Microstructural stability of ultrafine-grained niobium–zirconium alloy at elevated temperatures

Abstract

The present study reports on microstructural evolution upon static annealing treatment and elevated-temperature low-cycle fatigue (LCF) of an ultrafine-grained (UFG) body-centered cubic (bcc) niobium–zirconium (NbZr) alloy, processed by equal channel angular processing (ECAP) at room temperature.UFG NbZr showed recovery and recrystallization at homologous temperatures, which are in the same range as those of another UFG bcc material, i.e. interstitial free (IF) steel. Unlike the UFG IF steel, the UFG NbZr featured a distinct plateau of decreased hardness due to recovery at temperatures below the recrystallization limit. This was attributed to the absence of dynamic recovery during ECAP due to the low homologous temperature of Th=0.11 (Th=0.16 for IF steel) at room temperature processing.Strain-controlled elevated-temperature LCF tests performed in vacuum revealed stable cyclic deformation response up to 600°C (Th=0.32). At higher temperatures, but still below the static recrystallization limit (≈900°C, Th=0.43), cyclic softening, rapid decrease of mean stress and premature failure were observed. As compared to the UFG IF steel, cyclic stability is preserved up to higher Th due to the stabilizing effect of solid solution alloying elements, i.e. mainly Zr.In the case of the UFG IF steel, localized grain coarsening at the crack tip caused premature failure upon elevated-temperature LCF below the static recrystallization temperature. The more stable microstructure in the UFG NbZr did not show any localized alterations in the vicinity of the crack tip, but instead slightly coarsened throughout the whole gauge length.In combination with the results obtained on the UFG IF steel in previous studies, a comprehensive summary of the microstructural evolution of UFG bcc materials at elevated temperatures is presented.