Improving strength-ductility synergy in a TRIP metastable β-Zr alloy containing heterogeneous α precipitates

In this study, a simple two-step heat treatment is proposed to design a TRIP metastable β-Zr alloy with good strength-ductility synergy by introducing heterogeneous α precipitates. The heterogeneous α precipitates mainly consist of necklace-like α at β grain boundaries and a small amount of randomly distributed intragranular α. Compared with the sample consisting of single β phase with yield strength of 683 MPa and ductility of 14.6%, yield strength of the designed two samples with heterogeneous α precipitates increases to 781 MPa and 842 MPa respectively, while the ductility increases to 15.3% and decreases to 13.6% respectively. The microstructure analysis shows that the dominant deformation mechanisms of the sample containing single β phase are β to α′ martensitic transformation and domaination of α′ martensite, accompanied by kinking band, and {101¯1}_α′ twinning. For samples with heterogeneous α precipitates, the precipitation of α will increase the triggering stress of the kinking band, resulting in an increase in the yield strength at early stage of deformation. More importantly, the unique heterogeneous α structure brings a gently cutting effect of the necklace-like α on β grains and a traversability of the intragranular α to the growing α′ martensite, making the β to α′ martensitic transformation and domaination of α′ martensite slightly inhibited, together with the absorption and transfer of plastic deformation energy by α precipitates endow the alloy with high ductility. The present work can provide inspiration for improving the strength-ductility synergy of metastable β-Zr or β-Ti alloy through heterogeneous precipitate structure design.