The microstructure of Alloy 625, which has undergone prolonged (ϳ 70,000 hours) service at temperatures close to but less than 600 ЊC, has been characterized by transmission electron microscopy. The precipitation of an intermetallic phase Ni 2 (Cr, Mo) with Pt 2 Mo-type structure has been observed in addition to that of the ␥" phase. Six variants of Ni 2 (Cr, Mo) precipitates have been found to occur in the austenite grains. These particles exhibit a snowflake-like morphology and are uniformly distributed in the matrix. They have been found to dissolve when the alloy is subjected to short heat treatments at 700 ЊC. The occurrence of the Ni 2 (Cr, Mo) phase has been discussed by taking the alloy chemistry into consideration. Apart from the intermetallic phases, the precipitation of a M 6 Ctype carbide phase within the matrix and the formation of near continuous films, comprising discrete M 6 C/M 23 C 6 carbide particles, at the austenite grain boundaries have been noticed in the alloy after prolonged service.

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15, 327 (1969).In recent years there has been a widespread interest in the possibility of producing metallic hydrogen in the laboratory 1 by means of a pressureinduced transition from the molecular phase. To determine the feasibility of such a process, it is important to have some estimate as to the pressure needed. Previous calculations 2 " 5 have produced estimates ranging from 0.25 to 20 Mbar, the lower pressure being relatively easy to reach in the laboratory, and the higher pressure out of reach at least statically. 6 Recent experimenters 7 " 8 achieving high pressures dynamically suggest that they may have detected the transition at pressures of 2.0 to 2.8 Mbar. In this Letter we present preliminary results which are obtained from calculations more rigorous than those previously performed, and which support the experimental evidence of a possible transition.Theoretical estimates of the pressure required for the transition at zero temperature are obtained from the common tangent to the energyvolume curves for the two solid phases. Most of the previous estimates have been based on fairly similar equations of state for the metallic phase, but have used widely different and less reliable equations of state for the molecular phase. In fact, the metallic-phase equation of state recently obtained by Neece, Rogers, and Hoover 2 from the self-consistent calculation, using the Kohn-Sham local potential to approximate exchange and correlation effects, is not substantially different from the approximate cellular calculation by Wigner and Huntington 3 in 1935. The molecular -J. I. Pankove, J. E. Berkeyheiser, and E. A. Miller, J. Appl. Phys. 45, 1280 (1974). 7 R. H. Fowler and L. Nordheim, Proc. Roy. Soc. (London) U9, 173 (1928). 8 J. I. Pankove and M. A. Lampert, Phys. Rev. Lett. 33, 361 (1974). in-phase equation of state has been obtained by solvly-ing the Bethe-Goldstone equations using an apre-proximate analytically fitted curve for the H 2 -H 2 To interaction potential. 5 Forms such as the Lenis nard-Jones 6-12 or 6-8 potentials were used, 3-with parameters obtained either from experimen-)-tal virial coefficients and viscosities for molecular hydrogen, or from variational calculations of ich the interaction of two H 2 molecules. 9 The moof lecular-phase equation of state varies sharply rs 7 " 8 with the choice of parameters, with the result that estimates of the transition pressure based res-on slightly different H 2 -H 2 pair potentials vary 3-from 0.84 to 4.2 Mbar. 10 A main difficulty with om all the previous work is that the metallic and sly molecular crystals are not treated in an internal-L1 ly consistent degree of approximation. The calculations presented here are based on •ed the techniques developed by Harris and Monkhorst 11 for the computation of Hartree-Fock wave functions and energies. In their method each valof ence Bloch orbital 1ST) is expanded according to •lyW=E m cjft\k m ), (l) 3 where the | ET m ) are basis Bloch functions with Bloch wave vector fiT: ntly d the |£J = exp(i£.r)£2>Jr-J$ p -5"). ...

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