Quantitative TEM Investigation On Transformation Mechanisms And Behaviors Of Ti2448 Alloy

As a multi-functional metastable β titanium alloy,Ti-24Nb-4Zr-8Sn(wt.%,Ti2448)has excellent mechanical and physical properties such as low modulus,high strength and ductility,high recoverable strain,excellent biocompatibility and tunable thermal expansion,and thus has become an excellent candidate for advanced biomedical applications.The properties are usually related to the microstructure of materials,while the microstructure is controlled by the transitions of various equilibrium and non-equilibrium phases in the alloy.The mechanical properties depend on the interaction of deformation mechanism,microstructure and composition.However,key mechanisms of these unique properties have not been studied thoroughly,which greatly limit the optimization of the alloy for specific applications.In this paper,the deformation and transformation mechanisms of Ti2448 alloy were studied by means of aberration correction and in-situ transmission electron microscopies.Super-elasticity and shape memory of materials are typically associated with reversible phase transformations.The reversible phase transformations and their governing factors thus have long been research interests of materials scientists and physicists.Here,a novel reversible trigonal ω transformation has been observed in a metastable p-Ti alloy during tensile deformation with in situ aberration corrected transmission electron microscopy.We observed that the symmetry of the P parent is reduced during loading period,leading to a cubic to trigonal transition.Upon unloading the tensile force,the trigonal structure reverts to the original β parent completely.Moreover,continuous transition interfaces with no interfacial defects are formed between the ω and β phases,and they contribute essentially to the occurrence of the reverse transformation.This new reversible mechanism may also apply to other phase transformations induced by deformation and thus has great potential for developing super-elasticity and shape memory in materials.Diffusional-displacive transformations are generally associated with mechanical properties of materials such as high strength.Understanding structure evolution of these transformations is of great interests from both physics and application perspectives.By combining atomic resolution electron microscopy,energy dispersive spectroscopy and first principles calculations,a continuous β→α"→α transformation process has been quantitatively characterized at the atomic level in a metastable β-Ti alloy during aging treatment.The transformation is revealed to develop by a novel mechanism involving continuous structural and compositional changes towards the equilibrium assisted by compositional fluctuation in the β matrix.Moreover,the product phase induces a precipitate-matrix lattice mismatch,thus produces a coherency strain field surrounding the precipitates.The coherent strain field contributes significantly to the increasing hardness of the alloy after aging.These results have great potential for tailoring thermomechanical treatment routes and improving mechanical properties of materials.