Thermochemistry of the nickel–phosphorus system

In the first part, an extensive overview is given over the thermodynamic information for the nickel–phosphorus system which is available in the literature. In the second part, phosphorus vapor pressure measurements over nickel–phosphorus alloys are described for which an isopiestic method was employed. Data points were obtained between 33.82 and 44.40 at% P in the temperature interval from 977 to 1325 K. Based on these measurements partial thermodynamic properties and integral Gibbs energy values were determined. The standard molar Gibbs energy of formation of Ni₅P₄ for the temperature range 977–1042 K was obtained as:${\mathrm{\Delta }}_{\text{f}}{G}_{\text{m}}^{\text{o}}\left({\text{Ni}}_5{\text{P}}_4\right)\left(\text{s}\right)\pm 9.8/\left(\text{kJ}{\text{mol}}^{?1}\right)=?632.1+0.2136T/\text{K}$     

Orientation dependence of shock-induced twinning and substructures in a copper bicrystal

Shock recovery experiments have been conducted to assess the role of shock stress and orientation dependence on substructure evolution and deformation twinning of a [1 0 0]/$[01\overline{1}]$ copper bicrystal. Transmission electron microscopy of the post-shock specimens revealed that well-defined dislocation cell structures developed in both grains and the average cell size decreased with increasing shock pressure from 5 to 10 GPa. Twinning occurred in the [1 0 0] grain, but not the $[01\overline{1}]$ grain, at the 10 GPa shock pressure. The stress and orientation dependence of incipient twinning can be predicted by the stress and orientation conditions required to dissociate slip dislocations into glissile twinning dislocations. The dynamic widths between the two partials are calculated considering the three-dimensional deviatoric stress state induced by the shock as calculated using plane-strain plate impact simulations and the relativistic and drag effects on dislocations moving at high speeds.     

Grain boundary diffusion and segregation of Ni in Cu

Grain boundary (GB) diffusion of ⁶³Ni in polycrystalline Cu was investigated by the radiotracer technique in an extended temperature interval from 476 to 1156 K. The independent measurements in Harrison’s C and B kinetic regimes resulted in direct data of the GB diffusivity D_gb and of the so-called triple product P = s · δ · D_gb (s and δ are the segregation factor and the diffusional GB width, respectively). Arrhenius-type temperature dependencies for both the D_gb and P values were measured, resulting in the pre-exponential factors $D_{\mathrm{gb}}^0=6.93\times {10}^{-7}$ m² s⁻¹ and P₀ = 1.89 × 10⁻¹⁶ m³ s⁻¹ and the activation enthalpies of 90.4 and 73.8 kJ mol⁻¹, respectively. Although Ni is completely soluble in Cu, it reveals a distinct but still moderate ability to segregate copper GBs with a segregation enthalpy of about −17 kJ mol⁻¹.     

A model for the creep deformation behaviour of single-crystal superalloy CMSX-4

Constitutive equations are constructed for single-crystal nickel-based superalloys. Account is taken of dislocation glide in the channels of the matrix phase (referred to as $\mathrm{\gamma }$) of the face-centred cubic (fcc) type, dislocation climb at the interfaces with the reinforcing $\text{L}{1}_2$ precipitates (referred to as ${\mathrm{\gamma }}^{\prime }$) and the processes leading to cutting of the interfaces by dislocation ribbons via stacking fault shear of the $a〈112〉$ type. A treatment of $a〈112〉$ ribbons produced by the combination of $a/2〈110〉$ channel dislocations by an appropriate set of dislocation reactions is developed. The model allows the following features of superalloy creep to be recovered: dependence upon microstructure and its scale, effect of lattice misfit, internal stress relaxation, incubation phenomena, the interrelationship of tertiary and primary creep, and vacancy condensation leading to damage accumulation. Using the model, the creep deformation behaviour of the single-crystal superalloy CMSX-4 is studied, with emphasis on the interrelationship between primary and tertiary creep. It is shown that the values for the various parameters used in the modelling are physically reasonable and are related to the microstructure and its evolution during creep. The creep anisotropy prevalent in these materials due to primary creep is recovered correctly.