Preparation,Structure And Properties Of Boron Containing Zirconium Alloys

Zr and Zr alloys have potential application prospects in aerospace,energy,marine and biomedical industries due to their small thermal neutron absorption cross-section,oxidation resistance,anti-irradiation,corrosion resistance,small expansion coefficient,low density and many other excellent chemical and physical properties.However,the low strength of existing Zr alloys limits their large-scale engineering applications.Therefore,the development of new Zr-based alloys with higher strength is of significant importance.In this paper,a series of Zr-B binary alloys and Zr-Ti-B ternary alloys were prepared through different processes.The corresponding relationships among alloy composition,phase constitution,microstructure and mechanical properties were studied systematically.The as-cast Zr-B binary alloys were produced by the vacuum non-consumable electro-arc furnace and the electro-magnetic induction melting furnace.Microstructural observation indicated that both the prior-βgrain and theα-lath size were significantly refined with increased B content.The refinement of prior-βgrain was attributed to the constitutional undercooling,which was resulted from solute enrichment of B in the liquid-solid interface.The refinement of prior-βgrain will contribute to more grain boundaries acting as heterogeneous nucleation sites forαphase and reduce the growth space,and then result in the refinement ofαphase.Analysis of the microstructure of the hot-rolled Zr-B alloys shows that the hot-rolling process led to the alignment of the elongated prior-βgrain boundaries,theαlaths and the ZrB₂ phases that are predominantly parallel to the rolling directions.Meanwhile,the hot-rolling process also caused the breaking of some high-aspect ratio Zr B₂ phase into shorter segments.The hot-rolled Zr-0.8B（wt.%）alloy exhibited the maximum ultimate tensile strength,which increased by 28%compared with that of the hot-rolled pure Zr without B.The ZrB₂ phases and the grain refinement were considered to be responsible for the strengthening of the hot-rolled Zr-B alloys.With the aim to develop new Zr-based alloys with excellent mechanical properties,the Zr-0.8B alloy was manufactured through the following steps:casting,forging,hot-rolling and annealing treatment.The tensile strength of the annealed Zr-0.8B alloys was decreased with increase of annealing temperature,for the temperature values below 800℃.However,the ductility showed a contrary tendency.After annealing at 800℃,the room-temperature ultimate tensile strength and the elongation-to-fracture of the Zr-0.8B alloy were 691.91MPa and 21.63%,respectively.The strength of the annealed Zr-0.8B alloys were controlled by many factors,including the volume fraction of ZrB₂ phases,grain size,dislocation density,the amount of subgrains and solution strengthening.Through the research on the Zr-Ti-B ternary alloys,we could find that the enhancement of the strength of the Zr-Ti-B ternary alloys was mainly attributed to the solution strengthening of Ti and the presence of reinforcement phases.Both the Zr B₂phase and the TiB phase could be formed in the（50Zr-50Ti）-xB alloys.However,the EDS results revealed that the ZrB₂ phase dissolved some amount of Ti and the TiB phase dissolved some amount of Zr.The microstructure of the as-cast Zr-Ti-B ternary alloys and the size,morphology,orientation,and distribution of the reinforcement phases could be altered significantly by the hot-rolling process.The ultimate tensile strength and the elongation-to-fracture of the Ti-25Zr-1B alloy were 1000MPa and 8.82%,respectively.The hot-rolling process provides increases in ultimate strength of at least 7 pct and increases in elongation-to-failure of 135 pct relative to the as-cast sample.In comparison with pure Zr,boron containing Zr alloys have better microstructure stability.The reinforcement phases in the boron containing Zr alloys can pin grain boundary effectively to prevent the grain from growing at high temperature.The strengthening mechanism of the boron containing Zr alloys should include solution strengthening,fine-grain strengthening,load transfer strengthening and the dislocation strengthening.In addition,the increased strength after B addition usually comes at the price of decreased ductility.Therefore,it is necessary to control the B content when the boride reinforcement phases were applied to enhance the strength of Zr alloys.