Composites obtained by the chimeration of different materials through specific microstructures often exhibit unexpectedly excellent properties, which is also the mainstream microstructure form of natural materials after hundreds of millions of years of evolution. In the field of crystalline materials, two or three crystalline materials are mixed and melted according to specific compositions, and then cooled and solidified, and different crystals will automatically alternately compound from the melt in the form of eutectic reaction. If the solidification of the melt is further limited to a specific direction, and then the solidification speed is adjusted, it is possible to make the materials with different properties in situ self-generated and homogeneous recombination at the micrometer or even nanoscale according to specific growth orientations and binding patterns. The eutectic directional solidification technology derived from this has always been a research hotspot in the field of materials science, and has made key contributions to the development and improvement of high-performance composite materials in recent years.
The eutectic solidification theory is the basis of eutectic solidification technology, and the research of this theory has obtained a series of important results in the prosperous development of the past 60 years, and a systematic theoretical system has been established, which can clearly describe the directional solidification behavior of NF/NF and NF/F eutectic materials (according to the different crystal properties of each component, eutectic materials are divided into three categories: non-facet (NF)/non-facet (NF), non-facet (NF)/facet (F) and facet (F)/facet (F)). The rise and development of directionally solidified oxide eutectic ceramics (DSEC) composites at the beginning of this century has revealed the great application prospects of f/f eutectic materials in the field of high-temperature structural materials, and the importance of f/f eutectic materials has gradually attracted attention. However, it is difficult to accurately predict the high-speed directional solidification behavior of f/f eutectic materials using the existing eutectic solidification theory. Although the research on DSEC composites has made great progress in the past 20 years, it is still difficult to achieve the integration of the directional growth behavior of DSEC composites with the existing eutectic solidification theory. The reason is that the following two key problems have not been overcome, namely, the control of eutectic cell structure and the F-NF transition phenomenon under high-speed solidification conditions. The above two problems have become the main obstacles to the theoretical modeling of the growth behavior of the facet eutectic, which makes it particularly difficult to accurately predict and control the solidified structure of the facet eutectic composites.
In order to solve the above problems, the team of Professor Su Haijun of Northwestern Polytechnical University recently took Al2O3/YAG/ZrO2 ternary facet eutectic ceramic composites as the research object, combined with high-gradient laser suspension area fusion directional solidification technology and high-precision infrared imaging real-time temperature measurement, to realize the characterization of the temperature field evolution law of facet eutectic ceramics in the process of high-speed directional solidification at 300 μm/s, and the measurement error was reduced to 5.2 when the measurement temperature was close to 2000 °C On this basis, the internal relationship between the solidification parameters and the solidification structure of the material was revealed, and the critical supercooling degree of F-nf transition of Al2O3/YAG/ZrO2 ternary planar eutectic composite ceramics was discovered for the first time, and the eutectic cell structure of the planar eutectic ceramic composites was successfully optimized, and the micro-nano (~140nm) with excellent uniformity was obtained The planar eutectic structure lays an important theoretical and experimental foundation for the theoretical modeling and structural control of the directional solidification of planar eutectic composites. The results were published in Composites Part B under the title "Insight into faceted-nonfaceted transition of directionally solidified eutectic ceramic composites by laser floating zone melting and infrared imaging": Engineering.
Original link:
DOI: 10.1016/j.compositesb.2024.111372
Fig.1 Comparison of the microstructure of directionally solidified Al2O3/YAG/ZrO2 ternary facet eutectic composite ceramic rods obtained at different pumping speeds: (a1-a2) 25 μm/s; (b1-b2) 50 μm/s; (c1-c2) 100 μm/s; (d1-d2) 200 μm/s; (e1-e2) 300 μm/s.
Based on the ultra-high temperature gradient provided by the laser suspension zone fusion directional solidification technology, the directional solidification Al2O3/YAG/ZrO2 ternary planar eutectic ceramic composites were successfully prepared in a wide range of pull-speed range of 25~300 μm/s, and the microstructure spanned the three characteristic stages of uniform eutectic, non-uniform cellular eutectic, and uniform cellular eutectic, and the F-nf transition was successfully observed.
Fig.2 Three-dimensional analysis of the microstructure characteristics of Al2O3/YAG/ZrO2 samples after f-nf transition at a pull-out velocity of 300 μm/s: (a) 3D model of eutectic structure; (b) Comparison of the cross-section of the model with that of the 300 μm/s sample
The three-dimensional structure of the Al2O3/YAG/ZrO2 sample after f-nf transformation at a pumping speed of 300 μm/s was resolved, and the common unit cell center was triangular prism-like crystals, surrounded by layered crystals. At low pumping speed, the euunit cell of the sample has a coarse irregular structure, and this region is replaced by fine uniform layered crystals after the F-NF transition, and the cellular structure is optimized.
Fig.3 High-precision in-situ calibration strategy for real-time temperature measurement by infrared imaging: (a) the two-dimensional temperature field and the linear temperature distribution at the linear calibration at the suspended melt zone in a stable pumping state at a pull-out speed of 300 μm/s, (b-d) the evolution of the temperature field shown in (a) within 1.44 s after the laser is turned off, (e) the temperature field characteristics at the upper interface of the suspended melt zone during the stable pumping process at a rate of 300 μm/s, and its linear temperature curve and the corresponding second derivative curve; (f) The relationship between the eigentemperature of the re-emitter and the eigentemperature at the upper interface and the emissivity of the material
When the laser is turned off abruptly at a pumping speed of 300 μm/s, the re-luminescence phenomenon is unexpectedly observed in the suspended melting region, because the temperature of the re-luminescence region must be less than the melting point of the material, which is the characteristic temperature point 1, and when the steady-state pumping speed is 300 μm/s, the temperature of the upper liquid-solid interface must be higher than the melting point of the material, which is the characteristic temperature point 2. Through the in-situ calibration of temperature measurement at two characteristic temperature points, it is found that the ε range of material emissivity is between 0.3198~0.3223, and the ε measurement error can be reduced to 5.2 °C even though the measurement temperature is as high as 2000 °C.
Fig. 4 Comparison of the two-dimensional temperature field of the suspended melt zone at different pumping speeds (a), the distribution of the corresponding line temperature (lines are marked in the figure) at each pumping speed are as follows: (b) 25 μm/s, (c) 50 μm/s, (d) 100 μm/s, (e) 200 μm/s, (f) 300 μm/s
After the calibration of infrared temperature measurement, the evolution of the temperature field of Al2O3/YAG/ZrO2 samples in the range of 25~300 μm/s pumping speed was revealed. With the increase of the pumping speed, the suspended melt zone becomes longer, and the temperature gradient at the lower liquid-solid interface decreases from 2262.2 °C/cm to 818.9 °C/cm, and the supercooling degree increases significantly at 300 μm/s.
Fig.5 Analysis of temperature field evolution: (a-b) comparison of temperature field in suspended melt zone at 100 μm/s and 300 μm/s pumping speed, (c) schematic diagram of heat flux in suspended melt zone, (d) influence of supercooling degree on the length of suspended melt zone, (e) influence of latent heat of solidification on the length of suspended melt zone
The influence mechanism of the pumping velocity on the temperature field of the suspended melt zone is analyzed: the consistency of the pumping direction and the heat flow direction leads to the decrease of the temperature gradient with the increase of the pumping velocity, and the supercooling degree increases with the increase of the pumping velocity, which significantly affects the length and temperature distribution characteristics of the suspended melting zone, and reveals the reason for the formation of the ultimate pumping velocity.
Fig.6 Comparison of the supercooling degree of the S-L interface and the corresponding solidification structure in the suspended melt zone at 240~300 μm/s pumping speed: (a) the temperature field of the suspended melt zone, (b) the linear temperature distribution marked in Fig. (a), (c) the statistics of the interfacial supercooling during the 50 s stable pumping process, and (d) the cross-sectional microstructure of the corresponding solidified samples at different pumping speeds
The correlation between the evolution of the temperature field and the evolution of eutectic structure is analyzed in depth. The direct relationship between the F-NF transition and the solidification interface supercooling degree is revealed, and the critical supercooling degree of the F-NF transition is 34±13 °C, which reveals the importance of kinetic supercooling in the high-speed solidification of the facet eutectic.
In summary, this work aims at the two major problems of eutectic cell structure optimization and f-nf transition mechanism faced by the plane eutectic solidification theory. ZrO2 ternary facet eutectic ceramic composites are used as an example to systematically reveal the intrinsic mechanism of f-nf transition and its optimization effect on the eutectic cellular structure, and clarify the importance of kinetic supercooling in the rapid solidification process of plane eutectic from an experimental perspective, which has important reference significance for the microstructure control of DSEC materials and the expansion of the existing eutectic directional solidification theory.
About the Contact:
Su Haijun is a professor and doctoral supervisor of the School of Materials Science and Technology, Northwestern Polytechnical University. He is a national leading talent, a winner of the National Science Fund for Outstanding Young Scholars, a winner of the China Nonferrous Metals Innovation Program, one of the first batch of "Xiangjiang Scholars" in the country, a "young science and technology rising star" in Shaanxi Province, an academic leader of the youth innovation team of Shaanxi universities and a leader of the key scientific and technological innovation team in Shaanxi. He has long been engaged in the research and research of advanced directional solidification technology and theory and new materials, involving high-temperature alloys, ultra-high-temperature composite ceramics, structure-function integrated composite materials, and laser additive manufacturing. He has presided over more than 30 national, provincial and ministerial important scientific research projects, including 7 national funds such as the National Natural Science Foundation of China and Youqing, and published more than 160 papers in many well-known journals such as Nano Energy, Advanced Functional Materials, Nano Letters, Composites part B: engineering, Additive manufacturing, etc. It has been authorized more than 50 Chinese invention patents and 3 American invention patents. Participated in the compilation of 3 monographs. He has won the special prize of outstanding achievements in science and technology research in Shaanxi universities, the first and second prizes of Shaanxi Science and Technology, the first prize of Shaanxi Metallurgical Science and Technology, the National Nonferrous Metals Outstanding Youth Science and Technology Award and the Shaanxi Youth Science and Technology Award.
*Thank you to the team of authors for their great support.