Compared to conventional grain size (5-10 μ m) Compared to titanium alloys, ultrafine grained titanium alloys not only have higher strength and good plasticity matching, but also have higher wear resistance and better biocompatibility, making them highly attractive in many important application fields such as aerospace and biomedical fields. However, the preparation and processing of ultrafine grained titanium alloys are extremely difficult, and the thermal stability of the structure is poor. These two major bottleneck issues restrict the development and application of ultrafine grained titanium alloys.



Yang Ke's team from the Institute of Metals, Chinese Academy of Sciences has long been engaged in basic and applied research on new medical metal materials. Recently, team members Ren Ling, Wang Hai and others adopted the microstructure design idea of "two-phase shell wrapped with ultra-fine equiaxed grains" (Figure 1), improved the thermal stability of ultra-fine grained titanium alloy structure from both thermodynamics and dynamics, and realized the large-scale preparation of the above microstructures by using the process combination of conventional heat treatment and Hot working, solving the two bottlenecks of difficult preparation and processing of ultra-fine grained titanium alloy and poor structural stability, We have obtained ultrafine grained copper titanium alloys with excellent performance and high thermal stability. Recently, relevant research results have been published online in Nature Communications.



The research team has been committed to the integrated research and application of the structure and biological function of copper titanium alloys in recent years. On the basis of previous research work, the team proposed a preparation strategy for ultra-fine grained copper titanium alloy with "eutectoid element alloying → quenching → hot deformation" (EQD) (Figure 2), achieving the design concept of a microstructure with dual phase shell wrapped ultra-fine equiaxed grains. This strategy is realized through conventional Hot working equipment α- Large scale preparation of ultrafine grained Ti6Al4V5Cu alloy with Ti grain size in the range of 90-500 nm (Figure 2). At the same time, utilizing the β/ Ti2Cu dual phase honeycomb shell structure coating α The grain size significantly improves the thermal stability of ultrafine equiaxed grain structure, increasing the instability temperature of the material to 973 K (0.55 Tm) (Figure 3). The room temperature tensile strength of ultrafine grained Ti6Al4V5Cu alloy reaches a maximum of 1.5 GPa, and the elongation exceeds 10%. At 650 ℃ and a strain rate of 0.01 s-1, its tensile elongation exceeded 1000% (Figure 1), achieving superplastic deformation. In addition, the ultrafine grained Ti6Al4V-5Cu alloy did not undergo grain coarsening and growth under high-temperature tensile thermal coupling conditions (Figure 4). This EQD strategy not only achieves the preparation of high-performance and high thermal stability ultrafine grain structures of other titanium alloys such as TiCu and TiZrCu, but also extends to other alloy systems including steel materials, providing a new approach for the preparation of ultrafine grain metal materials. It is of great significance for the design and research of ultrafine grain metal materials.



The above work was jointly completed by the teams of Yang Ke and Ren Ling from the Institute of Metals, Qiu Dong from the Royal Melbourne Institute of Technology in Australia, and Chen Xingqiu from the Shenyang National Research Center for Materials Science at the Institute of Metals. Wang Hai, Assistant Researcher of Institute of Metals, is the lead author, and Qiu Dong, Researcher of Ling Project of Institute of Metals and Professor of Royal Melbourne University of Technology, Australia, is the corresponding author.



The research was supported by the National Key R&D Program, the National Natural Science Foundation of China (NSFC) Key and General Projects, the Chinese Academy of Sciences Key International Cooperation Project, and the Liaoning Provincial "Revitalizing Liaoning Talents Program".



Structure design and properties of nanocrystalline Ti6Al4V5Cu alloy with dual phase honeycomb shell structure, (a) Schematic diagram of structure design; (b) The instability temperature grain size diagram shows that the material has good structural thermal stability; (c) The room temperature strength elongation diagram shows that compared to other titanium alloys, the material has a good strength plasticity match; (d) The elongation of the material exceeds 1000% in the tensile Stress–strain curve at 650 ℃/0.01s-1.



Structure characterization and formation mechanism analysis of nanocrystalline Ti6Al4V5Cu alloy with dual phase honeycomb shell structure, (a) observation of HAADF imaging mode; (b) Energy spectrum surface scanning observation; (c) Schematic diagram of the formation mechanism of dual phase honeycomb shell structure; (d) XRD diffraction pattern; (e) β· Cos（ θ)- Sin（ θ) Figure, adding Cu increases the microstrain in the quenched alloy; (f) The addition of Cu refined the Martensite Flat noodles; (g) The addition of Cu is beneficial for the alloy to undergo cylindrical slip and form equiaxed crystal structures during hot deformation.



Thermal stability analysis of the microstructure of Ti6Al4V5Cu alloy with dual phase honeycomb shell structure nanocrystals, (a) EBSD microstructure after holding at different temperatures for 1 hour; (b) High resolution TEM observation indicates that α、β、 There is a specific Crystallography orientation relationship between Ti2Cu phases; (c) A model for calculating the phase boundary energy of materials based on first principles; (d) EBSD pole diagram, indicating α、β、 The Ti2Cu phase can still maintain its initial orientation after being kept at 700 ℃ for 1 hour; (e) 3DAP analysis of the initial microstructure of Ti6Al4V5Cu alloy; (f) 3DAP analysis of Ti6Al4V5Cu alloy after holding at 650 ℃ for 1 hour.



In situ SEM observation of the evolution during stretching at 650 ℃, (a) initial state SEM structure; (b) Local magnification shows that the material has a honeycomb shell structure; (c) ε= SEM organization at 0.4; (d) Partial magnification shows that the FIB notch around the phase boundary has deviated; (e) Calculate the microstrain distribution within the material based on the displacement of FIB notch nodes; (e) The interfacial slip plays an important role in the superplastic deformation of materials.



Source: China Daily website