Solid State Phase Transformation Behaviors In Zr-Cr-Fe Alloy And The Effect Of Mo,Bi Alloying Elements

In this paper,by means of X-ray diffraction(XRD),electron channeling contrast(ECC),electron backscatter diffraction(EBSD)and transmission electron microscopy(TEM)technologies,we focus on one novel Zr-1.0Cr-0.4Fe alloy and systematically investigate the formation mechanism of three-variant clsuster and the crystallography during martensitic transformation in Zr-1.0Cr-0.4Fe alloy,the crystallography and variant selection during massive transformation in Zr-1.0Cr-0.4Fe alloy,the effect of alloying element on microstructure of Zr-1.0Cr-0.4Fe-(0.4Mo)alloy during ?-quenching,the distribution trends and site preferences of alloying elements in Zr Cr2 Laves phase.Following conclusions can be obtained from the present study:The Zr-1.0Cr-0.4Fe alloy was mainly organized by ? martensite after ?-quenching by liquid nitrogen within interior of parent ? grains.Each of ? parent grains can transform into 12 ? martensitic variant following the Burgers orientation relationship.All 12 ? martensitic variant were not randomly distributed,but were formed into four different three-variant clusters.These three variants from individual cluster show self-accommodation triangular morphologies and are related to each other by an angle/axis pair 60°/(27)1120(29).The preferred formation of such three-variant cluster is ascribed to the self-accommodation effect and the purpose is to coordinate the shape strain produced during the phase transition process.A large amount of massive-shaped grains have formed along prior ? grain boundary in Zr-1.0Cr-0.4Fe alloy after ?-quenching by liquid nitrogen.The orientation between ?m massive phase and its ? parent phase during massive transformation satisfies Burgers orientation relationship.The nucleation and growth mechanisms of massive transformation in Zr alloy followed the so-called oriented nucleation and non-oriented growth modes,which results in the interlaced distribution of ?m massive grains along prior ?-? grain boundary.An obvious variant selection occurs during massive transformation due to pre-existing ?-? grain boundary.In-depth analysis indicates that the variant selection during massive transformation always occurs at both nucleation and growth stages.In general,the selection of different variants at the early nucleation stage is determined mainly by the activation energy of nucleus germination.The massive nucleus tends to select a variant that maintains an orientation relationship with the non-Burgers ? grain that has only a small deviation from the BOR(minimum activation energy of nucleus germination).In addition,it is shown that growth of initial nucleus is largely dependent on mobilities of the interphase boundaries.Mo and Bi elements have important effect on as-quenched microstructure of Zr-1.0Cr-0.4Fe-(0.4Mo)alloys.It is shown that,for the liquid nitrogen quenching case,Mo element can effectively refine the ? martensitic plates,while Bi element has no obvious effect on the width of martensitic plates.For the cold water quenching case,as the Mo content increased(from 0 to 0.6 wt.%),the phase composition of Zr-1.0Cr-0.4Fe-x Mo alloys changes as the order: ???+?+?? ?+?(trace)??.The trace addition of Bi element can improve the ? phase content and suppress the residual ? phase content.According to first-principle calculation and experimental results,Mo was identified as a ?-stabilising element and adding Mo element can significantly improve the ? phase stability.Similar to Sn element,Bi element was identified as an ?-stabilising element and adding Bi element tend to increase stability of ? phase.In addition the important effect on as-quenched microstructure of Zr alloy,the alloying elements also plays important role on precipitation.Therefore,we further study the distribution trends and site preferences of Fe,Mo and Bi elements with the same initial content in Zr Cr2 Laves phase.The STEM-EDS results indicate that the distribution trends of above three alloying elements in Zr Cr2 Laves phase is Fe>Mo>Bi.Subsequently,the equilibrium concentration of ternary alloying elements in Zr Cr2 Laves phase are calculated by first-principles.It is shown that the order of equilibrium concentration of above three alloying elements in Zr Cr2 Laves phase is Fe>Mo>Bi.The calculated results show a good consistence with the STEM-EDS results.In addition,it is also shown that Fe element has a strong preferences for Cr sites,while Mo and Bi elements have weak Cr site preferences in Zr Cr2 Laves phase.The reasons accounting for different distribution trends and site preferences of Fe,Mo and Bi in Zr Cr2 Laves phase are attributed to the pseudogap effect and hybridizations between atoms.