Microstructure Evolution Of TiB/TC25G Alloy During Thermal Processing

Beside the feature of reinforcement(TiB)containing the size,volume fraction and distribution,the microstructure and crystallographic orientation of matrix also are key factors in determining the final mechanical properties for TiB reinforced titanium matrix alloys(TiB/Ti).However,much of the present studies focused on the regulation of TiB reinforcements,while the research on the microstructure and crystallographic orientation of matrix is insufficient.In this work,TiB/TC25G alloys with different volume fraction of TiB reinforcements are produced via vacuum arc melting(VAR)and in situ reaction hot pressing(RHP),respectively.The aim is to investigate the microstructure and texture evolution during thermal deformation as well as the effects of TiB on the microstructure and crystal orientation of the matrix.In addition,the strengthening and fracture mechanisms of TiB/TC25G alloys are explored in brief.Based on the orientation relationship between α phase and β phase or equiaxed a phase and TiB,the effect of TiB on the a variant selection is analyzed firstly.The role of TiB reinforcements in the nucleation,crystallographic orientation,and eventually the morphology of a phase during heat treatment has been studied.The results show that the slow cooling process from β phase filed leads to the formation of equiaxed a grains which do not follow the Burgers orientation relationship with their parent β grains.Thus,the TiB/TC25G alloys with equiaxed structure could be obtained by annealing processes with slow cooling rate independently.The effects of TiB on the microstructure and texture evolution during thermal deformation in β phase field are studied as well.TiB is aligned along the plastic flow direction after deformation,forming a local high strain region around,which promotes the dynamic recrystallization of β phase.The main texture components ofβ phase are sharp<100>β∥CD fiber and weak<111>β∥CD fiber after severe deformation.In this case the transformed α texture is controlled by the deformation textures of β phase and variant selection,in which a variants precipitate with<1011>and<1120>parallel to CD form on either side of<100>/<100>and<100>/<111>prior β grain boundaries,respectively.The microstructure evolution of β phase during multi-directional forging process in β phase field is investigated.The texture analysis reveals that a sharp fiber texture with<100>β∥CD is inevitably formed under large uniaxial deformation,while the refinement of β grains and the weakening of β texture can be achieved by multi-directional compression.In this process the spatial distribution of TiB transformed into a disorder state with a single crystallographic orientation(<100>∥CD,<010>∥RD1,<001>∥RD2).The effect of TiB on the crystallographic orientation of primary a phase during the deformation in α+β phase field has also been studied.Continuous dynamic recrystallization occurs on the α phases adjacent to TiB owing to the large misorientation accumulation.The characteristic texture of the a phase in deformed TiB/TC25G alloy is {0001} perpendicular to the compression direction,with maximum intensities of 6.14 times random after 60%deformation.The a globularisation process is also proceeded by epitaxial recrystallization in which new equiaxed a grains formed maintaining the specific orientation relationship with neighboring TiB.Modification of microstructure and TiB distribution during thermal processing are two key methods in order to achieve satisfying mechanical properties of TiB/TC25G alloys.The laminated structure of TiB results in certain anisotropy of mechanical properties of deformed TiB/TC25G alloys.The results show a remarkable correlation between tensile properties and TiB distribution,in which the reinforcements work well in the case of TiB parallel to the stress direction.The formation of equiaxed non-BOR a grains during the β→α phase transformation with slow cooling rate significantly improves the ductility of TiB/TC25G alloys.Therefore,adjusting of the morphology and crystallographic orientation of a phase during thermal processes can improve the comprehensive properties of TiB/TC25G alloy.