In situ high temperature XRD studies of the thermal behaviour of the rapidly quenched Mg77Ni18Y5 alloy under hydrogen

The thermal behaviour of the rapidly quenched and electrochemically hydrogen-charged Mg77Ni18Y5 alloy was studied by means of in situ high temperature X-ray methods. Under high vacuum conditions and in hydrogen atmosphere up to 0.5 MPa without pre-charged hydrogen, the crystalline phase formation is observed at 130 < T A < 150 deg. C. A Mg2-xNi1-yY x+y phase and the metastable Mg6Ni phase are formed. Pure Mg is detected at T A = 200 deg. C. Mg6Ni decomposes at T A > 250 deg. C into Mg2Ni and Mg and Mg2-xNi1-yY x+y into Mg2Ni and YNi. A gas-solid reaction with hydrogen in the X-ray chamber could not be observed. Samples electrochemically hydrogen-charged with 0.3 < [H] < 0.6 wt% undergo the same phase formation during heat treatment. The formation of Mg6Ni is inhibited by an increasing hydrogen concentration [H] in the ribbons. Yttrium phases are observed only at T A > 250 deg. C. The grain sizes of the thermally formed phases decrease with increasing hydrogen content in the as-quenched samples caused by the hydrogen in the grain boundary regions around the Mg2-xNi1-yY x+y particles. The galvanostatic pre-charging with [H] 1.42 wt% leads at 250 deg. C under a hydrogen atmosphere of 0.5 MPa to the formation of the fcc complex Mg2NiH4 besides Mg2NiH0.2-0.3, YH2, YH3 and MgH2. Under desorption conditions (60 kPa hydrogen), the decomposition of the Mg2NiH4 phase to Mg2Ni and hydrogen at T A > 250 deg. C is detected. The sorption process is reversible