Study On Microstructure Evolution And Mechanical Behavior Of Ti-6Al-4V-0.1B Alloy

Titanium alloys with high specific strength and specific stiffness, excellent corrosion resistance, high temperature properties and processing properties, are widely used in the areas of aerospace, petrochemical, marine, automotive, medical and sports, and have become the important structure material and functional material. But the raw material costs and processing cost of titanium alloy are far higher than the aluminum alloys and steels, which limits the wider use of the titanium alloys. Therefore, it has critical significance to develop low-cost titanium alloys.In this research, the microstructural evolution and mechanical behavior of the Ti-6Al-4V alloy modified with0.1wt%Boron are systematically studied. Additionally, the low cost processing for the Ti-6Al-4V-0.1B alloy is investigated. The results are shown as follows:The microstructure character and formation mechanism of as-cast Ti-6Al-4V-0.1B alloy were analyzed firstly. The ingot of Ti-6Al-4V-0.1B alloy obtained through twice consumable melting in vacuum is qualified. The structures of as-cast alloys consist of α,β and TiB phases. The needle-like TiB phase, whose surface appears very smooth and flat, are mainly distributed along the boundaries of the original β phase and less in grains. After the addition of0.1wt%Boron, the original α and β phases with as-cast structures are obviously refined. During the solidification of as-cast alloy, the α,β and TiB phases appear the orientation relationships as (112)β//(010)Ti,(111)β//(1120)α,(001)TiB//(1010)α. The orientation relationships show that a part of a phase nucleates from the TiB phase and more cores for nucleation lead to the refinement of a colony.In this paper, the thermoplastic deformation mechanism and microstructure evolution character of the Ti-6Al-4V-0.1B alloy are studied. The results show that, the Ti-6Al-4V-0.1B alloy is sensitive to the deformation temperature and strain rates. The flow stress increased as the deformation temperature reducing and the strain rate increasing. During the deformation of the single-phase region, the main deformation mechanism of the as-cast alloy is dynamic recovery, but incomplete recrystallization occurs under the condition of high temperature, and the organization of the alloys is refined. When deformations occur in the two-phase region, the deformation mechanism of the as-cast alloy is DCRX (dynamic continuous recrystallization), so the lamellar a structure globularized. When deformations occur in the two-phase region, the deformation mechanism of the alloy with Widmannstatten structure is the same to as-cast alloy, and the structure is refined and globularized with the increasing of deformation amount.In low strain rate deformation, the extension of deformation time promotes the grain growth after the dynamic recrystallization. TiB phase reverses in the deformation process and distribute along the processing flow line. The TiB phase promotes the occurrence of the recrystallization and improves the spheroidizing rate of the alloys.Then the effect of heat treatment on the microstructure, tensile properties and dynamic properties are discussed and investigated. After simply annealing, β annealing and α+β annealing, the microstructure of rolling bar are transformed to equiaxed, Widmannstatten and bimodal structure respectively. During the cooling process from the temperature above the phase transition point, a phase nucleates and grows up in the β phase matrix and TiB phase. TiB phase as another nucleation position of a phase is conducive for the nucleation and precipitation of a phase. In the tensile deformation process, the strength of the alloys with Widmannstatten structure is higher than those with biomodal structure which have better plasticity however. Annealing cooling method has a greater impact on the properties of the alloy. The highest strength but the worst plasticity of the alloys is obtained by water-cooling. The strength is close comparatively by air-cooling and furnace cooling, and by both of which the alloys have the best plasticity. Under the condition of high-speed impact, the alloys with equiaxed structure and biomodal structure can carry higher strain rate in the process of dynamic compression deformation. The alloys with biomodal structure, equiaxed structure and Widmannstatten structure appear more and more sensitive to the adiabatic shearing in turn. The alloys with biomodal structure, in which the proportion of primary a phase is50%-55%, have the highest amounts of uniform plastic deformation, absorb the most energy in the deformation and show the best dynamic properties. Under different cooling rate (furnace-cooling, air-cooling and water-cooling) after two-phase annealing, the dynamic properties of the alloys perform better and better in turn.0.1wt%Boron addition affects little on the average flow stress of Ti-6Al-4V alloy, but obviously reduces the dynamic deformation plasticity, and the effects on the alloys with Widmannstatten, exquixed and biomodal structure show smaller and smaller in turn. During the dynamic loading process, the TiB phase doesn’t alter the principle of the sensitive of the alloys to ABS (Adiabatic Shear Band) and there are no direct corresponding relationship between the TiB phase and the ABS. Finally, the process of the preparation of the low-cost Titanium alloy by the method of α+β phase region rolling technology is investigated in this paper. It is indicated that, for the0.1wt%B addition in the Ti-6A1-4V alloy, the hot-working properties of the ingot alloy are critically improved. And the α+β phase region rolling of the alloy becomes possible. With the amount of α+β phase region deformation increasing, the microstructure of the alloy is refined remarkable, and the maximum deformation by direct rolling can reach70%. After α+β phase region thermal deformation processing and heat treatment, the biomodal microstructure with certain proportion of primary lath a phase can be observed. After direct-rolling, the mechanical properties of the alloy meet the requirements of index standard of GB/T3621-2007. The microstructures and mechanical properties of the alloy through two passes rolling are refined well than those through one passes rolling. After920℃/1h, FC of heat treatment which is low cost process for the preparation of titanium alloy plate, the properties of the alloy is equal to Ti-6Al-4V alloy.