Enhanced Mechanical Properties in LPBF-Fabricated Zr-Based Bulk Metallic Glass Composites through Martensitic Transformation

Significance 

Additive manufacturing (AM) is now considered an essential and transformative technology because it enables the fabrication of complex and customized components with unprecedented precision and flexibility. Among the myriad AM techniques, laser powder bed fusion (LPBF) stands out for its capability to produce high-density metallic parts with intricate geometries. LPBF uses a high-energy laser to selectively melt and fuse metal powder particles layer by layer, and creates fully dense parts with tailored microstructures. The process has been revolutionary in the production of bulk metallic glasses (BMGs). BMGs has unique combination of high strength, elasticity, and corrosion resistance due to their non-crystalline atomic arrangement which make BMGs highly desirable in aerospace, biomedical devices, sports equipment, and consumer electronics applications. Traditional fabrication methods for BMGs, such as mold casting, are limited by size and geometric constraints, which restricts the widespread adoption of these advanced materials. LPBF overcomes these limitations and allows the production of BMGs with complex shapes and larger dimensions, significantly broadening their application potential. Despite its advantages, LPBF-fabricated BMGs face significant challenges that impede their full utilization. One of the primary issues is the partial crystallization that occurs in heat-affected zones (HAZs) during the LPBF process. The localized heating and rapid cooling cycles inherent to LPBF lead to the formation of crystalline phases within the amorphous matrix. This crystallization deteriorates the mechanical properties of BMGs, and can result in reduced plasticity and fracture toughness compared to their cast counterparts. To manage the thermal history of the material to maintain its amorphous structure while minimizing defects such as porosity and brittle crystalline phases in HAZs is a complex task that need careful control over the process parameters and aborative selection of BMG systems. To address these issues, new study published in Journal of Materials Science & Technology and led by Professor Lin Liu from the Huazhong University of Science and Technology and conducted by Pengcheng Zhang, Cheng Zhang, et al.,  investigated a novel quaternary BMG system, Zr47.5Cu45.5Al5Co2, to suppress the formation of brittle intermetallic phases and promote the precipitation of a ductile B2-structured crystalline phase within the HAZs. They designed an alloy that stabilizes the desired phases and tailoring the LPBF process parameters which resulted in a balanced combination of strength, plasticity, and toughness in the resulting BMG composites.

The researchers first prepared the alloy ingot using arc melting in an argon atmosphere with high-purity elemental metals and ensured the production of a high-quality alloy with the desired composition. The ingot was then subjected to gas atomization, to create fully amorphous powders with diameters between 30 and 60 micrometers, suitable for LPBF processing. They confirmed the gas-atomized powders to be fully amorphous using different techniques including X-ray diffraction (XRD) analysis and confirmed that the alloy system had excellent glass-forming ability, essential for maintaining the amorphous structure during the LPBF process. Afterward, the authors conducted the LPBF experiments using a custom-built 3D printing device equipped with a fiber laser of maximum 500 W power, an 80-micrometer spot diameter, and a 1060 nm wavelength. The authors found that XRD analysis of the LPBF samples revealed a composite structure comprising an amorphous matrix and a B2-ZrCu crystalline phase. The proportion of the B2 phase increased with higher energy densities, with the highest energy density resulting in the appearance of a small amount of Cm-ZrCu phase, indicating excessive energy input. This variation demonstrated the ability to control the phase composition through precise adjustment of the LPBF parameters. Further analysis was performed such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to examine the microstructure of the LPBF samples. The SEM micrographs revealed an alternating structure of amorphous molten pools (MPs) and partially crystallized heat-affected zones (HAZs), with the B2-ZrCu phase forming cell-like structures within the HAZs while TEM can provide detailed images of these structures, including selected area electron diffraction patterns that identified the crystalline phases. After these analysis the authors showed microstructural analysis confirmed the presence of a layered structure with amorphous MPs and crystalline HAZs. This unique arrangement, resulting from the layer-by-layer construction of the LPBF process, was critical in enhancing the mechanical properties of the material. The B2-ZrCu phase in HAZs provided a mechanism for improved plasticity and toughness through strain-induced phase transformation during plastic deformation. Moreover, the authors performed detailed investigations of the structural changes of the samples after deformation by TEM. They found that martensitic phase transformation from B2 ZrCu to Cm-ZrCu occurred during plastic transformation, which contribute to the enhanced plasticity and fracture toughness.

The new study by Professor Lin Liu and his team is significant because, first, it demonstrated that LPBF can successfully overcome the current limitations and enable the production of BMGs with complex shapes and larger dimensions. Secondly, the study successfully resolved partial crystallization in HAZs with the design of Zr47.5Cu45.5Al5Co2 that promoted the formation of a ductile B2-ZrCu phase instead of brittle intermetallics. This approach significantly enhances the plasticity and fracture toughness of the material. Thirdly, Professor Lin Liu and colleagues research highlights the martensitic transformation from the B2-ZrCu phase to the Cm-ZrCu phase during plastic deformation, which is analogous to the TRIP effect observed in certain steels and can lead to significant strain-hardening, improve both the strength and plasticity of the BMG composites. This is a new mechanism to enhance the mechanical properties of BMGs. Finally, with varying the laser energy density during the LPBF process, the researchers were able to tailor the phase composition and optimize the mechanical properties of the BMG composites.  In conclusion, the innovative approach of combining alloy design, precise control of LPBF parameters, and comprehensive characterization allowed the researchers to successfully enhance the mechanical properties of LPBF-fabricated BMGs. The Zr47.5Cu45.5Al5Co2 alloy system demonstrated significant potential, with the optimal energy density producing a composite with an excellent balance of strength, plasticity, and toughness. These findings are critical for further advancements in the additive manufacturing of high-performance BMGs, and opens new avenues for their application in various engineering fields.

About the author

Dr. Pengcheng Zhang is currently a postdoc of School of Materials Science and Engineering at Huazhong University of Science and Technology, Wuhan, China. He received his PhD degree from Huazhong University of Science and Technology in 2022. He has published 10+ papers including Scripta Mater, J Mater Sci Technol, Mater Res Lett and J Alloys Compd. His specific research interests include bulk metallic glasses and additive manufacturing.

About the author

Prof. Cheng Zhang is currently a professor of School of Materials Science and Engineering at Huazhong University of Science and Technology, Wuhan, China. He received his PhD degree from Huazhong University of Science and Technology in 2012. Professor Zhang mainly works on amorphous coatings, 3D printing of advanced metallic materials. He has published 70+ papers in Mater Sci Eng Rep., Acta Mater, Scripta Mater, Corro Sci, ACS Appl Mater & Interface, J Mater Chem A and so on with total citations over 3700 times and H-index of 37 (google scholar). Prof. Zhang is also one of the Editor members of Surface & Coating and Technology. He has won the second prize of the Natural Science Award of the Ministry of Education of China.

About the author

Prof. Lin Liu is the group lead for Amorphous Alloys Lab, and currently a Huazhong-distinguished professor in Huazhong University of Science and Technology and chief professor in the State Key Laboratory for Materials Processing and Die & Mold Technology, China. Professor Liu mainly works on Metallic Glasses, High-entropy Alloys, Amorphous Coatings/Thin Films, 3D Printing of Metals and Nanoporous Metals. He has acquired 30 China’s innovation patterns and published more than 300 peer-reviewed papers in international journals with total citations over 11000 times and H-index of 58 (google scholar). He has won the second prize of the Natural Science Award of the Ministry of Education of China twice.

About the author

Dr. Di Ouyang completed his bachelor’s degree from Hefei University of Technology in 2013 and PhD degree from Huazhong University of Science and Technology in 2019. He is currently a postdoctoral fellow of Department of Industrial and Systems Engineering at The Hong Kong Polytechnic University, Hong Kong, China. He has published 20+ papers including Materials Science & Engineering R Reports, Additive Manufacturing, International Journal of Plasticity, Applied Materials Today, Corrosion Science, etc. His research interests include bulk metallic glasses, high-entropy alloys and additive manufacturing.

About the author

Jie Pan is currently a full professor of School of Materials Science and Engineering at Huazhong University of Science and Technology, Wuhan, China. He received his Ph.D. from HUST in 2011. His research interests include the design and mechanical behavior of bulk metallic glasses, high-entropy alloys, and gradient nanostructured metallic materials. He has published over 60 papers in SCI journals, including Nature, Nature Communications, Science Advances, and Acta Materialia.

Reference

Pengcheng Zhang, Cheng Zhang, Jie Pan, Di Ouyang, Lin Liu, Toughening additive manufactured Zr-based bulk metallic glass composites by martensite phase transformation, Journal of Materials Science & Technology, Volume 170, 2024, Pages 95-102,

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