Microstructure of Rolled Ti-46.5Al-4(CrNb,Ta,B)
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After the final rolling pass and cooling to room temperature, the grain size of the B-containing alloy is 2-10gm (Fig. 1(b)), significantly smaller than the B-free material with grain sizes 10-40Rm (Fig. 1(a)). The B-free material had a 20% less total deformation (fewer rolling steps) than the B-containing material which may partially contribute to the larger grain size. Since, however, the rolled material is reheated after each rolling pass to the a + ' phase field (so the next rolling step starts with a recrystallised microstructure), the main reason for the refined grain size is not the total deformation but the presence of boride particulate within the matrix. The boride particulate inhibits grain growth both in the a + ' phase field and on cooling. The SEM image of the B-containing alloy (Fig. 1(b)) shows a scattering of fine white horizontal lines which correspond to Ti-B particles. These borides are aligned with the rolling direction (left to right) and TEM shows that they are frequently located at grain boundaries (Fig. 3). The borides are predominantly 50-500nm in size (not resolved in the SEM image) with a rod-like morphology and 35 aspect ratio. Chemical mapping of the particles using EELS and EDX confirms that they are (Ti,Ta)B in composition, with trace amounts of Nb and no Cr. No B could be detected elsewhere in the microstructure. Diffraction studies and high resolution imaging of the borides shows that they have a complex internal faulted structure based on intermixing of the monoboride structures TiB B27 and Bf structures as observed in other Ti-Al-Ta-B systems [4]. (iii)Annealing heat treatments
After rolling a heat treatment of 1000°C for two hours in vacuum was applied, which flattens the sheets after rolling. Fig. l(c) shows the primary annealed B-containing sheet microstructure. The main effect of the primary annealing is to reduce the volume fraction of the lamellar domains by the growth of B2 and ', moving more towards the equilibrium of near ' with globular B2 and a 2.The density of dislocations in the primary '(-grains was also reduced forming low angle boundaries. TEM shows that not only is there significant growth of B2 grains surrounding the lamellar domains, which getters Cr, but B2 also precipitates along a 2-lamellae within the lamellar domains in KK3.12.2
Figure 1. As-rolled microstructures after rapid cooling from the a + Y phase field. Imaging is by back-scattered SEM, where -yis dark grey, % and a2 + y domains are light grey and B2 is white. (a) Ti-46.5AI-4(CrNb,Ta) alloy (b) Ti-46.5AI-4(CrNb,Ta,B). (c) Microstructure (b) after primary annealing at 10000C for two hours in vacuum. KK3.12.3
six orientations with the relationship B2 // a 2 and B2 H/ [00011 a 2 (Fig.4). No further B2 -*) o transition was observed. The lamellar oX 2 is additionally replaced by coarsening ylamellae growing into and along the a 2 lamellae, indicating that the volume of fraction % formed in the initial oa --4 oa2 + y transformation was much greater than the equilibrium value due to rapid cooling. Th
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