Abstract
Ni22 high entropy alloy (HEA) in Al-Co-Cr-Fe-Ni system has excellent corrosion resistance, especially in 3.5 wt. % NaCl(aq), and good strength-ductility combination after fine grain strengthening. This study further develops Al-Cr-Fe-Ni-Ti non-equal-mole HEAs on the basis of Ni22 in order to reduce the cost and achieve even better performance. In the first part, CT6, NT9 and NT6 are designed on the basis of Ni22. After homogenization at 1100 ℃ for 6 h and subsequent water quench, CT6 comprises of BCC + FCC + γ’ (Ni3Al) + Heusler (AlNi2Ti) phases, NT9 exhibits FCC + γ’ + Heusler phases, and NT6 nearly becomes a simple FCC+ γ’ phases. Due to the precipitation of σ phase, the present HEAs have excellent age hardening response (above HV 700) and resistance to overaging at 700 ℃, 800 ℃, and 900 ℃ for more than 400 h. However, NT6 shows slight overaging at 900 ℃ because the precipitation of η phase will consume the fine γ’ phase which could strengthen the matrix of NT6 at 900 ℃. NT6 displays excellent workability by rolling at ambient temperature. By aging at 700 ℃ for 30 min, the 85 %-rolled sample can be quickly hardens beyond HV 800 by forming fine and uniform σ phase which are smaller than 0.5 μm. Grain size of NT6 can be controlled by using thermo-mechanical processing, that is, cold-rolling plus annealing at 1000 and 1100 ℃. The results show that the Hall-Petch effect of NT6 is stronger than Ni22 HEA and 304 SS. The sample after 1000 ℃-2 min fine-grain treatment also has a high yield stress (1243 MPa) and a proper elongation (10.1 %). In Pin-on-Disk wear test, all test alloys with hardness above HV 800 display better wear resistance than SUJ-2 bearing steel and SKH-51 high speed steel and the as-aged CT6 shows the best wear resistance among them. For investigating the effect of Mo addition on corrosion resistance, the second part uses d-electrons alloy design theory (New PHACOMP) to design three Mo-containing FCC-type HEAs (M50, M25 and M14) on the basis of NT6. Corrosion test in sulfuric acid, sodium chloride, and their mixture are investigated. In sulfuric acid, both as-cast and as-homogenized HEAs have better corrosion resistance than 20Cb-3 and 316L SS. It is found that chloride ions tend to attack Heusler phase and cause very severe pitting and lower Eb (pitting potential) and Ep (protection potential). As-homogenized NT6 has very good pitting resistance because of containing very few Heusler phases. After adding Mo, Eb and Ep are raised, and M50 possesses the best pitting corrosion resistance in this study, which is better than that of 20Cb-3. In the comparison of as-homogenized and fine grain strengthened NT6, general corrosion resistances in sulfuric acid of specimens annealed at 1100 ℃ and 1000 ℃ are similar to as-homogenized specimen. In solution contained chloride ions, 1100 ℃-annealed specimen possess similar pitting resistance to as-homogenized specimen, but 1000 ℃-annealed specimen has sever pitting because of the existence of Heusler phase, which is a sensitive zone exhibiting a very low pitting resistance. However, 1000 ℃-annealed specimen still possesses suitable pitting resistance which is slightly better than as-homogenized specimen. In conclusion, Al-Cr-Fe-Ni-Ti HEAs have excellent corrosion resistance. Among them, NT6 and its fine grain strengthened states exhibit excellent combination of corrosion resistance and mechanical properties, whose performance-to-price ratio is better than Ni22. Therefore, NT6 has potential applications in marine environment. In addition, M50 can be applied in severe corrosion circumstances.