Role Of Boron On The Evolution Of Microstructure And Texture In Ti-6AL-4V-0.1B Alloy

Abstract

Titanium and its alloys constitute an important class of materials for aerospace, biomedical, and chemical industries, primarily due to their high specific strength and fracture toughness with good corrosion resistance. Owing to their hexagonal crystal structure at room temperature, both microstructure and texture play a crucial role in the processing and hence the properties of titanium alloys. The basis for tailoring the microstructure and texture in titanium alloys centers around the transformation of high temperature β (body-centered cubic) to the low temperature α (hexagonal close packed) phase. One of the most widely used titanium alloy is Ti-6Al-4V, which exists as two phase (α+β) alloy at room temperature. The cast structure of the alloy Ti-6Al-4V is generally coarse and has strong solidification texture that leads to inferior properties. Recently, trace boron addition has been reported to produce substantial refinement in cast microstructure for Ti-6Al-4V. Significant improvements in some of the mechanical properties have been reported for the Ti-6Al-4V-0.1B alloy in the as-cast condition. The reasons for microstructural refinement in the boron modified alloy and associated improvements in properties, however, needs to be investigated since the property attributes strongly depend on finer microstructural details including crystallographic texture. In addition, the titanium alloys are processed through thermo-mechanical treatments that involve deformation and annealing response of the alloy. The effect of boron modification on the processing response during thermo-mechanical treatments (TMP) has also not been studied. All these aspects shape the framework of the thesis, wherein microstructure and texture evolution is probed from starting cast condition through different stages of TMP. Micro-mechanisms are identified at every stage from the interrelation of these two intrinsic factors. In the first part of the study, the spatial variation of microstructure and texture in the cast ingot has been studied using SEM-EBSD technique. It has been found that trace boron addition (0.1 wt%) to Ti-6Al-4V alloy ensures excellent microstructural homogeneity throughout the cast ingot. A subdued thermal gradient due to constitutional undercooling persists during solidification and maintains equivalent β grain growth kinetics at different locations in the ingot. For Ti-6Al-4V-0.1B alloy, both high temperature β and room temperature α phase textures weaken. The microstructural attributes of boron addition manifests as the absence of grain boundary α-phase and the presence of TiB particles. Both these features strongly affect the mechanism of β→α phase transformation and consequently weaken the α phase texture. The evolution of microstructure and texture during β-processing of Ti-6Al-4V-0.1B alloy is examined in the second part of the work. Boron modified alloy shows the typical features of β worked microstructure with fine prior β grains, however without the formation of shear bands, which is generally observed in the microstructure of β worked Ti-6Al-4V alloy. The transformed α texture is almost same for the two alloys indicating similarity in the transformation behaviour of boron modified and boron free Ti-6Al-4V alloy due to complete dynamic recrystallization during β processing. The microstructural features as well as the crystallographic texture indicates dominant grain boundary sliding for the boron added alloy which leads to homogeneous deformation response without instability (shear band) formation. In the third part of the study, the deformation response in the (α+β) regime has been studied by carrying out hot compression tests at different temperature under constant true strain rate to simulate experimental processing conditions for the cast Ti-6Al-4V-0.1B alloy. The critical combinations of temperature and strain rate suitable for processing are identified based on flow curves and kinetic analyses. Microstructural features display dynamic recovery of the α-phase at low temperatures and softening due to globularization and/or dynamic recrystallization at high temperatures irrespective of boron addition. The transition temperature for the two mechanisms is comparatively lower for boron added alloy. Unlike Ti-6Al-4V, no sign of instability formation has been observed in Ti-6Al-4V-0.1B. The absence of macroscopic instabilities and early initiation of softening mechanisms has been attributed to microstructural features and texture of boron modified alloy in the initial as-cast condition. In the fourth part, the large strain deformation response has been studied for the Ti-6Al-4V-0.1B alloy by rolling in the (α+β) regime. Microstructure in near α rolling regime is characterized by a few kinked and bent α colonies while others are elongated along the rolling direction. Dynamic softening at higher temperatures is more dominant for the boron added alloy. Microstructural features are strongly orientation sensitive while relative differences are inherited from the starting cast alloys. Texture evolution, however, does not markedly vary for the two alloys and indicates little difference in the slip based deformation processes under plane strain condition. The influence of transformation texture appears early for the boron added alloy and affects the final texture in much stronger way at higher temperature. Subsequent to the (α+β) rolling, static annealing of warm rolled alloys has been carried out. A faster annealing kinetics for boron added alloy has been observed, which is related to deformation prior to annealing leading to additional diffusion pathways due to microstructural factors. Texture of the annealed material is similar to the deformed state for shorter annealing times but substantially modifies by epitaxial growth of primary α phase during long time annealing. The final part of the work deals with the deformation response of boron added alloy under superplastic conditions. Out of the two alloys with similar microstructure and texture, higher elongation for boron modified alloy is justified by the absence of slip based deformation and improved grain boundary sliding. Increase in α/β interfaces due to globularization during warm rolling and static annealing contribute to the grain boundary sliding. The outcomes of the thesis have been presented as a summary at the end and suggestions have been made indicating the scope for future investigations pertaining to this area.