Dilute Solution of H in bcc Ti 35 Cr 65-x V x Alloys
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1042-S03-26
Dilute Solution of H in bcc Ti35Cr65-xV x Alloys Giovanni Mazzolai Department of Physics, University of Perugia, Via A.Pascoli 5, Perugia, 06123, Italy ABSTRACT The solubility of H in the solid solution phase of the Ti35Cr47V18 alloy has been investigated at high and low temperature. The Sieverts law has been found to hold for 808 K≤T≤1204 K and fail for 302 K≤T≤335 K. For H contents nH ≤0.07 (nH=H/Me) the partial molar enthalpy and partial non-configurational entropy at high temperature are: ∆H H0 = −21 ± 2
kJ J nc ; ∆S H = −84 ± 2 . mol mol ⋅ K
On heating the material under H2 gas pressures lower than 0.1 MPa H absorption only occurs above 700 K. Below this temperature, surface oxide films prevent H uptake. Thermal decomposition spectroscopy of the α phase carried out for nH≤0.24 exhibits a single peak whose temperature increases with decreasing nH owing to selective occupancy by H of energetically non-degenerate interstitial sites.
INTRODUCTION The bcc TiVCr alloys have attracted a lot of attention in recent years due to their potential use as H storage materials [1-9]. With increasing the H content these alloys, first transform from the solid solution (α-phase) to a bcc hydride (β−phase) and finally to an fcc hydride (γ−phase) [6,7]. The P-C-T isotherms have usually been investigated for values of nH higher than about 0.5 MPa and the partial molar enthalpy and entropy for the β→ γ transition have been measured [9]. Little work, instead, has been carried out on the α−phase and on its reaction with the H2 gas. To the best of our knowledge no determination has been made of the thermodynamic features of this reaction. Therefore, the main aims of the present work were the determination of the enthalpy and entropy for the α-phase formation and the characterization of its thermal decomposition in the Ti35Cr47V18 alloy. EXPERIMENTAL DETAILS The alloy was prepared by arc melting under argon atmosphere, then annealed at 1273 K for 1 h and, finally, tested at room temperature by X-ray diffraction, which showed a homogeneous bcc solid solution of the constituent elements. Three samples were used whose characteristics are summarized in Table 1, together with the sequence of treatments subsequently given to the samples. Hydrogen was introduced into the material by the gas-phase/solid-surface reaction and its content nH (nH=H/Me) was determined volumetrically.
Table 1. Characteristics and history of the samples. Alloy
Sample number
Mass (g)
1
0.1472
2
0.0514
Ti35Cr47V18
3
0.2942
Thickness Series of treatments subsequently undergone by (mm) the sample 1. Surface activation cycles, followed by TDS measurement. 1.07 2. Series of absorption isotherms at increasingly higher pressures at 302 K, 310 K and 335 K, followed by TDS measurement. 1. Series of absorption isotherms (873K) at 0.56 increasingly higher pressures. Series of absorption isotherms at increasingly 0,57 higher pressure at 1204 K, 1017 K, 873 K and 808 K.
The calibrated experimental chamber was evacuated by a pumping system composed of rot
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