Existing thermodynamic data for the Cr‐N system were analyzed. High‐temperature heat contents measured by Sato were reevaluated, and the heat capacity of Cr2N was determined to be 15.05+6.58×10−3T (°K) and that of CrN 11.10+ 1.58×103T. Using these heat capacities and an estimated ΔS°298, f of 20×1 cal/°K‐g atom N for the formation of the nitrides, second‐ and third‐law calculations for all available vapor‐pressure data were made. The two calculations agreed very well for the data of Mills. The heats of formation of Cr2N and CrN are ‐31.8×1.0 and ‐28.4×1.5 kcal/mol, respectively. A partial phase diagram of the Cr‐N system is presented.
629ing strength trend occurs for the rods after shocks of 200 and 250C" AT. Statistical analysis indicates that this increasing strength trend is not significant; it may, however, be a reflection of the crack geometries. (Crack depth and density data are given in Table I.) The crack depths decrease between 200 and 250C" AT, and this trend is significant; it is related to the increase in crack densities and the strain energy available to drive the cracks. With shocks greater than 25OCO AT the cracks are driven deeper, and the strength decreases accordingly.The strength data dispersions (Fig. 2) of samples shocked a t 150C" AT do not differ significantly from those of the control group; however, the dispersions of the samples shocked at 175C0 AT increase appreciably. At shock levels above 175C" AT, however, there is a marked decrease in the dispersions for the rods and an appreciable decreasing trend in the data for the short cylinders.Daniels and Moore' induced flaws perpendicular to the direction of maximum bend stresses. They observed a marked decrease in data spread after the first flaws were introduced. The initial increase in dispersion noted in the present work can be explained as follows: At low shock levels there is a low density of randomly oriented and unconnected surface flaws; as the crack network becomes denser with increased shock, it provides, through interconnections, flaws which are effectively oriented perpendicular to the maximum stress. Thus, with higher shock levels and hence higher crack density levels, the dispersion data behave in a manner similar to that reported by Daniels and Moore.In these two studies, strength data dispersion decreased with increase in the density of the induced flaws, which in both instances were rather gross. This behavior is suggested by statistical theories of strength2 based on least value statistics. The flaw densities involved in this and the earlier work are low enough to cause the observed statistical behavior.' W. H. Daniels and R. E. Moore, "Fracture Behavior of a Model Brittle Solid Containing Artificial Surface Flaws," J . Am. Ceram. SOC., 48 [5] 274-75 (1965). recent diffusion measurements have been made with M radioisotopes.' In the present work, the diffusion rate of P b in a Pb0.2B203 melt and glass was measured using a thin sectioning technique and a stable isotope of lead, "'Pb, which was detected with a mass spectrograph. The toxic characteristics of the fission products of the radioisotopes of lead made the use of a stable isotope desirable.The glass was made from reagent-grade lead tetraoxide and boric acid. Chemical analysis showed that the glass contained 33.37 mol% PbO. The samples were made by pouring the molten glass into preheated aluminum oxide tubes 1 in. high and 1 in. in diameter with a wall in. thick. The samples were carefully annealed to avoid stress buildup between the alumina collar and the glass. The collar was used to prevent the glass from slumping during the long diffusion anneals above the glass transition temperature. After ...
The temperature dependence of the rate of growth of PbO/2B2O3 from lead borate melts with PbO/B2O3 mole ratios of 1/3,1/2, 2/3, and 1 was determined. The maximum growth rates occurred from those melts which contained a slight excess of PbO. The results are explained on the basis of a modification in the structure of the melt in the interfacial zone as the PbO content is changed. Such changes markedly affect the fluidity of the melt, alter the concentration of crystallizable species, and may reduce the driving force for crystallization.
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