| Ti2448(Ti-24Nb-4Zr-8Sn wt.%)alloy is a low electron concentration metastableβ titanium alloy that exhibits nonlinear elastic deformation behavior,which offers a good match between high strength,low modulus,and high toughness,and possesses multifunctional properties such as wide temperature range superelasticity and controllable thermal expansion.Previous studies have discovered a unique compositional decomposition mechanism coupled with a crystal structure transition,by which the ordering parameter η was defined based on in-situ SXRD data and quantitatively characterized the continuous transition from body-centered cubic structure to close-packed hexagonal structure.The Burgers path of the crystal structure transition at the atomic scale has been revealed,which explains the microstuctural transition from sponge-like to plate-like.These results provide a thermodynamic method for controlling the ordering parameter and provide guidance for exploring the integrated structural and functional properties of Ti2448 alloy.Using the ordering parameter η,this study investigates the nonlinear elastic deformation behavior and controllable thermal expansion performance of Ti2448 alloy,explores the feasibility of achieving wide-temperature Elinvar effect and wide-temperature constant resistivity,and reveal the root of compatibility between high strength and high toughness,providing fundamental data for understanding the integrated structural and functional properties of the alloy.In-situ investigations of the non-linear deformation process of the alloy were conducted using high-energy synchrotron X-ray diffraction.The evolution of the crystal structure in the nanoscale region with low and high Nb content under stress was continuously measured,revealing the continuous elastic lattice distortion behavior of the body-centered cubic structure with high Nb content and the continuous lattice reconstruction behavior of the orthorhombic structure with low Nb content along the Burgers path towards the close-packed hexagonal direction.The change in the ordering parameter η of the two-phase lattice during the nonlinear deformation process was characterized by measuring the atomic shear and atomic shuffle components along the Burgers path.Based on the continuous change in the lattice ordering parameter ηcontrolled by stress,a crystal structure continuous transition mechanism that differs from the first-order martensitic phase transition has been proposed,which explains the superelasticity dominated by nonlinear deformation of the alloy and provides fundamental data for using nonlinear deformation behavior to control material structural and functional properties.Through continuous control of the thermal expansion coefficient of Ti2448 alloy by tensile deformation,the Invar effect was achieved in a wide temperature range around 400 K.Using high-energy synchrotron X-ray technology and Rietveld fullspectrum fitting method,the microscale thermal expansion behavior of two-phase structures during the heating and cooling process of the Invar state alloy was characterized in detail.It was found that the crystal structures in the nanoscale Nb-rich and Nb-poor regions underwent continuous reversible elastic distortion and phase transformation,respectively,and the change in crystal structure interplanar spacing was linearly related to temperature.Based on the temperature-linearly-correlated ordering parameter η,a mechanism of continuous lattice reconstruction controlled by temperature was proposed for low-order orthorhombic crystals,revealing the origin of the Invar effect in the alloy and the negative thermal expansion effect in low-symmetry phases in such materials.The significant anisotropy of thermal expansion behavior of body-centered cubic crystals was discovered for the first time,and it was well explained by the intrinsic Poisson’s ratio anisotropy of single crystals.The evolution of decomposition microstructure induced by aging in Ti2448 alloy was studied,and the ultra-high number density and evolution of the α"+β dual-phase structure induced by decomposition were quantitatively characterized.Based on the continuously adjustable decomposition microstructure,the temperature coefficient of modulus and the temperature coefficient of resistance were continuously adjusted across positive and negative values,and the Elinvar effect and isoelectric resistance effect were achieved in a wide temperature range by finely controlling the ordering parameter and volume fraction of the composition-modulated structure.The temperature coefficient evolution of thermal properties controlled by the stability of each phase was quantitatively characterized using the mixture law,and the correlation between aging system,microstructure,and temperature dependence of thermal properties was established based on the ordering parameter η,providing important data for achieving wide temperature-insensitive functionality in such materials.Using high-energy synchrotron and room-temperature variable-temperature X-ray diffraction techniques,the wide-temperature-range Elinvar effect and its intrinsic mechanism in Ti2448 alloy were investigated.The evolution of ordering parameter ηof the α" phase lattice induced by aging during the heating process was continuously measured,and the lattice ordering behavior towards the densely packed hexagonal crystal structure during the evolution of the high initial ordered degree orthorhombic crystal was discovered.A continuousl crytal ordering mechanism in highly ordered orthorhombic crystals was proposed,explaining the wide-temperature-range Elinvar effect in the alloy.The lattice ordering and disordering directions controlled by the initial ordering parameter η were found,explaining the incompatibility of the Invar and Elinvar effects in the alloy and providing important basic data for achieving Invar and Elinvar effects.The study investigated the underlying mechanisms behind the high strength,low modulus,and high toughness properties of Ti2448 alloy.It was discovered that the alloy exhibited a high shear modulus to Young’s modulus ratio(-0.56)and a high Poisson’s ratio(～0.42),breaking the limitations of isotropic elasticity on the elastic properties of polycrystalline materials.From an elasticity standpoint,this overcame the trade-off between strength and toughness and allowed the alloy to possess a shear strength to Young’s modulus ratio of up to～1/80,a tensile strength to Young’s modulus ratio of up to～1/50,an elongation of 20%,and an impact toughness of～250 J/cm2,achieving a compatible match of high strength,low modulus,and high toughness plasticity.By comparing the elastic data of TiNb-based alloys with Ti2448 alloy,the study identified the characteristics of a covalent bond-like electronic cloud distribution in the alloy and proposed an electron density transfer mechanism based on this,revealing the root cause of the alloy’s exceptional elastic compatibility. |
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