF-TEM) (Figure 1e), highlighting the high continuity of network intermetallics in the MPI.It is also revealed that the dendrite network structures consist of closed or semi-closed submicron cells.Comprehensive characterizations were employed to identify all phases present, as detailed in our previous work [27].Additionally, the band contrast (BC) map combined with the inverse pole figure (IPF) provides clear evidence that the intermetallics are trans-granular, essentially forming a structural skeleton (Figure 1f).Similar network intermetallics were observed in LPBF Al-Ce alloys with high-Mg content, including the Al-7Ce-8Mg alloy [21], Al-11Ce-7Mg alloy [28], and Al-5.5Ce-8Mg alloy [29].In contrast, Al-Ce alloys without Mg alloying typically exhibited ribbonlike or lamellar intermetallic morphologies, as seen in the Al-10Ce alloy [30] and Al-10Ce-8Mn alloy [13].This disparity underscores the significant role of high Mg in the formation of network intermetallics.However, it is worth noting that excessive Mg content can lead to keyhole defects due to the unavoidable vaporization of Mg during the LPBF process, as evidenced in the Al-15Ce-9Mg alloy [28].Additionally, maintaining an adequate Ce content is essential for ensuring intermetallic volume; insufficient Ce content, as seen in the Al-5.5Ce-8Mgalloy [29], would result in a reduced continuity of the Al 11 Ce 3 intermetallic.Therefore, achieving a high Ce/La elemental composition with an appropriate Mg content is crucial for obtaining a highly continuous intermetallic structure while avoiding excessive defects.
Secondary electron-scanning electron microscopy (SE-SEM) image vividly reveals the intermetallics' intricate 3D dendritic network structure (Figure 2a), suggesting a complex and interconnected arrangement within the alloy.Moreover, a higher-magnification BF-TEM image offers additional insights into these network intermetallics, showing that they are decorated with dense nanoscale precipitates, as indicated by the red arrows (Figure 2b).These precipitates were identified as the Al 6 Mn phase [27].Furthermore, as evidenced in Fig. S1, the Al 11 La 3 intermetallics exhibited outstanding thermal stability, where the ultrafine network intermetallic remained despite undergoing thermal exposure at 300°C for 24 h.Similar results were also found in ultrafine Al 11 La 3 intermetallics fabricated by powder hot extrusion during thermal exposure at 400°C [31], signifying its ability to maintain structural integrity and performance characteristics even under relatively high-temperature conditions.This quality makes it suitable for applications involving elevated temperatures.
Schematic diagrams (Figures 2c, d) have been drawn to elucidate the intricate mechanism underlying the formation of the network intermetallic structure.As the molten alloy undergoes solidification, Mg atoms tend to be expelled in front of the solid phase, resulting in significant constitutional undercooling (Figure 2c).This undercooling effect leads to the solidification of α-Al in a cellular dendritic morphology, characterized by branched and interconnected structures (Figure 2d).Notably, within the inter-dendritic regions, substantial La enrichment occurs due to the limited solubility of La in the aluminum matrix.Consequently, the Al 11 La 3 phase solidifies from the remaining liquid and accumulates in the inter-dendritic spaces, forming a structural skeleton.Of note, the remaining liquid also consists of Mg and Mn atoms, causing the formation of a hierarchical structure via solute exclude effect.Further discussion about this effect can be found in our previous work [27].This phase transformation occurs with minimal energy consumption, giving rise to the intricate network intermetallic structure.This well-coordinated sequence of events during solidification provides valuable insights into the fascinating morphology of the network intermetallics in the material.

High-temperature strengthening mechanism
The engineering tensile stress-strain curves of the asprinted alloy (Figure 3a) present room-temperature engineering yield strength (YS) and ultimate tensile strength (UTS) values of 334 ± 9 MPa and 588 ± 11 MPa, respectively.These remarkable mechanical properties primarily stemmed from solid solution and Orowan strengthening, attributed to the high concentration of solute Mg atoms and the substantial volume fraction of intermetallics, respectively [27].Significantly, the as-printed Al-La-Mg-Mn alloy maintained its high strength even at elevated temperatures, exhibiting engineering YS values of 297 ± 10 MPa and 213 ± 9 MPa at 200°C and 300°C, respectively (Figure 3a).However, the alloy displayed varying ductility; it achieved excellent elongation of 19.3 ± 2.3% at 200°C but experienced a sharp drop to 9.0 ± 1.5% at 300°C (Figure 3a).This reduction in ductility at 300°C aligned with observations in LPBF Al-Cu-Ce alloy [15] and Al-Ce-Sc-Zr alloy [23], where the authors attributed the decline of elongation to strain  localization caused by heterogeneous structures.Furthermore, the stress-strain curves depict a steady deformation mode during high-temperature tensile testing, with no significant softening observed (Figure 3a).
For reference, various heat-resistant LPBF Al alloys were plotted (Figures 3b-d), highlighting the exceptional tensile mechanical properties of the as-printed alloy at elevated temperatures compared to other heat-resistant LPBF Al alloys [15,17,20,23,[32][33][34]. Figures 3b and c specifically illustrate the excellent tensile strength in the present alloy.Among them, the LPBF Al-Fe-Cr alloy, with a high-volume fraction of quasi-crystalline phase, achieved the highest strength at elevated temperatures but sacrificed ductility ( < 5%) at 200°C [34], significantly less than the present alloy (19.3 ± 2.3%) (Figure 3d).Despite the present alloy containing a high-volume fraction of continuous network intermetallics, it maintained appreciable ductility among various LPBF aluminum alloys.The diverse strength and ductility profiles highlight the unique advantages of both the present alloy and the LPBF Al-Fe-Cr alloy at elevated temperatures, with neither being replaceable by the other.Additionally, LPBF Al-Ce-Sc-Zr alloy exhibited a slightly higher YS at 300°C than the present alloy (Figure 3b).The elevated strength of the Al-Ce-Sc-Zr alloy results from the simultaneous addition of Sc and Zr, precipitating dense nanometer-sized Al 3 (Sc, Zr) particles; however, this enhancement comes at the cost of ductility ( < 5%) [23] (Figure 3d).Furthermore, the high price of Sc considerably inflates the overall alloy cost, rendering it less economically viable for widespread applications.
The thermally stable 3D network skeleton plays an indispensable role in enhancing high-temperature strength, particularly when grain boundaries weaken at elevated temperatures compared to the grain interior.Intriguingly, the present 3D network intermetallics exhibit a trans-granular structure comprising submicron cells significantly smaller than micron grains.The cell wall would effectively impede the movement of dislocations, as demonstrated by BF-TEM and transmission Kikuchi diffraction-Kernel average misorientation (TKD-KAM) images in the 300°C tensile fractured sample (Figures 4a and b), confining dislocations within the cells, especially along their boundaries.With such a 3D skeleton structure, the influence of grain boundaries on high-temperature mechanical properties is expected to be suspended, thereby negating the softening of grain boundaries at elevated temperatures, as illustrated in the schematic diagram (Figure 4c).Moreover, these cell walls can be regarded as stiffer and more stable boundaries, where the 'boundary strengthening mechanism' from the network remains effective at elevated temperatures.Furthermore, the present stiffer skeleton also effectively bears the load transferred from the matrix, as evidenced by the presence of numerous microcracks within the skeleton rather than long and straight cracks after the tensile test at 300°C (Figure 4a).Additionally, this phenomenon has been witnessed in the 200°C tensilefractured sample with a high density of microcracks also observed (Fig. S2).Quantitatively, the YS comparison between the as-printed alloy with network intermetallics and heat-treated alloy with granular intermetallics was employed to estimate the strength contribution of network intermetallic morphology at elevated temperatures, with detailed calculations shown in supplementary materials.The analysis reveals that the 3D network morphology accounts for approximately 31% and 40% of the YS at 200°C and 300°C, respectively.It is noteworthy that the YS contribution from the 3D network morphology is 9% higher at 300°C than that at 200°C.This phenomenon can be attributed to the diminishing dragging effect of Mg on dislocation movement as the temperature rises, leading the network intermetallics to bear a relatively higher load during deformation.

High-temperature toughing mechanism
The present stiff boundaries are supposed to result in reduced deformability compared to the typical grain boundaries.Notably, besides the conventional thin wall sites, partial microcracks were also observed near embedded Al 6 Mn precipitates in the skeleton (Figures 4d,  e), indicating that additional microcracks were induced by embedded Al 6 Mn particles.For visualization, a schematic diagram has been added to clarify the effect of these embedded Al 6 Mn particles on fracture behavior (Figure 4f).The interfaces between the precipitates and the skeleton feature high distortion energy, acting as potential weak sites during the tensile deformation at elevated temperatures.Consequently, additional microcracks form near the nanoprecipitates, isolating the intermetallics.More importantly, these additional microcracks would be buffered by the matrix rather than evolving into long and sharp cracks soon.The absence of decohesion along the interfaces between the intermetallics and the matrix indicates that these isolated intermetallics continue to strengthen the alloy and deform more coordinately with the matrix.The TKD-KAM map shows strong local plastic strain gradients occurring in the vicinity of the cell boundaries (Figure 4b), thereby resulting in additional hardening.The strain-hardened matrix can effectively buffer the microcracks by generating the plastic zone near the crack tips [35,36], as illustrated in the schematic diagram (Figure 4f).The accumulation of geometrically necessary dislocations (GNDs) was attributed to the plastic mismatch between the soft matrix and the stiff intermetallics.Importantly, the addition of Mg exerts a pronounced solute drag effect on dislocations, thereby retarding the occurrence of recovery [37].That is why no significant softening was observed during the tensile deformation (Figure 3a).Moreover, these induced additional microcracks also positively impact load transmission by relieving stress along the interfaces between intermetallics and the matrix, thereby promoting load transmission from the matrix to the skeleton.

Conclusions
In this study, we fabricated a heat-resistant Al-La-Mg-Mn alloy via LPBF, demonstrating outstanding strength at 200°C and 300°C.This LPBF alloy is characterized by a predominant high-Mg supersaturated α-Al matrix, reinforced with a trans-crystalline and Mn-doped 3D network intermetallic skeleton.This innovative structure has fundamentally transformed conventional grainboundary-structured aluminum alloy, rendering grain boundaries less significant while effectively impeding dislocation movement and bearing increased transferred loads to achieve high strength at elevated temperatures.These advancements hold great promise for developing cost-effective heat-resistant alloys and provide valuable insights into 3D intermetallic-structured alloys.

Figure 1 .
Figure 1.Phase composition and morphology of the as-printed Al-La-Mg-Mn alloy: (a) XRD pattern; (b) BSE-SEM image and locally enlarged images of MPB (c) and MPI (d) from the orange and red rectangles in (b); (e) BF-TEM image of network intermetallics; (f) IPF of the α-Al matrix in color combined with BC map of intermetallics in gray.

Figure 2 .
Figure 2. (a) SE-SEM morphology of 3D dendritic networks; (b) BF-TEM morphology of network intermetallics embedded with Al 6 Mn; (c, d) Schematic illustrations for dendritic network structure forming.

Figure 3 .
Figure 3. (a) Tensile stress-strain curves of the as-printed Al-La-Mg-Mn alloy at 25°C, 200°C and 300°C, with dashed and solid lines representing the engineering and true stress-strain curves, respectively; (b) YS, (c) UTS, and (d) total elongation comparison among various LPBF Al alloys at different temperatures.