Research Of Deformation Behaviour And Grain Refinement In Ti-55511Alloy

Near-β titanium alloys are the key material for long-life aerospace bearing structure manufacture. In this paper, the mechanical behavior during thermal deformation, the thermal processing map based on dynamic material model and the microstructure evolution of Ti-55511titanium alloy with4different types of initial microstructure (pure (3, equiaxial a, coarse a platelet and thin a platelet) were studied. A kind of Ti-55511ultra-fine grain alloy preparation method was developed by controlling the microstructure before thermal deformation. The microstructure and properties of the ultra-fine grain were studied. The main research results were as follows:(1) The differences of the peak stress, the steady stress, the flow softening degree and the average deformation activation energy in the true stress-strain curves between titanium alloys with4types of initial microstructure were analyzed. As for the peak stress, the coarse a platelet microstructure alloy was the highest, and the equiaxial a microstructure alloy was the lowest, while the pure β and the thin a platelet microstructure alloy were between them, and the peak stress of the pure β microstructure alloy was higher than the thin a platelet microstructure alloy. As for the steady stress, the law was the same as the peak stress when under high strain rate. When under low strain rate, however, the steady stress of the thin a platelet microstructure alloy was lower than the equiaxial a microstructure alloy. As for the flow softening degree,4alloys all decreased with the rise of the deformation temperature and the decrease of the strain rate. An obvious inflection point appeared during the decrease of the flow softening curves of all the alloys but the deformation conditions of the alloys corresponding to the inflection point were different.(2) The average deformation activation energies of4alloys with different initial microstructure were studied. The deformation activation energy corresponded to the peak stress of the alloys were between340-540kJ/mol, while the deformation activation energy of the pure (3microstructure alloy was the highest and the equiaxial a microstructure alloy was the lowest. With the increase of the strain, the deformation activation energy of all alloys gradually decreased. When the strain was0.7, the deformation activation energies of the pure (3, the equiaxial a and the coarse a platelet microstructure alloys were close (about300kJ/mol), and the deformation activation energy of the thin a platelet microstructure alloy decreased from380kJ/mol to220kJ/mol. In addition, besides the equiaxial a microstructure alloy, the deformation activation energies of other alloys emerged a mutation phenomenon. The mutation of the pure β, the equiaxial a and the coarse a platelet microstructure alloys appeared around the strain of0.2-0.3,0.3-0.4and0.4-0.5, respectively. The mutation strain of the deformation activation energy mentioned above was the critical strain for dynamic recrystallization.(3) The thermal processing maps of4types of Ti-55511titanium alloys with different initial microstructure were established. The best range of the thermal deformation parameters for different alloys was determined through the distribution of power dissipation efficiency factor η in the thermal processing map.(4) The microstructure evolution of the alloys with4types of initial microstructure during thermal deformation was investigated. With the increase of the deformation temperature and the decrease of the strain rate, the recrystallization of a phase increased, the degree of refinement of the α/β grains also increased and the microstructure homogeneity significantly improved. The volume fraction of a phase dropped as the deformation temperature rose, while the strain rate had a small effect on the volume fraction of a phase when deformed under the same temperature. During the thermal deformation process, dislocation increased in the platelet through plastic deformation of the a phase or dislocation cutting of β phase, which led the mechanical instability of the original stable a platelet. When dislocation increased to certain degree, a phase began to recrystallize and produced plenty of small grains. Under the traction of metal flow, the mechanical unstable a platelet or small grains partially separated from a platelet, which led the macroscopic plastic buckling.(5) Based on initial microstructure design, a ultra-fine grain microstructure with the size of α and β ranged between0.1-0.5μm was prepared through hot rolling. The tensile strength and elongation of the ultra-fine grain alloy after rolling were1304.7MPa and5.24%, respectively. The alloy possessed good thermostability after heat treatment. After heat treated at450℃for4h, the tensile strength reached1486.09MPa and the elongation reached8.54%. In addition, a kind of enhancing platelet phase a2based on Ti3Al was found in near-P titanium alloy, and the precipitation temperature interval was450℃-550℃.(6) The main strengthening mechanism of the ultra-fine grain Ti-55511alloy was fine grain strengthening and second phase dispersed strengthening. Recovery not only eliminated the work hardening, also promoted the stability of grain boundary and phase boundary, which improved the effect of fine grain strengthening. Phase transition of αâ†'α2and βâ†'ωâ†'α during annealing led the precipitation of a2and a from α and β, respectively. Thus the effect of second phase dispersed strengthening enhanced. When the second phase grain increased to a certain size, however, the elongation of the alloy significantly dropped.