α Phase Precipitation Mechanism During Thermo-mechanical Processing Of Ti-5Al-5Mo-5V-3Cr Alloy

Metastableβtitanium alloys are important structural materials for aeronautical applications due to their high strength to density ratio,good ductility and workability and excellent hardenability.Despite the efforts in resolving the complex microstructural evolution related to thermo-mechanical processes and in gaining knowledge on the produced phases and their contribution to the resultant mechanical properties,there are still some controversial and unresolved issues.The aim of the present PhD work is to determine precisely the metastable nature ofβphase and to characterize finely the characteristics of theβ→αtransformation during high and low temperature thermo-mechanical treatments.Investigations were performed on a Ti-5553 alloy with the singleβphase initial microstructure obtained by solution treatment followed by quenching using scanning and transmission electron microscopy(SEM/TEM)coupled to crystallographic orientation measurements and chemical analyses.The following main conclusions can be drawn:It was demonstrated experimentally that the structure of theβphase in the metastable titanium alloy is not“pure”body centered cubic.Diffraction diagrams presents streaking of theβdiffraction spots and additional spots at the 1/2,the 1/3 and2/3 diffraction positions.Also,striations are observed in TEM images.From this experimental evidence and crystallographic calculations,it was proved that atomic displacements on the{110}_βand{112}_βplanes formed a structure between that of the parentβphase and that of theαorωphase,demonstrating pre-phase transformation tendency.The striations could be both observed in theβphase in heat treatment and hot deformation microstructures.Its forming is not only insensitive to the solution temperature or cooling rate,but also irrelevant to thedeformation applied or not.The study of microstructure evolution during thermo-mechanical processing at higher temperature in theα+βregion revealed that the dominant softening mechanism during is dynamic recovery(DRV)of theβphase,with a certain degree of the dynamic recrystallization(DRX).Discontinuous equiaxed or short rod shapedαprecipitates(1~2μm)mainly form on the high angle and low angleβgrain boundaries but seldom inβgrain interiors,forming the“necklace”microstructure.Due to the continuous deformation and recrystallization that inducesβgrain migration and rotation.The coherency of theα/βinterfaces is reduced due to the different and uncoordinated lattice rotation atβ/βandβ/αboundaries.The Burgers orientation relationship(Burgers OR)between theαandβphases is destroyed gradually.The Burgers OR deviation of grain boundaryαis larger than that of intragranularα.And the deviation from the Burgers OR increases both with the increasing strain and decreasing strain rate.During the deformation at the lower temperature in theα+βregion,theαprecipitates exhibit different morphologies:such as lamellarα,equiaxedαand irregularα,forming the nano/ultrafine-trimorphic structure.Within the slip bands,equiaxedα/βgrains,in range of 50~200nm,with non-Burgers OR are present.However,between the bands,lamellarαandβphases maintaining the Burgers OR are distributed alternately,~20 nm in width.In that last case a strong variant selection is observed as only the two or three variants that form are those which can accommodate the macroscopic deformation.Comparatively,in absence of compression all 12variants are formed.This variant selection mechanism are dominated by the force field,i.e.,strain induce variant selection.Theβ→αphase transformation is retarded during the hot compression at higher temperature region,which is attributed to the competition between softening(DRV/DRX)and phase transformation.On the contrary,it is promoted during compression at lower temperature region due to the more inducted deformation defects acting asαphase nucleation sites and due to accelerating growth ofαprecipitates and retarded softening.Actually,it is a competitive mechanism between DRV/DRX andαphase nucleation,which depended on the deformation parameters and modes.This competition mechanism is not only applicable for the deformation temperature but also for strain and strain rate,which is not only found in the present work but also is often observed in other hot deformed Ti alloys.Dislocation slip is the leading deformation mechanism for the Ti-5553 alloy.Under the lower temperature deformation condition,single or multiple-slip bands with two or three different activated slip systems would form during the hot deformation process.Identification of these slip systems have been done by trace analysis.These results provide new insights into the structural nature ofβmetastable phase and valuable reference forβ→αphase transformation during thermo-mechanical treatment in metastableβtitanium alloys.