Formation of the B2-ZrCo phase and micro-hardness evolution in Zr-Co-Al BMGs via conventional and flash annealing

As a promising strategy to improve the ductility and work-hardening, the generation of bulk metallic glass (BMG) composites embedded with a B2 phase has sparked a great interest. In this study, we systematically explored the influence of annealing parameters on formation of the B2-ZrCo phase and micro-hardness evolution, and thus, we summarized the optimal annealing processes which can maximize the B2-ZrCo phase formation and maintain the glass matrix. We examined the microstructures of those annealed alloys via XRD, SEM plus EDS, DSC, FIB, TEM and Struers Durascan microhardness tester. Firstly, the amorphous alloys of Zr55Co31Al14 and Zr56Co28Al16 with 3 mm in diameter were heated to 753 K and 843/853 K for various times at a heating rate of 20 K/min via DSC and then naturally cooled to room temperature. It is found that for annealed Zr56Co28Al16 alloys, the ZrCoAl phase is the dominant phase regardless of the annealing temperature and time, while for annealed Zr55Co31Al14 alloys, at lower annealing temperature, the ZrCoAl phase is the dominant phase, whereas with increasing of annealing temperature and holding time, the B2-ZrCo phase is the dominant phase. In order to further understand the influence of annealing temperature on B2-ZrCo phase formation, we transiently heated those two amorphous alloys to 923 K and then naturally cooled to room temperature within DSC. It is found that the B2-ZrCo phase is still the dominant phase for annealed Zr55Co31Al14 alloy, while the ZrCoAl phase is still the dominant phase for annealed Zr56Co28Al16 alloy. However, the conventional annealing processes have notable disadvantages and significantly limit the retaining of amorphous matrix content due to its low heating rate. Based on the above analysis, the amorphous Zr55Co31Al14 alloy is a promising candidate to fabricate the Zr-Co-Al BMG composite incorporating high volume fraction of B2-ZrCo phase. The flash annealing was used to transiently heat the glassy Zr55Co31Al14 alloy at a heating rate of 1, 5 and 15 K/s via Gleeble. It is found that higher heating rate like 15 K/s can selectively promote formation of the B2-ZrCo phase and suppress the ZrCoAl phase and maintain a large volume fraction of amorphous content. As for the evolution of micro-hardness, we found that for all annealing conditions, the micro-hardness increases first and then decrease, and the micro-hardness of annealed Zr56Co28Al16 alloy is higher than that of annealed Zr55Co31Al14 alloy. The mixing enthalpy between the Zr-Al benefits the formation of a highly dense random packed structure which enhances the micro-hardness. The mechanism of micro-hardness evolution has been discussed in terms of volume fraction and types of crystalline phases, annihilation of free volume, transformation-induced hardening. (C) 2020 Published by Elsevier B.V.