Study Of The Microstructure Evolution And Hydrogen Permeation Performance Of The Nb-Ni-Ti Ternary Alloys

As-cast structures of19different Nb-Ni-Ti ternary alloys were analysed by scanningelectron microcopy （SEM） and X-ray diffraction （XRD）. The19compositions of alloyswere chosen with the design concept of “Share function with the polyphase structures” ofNb-Ni-Ti hydrogen permeation alloys. Variation of the as-cast structures with the changeof composition of Nb-Ni-Ti alloys was studied, and the ideal composition range ofhydrogen permeation alloy was obtained consequently. A Bridgman-type device was usedto study the directionally solidified growth morphology under near equilibrium freezingcondition with different cooling rates of Nb₁₉Ni₄₁Ti₄₀alloy. Hydrogen permeationexperiments of8different Nb-Ni-Ti alloys were conducted under various temperaturesand pressure differentials. The mechanisms of hydrogen permeation and hydrogenembrittleness of Nb-Ni-Ti alloys were analysed.The variation of phase constitutions and volume fractions are different in variouskinds of Nb-Ni-Ti alloys. In the case of alloy compositions in Nb-NiTi pseudobinarysystem and the Nb atomic content is higher than19at.%—the phase constitutions ofNb-Ni-Ti alloys are consist of primary α（Nb） solid solutions and the binary eutectics[α（Nb）+NiTi]—which are the ideal composition range for hydrogen permeation.Theexperiments of directionally solidified Nb₁₉Ni₄₁Ti₄₀alloys demonstrate that with thecooling rate lower than5μm/s, planar interface morphology of binary eutectics[α（Nb）+NiTi] obtained. The alloy composition of Nb₁₉Ni₄₁Ti₄₀proved to be the eutecticpoint sequentially. The highest volume fraction of the α（Nb） phase in binary eutectics[α（Nb）+NiTi] obtained in the directionally solidified alloy of the cooling rate of3μm/s.With the increase of cooling rate, eutectic spacing decrease obviously.Four alloys of different compostions, Nb₁₉Ni₄₁Ti₄₀, Nb₂₂Ni₃₉Ti₃₉, Nb₂₄Ni₃₈Ti₃₈andNb₃₀Ni₃₅Ti₃₅, show the persistent hydrogen permeability without hydrogen embrittleness.While with the brittle phase NiTi2intermetallic compounds embedded in structures,Nb₂₅Ni₃₀Ti₄₅, Nb₂₅Ni₃₅Ti₄₀, Nb₂₅Ni₄₀Ti₃₅and Nb₂₅Ni₄₅Ti₃₀alloys occur hydrogenembrittlement during the hydrogen permeation experiments. Correctness of the idealhydrogen permeation structure constitutions of α（Nb） solid solutions and the binaryeutectics [α（Nb）+NiTi] is confirmed in experiments. The results of hydrogen permeationexperiments show that hydrogen flow （J·L） permeates through the Nb-Ni-Ti alloy membranes showing a good linear relationship with the square root of the pressuredifference between two sides of the alloy memberane (Pu^0.5–Pd^0.5). The hydrogenpermeability of Nb-Ni-Ti alloys increase with the Nb content and temperature raising.Arrhenius equation of the relationship between hydrogen permeability and temperaturecan be fitted in different Nb-Ni-Ti alloys. The permeability of Nb₃₀Ni₃₅Ti₃₅alloy under673K is the same as pure Pd in same temperature, while all the Nb-Ni-Ti alloys showlower hydrogen permeability under lower temperatures.Hydrogen permeation process follows the rules of dissolving and diffusionregularity. It consists of dissolving and adsorption, atom diffusion and desorption.Hydrogen embrittleness belongs to the transgranular cleavage fracture. Precipitation ofhydride phases inside the solid solutions, which can decrease the interatomic bondingforce, and the adsorption of hydrogen on crack tips, which damage the rupture strength,are two reasons for hydrogen embrittleness.