Spinodal Decomposition And Its Effect On Mechanical Properties Of Ti2448 Alloy

Ti2448(Ti-24Nb-4Zr-8Sn,wt.%)alloy is a metastable β-type titanium alloy with not only excellent mechanical properties,such as high strength,low modulus and high toughness,but also functional properties,such as Invar,Elinvar and superelasticity in a wide temperature range,making it a novel structural and functional integrated metallic material.Previous studies showed that the alloy consist of a nano-scale sponge-like microstructure created by the spinodal decomposition.This leads to a continuous tuning on crystal structure from bcc,via orth,to hcp,as realized by the linearly variation of atomic shear and atomic shuffle component.In order to characterize the quantitative relationship between composition decomposition,crystal structure evolution,microstructure and mechanical properties,the thesis intends to investigate the effects of heat treatments on the composition decomposition,microstructure and crystal structure evolution,and mechanical properties of Ti2448 alloys by means of small angle neutron scattering,X-ray diffraction,scanning electron microscopy,transmission electron microscopy,tensile and cyclic loading tensile experiments.The aim of this study is to establish kinetic evolution equations and to construct a new method for orderly degree regulation of microstructure and mechanical properties was developed.The two-dimensional small-angle neutron scattering spectrum not only confirms that the hot-rolled Ti2448 alloy has a nanoscale sponge-like microstructure induced by spinodal decomposition,but also reveals a microstructural transition from the spongelike to the plate-like,which is different from the previously reported compositional decomposition alloy with a single type.In order to reveal the relationship between the microstructure and crystal structure transformation,the linear relationship between atomic shear and atomic shuffle component were used to define an ordering parameter of the BCC-HCP phase transition,the effect of heat treatment temperature and time on microstructure and ordering was measured,and kinetic equations for the evolution of microstructure and ordering were conducted.The results show that the two evolutionary processes are controlled by the uphill diffusion of Nb elements with an activation energy of 209 kJ/mol;the time index of microstructure evolution is 1/5,which is smaller than that of the conventional nucleation growth mechanism and plate microstructure(1/3);the time index of ordering is 1/16,which is much smaller than that of microstructure evolution.Based on the above kinetic equations,a quantitative relationship between the degree of ordering and microstructure was constructed,and a critical degree of ordering(approximately 0.5)for the regulated microstructure was found,which provides important fundamental data for cognition of the decomposition mechanism of the continuously ordering of the crystal structure.The effect of the nanoscale periodically distributed microstructure and the ordering of the crystal structure on the mechanical properties was investigated.Ultra-high density uniformly-distributed precipitation phases were obtained,and kinetic equations for the evolution of strength and modulus with heat treatment temperature and time were established,which increased the strength of the alloy by approximately 50%under high ductility conditions and significantly improved the strength-ductility trade-off of Ti2448 alloy.The strength to modulus ratio of the aged alloy is greater than 1.6%,close to that of amorphous alloys and much higher than that of other titanium alloys.As the aging time increases,the nucleation and growth mechanism of the precipitated phase is activated and the strength of the alloy can be further increased,but the room temperature ductility decreases gradually,maintaining about 8%room temperature with a high strength of 1400 MPa.Therefore,the continuous ordering of the crystal structure and the spinodal decomposition mechanism can significantly improve the contradictory relationship between strength and ductility of Ti2448 alloy,providing a feasible stragety for the development of high strength metallic materials.The effect of heat treatment temperature and time on the superelasticity of Ti2448 alloy was investigated using cyclic loading,and an evolutionary kinetic equation was established with an activation energy of approximately 83 kJ/mol,which is much smaller than that of microstructural evolution and crystal structure ordering,but is close to the activation energy of mechanical properties.A quantitative relationship between the ordering and superelasticity is constructed,and it is found that the superelasticity of the alloy remains stable when the ordering parameter of the crystal structure is below 0.5,which provides guidance for the heat treatments to regulate the reversible deformation strain of the alloy.