Hafnium - Periodic Table of Videos
The team at periodicvideos has created a TED-Ed Lesson for every element of the periodic table. Hafnium, with the symbol Hf, is element number 72.
Some superalloys, used for special applications, contain hafnium. C103, which consists of 89% Nb, 10% Hf and 1% Ti is an alloy used for liquid rocket thruster nozzles, such as those found in the main engine of the Apollo lunar modules.