Effects of alloying elements (Sn, Cr and Cu) on second phase particles in Zr-Sn-Nb-Fe-(Cr, Cu) alloys

In this investigation, the effects of Sn, Cr, and Cu on the precipitation of the second-phase particles (SPPs) in Zr-Sn-0.3Nb-0.3Fe-(Cr, Cu) alloys were investigated. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were jointly employed to carry out detailed analysis in the spatial distribution, crystal structure and chemical composition of second-phase particles. Most of the SPPs were homogeneously distributed, with clusters of SPPs occasionally being observed. The content of Sn shows no significant influence on the crystal structure of the SPPs, but the average size of the SPPs decrease as the content of Sn decreases from 1 wt% to 0.8 wt%. Zr(Fe,Cr,Nb)2 SPPs with C14 and C15 Laves structures were observed in the Cr-containing alloys, while in the Cr-free alloys, in addition to the Zr-Nb-Fe phase, the existence of Zr-Fe phases was confirmed. By comparing the average size of the SPPs between Cr-containing alloys and Cr-free alloys, it is revealed that the introduction of Cr element could remarkably refine the SPPs. The effects of Cu on the type of SPPs formed in Cr-containing and Cr-free alloys were also found different. In the Cr-containing alloys, in addition to hexagonal and cubic Zr(Fe,Cr,Nb)2, Zr3Fe SPPs was observed with the introduction of 0.05 wt% Cu. In contrast, Zr2Fe SPPs was observed but (Zr,Nb)2Fe disappeared in the Cr-free alloys with the introduction of 0.1 wt% Cu. With the adding of suitable amount of Cu, the number of precipitated SPPs significantly increased accompanied by a refinement in SPPs average size. In addition, stacking faults were founded in some Laves phases.