【作者】 徐萌；

【导师】 陈骏；

【作者基本信息】 北京科技大学 ， 冶金工程， 2022， 博士

【摘要】 固体材料的负热膨胀(NTE)是一种奇异的物理现象,它是由品格、电子和声子之间强烈的耦合作用导致。NTE材料在基础科学研究以及调控材料热膨胀等实际应用领域具有重要的价值。目前,在众多的功能材料中都观察到NTE性质;其中,金属基NTE材料由于具有良好的热导率、电导率以及优异的力学性能,因此更具有应用潜质。另外,成分简单的NTE材料有利于深入的揭示其物理机制,在实际应用中也利于通过成分调控其热膨胀性能;因此,本文选择在二元金属材料中设计新的NTE材料,深入揭示其NTE机制,并且实现二元NTE金属材料的热膨胀进行调控,以满足更为广泛和苛刻的应用需求。HfFe2为立方和六方结构相的混合相,但是过量Fe能够使Fe固溶进Hf亚晶格位点,在0.3 ≤δ ≤ 0.6成分区间HfFe2+δ可以形成单一的C14型六方相。通过同步辐射X射线衍射(SXRD)研究可知,在单相区随着Fe含量的增多,HfFe2+δ的热膨胀可以实现从NTE到零热膨胀(ZTE)再到正热膨胀(PTE)的连续调控。在HfFe2.5材料中能够实现高温ZTE(体积膨胀系数αv=1.25 × 10-6 K-1,433-583 K),这是目前磁性金属基材料可实现最高的ZTE温区。通过中子粉末衍射(NPD)解析其磁结构,发现Fe原子磁矩减小产生的磁体积效应与晶格振动产生的热膨胀相抵消是导致ZTE原因。第一性原理计算发现,占据Hf(4f)晶格位点的Fe原子与临近亚晶格位点(2a)和(6h)的Fe形成新的铁磁交互作用路径,增强铁磁相稳定性使TC升高,从而在高温区实现ZTE。研究发现金属基材料的NTE基本都与相变有关,导致NTE工作温区低且相对窄,而非金属框架NTE材料通常具有宽的NTE温区,因此在具有框架结构的金属基材料中将有可能实现宽温区NTE。本文在类框架金属材料FeZr2中实现巨大超宽温区的单轴NTE性能(αl=-34.18×10-6 K-1,93-1078 K),是目前金属材料中热膨胀系数最负和温区最宽的单轴NTE。通过XRD、SXRD、NPD和中子对分布函数(nPDF)分析,证明了 FeZr2铸锭在垂直方向(VD)的巨大NTE是由于其单胞参数c的强NTE以及(001)晶面具有强的织构VD//(001)导致。结合晶格动力学分析,在FeZr2中容易在光学声子区域贡献沿c轴方向负的格林艾森常数(γc)。第一性原理计算发现FeZr2的电子结构决定了其能够稳定为大轴比的CuAl2结构,为c轴产生巨大NTE提供必要条件。更重要的是,实验和计算研究发现FeZr2具有弱的键强,使其更容易受到声子振动影响,从而产生负的γc,因此诱导了巨大宽温区单轴NTE。FeZr2超宽温区的巨大单轴NTE为此类型材料的热膨胀调控提供了非常有利的条件。在同构型的MZr2(M=Fe、Co和Ni)体系可通过化学修饰实现热膨胀系数(CTE)的有效调控。本研究发现随着M元素3d电子数目增多,变温NPD观察到其c轴热膨胀逐渐从巨大NTE向PTE大范围转变。有意思的是,在Co0.5Ni0.5Zr2成分可实现宽温区的单轴ZTE(αl=-0.45 × 10-6 K-1,96-723 K)。结合不同成分MZr2的原子位移参数(ADPs),发现随着3d 电子增多,Zr原子U12的APDs(表示为:Zr-U12)从FeZr2的负值负增长逐渐转变为NiZr2中的正值增大。Zr-U12为负值倾向沿c轴方向负γc的振动模式,说明是声子振动导致的单轴NTE。进一步实验和计算发现,随着3d电子数增多,Zr-M键以及M-M键的强度均增强。化学键增强削弱了声子对品格的影响,难以产生负的γc,从而使c轴NTE受到抑制,因此向正热膨胀转变。复合相也能够调控FeZr2的单轴CTE,在纯Zr中加入少量的Fe能够实现Fe-Zr两元合金的热膨胀调控。本研究在Fe0.1Zr0.9成分中实现超宽温区单轴ZTE(αl=0.45×10-6 K-1,105-900 K),该ZTE材料同时具有高的压缩强度(δUs=1.40±0.13 GPa)以及大的极限应变22.4%。这是首次报道具有如此宽ZTE温区兼顾优异力学性能的非磁性ZTE合金。XRD、中子衍射、SEM、EPMA以及TEM发现在Fe-Zr合金为双相合金,并且特定相中均匀分布着FeZr2纳米析出相以及亚微米Zr相。该微结构有强的界面结合作用力,能够极大的缓解两相大热膨胀差异带来的晶格应变以及阻挡加载后带来的裂纹扩散。结合原位压缩中子测试揭示其优异的力学性能是由于强界面结合力和双相协同作用导致的。在ZrCox(0.5 ≤x ≤1)合金中通过改变Co含量可同时实现热膨胀调控和高温高强度力学性能。在成分x=0.82,能够实现宽温区单轴ZTE(110-760 K,αl=-0.35 × 10-6 K-1)以及随着温度升高增大的高温高极限强度(δ573K,US=2.56 GPa)。XRD和中子衍射发现Co-Zr合金是由软相ZrCo以及具有强单轴NTE的CoZr2脆相组成的双相合金。结合SEM、EPMA、EBSD以及TEM发现ZrCo和CoZr2两相结合紧密。原位应力应变中子测试发现两相的协同作用导致其具有良好的压缩强度以及塑性,ZrCo相的高温高压缩强度决定了其具有随温度升高的优异力学性能。更多还原

【Abstract】 Negative thermal expansion(NTE)of solid materials is an interesting physical phenomenon closely related to the coupling between the lattice,electrons,and phonons.NTE materials are of great value in basic science research and practical applications such as regulating the thermal expansion of materials.To date,NTE has been observed in various types of materials.However,metal-based NTE materials have promising potential applications compared to other types of NTE materials because metal materials feature high thermal and electrical conductivity and good mechanical properties.On the other hand,the simple composition of NTE materials is favorable to insight into revealing its physical mechanism and regulating its thermal expansion by chemical modification,Binary NTE metalbased materials are simple in composition and rich in physical properties.Therefore,in this paper,binary metal-based materials are chosen to design new NTE materials,reveal the mechanism of NTE,and regulate the thermal expansion of binary metallic materials to meet the requirements of more extensive and demanding applications.HfFe2 is a mixture of cubic and hexagonal structure phases,but the excess Fe can make partial Fe atoms solid solution into Hf sublattice sites,and HfFe2+δ can form a pure C14-type of Laves phase between 0.3≤δ≤0.6.Synchronous Xray powder diffraction(SXRD)shows that the thermal expansion in the singlephase region of HfFe2+δ changes continuously from NTE to ZTE to PTE with increasing Fe content.Interestingly,high temperature ZTE(αv=1.25 ×10-6 K-1,433K-583K)can be achieved in composition HfFe2.5.This is the highest ZTE temperature region achievable for metal-based magnetic materials to date.The magnetic structure is analyzed by neutron powder diffraction(NPD).And it is found that the magnetovolume effect due to the reduction of the total magnetic moment of Fe produces a negative volume contribution,offset by the lattice thermal expansion is responsible for its ZTE.First-principles calculations reveal that Fe occupies Hf sublattice sites,which can stabilize the ferromagnetic phase.At the same time,the excess Fe forms new ferromagnetic interactions with the adjacent Fe sublattice sites(2a)and(6h).This can raise the TC.Therefore,high-temperature ZTE can be achieved.It is known that metal-based NTE materials are associated with phase transitions,with low NTE temperature windows and narrow temperature regions.It can be found that framework NTE non-metallic materials possess broad NTE temperature regions.Therefore,the design of framework systems in metal-based materials will make it possible to achieve a wide NTE temperature range.Fortunately,framework-like FeZr2 ingot features a colossal uniaxial NTE over an ultrawide working temperature region(αl=-34.18 × 10-6 K-1,93K-1078K).Such an excellent uniaxial NTE is the largest and widest NTE in all metallic materials so far.It was demonstrated by XRD,SXRD,NPD,and neutron pair distribution function(nPDF)that FeZr2 ingots exhibit colossal NTE along the vertical direction(VD)is due to the colossal NTE of its lattice parameter c and the strong texture of the crystal direction(001)//VD.Combined with lattice dynamics analysis,it is found that the mechanism of colossal uniaxial NTE is related to phonon vibrations,which are more likely to contribute negative Grüneisen parameters along the c-axis(γc)in the optical phonon region for FeZr2.The special electronic structure of FeZr2 was found by first-principles calculations,which can stabilize FeZr2 into the CuAl2 structure with a large axial ratio.The large axial ratio provides the necessary conditions for generating large space for huge NTE.More importantly,its weak chemical bonding is more susceptible to phonon influence to produce negative γc,which determines its colossal and ultrawide temperature region uniaxial NTE.More importantly,the experimental and computational studies indicate that FeZr2 has a weak bond strength,making it more susceptible to phonon vibrations that produce negative γc,determining its huge broad-temperature region uniaxial NTE.In the isostructure MZr2(M=Fe,Co,and Ni)systems with the increasing number of 3d electrons of M elements,the c-axis coefficient of thermal expansion(CTE)was found to gradually regulate from a colossal NTE to PTE according to the variable temperature NPD.Interestingly,it was found that the Co0.5Ni0.5Zr2 ingot shows a wide temperature region uniaxial ZTE(96-723 K,αl=-0.45 × 10-6 K-1).Combining the atomic displacement parameters(ADPs)of the different MZr2 components,it can be found that the APDs of Zr-U12 gradually decrease from negative in FeZr2 to positive in NiZr2 as the number of 3d electrons increases.The negative value of Zr-U12 corresponds to the vibrational mode with a negative γc.It indicates the uniaxial NTE is due to the phonon vibrations.Further experiments and calculations reveal that the chemical bond strength is also directly related to the thermal expansion of the c-axis.With the increase of 3d electrons in the MZr2 system,the strength of the Zr-M and M-M bonds both increases.The strong chemical bonding weakens the influence of phonons on the lattice.It makes it difficult to produce negative γc,which leads to the suppression of the c-axis NTE,eventually producing positive thermal expansion.The composite phase is also capable of modulating the uniaxial CTE of FeZr2.Adding a small amount of Fe to pure Zr can regulate the thermal expansion of FeZr binary alloys.Interestingly,the composition Fe0.1Zr0.9 achieves uniaxial ZTE over an ultrawide temperature range(αl=0.45 × 10-6 K-1,105K-900K)and the ZTE composition with both high compressive strength(δUS=1.40 ± 0.13 GPa)and a large ultimate strain of 22.4%.This is the first report of a nonmagnetic ZTE alloy with high mechanical properties in such a wide ZTE temperature range.XRD,neutron diffraction,SEM,EPMA,and TEM reveal that the Fe-Zr alloy is a biphasic alloy with uniform distribution of FeZr2 nanoprecipitation phase and submicron Zr phase in the specific phase.The unique microstructure distribution not only has strong interfacial bonding forces but also can significantly relieve lattice strain.In combination with in situ neutron compression tests,the high mechanical properties are revealed due to the strong synergistic effect between the interfacial bonding force and the two-phase interaction.ZrCox(0.5 ≤x≤1)alloys achieve high-temperature high-strength performance and modulate its thermal expansion by varying the Co content.At composition x=-0.82,a wide temperature region uniaxial ZTE(110 K-760 K,α=-0.35 × 10-6 K-1)and high-temperature ultimate strength(δ573K,US=2.56 GPa).XRD and neutron diffraction revealed that the Co-Zr alloy is a dual-phase alloy composed of soft phase ZrCo and brittle strong uniaxial NTE of CoZr2.The combination of SEM,EPMA,EBSD,and TEM reveals that the ZrCo and CoZr2 phases are strongly combined,and the synergistic effect of the two phases is found to have good compressive strength and plasticity in combination with in situ neutron stress-strain tests.The ZrCo phase has high compressive strength at high temperatures,which determines its high mechanical properties with increasing temperature.更多还原