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;.
