Effect Of Boron On Microstructural Evolution And Mechanical Property Of The Ti-2Al-9.2Mo-2Fe Beta Titanium Alloy

In this study,0.1 wt.%boron was added to the new beta Ti-2Al-9.2Mo-2Fe(2A2F)alloy to improve formability and reduce processing cost.The effects of boron on the microstructure evolution,process,and microstructure-property relationship of the 2A2F alloy were studied.The effect of adding 0.1 wt.%boron on the as-cast microstructure of the 2A2F alloy ingot melted through vacuum arc re-melting was investigated.The grains of three zones in the as-cast microstructure of the Ti-2Al-9.2Mo-2Fe-0.1B(2A2F10B)alloy were refined to approximately 1/3 of-the 2A2F alloy.The refinement mechanism comprised inhibition of beta crystal nucleus growth and promotion of crystal nucleation by the boron-rich layer produced in front of the liquid/solid interface.However,adding 0.1 wt.%boron did not lead to sufficient columnar-equiaxial transition in the 2A2F10B alloy,and can not obtain the uniform as-cast microstructure in the 2A2F10B ingot melted by vacuum arc remelting.Moreover,TiB particles were also formed in inter-dendrites and grain boundaries through eutectic reactions at the end of solidification.These particles exhibited orientation relationship to the surrounding beta phases.In addition,the yield strength of the as-cast 2A2F10B alloy was improved by grain refinement and reinforcement of TiB particles.Adding boron reduced the ductility of the alloy,resulting in void formation by TiB fracture beyond the yield point.The effects of adding 0.1 wt.%boron on the deformation behavior of the 2A2F alloy were investigated through hot compression test at 850 ?-1000 0C and strain rates ranging from 0.01/s to 10/s.Dynamic recrystallization was promoted in both 2A2F and 2A2F10B alloys with decreasing strain rate and increasing deformation temperature.The nucleation mechanism of dynamic recrystallization involved grain boundary bulging and sub-grain rotation.Deformation occurred at low temperatures,high strain rates,and different softening mechanisms in both alloys.Moreover,the boron-free alloy showed dynamic recovery.We found that boron-containing alloy was formed through dynamic recrystallization at low temperatures and high strain rates because of particle-stimulated nucleation(PSN)mechanism of TiB particle recrystallization.Dynamic recrystallization grain nuclei were formed by lattice rotation from the initial grains and were located near the TiB particle.Moreover,the flow stress of boron-containing alloy was lower than that of boron-free alloy during hot deformation because of grain refinement as a result of boron addition.In addition,the constitutive model of both alloys was obtained using kinetics calculations.The beta-grain growth behavior of 2A2F,2A2F05B(Ti-2Al-9.2Mo-2Fe-0.05B),and 2A2F10B(Ti-2Al-9.2Mo-2Fe-0.1B)alloys were examined at different temperatures.The grain growth of boron-containing 2A2F alloy was restricted effectively after adding trace amounts of boron.Kinetic calculations and microstructural analysis suggested that the TiB phase suppressed grain boundary migration above the beta transus temperature through Zener pinning.Based on the Zener pinning model,the theoretical maximum grain size range was similar to the experimentally measured grain size.The addition of 0.1%boron effectively refined beta grain size even above the beta transus.Therefore,the good ductility and strength of boron-containing alloys were retained after 1000 ?/12 h/WQ treatment.During aging,the secondary alpha phase grew from the surface of TiB.The TiB particles provided nucleation sites for precipitation of the alpha phase and showed orientation relationship to the secondary alpha phase.Therefore,the time for attaining the peak aging of the 2A2F10B alloy was shorter than that of the 2A2F alloy at high temperatures.Moreover,after solution treatment and aging,the strength and elongation of the 2A2F10B alloy were lower than those of the 2A2F alloy.The 2A2F10B alloy aged at 550 ? and 600 ? for 2 h exhibited satisfactory strength and ductility,with 1100?1300 MPa of ultimate strength and elongation exceeding 7%.The presence of the TiB phase provided source of cracks during tensile deformation,thereby decreasing the ductility of the alloy.By the PSN mechanism,adding 0.1 wt.%boron resulted in finer recrystallized microstructure,with more than 90%recrystallization level,than the as-rolled 2A2F10B alloy round bar(less than 40%recrystallization level).Therefore,directly aged 2A2F10B alloy round bar exhibited similar microstructure to that of solution-and aging-treated 2A2F10B alloys because of the high recrystallization level.Furthermore,the ductility of the as-rolled bar in boron-containing alloy subjected to direct aging treatment was higher than that in the sample under solution treatment.and aging.Under direct aging at 600?/2h/WQ,the 2A2F10B alloy bar exhibited satisfactory strength and better ductility,with 1093 MPa of ultimate strength and 15%of elongation.The direct aging treatment could reduce the processing cost of existing alloy products.