Influence of alloying and microstructure on the electrochemical hydriding of TiNi-based ternary alloys

Nanostructured ternary TiNi-type alloys, namely Ti0.8M0.2Ni (M = Zr, V), TiNi0.8N0.2 (N = Cu, Mn) and TiNi1-x Mn (x) (x = 0.2, 0.4, 0.6, 1.0), were synthesized by mechanical alloying. Depending on the intensity and time of milling alloys with different microstructure were obtained. The as-milled TiNi1-x Mn (x) alloys contain substantial amount of amorphous phase, which crystallizes during annealing. Annealing of the as-milled fine nanocrystalline materials at 500 degrees C results only in slight coarsening of the microstructure, which remains still nanocrystalline. Fully crystalline material (with crystal size larger than 50 nm), consisting of mainly cubic TiNi was obtained by annealing the ball-milled alloys at T >= 700 degrees C. Electrochemical hydrogen charge/discharge cycling of the as-milled as well as of annealed alloys were carried out at galvanostatic conditions. It was found that among the nanocrystalline Ti0.8M0.2Ni0.8N0.2 (M = Zr, V; N = Cu, Mn) alloys TiNi0.8Mn0.2 revealed the highest discharge capacity of 56 mAh g(-1) in the as-milled state and 75 mAh g(-1) after short-time annealing at 500 degrees C. Annealing at higher temperature does not increase the capacity further. The as-milled TiNi1-x Mn (x) alloys with x <= 0.4 reveal noticeably higher discharge capacity and better cycle life than the Mn-richer alloys. Based on potentiostatic experiments the diffusion coefficients of hydrogen into TiNi alloys in two different microstructural states (fine and coarser nanocrystalline) as well as in as-milled amorphous/nanocrystalline and nanocrystalline TiNi0.8Mn0.2 were determined. The hydrogen diffusion coefficients of the TiNi alloys are comparable (1.9-2.7 x 10(-12) cm(2)s(-1)). The diffusion coefficient in the as-milled amorphous/nanocrystalline TiNi0.8Mn0.2 was found to be 3-4 times higher than that of the as-milled nanocrystalline alloy.