A Newly Generated Nearly Lamellar Microstructure in Cast Ti-48Al-2Nb-2Cr Alloy for High-Temperature Strengthening

Alloy 4822 (Ti-48Al-2Cr-2Nb at. pct) cast material was given a controlled heat treatment cycle to generate a casting nearly lamellar (CNL) microstructure that enhances the temperature capability over its current engineering casting duplex (CDP) microstructure form. The cycle consisted of three steps: a short alpha field annealing, an alpha + gamma field annealing, and then aging at a low temperature, with each step being followed by controlled cooling. The resulted microstructure is shown to be a mixture of non-uniformly distributed similar to 250 mu m size lamellar colonies containing similar to 0.15 mu m spaced laths. Standard tensile testing at 700 degrees C shows a yield stress of 344 MPa that is similar to 55 MPa greater than that of the current engineering CDP form. The sequential microstructure evolution processes following the three-step thermal cycle are assessed and explained in terms of phase transformations taking place across and below the alpha transus upon isothermal treatment and subsequent cooling. The resulted increases in high-temperature strengthening are explained by the colony and gamma grain size distributions. The strengthening mechanism along with the significance is discussed.