CHEMICAL SCIENCE

Tap the performance potential of traditional alloys – additive manufacturing of ultra-high strength nano twin crystal titanium alloys


With a high degree of design freedom and the ability to manufacture parts with almost any complex geometry, 3D printing technology is leading a new era of metal parts manufacturing in aerospace, automotive, biomedicine and energy. At present, titanium alloy is the most used 3D printing metal material in the aviation industry.

Monash University in Australia, the Institute of Metal Sciences of the Chinese Academy of Sciences, the University of Shanghai for Science and Technology, the Australian National University, Deakin University in Australia and the Ohio State University in the United States have carried out a comprehensive cooperation to use 3D printing technology to greatly enhance the strength of existing commercial titanium alloys, making them have the highest specific strength of all existing 3D printed metals. The researchers took advantage of the unique thermal cycling and rapid solidification characteristics of the 3D printing process to form a unique nano-precipitation microstructure of dense, stable and multiple internal twin crystals in the material, thereby obtaining unprecedented mechanical properties. While existing work has demonstrated that achieving high density of nano-twin crystals and nano-precipitated phases in pure metals can achieve exceptionally high strength and sufficient ductility, this nanoprecipitated phase with dense internal twin crystals has been reported for the first time in existing commercial alloys.

On September 15, 2022, the research was published in the journal Nature Materials under the title “Ultrastrong Nanotwinned Titanium Alloys through Additive Manufacturing”.

Professor Huang Aijun, Senior Researcher Zhu Yuman and Professor Wang Hao of the University of Shanghai for Science and Technology of Monash University are co-corresponding authors, the co-first authors are Zhu Yuman, Senior Researcher of Monash University, Dr. Zhang Kun and Dr. Meng Zhichao of the Institute of Metal Sciences, Chinese Academy of Sciences, Yang Rui, Researcher of the Institute of Metal Sciences/ShanghaiTech University, and Associate Professor Zhang Kai of the University of Shanghai for Science and Technology as co-authors.

In this work, the researchers prepared a commercial titanium alloy (Beta-C) using the commonly used laser powder bed 3D printing technology. The printed specimens were subjected to direct aging heat treatment at two different temperatures. Figure 1a shows a tensile stress strain curve showing that samples treated with heat at 480°C and 520°C have unexpected high strength. After heat treatment at 480°C, the ultimate strength reaches 1611 MPa and maintains a uniform elongation of 5.4%. This strength is higher than all 3D printed titanium alloys, steels, aluminum alloys, and nickel-based superalloys reported to date, as shown in Figure 1b. In addition, the strength and ductility of this alloy can be regulated by adjusting the heat treatment scheme to meet specific application needs.

Figure 1: Tensile properties of a commercial Beta-C titanium alloy prepared by laser powder bed 3D printing and subsequent heat treatment.

To reveal the root causes of the special reinforcement mechanism of this laser powder bed 3D printing ultra-high strength titanium alloy, the researchers conducted a meticulous microstructure study of the printed samples before and after heat treatment (Figure 2). The results show that the microstructure of the morphology is a pure body-centered cubic β and a high-density spiral dislocation configuration. The microstructure (nanoscale α-precipitates in the 10-50 nm range) formed by heat treatment on this basis is very different from the titanium alloy prepared by the traditional process and inhibits the precipitation of the grain boundary α phase during the heat treatment process, as shown in Figure 2c.

Figure 2: Microstructure of Beta-C titanium alloys by laser powder bed 3D printing and subsequent heat treatment.

Further observation of these nanoscale α-precipitated phases revealed dense triple twin substructures. Further analysis determined that they were {10-11} twins. The presence of these twin crystals results in a higher thermal stability of the α-precipitated phase. More importantly, these twin interfaces can act as slip surfaces to release internal stresses and increase the number of slip lines in the hexagonal structure α precipitated phase. In addition, periodic solute partial polymerization was observed at the {10-11} twin interface, as shown in Figure 3f. This partial gathering has a pinning effect on the twin grain boundaries and further increases its stability.

Figure 3: Nano twin α-precipitate and twin grain boundary atoms in the microstructure of the 3D printed Beta-C alloy after subsequent heat treatment (480 °C/6 h).

Further molecular dynamics (MD) simulations revealed the effect of printed dislocation configuration on nano twin α-precipitation during subsequent heat treatment. By applying a three-way tensile stress to the β-phase matrix containing a dense 1/2 screw dislocation to simulate the micro-strain structure of the printed state, it was found that the α-precipitated phase nucleated along these dislocations, as shown in Figure 4. This is because local strain around the dislocation core can significantly reduce the energy barrier required for nucleation of the precipitated phase. What’s more, all three twin-related α variants can form nuclei individually at different misalignment positions. These α variants grow up as heating time increases and form multiple multiple twin variants. This simulation results are consistent with the experimental results observed in Figure 3.

Pic 4 .png

Figure 4: Molecular dynamics simulation of nano twin precipitation around dense screw dislocation.

This work uses additive manufacturing techniques to introduce high-density nano twin precipitates into the material structure, resulting in super-strong titanium alloys. This unique microstructure and properties achieved in commercial titanium alloys may lead to practical industrial applications. At the same time, the results of this work also bring new perspectives to the traditional precipitation strengthening mechanism and dislocation engineering in the field of physical metallurgy. (Source: Science Network)

Related paper information:https://doi.org/10.1038/s41563-022-01359-2



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