Effect of Cold Rolling and Heat Treatment on Impact Property of TiVTaNb Refractory High-entropy Alloy for Nuclear Reactor
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摘要:
难熔高熵合金是由多种难熔元素形成等原子比或近等原子比的多主元合金，由于其具有优异的高温力学性能、耐腐蚀性能和抗辐照性能，有望作为新型抗辐照结构材料应用于新一代先进核能反应堆系统。本文针对具有强塑性匹配的TiVTaNb高熵合金，研究了冷轧及热处理对TiVTaNb难熔高熵合金冲击性能的影响。试验结果表明冷轧及热处理可显著提升合金的冲击性能，均匀态TiVTaNb合金的冲击吸收能为11.92 J，为铸态TiVTaNb合金冲击吸收能（5.15 J）的2.3倍，均匀态TiVTaNb合金的裂纹形成能及裂纹扩展能均提高，分别为铸态TiVTaNb合金的1.33、2.88倍。均匀态合金的冲击断口上出现了明显的纤维区，且剪切唇面积及弯曲程度均明显大于铸态。铸态合金冲击断口上的韧窝小且浅，而均匀态合金断口上的韧窝相对大且深，并且在大韧窝中还分布着许多小韧窝，可更多的耗散冲击能量。铸态及均匀态合金的冲击变形均由位错和孪晶协同主导，但是均匀化后的合金在冲击变形过程中位错密度明显升高，变形孪晶数量增多，为合金在冲击载荷作用下冲击性能改善和抗裂性能增强的主要原因。相关结果将为核反应堆先进结构材料的研发提供重要的理论参考和设计依据。
Abstract:The refractory high-entropy alloy (RHEA) usually forms a multi-principal element alloy with atomic ratio or near equal atomic ratio via adding a variety of high melting point elements. It is expected to be used as a new type of anti-irradiation structural material in the new generation of advanced nuclear reactor system due to the excellent mechanical properties at high temperatures, corrosion resistance and radiation resistance. In the present study, the effect of cold rolling and heat treatment on Charpy impact properties of TiVTaNb RHEA with high strength-ductility trade-off was systematically studied by using the instrumented Charpy impact testing machine. The experimental results show that after cold rolling and heat treatment, the segregation of elements in the as-cast alloy disappears, and the alloy microstructure is equiaxed grains with uniform composition. No element segregation or second phase precipitation is observed inside the grains and at the grain boundaries of the homogenized alloy under SEM. Cold rolling and heat treatment can significantly improve the impact absorbed energy of the alloy. The impact absorbed energy of homogenized TiVTaNb alloy is 11.92 J, which is 2.3 times that of as-cast TiVTaNb alloy (5.15 J). The fracture mode of as-cast TiVTaNb alloy under impact load is a ductile-brittle mixed fracture mode dominated by ductile fracture. However, the fracture mode of homogenized TiVTaNb alloy is ductile fracture, with obvious fiber regions in the lower part of the V-shaped notch, larger bending degree and area of shear lips, deeper and larger dimples with small dimples distributed in it for both the crack initiation region and the crack propagation region, which all effectively dissipate more impact energy. The crack initiation energy and propagation energy of homogenized TiVTaNb alloy are both higher than those of the as-cast TiVTaNb alloy, especially the increase of crack propagation energy is larger, which is 2.88 times that of as-cast TiVTaNb alloy. This indicates that the cold-rolled and heat-treated TiVTaNb alloy has higher resistance to crack initiation and crack propagation, and its crack propagation rate is slower. The cold rolling and heat treatment processes do not change the impact deformation mechanism of the alloy. The deformation mechanism is synergized by dislocation activities and deformation twinning for both as-cast and homogenized alloys. However, the dislocation density and the number of deformation twins increase significantly during the impact deformation process of homogenized TiVTaNb alloy, which is the main contribution for the improved impact properties and the stronger crack resistance under impact loading.
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图 3 冲击试验TiVTaNb合金载荷-位移曲线
a——铸态TiVTaNb合金；b——均匀态TiVTaNb合金
Figure 3. Load-displacement curve of TiVTaNb alloy during impact testing
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图 4 TiVTaNb合金微观组织
a——铸态TiVTaNb合金微观组织；b——铸态TiVTaNb合金EDS；c——铸态TiVTaNb合金XRD；d——均匀态TiVTaNb合金微观组织；e——均匀态TiVTaNb合金微观组织局部放大；f——均匀态TiVTaNb合金XRD
Figure 4. Microstructure of TiVTaNb alloy
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图 5 冲击前后试样IPF图
a——冲击前铸态TiVTaNb合金；b——冲击后铸态TiVTaNb合金；c——冲击前均匀态TiVTaNb合金；d——冲击后均匀态TiVTaNb合金
Figure 5. IPF map of impact sample before and after impact testing
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图 6 裂纹扩展区域的KAM图
a——铸态TiVTaNb合金；b——均匀态TiVTaNb合金
Figure 6. KAM map of crack propagation region
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图 7 断口侧面的宏观形貌
a——铸态TiVTaNb合金；b——均匀态TiVTaNb合金
Figure 7. Macroscopic morphology of fracture surface
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