【作者】 张娜；

【导师】 沈同德；

【作者基本信息】 燕山大学 ， 材料学， 2019， 博士

【摘要】 对于传统粗晶材料,退火通常会引起强度和硬度下降。然而,在许多纳米/超细晶金属和合金中发现,退火能引起强度和硬度的上升,称为反常“退火硬化”现象。这种退火引起的反常硬化现象已有许多理论解释,例如:溶质在晶粒边界的偏聚、晶粒边界弛豫、第二相钉扎晶界等等。为证明何种机制对退火硬化起着主要作用,本文用电沉积的方法制备了单相的Ni（Fe）合金,Fe原子均匀分布在Ni的基体中,Fe原子的含量约为1%。该合金是Ni-Fe固溶体,没有第二相存在,这样可以排除第二相对退火硬化的影响。退火过程中,在晶粒开始长大的温度之前,硬度确实有稍微升高。另外,在退火硬化的温度进行长时间退火晶粒发生异常长大,硬度下降。晶格常数和原子探针的结果均表明:纳米晶Ni（Fe）合金的退火硬化是由溶质原子和杂质原子在晶界的偏聚引起的。纳米/超细晶金属较传统粗晶材料具有高得多的强度/硬度,超细的甚至是纳米尺寸的晶粒也使它们具有非常好的电学、磁学、耐腐蚀性能等,这些优越的性能使其具有很好的工程应用能力。但是,纳米/超细晶金属中高密度的晶界增加了其储存能,导致它们的热稳定性很差,在低温甚至室温下晶粒就会长大,这些问题严重制约了其实际应用。因此,研究纳米/超细晶金属的热稳定性和探索提高其热稳定性的途径对在高温下制备和应用都至关重要,具有重要的学术价值和应用意义。本论文设计制备了纳米晶Ni₉₉Fe₁合金,该合金以纯镍（99.98%）为基体,在基体中加入原子比为1%的铁,利用固溶元素铁稳定镍合金的纳米结构。通过对纳米晶Ni₉₉Fe₁合金在宽温度范围内（66¹000℃）退火研究发现,Ni₉₉Fe₁合金的硬度随着退火温度的升高呈阶梯状陡降。在≤216℃,256⁶26℃和646¹000℃三个宽的温度范围内,硬度保持几乎恒定或缓慢下降的趋势;然而,在216²56℃和626⁶46℃仅仅40℃和20℃的温度范围内,硬度却发生急剧下降,分别下降约40%和30%。通过对微观结构研究和热力学分析表明,铁的添加对Ni₉₉Fe₁合金表现出独特的硬度随着退火温度变化的阶梯状曲线起着关键作用。主要结论如下:（1）低温退火时,固溶到Ni基体中的Fe和杂质原子不断地向晶界偏聚,抑制了晶粒长大。当大部分或全部Fe和杂质原子偏聚到晶界后,继续升高退火温度时,没有更多的原子可以偏聚到晶界抑制晶粒长大,发生了晶粒异常长大,硬度出现第一个急剧下降。随着晶粒长大,晶界上的Fe和杂质原子反偏聚到Ni的晶格内。（2）晶粒长大后,随着退火温度的继续升高,反偏聚到Ni基体中的Fe和杂质原子又不断地向晶界偏聚,抑制了晶粒长大,硬度缓慢下降。（3）当退火温度升高到626℃时,热力学上达到了Ni与Fe形成Ni₃Fe化合物的温度。这时,晶界上的Fe与Ni形成Ni₃Fe相,偏聚到晶界上的Fe原子被消耗掉,不能再起到抑制晶粒长大的作用,晶粒发生长大,硬度出现第二个急剧下降阶段。（4）随着退火温度的继续升高,晶粒发生急剧长大。由于晶界上的Ni₃Fe纳米析出相阻碍位错运动,增加位错运动的剪应力,硬度没有急剧下降而出现平稳阶段。该研究结果表明,低温退火时Fe和杂质原子在晶界的偏聚和高温退火时Ni₃Fe颗粒阻碍位错运动是Ni₉₉Fe₁的硬度随着退火温度的升高呈现阶梯状变化的原因。同时也表明,热力学上的偏聚比动力学上Ni₃Fe对晶界的钉扎稳定纳米晶Ni₉₉Fe₁合金更有效。本研究设计了添加不同原子类型和浓度的溶质原子到镍基体中,通过计算溶质偏聚对吉布斯自由能和晶粒边界自由能的改变,预测合金的热稳定性。同时,用熔炼和高压扭转变形的方法制备了相应的超细晶合金,通过实验测试硬度随着退火温度的变化进行这些合金的热稳定性研究。本论文中设计制备了超细晶Ni和Ni₉₉Fe₁,Ni₉₉Cr₁,Ni₉₉V₁,Ni₉₇Cr₃合金,热稳定性的实验结果与理论计算结果一致,证明了热力学模型预测的可靠性。与超细晶纯Ni相比,Ni₉₉Fe₁的热稳定性从140℃提高到220℃,Ni₉₉Cr₁的热稳定性提高到240℃,Ni₉₉V₁的热稳定性提高到300℃,Ni₉₇Cr₃的热稳定性提高到320℃。该研究结果表明,通过偏聚降低吉布斯自由能和晶粒边界自由能越多的合金的热稳定性越好。当添加相同原子Cr时,添加3%原子的Ni₉₉Cr₃合金的热稳定性明显好于添加1%原子的Ni₉₉Cr₁合金。该研究对设计提高热稳定性的纳米/超细晶合金具有重要的应用意义。更多还原

【Abstract】 Annealing often causes softening in conventional large-grained materials.However,annealing has been found to cause hardening in many nanocrystalline metals and alloys.This abnormal hardening by annealing has been explained by such factors as solute segregation at grain boundaries,grain boundary relaxation,grain boundary pinning by second phase particles,et al.To shed light on which mechanism dominates annealing hardening,we have prepared a homogenous single-phase nanocrystalline Ni（Fe）alloy with approx.1 at%Fe by electrodeposition.This alloy is a solid solution of Fe in Ni without any precipitated phase.Thus,the hardening caused by second phase particles pinning can be excluded.We indeed found that microhardness of the alloy increased slightly just before the grains started to grow during annealing.In addition,long time annealing at the hardening temperature resulted in the decrease of hardness due to grain growth.Both lattice parameter and atom probe tomography studies suggest that the annealing hardening is caused by the segregation of solute and impurity atoms on grain boundaries in our nanocrystalline Ni（Fe）alloy.Nanocrystalline and ultra-fine-grained metals and alloys show superior strength as compared to their conventional coarse-grained counterparts.These materials also exhibit attractive electrical,magnetic,and corrosive properties owing to their ultra-fine or nano-sized grains.These superior properties make them desired materials for structural engineering applications.However,the poor thermal stability of nanocrystalline and ultra-fine-grained metals and alloys,which is related to the high stored energy associated with the high density of grain boundaries,seriously restricts their practical application.The grains start to grow even at low and room temperature.It is important to investigate the thermal stability of nanocrystalline and untra-fine-grained metals fabricated and applicated at high temperature and explore new approaches to improve their thermal stability.The corresponding study should be of scientific and practical importance.In this study a nanocrystaline Ni₉₉Fe₁ alloy is designed and produced.High purity Ni（99.98%）is used as base metal and 1 at.%Fe is added as solute to make the alloy.The objective of the present study is to explore the effects of solid solute Fe on the thermal stability of the nanocrystalline Ni₉₉Fe₁ alloy.The thermal stability of the nanocrystalline Ni₉₉Fe₁ alloy is investigated over a wide range of annealing temperature from 66℃to 1000℃.It is found that the microhardness exhibit a step drop with increasing annealing temperature.The microhardness remains almost constant or drops slightly in the wide annealing temperature range of≤216℃,256⁶26℃and 646¹000℃.However,the microhardness drops sharply in the narrow annealing temperature range of 216²56℃and 626⁶46℃.The microhardness drops about40%and 30%in the narrow temperature range of 40℃and 20℃respectively.After investigating the microstructure and thermodynamic data,it has been found that the addition of Fe plays a vital role on the unique step change of microhardness with annealing temperature.The main conclusions are:（1）In the low annealing temperature,Fe and impurity atoms dissolved in Ni base segregate to grain boundaries gradually,which inhibits grain growth.After most or all of the Fe and impurity atoms segregate to grain boundaries,the grains grow abnormally with increasing annealing temperature because there isn’t any more atoms segregated to grain boundaries.Fe and impurity atoms segrate at grain boundaries dissolve back to Ni lattice while the grains is growing.（2）With increasing annealing temperature continuously after the grains growing up,Fe and impurity atoms dissolve back to Ni lattice and finally segregate to grain boundaries again which inhibites grain growth.The microhardness dropped slightly.（3）When the annealing temperature is increased to 626℃,the high temperature allows to form Ni₃Fe particles thermodynamicly.Fe and Ni atoms at grain boundaries form Ni₃Fe,therefore the grains grow and microhardness drops sharply again because there isn’t Fe atoms segregated at grain boundaries inhibitting grain growth.（4）With increasing annealing temperature continuously,grain grow rapidly.The nanoprecipitates of Ni₃Fe act as barriers for dislocation motion,increasing the shear stress required for dislocation motion,therefore the microhardness doesn’t drop but keeps stable.This investigation makes clearly that step change of microhardness of Ni₉₉Fe₁ alloy is attributed to Fe and impurity atoms segregated to grain boundaries at low and medium annealing temperatures and Ni₃Fe particles act as barrier for dislocation motion at high annealing temperature.In this study,it also indicates that atoms segregation is more effective for stabilizing nanocrystalline Ni（Fe）alloy than particles pinning.In this study nanocrystaline Ni and Ni alloys with different types and concentration of solutes are designed.Thermal stability of the alloys is predicted based on their Gibbs free energy and grain boundary energy changed by solute segregation.Thermal stability of Ni and Ni alloys are also experimentally studied by their microhardness changes with annealing temperature.Ultra-fine-grained pure Ni and Ni₉₉Fe₁,Ni₉₉Cr₁,Ni₉₉V₁,Ni₉₇Cr₃alloys are designed and produced.The experimental results agree well with the theoretical predictions which testify the reliability of the thermodynamic model.Compared to ultra-fine-grained pure Ni,thermal stability of Ni₉₉Fe₁ is improved from140℃to 220℃.Ni₉₉Cr₁ is improved to 240℃and Ni₉₉V₁ is improved to 300℃.Thermal stability of Ni₉₇Cr₃ is improved to as high as 320℃.This study indicates that thermal stability of the alloys with lower Gibbs free energy and grain boundary energy after segregation is better.With the addition of Cr,the thermal stability of Ni₉₇Cr₃ is much better than Ni₉₉Cr₁.This study should help design UFG Ni alloys with improved thermal stability for practical applications.更多还原