Effect of Hf addition on solidification and hot isostatically pressed microstructures of the Ti-48Al-2Cr-2Nb (at%) intermetallic alloy

The microstructural evolutions during solidification and hot isostatic pressing (HIP) of Ti-48Al-2Cr-2Nb-xHf (4822-xHf, x = 0, 2, 4, 6 at%) intermetallic alloys were investigated using both experimental observations and thermodynamic calculations. The primary solidification phase of all the studied alloys was beta, albeit it was predicted by Thermo-Calc to be alpha for the alloys containing 4% and 6%Hf. In agreement with the ThermoCalc calculations, the solidification path of the base 4822 alloy encountered a peritectic reaction. Although alloying by 2%Hf did not change the solidification path, further increase in Hf content caused an additional eutectic reaction following the peritectic one. The eutectic microstructure was composed of two orthorhombic phases with TiAl2 and Al3Hf2 compositions and this finding was the first evidence of such phases in a quinary alloy with Cr, Nb, and Hf. It was found that Hf acted as a weak beta stabilizer as it showed a low partitioning tendency in beta and the beta single-phase field was not formed in the high-Hf alloys. A finer dendritic microstructure was characterized in the high-Hf alloys due to the Hf solubility limit in the 4822 alloy. Interestingly, alpha(2) laths dissolved in the Hf-containing alloys after HIP treatment and instead rode-like precipitates of Al3Hf2 emerged at newly formed gamma/gamma interfaces. The precipitation mechanism of Al3Hf2 particles was discussed. (C) 2020 Elsevier B.V. All rights reserved.