Interaction Between Deformation And Phase Transformation And Texture Control During Hot Processing In Titanium Alloys

Phase transformation is always accompanied with deformation during hot process of titanium alloys.Deformation can not only change the microstructure or refine the grain sizes of materials,but also can promote or restrain the occurance of phase transformation.Further more,deformation can also cause variant selection during subsequent phase transformation which will affect the characteristic of textures of the materials.There are three main types of titanium alloys including near-a type,a+p type and near-? type.The relationship of a and P phase during thermal processing would influence the ultimate microstructure and property of titanium alloys.The influence of deformation on microstructure,phase transformation and variant selection are investigated in this work.Firstly,the inhomogeneity of microstructure and texture in forged TC18,Ti-55531 and TA15 alloy bars are investigated.The formation of the coarse {100}-oriented grains in the center of the TCI 8 forged bar is closely related to strain status and cooling rate during forging.And it is difficult to be eliminated after subsequent deformation and heat treatment.In addition,the inhomogeneity of microstructure and properties in near a alloy is also originated from the inhomogeneity of ? phase.The inhomogeneity of microstructure and micro-texture are inherited after phase transformation.Thermal simulation of Gleeble compression is conducted to investigate the growth of {100}-oriented grains of ? phase during hot compression in TC18 alloy.It is found that recovery is more easily happened in {100}-oriented grains than in other grains.As a result,the {100}-oriented grains would merge {111}-oriented grains and other grains due to the dynamic strain-induced boundary migration mechanism.Therefore,the increasing deformation rate,various strain status and increasing deformation amount during hot process are proposed to reduce the formation of coarse {100}-oriented grains.Secondly,the variant selection of TA2,TA15 and TC18 alloys caused by specific stress during deformation and annealing is investigated.The variant selections of TA2 alloy and TC18 alloy during hot compression are investigated.It is found that extrenal stress would induce preferential orientations of variants.The a grains which have a {11-20} planes paralleling to compressing direction will develop preferentially.The variant selection in TA15 forged bar is investigated to illustrate the formation of the so called'Macrozones,as an effect of partial orientation preference.And the coarse "Macrozones"in the center of the TA15 forged bar is the main reason that cause the inhomogeneous properties.The cold rolling of TA2 alloy and following annealing are conducted to control the formation of different textures.It is found that the nucleation from the suface region of the TA2 sheets during annealing could promote the occurance of columnar grains and strong {11-20}//RD can be obtained after annealing.Various kinds of textures can be controlled by rolling and annealing in TA2 sheets.In addition,it is found that the special grain boundaries will cause the orientation preference of nucleation during phase transformation in a hot compressed TC18 alloy.The nucleated a phase on grain boundary would keep Burgers orientation relationship with both grains to reach the minimum strain energy during phase transformation.Finally,the interaction between deformation and phase transformation has been investigated in a hot compressed TC18 alloy.The compression below the ?(?)?+?*a+p transus temperature can promote the ??? transformation.Compared with the thermodynamic equilibrium,the free energy of both phases has been changed during hot compression which makes a thermodynamic possibility for the transformation.The re-distribution of the solute elements in both phases breaks the thermodynamic equilibrium.And variation of free energy provides the driving force for the promotion of phase transformation.Orientation dependence is found between {100} and {111}grains,the promotion is more easily in {111|-oriented grains than in {100}-oriented grains.Different compressing parameters have been studied and it is found that the long-range diffusion is a main reason that influence the occurance of transformation.In conclusion,the mechanism of {100} texture in P phase and the transformation texture in a phase have been investigated.And the methods for textures control have been established to improve the properties of various titanium alloys.