Variant selection and morphology by the different cooling rates in Zr-xNb-0.4Mo alloys

Abstract

Although many efforts have been made on understanding the variant selection rules, the interplay of these rules and their combined effects on variant selection have not been well addressed. In this work, variant selection and morphology by the different cooling rates in Zr-xNb-0.4Mo (x=0.2, 0.6 and 1.0 wt.%) alloys are characterized using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD) technique. The results show that the variation of variant selection and morphology is attributed to the Nb content, the parent β grain size, the cooling rate, the β grain boundary (GB) characteristics and the interplay among them. The Nb addition (Nb≤0.6 wt.%) decreases the average grain size of parent β grains, and the smaller parent β grain size generally results in the larger degree of variant selection (DVS) values in the individual parent β grain due to their resistance to the uniform nucleation and growth of α-variants. The slow cooling rate promotes the diffusional transformation and favors the formation of special β GBs and the β GBs with high dislocation density, which usually lead to a stronger DVS in the individual parent β grains. The very large weighted area percentage of α-variant in the individual parent β grains, which accounts for the stronger DVS in the individual parent β grains, is due to the special β GBs, the β GBs with high dislocation density and the incomplete martensitic transformation (martensitic-diffusional mixed transformation). The β GB with high dislocation density has a larger contribution to the strong DVS of individual parent β grains than the special β GB. In addition, the result shows that the low density of statistically stored dislocations (SSDs) can inhibit the nucleation and growth of the α-variant produced by the special β GB.