Hetero-deformation induced (HDI) strengthening generally yields a weak effect on the mechanical property improvement of particle-reinforced metal matrix composites (MMCs). In the present work, a novel strategy was reported to induce remarkable HDI strengthening in MMCs by selecting a reinforcing material with excellent geometrically necessary dislocation (GND) storage ability. The viability of the proposed strategy was tested on additively manufactured nickel matrix composites consisting of Inconel 625 alloy (IN625) as the matrix and high-entropy alloy VNbMoTa as the reinforcing material. It was found that the average grain size and dislocation density of the additively manufactured MMCs gradually decreased with the increase in the additional amount of VNbMoTa. All the samples possessed a similar two-layer VNbMoTa-matrix interface structure containing a high-entropy alloy layer and a Laves phase layer; however, the interface width varied. This two-layer interface could hold GND pile-ups without breaking to ensure a good load transfer effect, and ductile VNbMoTa particles demonstrated excellent GND storage capacity to induce significant HDI stress. The HDI stress for the IN625-(10 wt%)VNbMoTa sample was approximately 200 MPa higher than that for the pure IN625 alloy, resulting in an excellent strength-ductility synergy. The yield strength and elongation of the IN625-(10 wt%)VNbMoTa sample reached (1032.5 ± 18.8) MPa and (11.8 ± 1.2)%, respectively. In addition, the IN625-(10 wt%)VNbMoTa composite also demonstrated superior mechanical properties at 650 ℃ that were comparable to those at room temperature, implying that VNbMoTa addition remarkably limited strength reduction caused by temperature. Deformable VNbMoTa particles effectively alleviated the stress concentration, delayed the crack initiation, generated more dislocations and pile-ups, and, in turn, improved the overall high-temperature strength of composites.

Customized heat treatment is essential for enhancing the mechanical properties of additively manufactured metallic materials, especially for alloys with complex phase constituents and heterogenous microstructure. However, the interrelated evolutions of different microstructure features make it difficult to establish optimal heat treatment processes. Herein, we proposed a method for customized heat treatment process exploration and establishment to overcome this challenge for such kind of alloys, and a wire arc additively manufactured (WAAM) Mg-Gd-Y-Zn-Zr alloy with layered heterostructure was used for feasibility verification. Through this method, the optimal microstructures (fine grain, controllable amount of long period stacking ordered (LPSO) structure and nano-scale β′ precipitates) and the corresponding customized heat treatment processes (520 ℃/30 min + 200 ℃/48 h) were obtained to achieve a good combination of a high strength of 364 MPa and a considerable elongation of 6.2%, which surpassed those of other state-of-the-art WAAM-processed Mg alloys. Furthermore, we evidenced that the favorable effect of the undeformed LPSO structures on the mechanical properties was emphasized only when the nano-scale β′ precipitates were present. It is believed that the findings promote the application of magnesium alloy workpieces and help to establish customized heat treatment processes for additively manufactured materials.

Magnesium and its alloys, as a promising class of materials, is popular in lightweight application and biomedical implants due to their low density and good biocompatibility. Additive manufacturing (AM) of Mg and its alloys is of growing interest in academia and industry. The domain-by-domain localized forming characteristics of AM leads to unique microstructures and performances of AM-process Mg and its alloys, which are different from those of traditionally manufactured counterparts. However, the intrinsic mechanisms still remain unclear and need to be in-depth explored. Therefore, this work aims to discuss and analyze the possible underlying mechanisms regarding defect appearance and elimination, microstructure formation and evolution, and performance improvement, based on presenting a comprehensive and systematic review on the relationship between process parameters, forming quality, microstructure characteristics and resultant performances. Lastly, some key perspectives requiring focus for further progression are highlighted to promote development of AM-processed Mg and its alloys and accelerate their industrialization.