On high creep activation energies for dispersion strengthened metals

Creep studies on dispersion strengthened metals have often resulted in inexplicably high creep activation energies and high stress exponents. An analysis is presented which shows that the contribution of the temperature dependent elastic modulus to the apparent activation energy can be very large. Two cases are examined in which the difference between the apparent activation energy and the activation energy of self diffusion is almost entirely accounted for by the contribution of the temperature dependent elastic modulus. The modulus correction is particularly important at high temperatures and for materials with high stress exponents.     

On high creep activation energies for dispersion strengthened metals

Creep studies on dispersion strengthened metals have often resulted in inexplicably high creep activation energies and high stress exponents. An analysis is presented which shows that the contribution of the temperature dependent elastic modulus to the apparent activation energy can be very large. Two cases are examined in which the difference between the apparent activation energy and the activation energy of self diffusion is almost entirely accounted for by the contribution of the temperature dependent elastic modulus. The modulus correction is particularly important at high temperatures and for materials with high stress exponents.     

Determination of standard gibbs energies of formation of CaC2, SrC2, and BaC2

The standard Gibbs energies of formation of CaC₂, SrC₂, and BaC₂ were determined by an equilibration technique at temperatures ranging from 1223 to 1673 K, yielding the following results: Ca (l)+ 2C (s)= CaC₂\GDG\dg = -90,100(\+-500)- 11.0(\+-1.1)T [J/mol]Sr (l) + 2C (s) = SrC₂ (s)\GDG\dg = -104,000(\+-1000) - 8.04(\+-2.76)T [J/mol]BA (l) + 2C(s) = BaC₂ (s)\GDG\dg = -115,000(\+-1000) + 3.53 (\+-1.69)T [J/mol]     

Kinetics and mechanism of the desulfurization of liquid blast furnace slags by Ar-H2O gas mixtures

The rate of evolution of sulfur-bearing gases from liquid silicate slags at 1400 ‡C when exposed to Ar-H₂O gas mixtures has been studied. The rate was first order with respect both to the concentration of sulfur ions adsorbed on the surface of the slag and to the partial pressure of water vapor when P_{H 2}O was greater than 0.15 atm. When the partial pressure of water vapor was less than 0.15 atm, the rate was second order with respect to the concentration of sulfur ions adsorbed on the surface early in the reaction. At longer times in the latter case, the rate was independent of the partial pressure of water vapor, but first order with respect to the concentration of sulfur ions adsorbed on the surface. It was concluded, based on the kinetics of desulfurization, that the sulfur-bearing species evolved from the surface of the slag was S₂ when the rate was second order with respect to the concentration of adsorbed sulfur ions, but SO₂ was evolved when the rate was first order with respect to the concentration of adsorbed sulfur ions. Under the conditions employed in the experiments, H₂S never evolved from the surface of the slag, although it did form, in some instances, in the gas phase. B. AGRAWAL, formerly a Graduate Student at Massachusetts Institute of Technology     

Calculation of the surface energies,energies of rupture,and theoretical strengths of cubic monocarbides

Conclusions An analysis is made of several thermodynamic methods of calculation of surface energy and of bond models for cubic monocarbides of transition metals. The specific surface energies of the carbides were calculated taking into account some characteristic features of atomic interactions in these compounds. In addition, the energies of rupture of the carbides are given, together with their estimated theoretical strengths. The variation of these properties with temperature is examined. Calculated results are compared with literature data.Translated from Poroshkovaya Metallurgiya, No. 1(157), pp. 70–74, January, 1976.     

Essential iris atrophy, pigment dispersion, and glaucoma in DBA/2J mice

PURPOSE: To characterize ocular abnormalities associated with iris atrophy in DBA/2J mice and to determine whether mice of this strain develop elevated intraocular pressure (IOP) and glaucoma. METHODS: Different approaches, including slit-lamp biomicroscopy, ophthalmoscopic examination, ultrasound backscatter microscopy, and histology were used to examine the eyes of DBA/2J mice ranging from 2 to 30 months old. IOP was measured in DBA/2J mice of different ages. RESULTS: DBA/2J mice were found to develop pigment dispersion, iris transillumination, iris atrophy, anterior synechias, and elevated IOP. IOP was elevated in most mice by the age of 9 months. These changes were followed by the death of retinal ganglion cells, optic nerve atrophy, and optic nerve cupping. The prevalence and severity of these lesions increased with age. Optic nerve atrophy and optic nerve cupping was present in the majority of mice by the age of 22 months. CONCLUSIONS: DBA/2J mice develop a progressive form of secondary angle-closure glaucoma that appears to be initiated by iris atrophy and the associated formation of synechias. This mouse strain represents a useful model to evaluate mechanisms of pressure-related ganglion cell death and optic nerve atrophy, and to evaluate strategies for neuroprotection.