Microstructure and tensile properties of cast Ti-44Al-4Nb-4Hf-0.1Si-0.1B alloy with refined lamellar microstructures

The tensile properties of a cast Ti-44Al-4Nb-4Hf-0.1Si-0.1B alloy with fine lamellar structures have been measured at room temperature. The average lamellar colony size was in the range of 70–130 μm, depending on heat treatment condition. All the tensile ductilities measured at room temperature are below 0.45% with the majority between 0.3 and 0.4%. Microstructural examination and fractographic examination revealed that there are many lamellar colonies with their lamellar interfaces parallel to each other, in contrast to observations on an extruded sample, which showed 1% ductility. Cleavage along the lamellar interfaces in those parallel colonies in the cast samples forms clusters of small parallel cracks close to each other and their collective effect, together with that from the debonding between boride ribbons and the metal matrix, may contribute to the low ductility in the cast alloy. Analysis shows that those parallel lamellar colonies probably originate from the Burgers alpha grains (obeying Burgers OR with the beta phase) formed during the beta-to-alpha solid phase transformation.