Alumina forming alloys (steels, high entropy materials) for the mitigation of compatibility issues with liquid metals and steam in energy related, high-temperature applications

Energy-related applications with aggressive environments at high temperatures require the development of advanced structural materials. Alumina forming alloys (e.g. Ni-based, Fe-based) have received a great attention for high temperature applications because of their excellent oxidation resistance and adequate mechanical properties. By adding appropriate amounts of Al and Cr, these alloys are able to form protective alumina-rich oxide scales when exposed to the oxygen-containing extreme conditions (e.g. high temperature, corrosivity). Ferritic FeCrAl alloys have been successfully developed for use in liquid Pb environment. However, ferritic steels as well as ferritic FeCrAl alloys suffer from liquid metal embrittlement in liquid Pb. Alumina-forming austenitic alloys (AFA) have the potential to mitigate the corrosion issues in molten Pb and to avoid liquid metal embrittlement at the same time, due to the austenitic structure. Another promising material type is high entropy alloys (HEA). Typically, they have five or more principle elements with the atomic fraction of each element in the range of 5-35 at.%. By alloying passive layer forming elements like Al and Cr, the alloys are able to withstand oxidizing conditions at temperatures above 1000 °C by the formation of a protective alumina-rich scale. In this dissertation, AFA and HEA model alloys, based on the backbone composition of Fe-Ni-Cr-Al are designed in order to search for the composition map of target materials that are compatible with the selected aggressive environments (molten Pb and steam). AFA model alloys based on Fe-(20-29)Ni-(12-16)Cr-(2-4)Al (in wt.%) have been designed based on equilibrium phase calculations (Thermo-Calc) and the Schaeffler diagram. All the annealed alloys are single FCC phase with specific heat capacity, thermal conductivity and thermal expansion comparable with SS316. Nine alumina-forming HEA alloys have been designed based on the empirical parameters, including enthalpy of mixing (ΔHmix), atomic size difference (δr), parameter $\Omega$ ((Ʃc${}_i$T${}_{m,i}$)ΔSmix/|ΔHmix|) and valance electron concentration (VEC). The designed alloys include four quaternary alloys with dual phase (Al${}_{8.91-11.7}$Cr${}_{22.40-30.28}$Fe${}_{32.62-34.50}$Ni${}_{26.91-34.28}$, FCC+B2 or BCC), two quaternary alloys with single FCC phase (Al${}_{6.02-7.96}$Cr${}_{23.23-25.01}$Fe${}_{33.99-34.06}$Ni${}_{34.75-34.98}$), and three quinary alloys alloyed with Nb/Ti/Cu (Al${}_{7.90-8.24}$Cr${}_{21.37-22.04}$Fe${}_{30.29-31.96}$Ni${}_{33.01-35.02}$Nb${}_{5.08}$/Ti${}_{5.01}$/Cu${}_{5.00}$, FCC+Laves or γ' phase and single FCC). The thermo-physical properties of as-cast HEA alloys have comparable thermal physical properties like SS 316 at temperatures below 800 °C. Compatibility tests have been performed in 10${}^{-6}$ wt.% oxygen containing molten Pb at 550 °C and 600 °C for 1000 h and 2000 h. The excellent corrosion resistance of the AFA alloys observed in these tests is due to the formation of a protective oxide scale (<200 nm) based on an outer layer of II Cr${}_2$O${}_3$ and an inner layer of Cr${}_2$O${}_3$-Al${}_2$O${}_3$ solid solutions. The passivated alloys also preserve their austenitic matrix. By adding yttrium, the uniformity in scale thickness and in Al and Cr distribution has been improved. In case of Nb containing samples, TEM evaluation of the alloy matrix indicates the formation of B2-NiAl and Laves (Fe${}_2$Nb) phases in addition to the austenite phase. The mechanism of the oxide layer passivation on HEA alloys is almost identical to that of the AFA alloys. A continuous oxide scale is formed based on Cr${}_2$O${}_3$ or (Fe,Cr)${}_3$O${}_4$ (in case of BCC phase) or TiO${}_2$ (HEA alloyed with Ti), which acts as a first corrosion barrier. Then, alloys with sufficient Al addition form a protective oxide scale underneath the first corrosion barrier, based on Cr${}_2$O${}_3$-Al${}_2$O${}_3$ solid solution or α-Al${}_2$O${}_3$. Four HEA model alloys have shown their microstructure stabilities during exposure in low-oxygen containing molten Pb at 550-600 °C. Three HEA alloys show precipitations of B2-NiAl phases at the grain boundaries. Sample with Ti addition exhibits phase transformations, namely FCC $\to$ sigma and γ' $\to$$\eta$ phase. For the tests in steam at 1200 °C three AFA alloys based on the compositions with Y or Nb addition and four HEA alloys, two with Nb and Ti have been selected. The AFA alloys containing Y (Fe-15Cr-2.5Al-20Ni-0.5Y, Fe-16Cr-2.5Al-22Ni-0.5Y) and the three HEA alloys not containing Ti have formed protective α-Al2O3 scales on the alloy surface. Considering both test environments, a general formula of AFA alloys that are compatible with oxygen containing Pb at 550-600 °C and steam at 1200 °C is derived: Fe-(20-29)Ni-(15.2-16.5)Cr-(2.3- 4.3)Al (wt.%). A formula of HEA alloys based on the backbone composition of Fe-Ni-Cr-Al that are compatible with oxygen containing molten Pb at 550-600 °C and steam at 1200 °C, and maintaining the FCC structure in the matrix, is defined as: (30.29-34.50)Fe-(33.15-35.02)Ni-(21.37-25.01)Cr-(6.02- 11.69)Al (at.%). In addition, elements like Nb or Y used either as principle element (Nb in HEA) or as minor additions (AFA) have an additional positive effect on the alumina scale formation in aggressive environments. The addition of Nb foster in addition the mechanical strength at high temperatures.