Microstructural Characterization And Phase Transformation Behavior Of A Hot-rolled Ti-7333 Alloy

Ti-7333 alloy is one of the promising high-strength titanium alloys for aerospace application owing to its high special strength,high elasticity modulus and considerable ductility and workability.In order to explore its application for high-strength fastener,the studies about its hot-rolled microstructure and the influence of the thermal treatments on the microstructure and the phase transformation are needed.In this work,the hot-rolled Ti-7333 would be investigated.The SEM,EBSD,TEM and Crystal calculation would be carried out to characterize the microstructural characteristics of the rolled Ti-7333,to study the influence of the solution treatment on the microstructure evolution and to investigate the phase transformation under the different thermal conditions.The following conclusions can be drawn:After the rolling deformation,theβmatrix is elongated and presents obvious preferred orientations,i.e.texture.The main texture consists of theα_bcc-fiber components（{112}<110>and{111}<110>）and the cubic component of{100}<001>.It is found that the dislocations distributed within the rolled matrix belong to the{110}<111>edge dislocation.Within the deformedβgrain only twoαvariants are detected and the orientation relationship between them is＜1120＞_α/60°.Meanwhile,the recrystallization occurs during the rolling deformation.Most of the recrystallized grains nucleate at the bulged boundaries of the deformed grains,and they hold the similar orientations with respect to the deformed matrix,which eventually results in that the texture of the recrystallized grains is similar with the texture of the deformed grains.Thus it is concluded that the strain-induced boundary migration is the dominated mechanism accounting for the recrystallization.The effect of the solution treatment on the microstructure evolution was investigated.It is found that during the beginning stage of the solution treatment in the（α+β）field the recovery is dominant.With the solution time extending,the recrystallization occurs and it is found that the recrystallized grains hold their<110>parallel to RD invariant.The main mechanism controlling the recrystallization is the strain-induced boundary migration.After the solution treatment in the（α+β）field for30 min,the volume fraction of the recrystallization is about 72%.While solution treatment in theβfield for 3 min,the recrystallization is dominant and the fraction is about 72%and the texture of the matrix tends to present random orientation.The recrystallization kinetics were investigated.The results indicate that the nucleation of the recrystallized grains is the site-saturated nucleation and the apparent activation energy for the recrystallization is about 255.74 k J/mol.The driving force for boundary migration decreases with time,eventually resulting in the decreasing recrystallization.The precipitation behavior ofαphase in theβ-quenched Ti-7333 during the aging treatment was investigated.Firstly,the microstructure of theβmatrix after theβsolution treatment was characterized.It is found that there are numerous dislocations distributed within the matrix and they belong to the{110}<111>edge dislocation.During the aging treatment,the grain boundaryαphase(α_GB)firstly precipitate,then the intragranularαphase and the widmanst（?）ttenαphase(α_WGB).It is found the strain in the direction of<111>_βand on the{112}_βplane is dominant during theβ→αtransformation,which intrinsically results in the formation of V-shape or triangle-shape group ofαvariants.The variant selection ofα_GB was investigated.It is found that every variant corresponds to one of{112}<111>system and the one whose corresponding{112}_βplane and<111>_βdirection tend to parallel to the grain boundary plane is preferentially selected in order to eliminate the fraction of grain boundary area and the phase transformation resistance.The selection ofα_WGB is associated with theα_GB phase and the dislocation nearby the grain boundary.It is found that when single dislocation of{110}<111>distributes nearby the grain boundary the orientation ofα_WGB is different from theα_GB,the formation of which is controlled by the mechanism of the interface instability.While there are dislocation variant of{110}<111>distributed nearby the grain boundary,the orientation ofα_WGBpresent the same orientation with theα_GB,the formation of which is dominated by the sympathetic nucleation mechanism.The precipitation ofαphase and its morphological evolution during continuous heating was investigated inβ-quenched Ti-7333.It is found that theωvariants with an ellipsoidal shape precipitate within theβmatrix at the low temperature stage of continuous heating process.The lattice misfit at theω/βinterface is determined to be2.824%.When heating to 450 ℃,theαphase precipitates within the matrix.The large transformation strain of theω→αtransformation prevents the direct formation of theαphase from theωstructure.However,theβlattice deformation induced by theωstructure can be well accommodated by the formation of theαphase,i.e.the lattice elongation in the<112>_βdirection induced by theβ→ωtransformation could be accommodated by the lattice contraction of theβ→αtransformation in this direction and in the directions of<110>_βand<111>_β,the lattice elongations induced by theβ→ωtransformation could annihilate the lattice elongation from theβ→αtransformation.Thus the vicinities of theω/βinterfaces are the favorable sites for the nucleation of theαphase.According to this accommodation,the stain induced by theβ→ωtransformation has an selection effect on theαphase.It is found that theαvariant holding the[0001]/10.53^o relationship with respect to the forming one is detected.By virtue of theω-assisted nucleation,the ultrafineαprecipitates could be obtained.The lattice misfit ofα/βinterface is determined to be 5.34%.Eventually,The morphological development sequence ofαprecipitates during continuous heating is concluded:platelet-likeαprecipitates with the assistance ofωphase,lenticular-likeαprecipitates,rod-like morphology;eventually ellipsoidal morphology.