Strengthening mechanism of Nb addition in Fe–13Cr–4.5Al–2Mo alloys assessed by internal friction measurement

Abstract

The Fe–13Cr–4.5Al–2Mo (FeCrAlMo) alloys, one of the most promising candidates for the accident tolerant fuel cladding in the light-water reactors, were strengthened with Nb addition and the strengthening mechanism were assessed by an internal friction (IF) measurement method combined with microstructure characterization. The FeCrAlMo alloy with 1.0 wt% Nb addition exhibited the highest ultimate tensile strength (UTS) and an acceptable ductility at each tested temperature (room temperature, 300 °C or 700 °C). For example at 700 °C, the UTS of FeCrAlMo −1 wt%Nb was about 215 MPa, which was 53.5% higher than that of Nb-free FeCrAlMo alloy. For Nb-free FeCrAlMo alloy, a high IF peak P2 appeared at about 680 °C was related to the recovery and recrystallization process during the first heating cycle, which was characterized by an internal friction measurement method. Another small IF peak P1 corresponding to Zener relaxation process and a predominant IF peak P3 corresponding to the grain boundary relaxation process were observed at 560 °C and 720 °C, respectively, during the second heating cycle. However none of IF peaks was detected for the Nb containing FeCrAlMo alloys up to 800 °C. Based on the IF theory, the disappearance of P1 (Zener peak), P2 (recrystallization peak) and P3 (grain boundary peak) in the first and second heating cycles was owing to the pinning of dislocations and grain boundaries by high density Fe2Nb nanoscale particles dispersed in the FeCrAlMo matrix, which enhanced the tensile strength of the Nb containing FeCrAlMo alloys.