Regulation And Mechanism Of Thermal Expansion Of Binary Metal-based Compounds

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.