Research On Preparation And Hydrogen Storage Properties Of Ti-Zr-Hf-Mo-Nb High Entropy Alloys

Tritium is the key fuel in nuclear fusion reactors.With the development of the International Thermonuclear Experimental Reactor(ITER)project,the annual requirement of tritium has increased up to several kilograms.The candidate materials for tritium storage have many shortcomings such as insufficient kinetic performance,disproportionation effect,poor oxidation resistance,and poor helium(He)retaining ability,etc.Therefore,it is urgent to develop a novel material system which satisfies all the requirements of tritium storage materials.High-entropy alloys(HEAs),which are composed of many main metal elements,have attracted much attention as hydrogen storage materials due to their high storing capacity and great hydrogenation performance.The distorted lattice caused by lattice distortion effect helps to provide more interstitial sites for accommodating H atoms and enhance the He retaining ability by slowing down the He diffusion in the HEA lattice.The precondition of developing new tritium storage HEA is to have a deep understanding of its hydrogen storage performance.Based on this,a new Ti-Zr-Hf-Mo-Nb HEA system was designed and prepared in this paper,and their hydrogen storage performance were systematically studied.The characterization of Ti_0.2Zr_0.2Hf_0.2Mo_xNb_0.4-x(x=0,0.1,0.2,0.3,0.4)HEAs showed that the HEAs system was a Body-Centered Cubic(BCC)single phase with no other precipitates,and the principal elements of Ti,Zr,Hf,Mo and Nb were evenly distributed.The study on the hydrogen storage performance of Ti-Zr-Hf-Mo-Nb HEA system mainly includes the hydrogen storage thermodynamic property,the hydrogen storage kinetic property,and the mechanism of phase transition in the process of hydrogen absorption and desorption(Ab/Des).The results show that the maximum hydrogen storage capacity of Ti_0.20Zr_0.20Hf_0.20Nb_0.40HEA is 1.5 wt.%at 300?.During the hydrogen Ab process of Ti_0.20Zr_0.20Hf_0.20Nb_0.40HEA from BCC to the hydride phase of Face-Centered Cubic(FCC),there existed a series of body-Centered Tetragonal(BCT)intermediate phase with different cell volumes.The presence of these BCT intermediate phase may be cause the increasing of platform pressure in the PCI curve.The platform pressure of Ti_0.20Zr_0.20Hf_0.20Nb_0.40HEA gradually increase with the enhance of hydrogen capacity.Ti_0.20Zr_0.20Hf_0.20Nb_0.40HEA has excellent hydrogen Ab/Des kinetics performace.It can exceed 84%/92%of the equilibrium hydrogen Ab/Des capacity with only 50 s/150 s.Before hydrogen absorption,the Ti_0.2Zr_0.2Hf_0.2Mo_0.1Nb_0.3 or Ti_0.2Zr_0.2Hf_0.2Mo_0.2Nb_0.2HEA is BCC phase,after completely hydrogenation they trainsform to FCC hydride phase.Nevertheless,their phase transition back to BCC after their hydride complete decomposition.Among them,the peak decomposition temperature of Thermal Desorption Spectroscopy(TDS)of Ti_0.2Zr_0.2Hf_0.2Mo_0.2Nb_0.2 HEA hydride is 302?,it is consistent with in situ heating X-ray Diffraction(XRD)results.It is concluded that when the temperature reaches to 300?,the desorption of Ti_0.2Zr_0.2Hf_0.2Mo_0.2Nb_0.2 HEA hydride began to happen,which led to the phase transition from FCC to BCC.The maximum amount of hydrogen released by Ti_0.2Zr_0.2Hf_0.2Mo_0.2Nb_0.2 HEA hydride is 1.18wt.%.The Ti_0.2Zr_0.2Hf_0.2Mo_0.3Nb_0.1 or Ti_0.2Zr_0.2Hf_0.2Mo_0.4 is BCC phase before hydrogen absorption,after completely hydrogenation their trainsform to BCT hydride phase.After all,their phase transition back to BCC when their hydride complete decomposition.In the last chapter of this work,the future research directions are prospected.