Preparation,Strengthening And Toughening Of Rare-Earth-based Zero Thermal Expansion Alloys

Zero thermal expansion(ZTE)materials exhibiting dimensional stability under the condition of temperature fluctuation,which are utilized in supporting parts of optical systems,ultra-stable components for space gravitational wave detection,and liquefied natural gas carriers(LNG)internal shell materials,etc.Among them,rare-earth-based zero thermal expansion intermetallic compounds have potential application due to their excellent thermal conductivity,electrical conductivity,and structural stability.However,intermetallic compounds are inherently brittle,making it challenging to meet the requirements under complex service conditions.In this thesis,several typical rare-earth-based negative thermal expansion(NTE)intermetallic compounds are selected as the primary candidates.Then realizing ZTE and excellent mechanical properties by the strategy of in-situ composite construction of heterogeneous dual-phase alloys.Based on the methods of in situ synchrotron radiation X-ray diffraction(SXRD),in situ neutron pattern diffraction(In-situ NPD),and microstructure characterization,the evolution mechanism of the dual phases in an alloy under temperature and stress fields was revealed.Preparation of plastic one-dimensional ZTE dual-phase alloys via eutectic reaction.The Ho2Fe17(H phase)/α-Fe(α phase)heterogeneous dual-phase alloy was constructed simply and efficiently in the Ho-Fe binary system.And a onedimensional ZTE characteristic was achieved in the Ho0.04Fe0.96 composition.The introduction of the a phase can effectively improve the intrinsic brittleness of the H phase,enhancing its plasticity.In-situ NPD and microstructural analysis demonstrate that the two phases produce coordinated deformation during loading.The α phase can suppress the strain localization and crack propagation in the H phase by relying on the stable phase interface.The movement and production of dislocations in the a phase can further weaken the interface stress concentration and effectively achieve the strengthening and toughening of mechanical properties.In addition,high-resolution transmission electron microscopy analysis shows that the two-phase interface is connected by a disordered transition layer of about 1 nm,which makes it have excellent thermal shock resistance.Construction of "plum-pudding" structure to realize isotropic ZTE dual-phase Er2Fe14B(E phase)/α-Fe(α phase)alloy.In the Er-Fe-B ternary system,the microstructure is finely controlled by a boron-migration-mediated solid-state reaction(BMSR),and a dual-phase alloy with a "plum pudding" structure is constructed.This unique "plum pudding" structure strengthens the grain boundary brittleness of the matrix phase and inhibits the crack propagation within the grain,thereby significantly enhancing its mechanical properties.In-situ NPD and microstructural characterization clarified the key mechanism for the two optimized microstructures to promote the uniform deformation of the matrix phase(E phase),thereby improving the mechanical properties.In addition,the BMSR greatly weakens the crystallographic texture of the precursor matrix alloy and achieves the isotropy of the macroscopic thermal expansion performance.Further local structure and first-principles calculations show that the selective directional migration of boron atoms and the chemical short-range order(SRO)in the ErFe10 phase are the keys to the efficient occurrence of the BMSR.Preparation of ultra-plastic isotropic ZTE dual-phase alloys using the partition coefficient rule.In the La-Fe-Co-Si quaternary system,the distribution coefficient of elements in the La(Fe,Co,Si)13(L phase)/α-(Fe,Co,Si)(α phase)was subsequently determined.The eutectic reaction was extended from the binary system to the multi-component system.The thermal expansion was continuously tailored,and the isotropic zero thermal expansion performance was realized in LaFe54Co3.5Si3.35 composition.The comprehensive mechanical properties are superior to other ZTE dual-phase alloys reported so far,which have broad application prospects.In-situ NPD and microstructure analysis illustrate that the dual phases have orderly transfer of stress at different stages during the deformation process,and the coordinated deformation of their substructures is a necessary condition for their orderly transfer.Among them,the generation and movement of a large number of dislocations({110}<111>)in the a phase during the deformation process can reduce the strain localization at the interface.In-situ NPD and microstructural characterization revealed that heterostructures,phase boundaries,and multilevel buffering and hindrance of lattice symmetry are key to achieving ultra-large plastic deformation.