Reactive element RE optimization

Too little dopant (0.018 Hf but no detectable Y) also produced little benefit. Fig. 13.9. Empirically, it has been observed that effective dopant levels are 

9 Lifetime (time to FeO formation) for various FeCrAI alloys with 10% Ai and different dopant additions during 1 h cycles at 1200°C in dry O2. The highest lifetimes are with alloy additions with combinations of Hf and Y. However, adding too much or too little dopant results in reduced lifetime. For the alloys marked (Y), a Y addition was made but none was detected in the casting. Initial data from Pint (2003b). 

A variety of alloys introduced in the past 20 years have excellent oxidation performance due to RE dopant optimization. Alloy 22APU, discussed in 

10 Specimen mass change during 1 h cycles at 1100°C in dry O2 for various Ni-7Cr-13AI alloys with various Hf/C and Y/S contents. Data from Pint (2003b). 

11 Total mass gain (specimen + spalled oxide) during 100 h cycles at 1250°C in laboratory air for various ferritic Fe-(17-18)AI alloys. Additions of only Y showed relatively short times to breakaway oxidation due to broccoli formation. Addition of Hf or Y and Hf or Ti prevent this form of attack. The co-doped alloy specimens have not reached breakaway oxidation to date. 

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