Lee M. Pike

Enthalpies of formation of binary Laves phases

Abstract Enthalpies of formation of binary Laves phases have been critically surveyed and reviewed in this paper. The enthalpy-of-formation data indicate that both geometric and electronic factors are important in stabilizing Laves phases. Analysis of enthalpy data reveals that there are metallic, covalent, and ionic bonds, or a mixed metallic-covalent-ionic bond, in Laves phases. The enthalpies of formation for binary transition-metal lanthanide Laves phases including ReCo 2 , ReNi 2 , ReFe 2 , ReRu 2 , ReRh 2 , ReOs 2 , ReIr 2 , and RePt 2 (Re—lanthanide element), as calculated by the semiempirical Miedema model, are found to be in good agreement with the available experimental data. This indicates that Miedemas theory is capable of predicting the enthalpy of formation of transition-metal lanthanide Laves-phase systems.

Thermal Stability of a Ni-Cr-W-Mo Alloy - Long-Term Exposures

HAYNES® 230® alloy, based on the Ni-Cr-W-Mo system, is extensively used in gas turbines and other applications at high temperatures. It is recognized that the microstructure of many alloys under long-term service conditions can change. Microstructural changes may affect mechanical properties which in turn may determine component design life in service. Given the importance and widespread usage of 230 alloy, a number of studies on its thermal stability have been conducted. This study will focus on the effect of long-term thermal exposures at temperatures ranging from 649 to 871°C (1200 to 1600°F) on the microstructure and properties of 230 alloy. Room and elevated temperature tensile, and room temperature Charpy impact toughness properties of thermally exposed material are determined and correlated to observed changes in the microstructure. Microstructural analyses were performed using extraction/X-ray diffraction together with optical and scanning electron microscopy. The mode of fracture was ascertained by fractography of the thermally-exposed and broken test samples. The paper presents previously unpublished results for long-term exposures with durations ranging up to 50,000 hours (~ 6 years).

A Nondestructive Study Using Lattice Plane Specific Analysis on a Nano-Precipitate Strengthened Alloy

The HASTELLOY C-22HS alloy is a corrosion-resistant nickel-based alloy in a faced-center-cubic (FCC) crystal structure. The aging process produces the nano-precipitates in the matrix and the yield strength of the aged alloy is doubled at both room temperature and 595C. In the present study, the tensile behavior of the alloy was examined by in-situ neutron-diffraction at room temperature. The peak-width of the hkl plane from both the nano-precipitates and the matrix were determined as a function of the applied stress. The evolution of the peak widths from the matrix and the precipitates during the monotonic-tension-loading experiments clearly demonstrated that the effect of the nano-precipitate strengthening mechanism was active. It reflects clearly the importance of the nondestructive in-situ neutron experiments examining the diffraction profile evolutions coupled with the mechanical measurements.

Point defects in binary NbCr{sub 2} Laves-phase alloys

Point defects in the binary NbCr{sub 2} alloys have been studied and clarified combining both bulk density and X-ray lattice parameter measurements. It was shown that the previous postulation that constitutional vacancy is the defect mechanism at the Nb-rich side is not accurate. The constitutional defects on both sides of stoichiometry for the NbCr{sub 2} system are of the anti-site type. Thermal vacancies exhibiting a maximum at the stoichiometric composition were obtained in samples quenched from 1,400 C. These could be completely eliminated by annealing at 1,000 C.