Roger Chang

Low‐Temperature Elastic Properties of ZrC and TiC

The elastic constants of zirconium carbide and titanium carbide were determined over the temperature range of 4.2° to 298°K using a pulsed‐ultrasonic phase comparison technique, and are compared to previously determined values for uranium carbide. Since these carbides are of the NaCl structure, only three independent constants are required to describe their elastic behavior. Their values at 298°K are ZrC0.94TiC0.91C114.720±0.0055.145±0.005C120.987±0.0051.060±0.002C441.593±0.0021.788±0.002 all in units of 1012 dyn/cm2. The differences in the elastic behavior of these group IV, transition metal carbides and the monocarbide of uranium are discussed in reference to their different electronic structure.

High temperature creep and anelastic phenomena in polycrystalline refractory oxides

Abstract High temperature creep and anelastic phenomena in polycrystalline Al 2 O 3 and BeO were studied. The activation energies for both “steady-state” creep and grain boundary relaxation are approximately 2.0 × 10 5 calories per mole for Al 2 O 3 and 1.2 × 10 5 calories per mole for BeO. Addition of small amounts of Cr 2 O 3 or La 2 O 3 to Al 2 O 3 and of MgO to BeO introduces additional grain boundary relaxation peaks at lower temperatures and reduces grain boundary viscosity. The beneficial effects of these additives in improving the high temperature ductility of refractory oxides are briefly discussed. A correlation between the experimentally observed “steady-state” creep rates and those calculated according to the Nabarro-Herring mechanism from self-diffusion data for BeO suggests that the Nabarro-Herring mechanism may predominate in the creep of polycrystalline BeO and possibly other oxides at high temperatures and low stresses. Further verification of the observation is needed.

Screw dislocation core structure in body-centred cubic iron

Abstract The core structure of a screw dislocation with Burgers vector along [111] in body-centred cubic iron was evaluated numerically using an anharmonic potential matching the second and third-order elastic properties of the material. The core radius is found to be 3 to 4 A and the corresponding core energy per identity distance along the dislocation line (three atom planes) is 0.25 to 0.35 ev. The core structure consists principally of three planar imperfections, corresponding to three intrinsic faults (of atomic dimensions) of the type 123456/56/123456 discussed by Frank and Nicholas (1953). There is a maximum energy barrier of 0.07 ev for motion of the screw dislocation in the [112] direction. The barrier is asymmetric and corresponds to ‘Peierls’ stresses at absolute zero of 4.1×109 dynes/cm2 and 2.7×109 dynes/cm2, respectively, for motion of the screw dislocation in the [112] and [112] directions.

Flow and Recovery Properties of Nearly Stoichiometric Polycrystalline Uranium Carbide and the Mechanism of Work Hardening of Crystalline Solids

The flow and recovery properties of nearly stoichiometric polycrystalline uranium carbide were studied in the temperature range 1500° to 1900°C at strain rates varying between 2×10−5/sec to 2×10−3/sec. The flow data seem to follow a relationshipe≅Kσ5exp(−37 500/RT), wheree is the strain rate, K a proportionality constant, R the gas constant, T the absolute temperature, and σ the flow stress at strain =0.001. A mathematical evaluation from recovery data of the activation volume associated with dislocation motion is presented and its potential to yield basic information on the mechanism of work hardening is discussed.

RELATIONSHIPS BETWEEN THE NONLINEAR ELASTIC CONSTANTS OF MONOCRYSTALLINE AND POLYCRYSTALLINE SOLIDS OF CUBIC SYMMETRY

The relationships between the third‐order elastic constants of monocrystalline and polycrystalline (quasi‐isotropic, randomly oriented) solids of cubic symmetry are derived according to elastic strain energy balances assuming negligibly small elastic strain energy contribution from mismatching of the grain boundaries. The relationships between the three third‐order isotropic Lame coefficients (γ1, γ2, γ3, Brugger convention) and the six third‐order single‐crystal elastic constants are given by: γ1=135(C111+18C112−30C144−12C166+16C123+16C456) γ2=135(C111+4C112+19C144+2C166−5C123−12C456) γ3=135(C111−3C112−9C144+9C166+2C123+9C456). Good agreement was found between the experimentally evaluated γ1, γ2, γ3 values and those derived from the above equations using the six third‐order single‐crystal elastic constants for bcc columbium at room temperature.

Phase transformation, twinning and anelastic phenomenon associated with zirconium dihydride☆

Abstract The atom movements and crystallography of cubic to tetragonal transformation in ZrH 2 are studied and discussed according to the phenomenological theories of Bowles and MacKenzie and of Wechsler, Lieberman, and Read. The large internal friction peak found in ZrH 1.92 (but absent in ZrH 1.6 ) suggests that it is associated with a stress induced twin interface motion. The relaxation phenomenon appears to be controlled by a diffusion process, presumably that of hydrogen, with an activation energy of about 20 000 calories per mole.

Mechanical relaxation associated with paired point defects in cubic lattices of Oh point group symmetry

Abstract The mechanical relaxation modes associated with paired point defects in cubic lattices of O h point group symmetry are derived from group theory considerations. The following paired point defects are considered, s - s , i - i , v - v , s - i and s - v , where s is a substitutional impurity, i is an interstitial and v is a vacancy. The following crystal lattices are studied, face-centered cubic, body-centered cubic, diamond cubic, NaCl and CaF 2 . Relaxation frequencies for each of the mechanical relaxation modes are evaluated and tabulated for the paired defect in nearest neighbor and higher order nearest neighbor (second, third, etc.) configurations.

Changes in the electronic states of vitreous selenium upon light radiation and plastic deformation

Abstract The changes in the electronic states of vitreous selenium by light radiation and plastic deformation and its subsequent recovery behavior were studied experimentally. The low temperature recovery experiments of vitreous selenium after light radiation indicate two distinct stages of recovery at about 100 and 200°K. The plastic deformation and associated experiments suggest a correlation between the localized electronic states and the intermolecular structure of vitreous selenium. A more rigorous theoretical approach to the transport properties of amorphous materials than that is presently available is recommended.

Long-range elastic interaction between dislocations and dislocation loops

The long-range elastic interaction between dislocation loops and dislocations in cubic lattices is studied, using anisotropic elasticity. Analytical solutions of both the interaction energies and f...