The migrational characteristics of the cusp-oriented α/κ interface in the Cu-Si system

The mobility of the coherentα/κ boundaries in the copper-silicon system was studied in a Cu plus 5.16 wt pct Si alloy with a lamellar microstructure. A new resistometric technique was employed to study the isothermal kinetics ofα/κ interphase boundary migration as a function of driving force at two reaction temperatures, 702° and 568°C. The driving force was shown to be linearly related to the temperature change, δT, and the experimental results show that the functional relationship between mobility and δT exhibits an initial linear dependence followed by a sharp rise in mobility at higher values of δT. Upquench and down-quench results were found to be equivalent, implying that the mechanism for boundary motion is independent of whether the boundary is moving into theα or into theκ phase. The results demonstrate that theα/κ interface does not move normal to itself by the long range bulk diffusion of silicon at low values of δT during the initial periods of the reaction times. A pole mechanism with its associated dislocation node configuration, consisting ofa/2〈112〉 andc[00.1] pole dislocations anda/6〈112〉 transformation partial, is proposed as the mechanism for the migration of the coherent fcc/hcp boundary in the range of small δT. The results obtained at the different reaction temperatures are consistent with the mechanism proposed, and quantitative agreement exists between the experimental and predicted mobilities. The sharp rise in mobility at larger values of δT could not be explained in terms of the pole mechanism. Attempts to rationalize this behavior in terms of a two dimensional nucleation model were not successful since unrealistically low values of surface energy would be required. Formerly with Carnegie-Mellon University Formerly Professor, Carnegie-Mellon University This paper is based on a portion of the Ph.D. Thesis submitted by J. K. TIEN to the Department of Metallurgy and Materials Science, Carnegie-Mellon University, in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Metallurgy and Materials Science.     

Hydrogen induced interior crack-tip morphologies in high strength steel

In this study WOL specimens of high strength 4340 steel were used to obtain information on the interior, or near plane strain, crack-tip morphology resulting from hydrogen assisted crack growth, including the position of secondary cracks in relation to the region of maximum triaxial stress, crack-tip plastic zone size, and grain size. The interior, or near plane strain, regions were revealed by removing thin sections, by a grinding and polishing procedure, from the surfaces of specimens that underwent hydrogen assisted crack growth. The results appear to indicate that advance of the crack occurs by growth of protrusions, or steps, from the main crack front, followed by linkup of these protrusions by sideways growth along grain boundaries.     

High temperature reaction zone growth in tungsten fiber reinforced superalloy composites: Part I. Application of the moving boundary equations

The nature, kinetics, and rate-controlling process for high temperature reaction zone growth in tungsten fiber reinforced superalloy (TFRS) composites are determined. The matrix alloys considered in this study are INCOLOY 903 and a W-modified INCOLOY 903. The kinetics and rate-controlling process for reaction zone growth are determined by direct application of the moving boundary equations. The results show that reaction zone growth is a results of longrange interdiffusion and that the rate-controlling process for reaction zone growth is interdiffusion across the reaction zone. Further, the results show that the interdiffusion coefficients of the reaction zone can be calculated from the parabolic rate constants for the reaction zone growth without directly considering the kinetics of diffusion in the matrix. The activation energy,Q, and the frequency factor,D _o, for the interdiffusion across the reaction zone in the INCOLOY 903 matrix composite are experimentally determined to be 268 kJ/mole and 0.22 cm²/s, respectively. Further, the relative chemical stability of the reaction zone is discussed with respect to reaction zone chemistry. T. CAULFIELD, formerly Postdoctoral Research Scientist at Columbia University, is Senior Member of Research Staff, Philips Laboratories, Briarcliff Manor, NY 10510. J. K. TIEN is Henry Marion Howe Professor and Director, Center for Strategic Materials, Henry Krumb School of Mines, Columbia University, New York, NY 10027.     

Hydrogen transport by dislocations

A kinetic model is presented for the transport of hydrogen, as Cottrell atmospheres on dislocation, at a rate appreciably in excess of that for lattice diffusion. The particular destinations for the hydrogen which are modeled here are ductile fracture initiation sites such as inclusions and microvoids. The functional predictions of the mechanism are shown to be consistent with available experimental evidence on ductile fracture behavior in the presence of hydrogen.     

Surface acoustic wave measurements of surface cracks in ceramics

An investigation of scattering from surface cracks has been conducted. In particular, the change in the reflection coefficient of a Rayleigh wave incident on a surface indentation crack has been measured as the sample is stressed to fracture. The acoustic measurements have been correlated with the stable crack extension that precedes final failure. The crack extension behavior of as-indented specimens was found to differ appreciably from that of annealed specimens. Cracks in the annealed samples exhibited partial crack tip closure, but little stable extension, whereas cracks in the as-indented samples displayed both crack closure and irreversible crack growth. This behavior has been rationalized by invoking concepts based upon the residual stresses created by indentation.     

Determination of phase equilibria in the system Si-Al-Zr/N-O by experiment and thermodynamic calculation

The phase relations in the system Si-Al-Zr/N-O are studied by experiment and by thermodynamic calculations. The results show the conditions under which such ceramics can exist and thereby explain contradictions between earlier results. The knowledge of these phase equilibria makes it possible to choose suitable material compositions in order to apply the toughening effects of ZrO₂ on a SiAlON matrix.