Enhanced strength and ductility in Ti46Al4Nb1Mo alloys via boron addition

To improve the strength and ductility of TiAl alloys by second phase, Ti46Al4Nb1Mo alloys doped with different B content (0.4%, 0.8%, 1.2%, 1.6% and 2.0%, atomic percent, hereafter in at.%, referred to as TNM-xB) were prepared. Macro/microstructure evolution, mechanical properties and deformation mechanisms of the alloys were studied systematically. Results showed that the microstructure of TNM-0.4B and TNM-0.8B alloy remained columnar dendrites, and the secondary dendritic arms of columnar grains were more obvious. When the content of B is 1.2%, the columnar dendrites transformed to equiaxed grains, and the α₂/γ lamellae colony size was further refined in TNM-1.6B and TNM-2.0B alloy. The morphologies and kinds of the borides were changed with increasing B content, XRD results showed that TiB phase appeared with 1.6%B content, and both TiB and TiB₂ phase formed in TNM-2.0B alloy. There were straight and curved TiB phases located around grain boundaries in TNM-0.4B and TNM-0.8B alloy, and when the content of B increased to 1.2%, the curved TiB phases were reduced, while the tiny and straight TiB phases increased. With further increasing B content to 1.6% and 2.0%, the tiny and straight TiB phases were coarser. Compressive testing results showed that the mechanical properties of the TNM alloy were enhanced with increasing B content. The maximum strength and strain of TNM alloy were 2339MPa and 33.7% with 1.6% B addition. The compressive strength and strain were mainly enhanced via refinement of lamellar colony and formation of TiB, and it is found that pile-up of dislocations and deformed twins promoted by TiB are predominant in improving the mechanical properties of TNM alloys with higher strength and strain.