Effects Of Boron On Microstructures And Mechanical Properties Of Zr₃Al-based Alloys

In order to improve brittleness of ZrAl intermetallic compounds at room temperature and obtain good cooperation of strength and ductility of ZrAl-based alloys, this paper mainly studied the effects of trace element boron on microstructures and mechanical properties of Zr-6Al-xB and Zr-8Al-xB（x=0wt%、0.02wt%、0.05wt%、0.10wt%、0.15wt%、0.20wt%）. Refined microstructure was expected to be obtained by the addition of boron, and room-temperature ductility was desired to be noticeably improved while guaranteeing the high strength. By changing boron amount, the effects of boron on microstructures and mechanical properties were systematicly researched, and the best boron content was optimized at the same time. ZrAl alloys with high strength and good ductility were thus achieved.Raw materials were respectively smelt in high vacuum non-consumable electric arc furnace and vacuum induction levitation furnace. Then the ingots annealed at 750℃ and 800℃ for 4h after rolling and forging. The phase structures of as-cast and annealed alloys were detected and analyzed by X-ray diffraction, and samples in different processing conditions were tested and observed via optical metallographic microscope. The results showed that as-cast alloys Zr-6Al-xB and Zr-8Al-xB which consist of αZr and Zr2 Al, have basically completed the peritectoid transformation βZr+Zr2Al?Zr₃Al after separately annealing at 750 and 800 for 4h,℃ ℃ and obtained the phase structure of αZr and Zr₃Al. The addition of B element had no dramatical influence on peritectoid transformation and phase structures.Metallographic examination indicated that in annealed alloys Zr-6Al-xB and Zr-8Al-xB, the average grain size of Zr₃Al decreased at first and then increased with the increasing B. As B content reached 0.1wt%, the raw grain size of Zr₃Al refined to the minimum from 400⁶00μm to 20⁵0μm. Therefore the alloying element B was the effective approach to refining ZrAl grain size.Mechanical test showed that under different conditions of hot working（rolling or forging）, tensile strength and ductility performed increase previously and decrease later in annealed alloys Zr-6Al-xB and Zr-8Al-xB. When B content was less than 0.1wt%, tensile strength and ductility increased gradually with the increasing B content; When B content was 0.1wt%, tensile strength and ductility reached the maximum; As B content was more than 0.1wt%, tensile strength and ductility decreased for the continue increasing B. Hence, 0.1wt%B was the best boron content to improve the tensile strength and room-temperature ductility of Zr-6Al-xB and Zr-8Al-xB alloys. Under the guidance of this principle, Zr-6Al-0.1B possessed good strength and better ductility which respectively reached 1173.05 MPa and 18.62%, through annealing at 750℃.TOF-SIMS result indicated that in the 750 ℃annealed alloy Zr-6Al-0.1B, boron that located in grains was almost zero, and mainly segregated in grain boundaries. Hence, it can be inferred that B gathered in grain boundaries, reduced the interface energy and made boundaries more stable. In this case, it increased the migration resistance of grain boundaries and refined the grain size, which leading to significant improvement of strength and room-temperature ductility of ZrAl alloys.