| Ti-6Al-4V alloy has the advantages of high specific strength,good high temperature performance,low density,good corrosion resistance,and its constantly used in aerospace military equipment.The microstructure of titanium alloy has diversified characteristics,and the room temperature strength of the tri-modal structure is higher than that of the equiaxed structure,the plasticity and thermal stability are at the same level as the equiaxed structure,and the fracture toughness and fatigue performance are higher than bimodal structure,which can meet the urgent demand for higher performance of high-end products in aerospace,defense and military industry.In this paper,the hot isostatic pressed Ti-6Al-4V alloy is used as the research object.The the multi-pass deformation at decreasing temperature(950?-900?-850?)and decreasing and increasing temperature(950?-850?-900?)under different strain rate and strain amount matching are carried out.Combined with the hot deformation and heat treatment system to analyze the microstructure evolution process,and a tri-modal structure with high-performance is obtained.The main research contents and results are as follows:During multi-pass deformation at decreasing temperature,the flow stress is very sensitive to the deformation parameters,which increases with increased temperature and decreased strain rate,and shows different softening degrees in different deformation passes.The main softening mechanisms of one-pass,two-pass and three-pass deformation are DRX of ? phase,fragmentation and spheroidization of lamellar ? phase,DRX and spheroidization of ? phase.During the one-pass deformation(950?),DRX of ? phase occurs at the intersection of grain boundaries,and the ? phase in the grains was only deflected and kinked,and the increased strain broke the grain boundary ? phase.The needle-like phase was precipitated during two-pass deformation(900?),the locally oriented lamellar ? phase spheroidized,and the fraction of DRX was only 2% which mainly occurred DDRX.During three-pass deformation(850?),the spheroidization of ? phase increases with increased deformation and decreased strain rate,and the fraction of DRX increases from 3.2% under the deformation amount of 10% to 20.1% under the deformation amount of 40%,which mainly occurred CDRX and DDRX.During multi-pass deformation at decreasing temperature,the lamellar ? phase deflected with the increase of the deformation pass,and gradually evolved from the initial bimodal distribution structure to a single peak structure at 95°to the compression axis.The length of lamellar ? phase gradually decreases,but its thickness is gradually increasing,and spheroidization occurred with kink and fragmentation of lamellar ?.And,a tri-modal microstructure which average grain size is 5?m was formed under the deformation amount of 20%-40%-10% in each pass,in which the contents of equiaxed ?phase,secondary ? phase and ? transformation are 22%,50% and 28%,respectively.And the tri-modal structure with an average grain size of 4.5?m is formed under the deformation parameters of 20%-20%-30%,corresponding to 13%,45%,42%.In the process of decreasing and increasing temperature deformation,the decreased temperature(850°C)promoted the transformation of ? phase to ? phase during two-pass deformation,resulting in the precipitation of more needle-like ? phase in the matrix.And the dynamic recrystallization increases from 4.9% under the deformation of 20% to 9.6% under the deformation of 30%,which mainly occurred CDRX.At this time,the lamellar ? phase mainly undergoes deflection and twisting.The three-pass deformation temperature rises to900°C,and the large number of lamellar ? phase occurred spheroidization.Through Schmidt factor analysis,it is found that the difficulty of starting slip surface of the lamellar ? phase is different,and the conical and cylindrical slip are easier to start than the base slip.The DRX statistics show that when three-pass deformation are 10%,20% and 30%,the fraction of DRX are 18.8%,5.6% and 7.8% respectively.The recrystallization does not increase significantly,which is mainly due to the large consumption of dislocations caused by spheroidization.And,when deformation amount of each pass is 20%-40%-10%,a tri-modal microstructure with the average grain size of 6.8?m is formed,in which the contents of equiaxed ? phase,secondary? phase and ? transformation are 10%,68% and 22%,respectively.Under the parameters of20%-10%-40%,when the strain rate is 1s-1,the corresponding tri-modal microstructure contents are 8%,70%,and 22%,respectively.When strain rate is reduced to 0.01s-1,the content of the tri-modal microstructure changes to 14%,66%,and 20%.It can be seen that the influence of deformation and strain rate to microstructure are obvious,and reasonable matching of deformation parameters is very important for the formation of the tri-modal structure.The relationship between microstructure and properties shows that the primary deformation microstructure is mainly composed coarse ? grain,and the comprehensive hardness is 3.21 GPa.Its contribution to the hardness is mainly the interface between ? and ?phase,that is the interface strengthening effect;The comprehensive hardness of the two-pass is about 3.26 GPa,and its main contribution is the interface between ? and ? phase and the structure of ? transformation.The comprehensive hardness of the three-pass increased to 3.37 GPA,and the main contribution of the comprehensive hardness is transformed into the result of the joint action of lamellar ? phase,? transformation and equiaxed microstructure.At the same time,after aging at 550 ? + 6h,the microstructure in different deformation and solution treatment states becomes more uniform,more acicular phase is precipitated at higher solution temperature,and the content of equiaxed ? phase is increased at lower deformation temperature in the two-pass,forming a uniform tri-modal structure.The tri-modal structure of equiaxed ? phase+lamellar ? phase+? transformation was obtained in the two-pass deformation at decreasing and increasing temperature deformation after solution 950°C for 60 min and aging at 550°C for 6h.. |
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