| The strength of tri-modal titanium alloy is higher than that of equiaxed titanium alloy at room temperature,its plasticity and thermal stability are close to that of equiaxed titanium alloy,and its high-temperature performance is higher than that of bimodal titanium alloy.It has excellent comprehensive properties and can meet the high performance requirements of aerospace field.In this paper,TC4 titanium alloy as the research object,the hot compression deformation experiment was conducted to analyze the influence of different hot compression parameters on the microstructure.Combined with the subsequent solution aging heat treatment to adjust the microstructure of the alloy,the tri-modal microstructure of TC4 titanium alloy was finally obtained.The main research contents and conclusions are as follows:When the strain rate and deformation are the same,the content of equiaxedαphase decreases and the content of secondary lamellarαphase increases with the increase of deformation temperature.The bending,folding and twisting of lamellarαphase occur atα+βdeformation,and the main spheroidization mode is grain boundary grooving.At nearβphase field,the equiaxedαphase is mainly formed by discontinuous dynamic recrystallization（DDRX）.The strain rate mainly affects the content and diameter of equiaxedαphase,that is,under low strain rate condition,the content and diameter of equiaxedαphase increase.The deformation inhomogeneity is analyzed,from small deformation zone to large one,the microstructure changes from lamellarαphase andβt to widmanian microstructure.Moreover,in the large deformation region,the acicularαphase andβphase keep a certain orientation relationship and intensives around 60°,and the grain boundaryα（GBα）keeps BOR relation with the adjacentαphase,which is beneficial to reduce the nucleation of GBαphase.It is found that pyramidal slip and prismatic slip are easier to start than basal slip.The dislocation density of the sample was analyzed at 940℃/0.001s-1/60%.It was found that the dislocation density in the large deformation region was serious,and the dislocation density on the base plane was twice that pyramidal slip for the same region.By analyzing the orientation relationship of variantα,it was found that there were threeαvariants at 940℃/0.001s-1/60%.Through texture analysis,it is found that the preferred grain orientation of 45°direction with CD and 45°direction with RD is enhanced in the large deformation zone at 940℃;The preferred grain orientation exists in the line direction between<0001>and<10-10>,and the extreme density of texture is larger.This is due to the preferred orientation of the grains caused by the phase transformation.After solid solution treatment at 950℃/1h/WQ,it is found that the diameters of equiaxialαphase are 7.57μm and 6.89μm respectively at 850℃and 920℃after solution,and 5.46μm and 6.62μm respectively at 940℃/0.1s-1 and 940℃/0.001s-1.The volume fraction and diameter of equiaxedαphase change greatly before and after solution treatment.Taking 940℃/0.1s-1/60%and 940℃/0.001s-1/60%as examples,for example,before heat treatment,the volume fraction and diameter are 0.8%,9.1%,2.89μm and 6.00μm,respectively.After heat treatment,the volume fraction and diameter were 6.1%,18.3%,5.46μm and 6.62μm,respectively.It was noted that the tri-modal microstructure appeared after heat treatment at 940℃/0.001s-1/60%and 950℃/1h/WQ.Moreover,heat treatment does not change the phenomenon of orientation intensity during thermal deformation.After solid solution at 1050℃/1h/WQ,mainly consisting of Widmannstatten structure,which consists of partial initial lamellar cluster surrounded by largeβgrains and fine martensite.With the increase of thermal deformation temperature,the initial lamellar cluster inβgrains decreases and acicular martensite increases.Theβgrains grow rapidly with the average diameter of 200μm,while the diameter of the thermal deformationβgrains is 16μm.After solution aging treatment at 950℃/1h/WQ,500℃/6h/AC and950℃/1h/WQ,530℃/6h/AC,the content of equiaxedαphase decreases and the content of lamellarαphase increases with the increase of thermal deformation temperature at the same aging temperature.When the aging temperature increases,the equiaxedαphase increase at the same deformation temperature,that is,increasing the aging temperature is beneficial to the spheroidization ofαphase.In addition,the grain boundaryαformed during the deformation process is partially spheroidized after heat treatment.After heat treatment at 1050℃/1h/WQ and 530℃/6h/AC,the microstructure is Widmannstatten structure,but theαphase at grain boundary partially disappears,and the lamellar clusters inβgrains at solution state disappear,and are replaced by interlacing or parallel fine lamellarαphase. |
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