Effects of β-cooling rates on microstructural characteristics and hardness variation of a dual-phase Zr alloy

In this study, a typical dual-phase Zr alloy (Zr-2.5Nb) was subjected to beta-solution treatment at 1000 degrees C for 10 min and then cooled down to room temperature at different rates (in water (WC), air (AC) and furnace (FC)). Microstructural characteristics of the specimens were thoroughly analyzed by jointly using electron backscatter diffraction (EBSD), electron channeling contrast (ECC) imaging, X-ray diffraction (XRD) techniques and transmission electron microscopy (TEM). Specimen hardnesses were measured by a Vickers indentation tester and well correlated with the revealed microstructural characteristics. Results show that the initial dual-phase microstructure is replaced by twinned martensite, basket-weave structure and lenticular Widmanstatten structure after water cooling, air cooling and furnace cooling, respectively. Internal twins in the WC specimen are determined to be {10 (1) over bar1} compressive twinning, while inter-plate films in AC and FC specimens are Nb-enriched residual beta phases. Orientation analyses show that the a phase exhibits the Burgers misorientation characteristics in all the beta-cooled specimens and a single beta orientation can give birth to all 12 alpha variants at relatively high cooling rates (both in water and air). Hardness analyses reveal that faster cooling always results in higher hardness, increasing from 216.5 HV of the FC specimen to 285.9 HV of the WC specimen (harder than the asreceived material (221.9 HV)). Such variation is related to hardening contributions from specific microstructural (grain refinement, nanotwins, and solid solution) and orientation characteristics (angles between c-axes of alpha grains and the loading direction).

Automated summary

The study found that under different cooling rates, the microstructure and hardness of a dual-phase Zr alloy undergo significant changes, with water cooling resulting in twinned martensite, air cooling resulting in basket-weave structure, and furnace cooling resulting in lenticular Widmanstatten structure. Furthermore, hardness increases with faster cooling rates.