【作者】 王振宇；

【导师】 陈正；

【作者基本信息】 中国矿业大学 ， 材料科学与工程， 2021， 硕士

【摘要】 纳米晶合金（<100 nm）具有优异的力学性能。但因其晶界比例大,能量高,热稳定性较差,晶粒在外界环境影响下极易失稳。溶质偏析是提升纳米晶合金热稳定性的有效手段,高熵晶界修饰是基于溶质元素偏析的基础上,在纳米晶合金中引入多种溶质元素,进一步强化纳米晶热稳定性。本文引入高熵晶界修饰新理念,通过机械合金化制备铁基纳米晶合金Fe-ZrNbMoTa和Fe-Zr Nb Hf Ta,并研究其高温热稳定性。采用高温高压烧结技术将稳定性优异的纳米晶合金粉末制备成块体合金。采用X射线衍射仪,扫描电子显微镜、透射电子显微镜、材料试验机等研究了纳米晶合金的显微组织和力学性能。结合纳米晶合金热稳定性模型,分析了纳米晶合金热稳定机制及力学性能。得到以下结论:（1）采用机械合金化法制备纳米晶铁基合金时,粉末粒径随球磨时间的延长逐渐降低,并形成了单相过饱和固溶体。球磨50 h时,Fe-Zr_0.2Nb_0.2Mo_0.2Ta_0.2合金晶粒尺寸为17 nm。对单相纳米晶固溶体进行等温退火处理时,多组元溶质不断向晶界偏析,增加了合金的热稳定性。纳米晶Fe-Zr_1.0Nb_1.0Mo_1.0Ta_1.0合金,在900 ℃下退火10 h,平均晶粒尺寸为55 nm。添加Hf元素的Fe-Zr_0.2Nb_0.2Hf_0.2Ta_0.2比Fe-Zr_0.2Nb_0.2Mo_0.2Ta_0.2合金表现出更好的热稳定性。（2）依据热稳定性实验结果,在800 ℃下采用高压烧结制备了铁基纳米晶合金块体。纳米晶Fe-Zr_0.5Nb_0.5Mo_0.5Ta_0.5合金的硬度可达738.6 HV（7.238 GPa）,屈服强度和抗压强度分别为3107 MPa和3596 MPa。用溶质元素Hf替代Mo,纳米晶Fe-Zr_0.2Nb_0.2Hf_0.2Ta_0.2合金表现出更小的晶粒尺寸（79 nm）,更高的强度（抗压强度3523 MPa）和硬度值（729.5HV）。（3）基于热力学极值原理推导了多元纳米晶合金晶粒生长模型,并与二元简单纳米晶合金体系对比分析了晶界能以及退火时的温度-尺寸关系。从热力学上,多组元溶质偏析相比单元素显著降低了晶界能,提高了纳米晶热稳定性。从动力学上,溶质偏析产生的溶质拖拽效应和高熵晶界带来的迟滞扩散协同作用阻碍了晶界迁移,抑制晶粒长大。在较低温度下,纳米晶合金的热稳定性主要受溶质偏析带来的晶界能的降低和溶质拖拽效应的影响。在较高温度下,沿晶界区域析出第二相沉淀,纳米晶合金的热稳定性主要受高熵修饰和第二相钉扎的共同作用。更多还原

【Abstract】 Nanocrystalline alloys（<100 nm）have excellent mechanical properties.However,due to the large grain boundary proportion,high energy and poor thermal stability,the grain is easily unstable under the influence of external environment.Solute segregation is an effective means to improve the thermal stability of nanocrystalline alloys.The high entropy grain boundary decoration is based on the segregation of solute elements and a variety of solute elements are introduced into the nanocrystalline alloy to further enhance the thermal stability of nanocrystalline alloys.In this paper,Fe-ZrNbMoTa and Fe-Zr Nb Hf Ta nanocrystalline alloys were prepared by mechanical alloying with the introduction of a new concept of high entropy grain boundary decoration,and their high temperature thermal stability was studied.Nanocrystalline alloy powder with excellent stability was prepared into bulk alloy by high temperature and high pressure sintering technology.The microstructure and mechanical properties of nanocrystalline alloy were studied by X-ray diffractometer,scanning electron microscope,transmission electron microscope and material testing machine.The thermal stability mechanism and mechanical properties of nanocrystalline alloy were analyzed with the thermal stability model of nanocrystalline alloy.The following conclusions are drawn:（1）When the nanocrystalline iron-based alloy is prepared by the mechanical alloying method,the grain size of the powder gradually decreases with the extension of the milling time,and a single-phase supersaturated solid solution is formed.The grain size of nanocrystalline Fe-Zr_0.2Nb_0.2Mo_0.2Ta_0.2 alloy can be refined to 17 nm after ball milling for 50 h.When the single-phase nanocrystalline solid solution is subjected to isothermal annealing treatment,the multi-component solute continuously segregates to the grain boundary,which increases the thermal stability of the alloy.Nanocrystalline Fe-Zr_1.0Nb_1.0Mo_1.0Ta_1.0 alloy,annealed at 900 ℃ for 10 h,has an average grain size of55 nm.Fe-Zr_0.2Nb_0.2Hf_0.2Ta_0.2 with Hf element shows better thermal stability than Fe-Zr_0.2Nb_0.2Mo_0.2Ta_0.2 alloy.（2）According to the study of thermal stability of ball milling nanocrystalline alloys,Fe-based nanocrystalline alloy bulk was prepared by high pressure sintering at 800 ℃.The hardness of nanocrystalline Fe-Zr_0.5Nb_0.5Mo_0.5Ta_0.5 alloy with high thermal stability can reach 738.6 HV（7.238 GPa）,and the yield strength and compressive strength are 3107 MPa and 3596 MPa,respectively.The solute element Hf is used instead of Mo,the nanocrystalline Fe-Zr_0.2Nb_0.2Hf_0.2Ta_0.2 alloy exhibits a smaller grain size（79 nm）,higher strength（compressive strength 3523 MPa）and hardness value（729.5 HV）.（3）Based on the principle of thermodynamic extreme value,a multi-element nanocrystalline alloy grain growth model was derived,and compared with the binary simple nanocrystalline alloy system,the grain boundary energy and the temperature-size relationship during annealing were analyzed.Thermodynamically,the multi-element solute segregation significantly reduces the grain boundary energy compared to a single element,and improves the thermal stability of nanocrystals.Dynamically,the solute drag effect caused by solute segregation and the delayed diffusion caused by high-entropy grain boundaries have a synergistic effect that hinders grain boundary migration and inhibits grain growth.At lower temperatures,the thermal stability of nanocrystalline alloys is mainly affected by the reduction of grain boundary energy caused by solute segregation and the solute drag effect.At higher temperatures,the second phase precipitates along the grain boundary region,and the thermal stability of the nanocrystalline alloy is mainly due to the combined effects of high entropy decoration and second phase pinning.更多还原