Hot Deformation Behavior And Microstructure And Mechanical Properties Of Boron Containing Titanium Alloy

Titanium alloy possesses good combined properties, which can be applied to very extensive fields like aerospace, automobile, ship and biomaterials industry etc. Grains of as-cast titanium alloy are usually coarse, which are broken by cogging in β phase, and then refined equiaxed microstructures are obtained by recystallization. However, the process of cogging increases the production time, cost and the consumption of energy. The ingot grain size is refined by certain means, and its growth at high temperature is also limited to improve its formability. The plate can be obtained by directly rolling as-cast titanium alloy without the cogging process, which can reduce the cost, time and the energy consumption. According to this idea, the following studies are conducted in the paper: The function of grain refinement that trace B/TiB2has on two typical titanium alloys,+alloy (Ti-6Al-4V) and nearalloy (Ti-B20); the restrictive effect to the grain growth at heat exposure from TiB whiskers, which are produced by the reaction between B/TiB2and Ti; the formability of boron containing titanium alloy during the direct rolling; the microstructure and property obtained through direct rolling the plate, and the influence that the general forging/heat treatment have on the microstructure and property of boron containing titanium alloy. Modern testing technology and methods such as XRD, SEM, TEM, EBSD and universal strength tester are used in the study, characterizing the microstructure morphology, phase structure, texture and mechanical properties of as-cast and as-rolled alloys.According to the result of study on the grain refinement and microstructural stability, trace TiB2significantly refined grains and lath of as-cast Ti-6Al-4V alloy and Ti-B20alloys. The grain size can be reduced by one order of magnitude by adding TiB2with a mass fraction of0.32%. The0.32%mass fraction is the inflection point of refinement, which means the grain size can’t be further reduced with more addition of TiB2. The grain refinement of titanium alloy is attributed to the constitutional undercooling built up by element B in front of the liquid-solid interface. The constitutional undercooling causes the increase of nucleation site in alloy melt in front of the liquid-solid interface, which contributes to the grain refinement. When adding more TiB2, reaction between Ti and element B enriching between secondary dendrite arms produces TiB whiskers which describe the morphology of dendrite in the room temperature structure.TiB whiskers can pin grain boundary effectively to prevent the grain from growing at high temperature. When its major axis is parallel to the grain boundary, TiB whisker has the most remarkable pinning effect on the grain growth. According to the kinetics analysis of grain growth, compared with matrix alloy, boron containing titanium alloy possesses relatively higher grain growth exponent and activation energy.For boron containing as-cast titanium alloy has refined grain size, and the grain size does not grow obviously at high temperature, which improves the formability of the plastic deformation and makes direct rolling possible. A sheet without macroscopic cracks can be obtained after rolling the as-cast titanium alloys directly without going through cogging process. Analysis of the sheet microstructure and property shows that direct rolling makes TiB whiskers and primary α phase distribute along the rolling direction. With the increase of reduction ratios, significant recrystallization occurs on primary α phase of Ti-6Al-4V alloy and Ti-B20alloy. When true strains reaches1.81, the dislocation density of Ti-6Al-4V-0.1B alloy decreases obviously. Equiaxed microstructure is obtained after the complete recrystallization of alloy. The alloy grains mainly distribute in the range of13μm. Due to the refinement of matrix structure and the carrying role of TiB whiskers, direct rolling remarkably increases the room temperature and high temperature mechanical properties of boron containing titanium alloy. The room temperature tensile strength and high temperature yield strength of Ti-6Al-4V-0.1B alloy increased by193MPa and182MPa respectively when rolling in two phase region with84%reduction ratio. Compared with Ti-B20alloys, the predicted value of as-rolled Ti-B20-0.1B alloys conform to its experimental value. As shown in pole figure analysis, different mechanical properties of Ti-6Al-4V-0.1B alloy in different directions are not caused by texture, but by the directional alignment of the matrix structure and TiB whiskers.Forging/heat treatment has remarkable effect on the microstructure and property of Ti-B20-0.1B alloy. After solution treatment and then short-time aging, precipitate free zone appears in the alloy matrix structure, which reduces the alloy strength and ductility. With the rising solution treatment temperature and the increasing aging time, the precipitate free zone disappears. The mechanical properties of alloys are determined by both solution treatment temperature and the aging parameter (aging temperature and aging time). The study shows that when solution treated at780°C and aged at550°C for1–8h, alloys possess an excellent combination of strength and ductility.