The absorption isotherms of hydrogen in the cubic intermetallic phase Ti3Sb were measured in the range 400 ≤ 77°C ≤ 800; 0.001 ≤ pH2/bar ≤ 0.925. The relative partial molar enthalpy of hydrogen solution at infinite dilution was found to be ΔH0H/J(K mol)−1 = −55.0 ± 1.5. The relative partial molar excess entropy of the solution at infinite dilution has been calculated to be ΔSE,0H/J(K mol)−1 = −63 ± 1, assuming the number of available interstitial sites per metal atom to be 0.75. The results are compared with data on other similar titanium alloys.

Crystal Structure / Hydrogen in Metals / Metals / Phase Transitions / ThermodynamicsApplying the manometric method, the solubility of hydrogen in alloys Ti, _,yGa, was studied in the range 0 5 x 5 0.25; 5 . 5 pH,/bar 5 1; 773 5 T/K 5 1298. Within the given range of p and T the phase diagram of Ti, -,Ga,H, was investigated for 9 compositions x . In the range of solid solutions, 0 x 5 0.11, with increasing x the range Ar of the hcp phase a-Ti,_,Ga,H, increases slightly, whereas the range of the bcc phase P-Ti, -,Ga,H, narrows strongly with increasing x . The width Ar of the two phase region a + P is nearly independent of x and the phase boundary between 0-phase and (0 + y) region shifts with decreasing A r of the P-phase to smaller r. In a-Ti,-.Ga,H,, the enthalpy of hydrogen solution AHH is practically independent of r but depends slightly on x, decreasing from -46.0 kJ mol-' ( x = 0) to -46.5 kJ mol-' ( x = 0.10). In the bcc phase P-Ti, .,Ga,H,, AHH decreases markedly from -57 kJ mol-' ( r -+ 0) to -70 kJ mol-( r = 1). A H H is independent of the gallium content x within the limits of error. -Assuming a plausible blocking model for the occupancy of interstitial sites in the metal matrix by hydrogen, the excess entropies of dissolved hydrogen were evaluated. In a-Ti, -xGa,H, a n increase of ASP is found with increasing r from -55 JK-' mol-' ( r + 0) to -53 J K -' mol-' ( r = 0.1). ASE depends much stronger on r in P-Ti,_,Ga,H,:AS: = -70 J K -* mol-' (r + 0); AS: = -66.5 J K -' mol-' ( r = 0.8). There is negligible influence of x on ASE in both phases. -In the ordered hcp phase a,-Ti,GaH, there is an increase of exothermicity compared to a-Ti, -&ia,H,. AHH (r 4 0) is -51.3 kJ mol-' for the ordered phase and it depends on r, increasing to -50 kJ mol-' at r = 0.1. ASP for this phase is independent of r , AS: = -53 J K -' mol-'. -Additional data on the hydrogen solubility in a-Ti,Sb are given, resulting on more accurate thermodynamic data. AHH. ASH, and AS;.

Experimentelle Daten für in Ti und Ti‐Legierungen gelösten Wasserstoff werden im Sinne eines geometrischen Modells mit gegenseitiger (H‐ID‐Blockierung verglichen.

From the absorption isotherms in the ranges 935 ä T ä 1175 Kand 0.009 ä p(H2) g 1 bar AH° H = ‐42.2 kJ mol‐1 and a linear dependence of AHH on the hydrogen concentration are found.

Assuming a geometrical model with mutual (H‐H) blocking, for hydrogen dissolved in titanium and titanium alloys experimental data are compared. It is found that interstitial sites with pure titanium coordination only host hydrogen atoms. In alloys with bcc crystal lattices only tetrahedral interstitial sites are occupied, whereas in hep alloys both tetrahedral and octahedral interstitials host hydrogen. A linear relation is found which connects the partial molar absorption enthalpy ΔHOH and the partial molar excess absorption entropy ΔSE, OH at infinite dilution of dissolved hydrogen. The enthalpy of absorption ΔHOH can be rationalized as a function of interstitial hole size and an optimum hole radius of 47 pm is found for titanium and titanium alloys.