The Microstructure And Hydrogen Permeation Performance Of Nb-Hf-Ni(Co) Alloys

In order to explore the influence of the substitution of Ni by Co on the structureand hydrogen permeation performance of Nb-Hf-Ni alloy, as-cast structure of fourgroups of different composition of alloy system, namely NbxHf(100-x)/2Ni(100-x)/2(x=14...40) alloys marked as group S, Nb20Hf40CoxNi40-x(x=0,5...40) alloys marked asgroup A, Nb30Hf35CoxNi35-x(x=0,5...35) alloys marked as group B andNb40Hf30CoxNi30-x(x=0,5...30) alloys marked as group C, were analyzed by scanningelectron microcopy(SEM) and X-ray diffraction(XRD). Evolution of the as-caststructures with the change of composition of each group of alloy system wasstudied.and Hydrogen permeation experiments of the alloy chips with the potential ofhydrogen permeability were conducted under various temperatures and pressuredifferentials. The relationship between hydrogen permeability and composition ofNb-Ti-Ni(Co) alloys was investigated as well as the mechanisms of hydrogenpermeation and hydrogen embrittlenessThe position of the eutectic point of each group of alloy system was achievedthough the analysis of the phase composition and phase volume fraction of each groupof alloy system. With the increase of Co content, the eutectic point of alloy systemgradually moves in the direction of high Nb. And the content of impurities ofcompound phase in the alloy gradually reduce as well as the volume fraction of theprimary bcc-Nb phase. But the volume fraction of the bcc-Nb in the eutectic phaseincreases gradually with the increase of Co content. Meanwhile the eutecticmorphology evolves into the layer of lamellar eutectic from the rod-like eutectic.The alloy made up of primary bcc-Nb solid solution and the binary eutecticsphase or eutectic phase completely was used for hydrogen permeation experiment. Thealloy with low Nb content in group S is prone to hydrogen embrittlement at lowtemperature, while the alloy with high Nb content has good hydrogen permeability;The two alloys selected in group A occur hydrogen embrittlement at every temperature.The alloys in group B and C show the good perfoemence without hydrogenembrittleness. The hydrogen permeability coefficient was used to characterize alloy hydrogen permeability property and increase gradually with the increase oftemperature. The hydrogen permeability coefficient of alloys in group S, B, and Cshow a good linear relationship with the reciprocal of temperature1000T-1. Thehydrogen permeability coefficient of alloys in group B varies with the increase of Cocontent in the trend of increase, while decreases firstly and increases subsequently ingroup C.Bcc-Nb phase is the main path of dissolution and diffusion of hydrogen ratherthan compounds such as HfNi phase by comparing the XRD spectrum of the alloybefore and after hydrogen permeability experiment. Hydrogen permeation processfollows the dissolving and diffusion mechanism. The dissolving and diffusion behavioris mainly occurred in the bcc-Nb phase. The substitution of Ni by Co will increaseslightly the size of the tetrahedral gap in Nb metal and change the morphology of theeutectic phase, and then improve the hydrogen permeability coefficient of the alloys.Hydrogen embrittleness belongs to the cleavage fracture by analyzing the SE images ofthe Nb18Hf41Ni41alloy and Nb20Hf40Co5Ni35alloy.