Elastic constants of single crystals of yttria-stabilized zirconia were determined through the temperature range 20" to 700°C. Crystals containing 8.1, 11.1, 12.1, 15.5, and 17.9 mol% YzO3 were measured. The elastic constant CI1 was found to decrease and CI2 and C, to increase with increasing YzO, content; this appears to be due to decreasing coulombic interaction as Y" replaces Zr"'. Except for the 8.1 mol% Y203 crystal, the conventional elastic constants all showed normal monotonic decreases with increasing temperature. In the case of the 8.1 mol% YzO, crystal, measurements as a function of temperature were not reproducible, and it is likely that this composition at room temperature is below the composition limit of thermodynamic stability of the cubic fluorite phase.
The six independent elastic constants of single crystals of tetragonal MgF2 have been measured over the temperature range 4.2–300 K. All six of the conventional constants show normal negative temperature dependences. The shear constants associated with normal modes of deformation tend to be weak, and one of these, (1)/(2) (C11−C12), also exhibits a weak positive temperature dependence as do all other fluorides with the rutile structure for which data are available. The data indicate a high degree of anisotropy in the atomic force interactions. A Debye temperature evaluated from the single-crystal elastic constants when combined with two empirical Einstein temperatures satisfactorily describes tabulated heat-capacity data to within 1%.
The age hardening behavior of the commercial 2024 aluminum alloy was the interest of the present investigation. The mechanical properties of alloys in the two temper conditions; T6 and T86, were susceptible to the aging processes. Artificial aging at different temperatures appreciably improved both hardness and tensile properties of the present alloy, however, at the ductility. Strain introduced by stretching prior to the precipitation hardening process further increased the hardness and tensile parameters. This alloy is characterized by a high ratio of yield to tensile strength. In the T6-temper condition, a maximum tensile strength of 482 MPa and a yield strength of 444 MPa were attained after 12h aging at 190°C. In addition, the alloy in the T86-temper condition attained even higher tensile strength of 506 MPa and a yield of 467 MPa after aging only for 6h at the same aging temperature. The initial age hardening was found to be a diffusion-controlled process with an activation energy of 0.83 ± 0.01eV for the T6-temper and 0.90 ± 0.01eV for the T86-temper. The present precipitation hardening treatments have, however, a deleterious effect on both ductility and notch toughness.
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