Study On Hot Deformation Mechanism And Microstructure Evolution Of Low-cost Ti-4Al-2.5V-1.5Fe-xB Allo

Ti-4Al-2.5V-1.5Fe alloys,which have comparable properties to Ti-6Al-4V alloys in terms of strength and elongation,are more attractive because they can be both hot and cold worked,and can be processed into plates,strips,tubes,and other titanium mill products.Besides,the alloy has a lower cost due to V being replaced by Fe as theβ-stabilizing element.At present,high-performance and low-cost titanium alloys are one of the promising studies in the development and application of titanium alloys.However,due to the coarse grain size of the ingot,a series of thermo-mechanical processing such as forging-cogging are required to obtain products with excellent performance for practical applications,which not only the process is complicated,but also the yield is low,increasing the cost of the products.Several studies show that the final titanium product is directly related to the initial microstructure of the ingot,adding boron or boride can not only refine the grains of the ingot but also theoretically it is possible to improve the alloy properties and reduce the difficulty of subsequent thermo-mechanical processing to achieve a good match between low cost and high performance.However,the studies on the thermal deformation response of borides on titanium alloys during thermo-mechanical processing and the strengthening mechanism are limited.For this reason,the study prepared boron-modified titanium alloys using TiB₂ as the boron source,characterized the microstructure,phase structure,texture,and mechanical properties of bare and boron-modified alloys by using modern analysis and testing methods such as XRD,SEM,TEM,EBSD,and finite element simulation.The effect of TiB₂ addition on the grain refinement mechanism of ingots,thermal deformation behavior,and strengthening mechanism of boron-modified titanium alloy during thermo-mechanical processing was investigated in detail and the following findings were obtained:The Ti alloy with TiB₂ as the boron source has the same effect as the direct addition of B in the alloy,i.e.,significant refinement of the grain.The boron with a weight percent of 0.04 is the inflection point at which the grains of Ti-4Al-2.5V-1.5Fe alloy starts to refine significantly.Below this content,the effect of TiB₂ addition on the grain refinement of ingots is not obvious.The grain refinement of the ingots with TiB₂ as the boron source is attributed to the composition subcooling caused by the B atoms generated by the in-situ reaction to produce TiB decomposition and the limitation of grain growth by the resulting boron-rich layer.The formation of TiB whiskers with chain-like structures at the grain boundaries of ingots after melting effectively avoids abnormal growth of grains during high-temperature deformation through the Zener pinning effect.Compared with the coarse grains exhibited by the bare alloy after forging,the boron-modified titanium alloy preserved the fine grain structures due to the presence of TiB,which will improve the formability of the alloy for plastic deformation during further thermo-mechanical processing.The effects of thermal deformation parameters and borides on the microstructure and deformation mechanism were investigated by multi-stage thermal deformation experiments of Ti-4Al-2.5V-1.5Fe（B）alloy.During the construction of the strain–compensated Arrhenius–type constitutive model for titanium alloys,it was found that the addition of borides had a dramatic effect on the thermal activation energy,such as a significant decrease in the thermal activation energy,which may be directly related to the greater recrystallization ability of the boron-modified titanium alloys.Moreover,it was indeed observed in further studies after multi-stage thermal deformation that the boron-modified titanium alloy has a greater recrystallization ability,which is attributed to the particle–stimulated nucleation（PSN）mechanism.Based on the finite element simulation,the effect of boride on the equivalent strain effect during deformation was investigated.Combined with the simulation results and the corresponding microstructure analysis,it is shown that the boron-modified titanium alloy can effectively alleviate the local strain and avoid the occurrence of microstructure inhomogeneity and flow instability during the deformation.Finally,the wrought alloy was rolled,and the boron-modified titanium alloy had a more excellent combination of strength and plasticity than the bare alloy due to various strengthening mechanisms such as fine grain strengthening and second phase strengthening brought about by the boride addition.In summary,while obtaining boron-modified titanium alloys with better properties through conventional processes,the boron-modified titanium alloys can effectively inherit and retain the fine grain structure after thermal deformation compared to the bare alloys,improving the formability of the alloy and reduce the difficulty of thermomechanical processing.This provides a new strategy for the manufacturing of titanium alloys with superior properties in a cost-effective way.