Stephen M. Copley

The constitution and phase stability of overlapping melt trails in Ag- Cu alloys produced by continuous laser melt quenching

Coatings consisting of overlapping trails melted with a scanning CW CO2 laser have been produced on Ag-Cu alloys with the following compositions: Cu 17 at. pct Ag; Cu 37 at. pct Ag; Cu 61.7 at. pct Ag; Cu 71.8 at. pct Ag; and Cu 82 at. pct Ag. The laser beam was scanned at a velocity of 34 cm s1 and with an intensity of 3.6 MW cm-2. Selected trails were examined by X-ray diffractometry, optical microscopy, and scanning electron microscopy in the as-irradiated condition and after annealing for various periods of time in the temperature range 100 to 450 ° C. Time-temperature-transformation diagrams based on the annealing studies are presented. Significant amounts of the metastable extended solid solution (γ) were observed in the Ag-rich alloy trails. The silver rich terminal solid solution (α) was also detected, formed probably by solid state precipitation. An α’ phase with lattice parameter lying between that ofy and α was also observed in the Cu 61.7 at. pct Ag alloy. A metastable equilibrium diagram has been constructed and is employed to interpret these observations. The most striking microstructural feature of the trails are bands marking sequential positions of the melt-solid interface. We propose that these bands are evidence for planar, oscillating - steady-state, interface motion. The observation of a periodic cellular breakdown of the planar interface in the Cu 61.7 at. pct Ag alloy is attributed to a diffusional instability previously predicted by Baker and Cahn.

The microstructure of metastable phases in Ag-Cu alloys generated by continuous laser melt quenching

A scanning high power CW CO2 laser with an average power density of 13.4 MW cm-2 was used to produce metastable phases in three Ag-Cu alloys; Cu 25 at pct Ag, Cu 50 at pct Ag, and Cu 75 at pct Ag. Scanning, at traverse rates of 10 to 115 cm s-1, was performed in an inert atmosphere to prevent oxidation. Inspection of the trails by scanning electron microscopy showed the microstructure to be either cellular dendritic or featureless. X-ray examination of the trails verified the presence of extended metastable solid solutions. After etching, the transverse section showed alternating light and dark bands perpendicular to the local growth direction. Through the use of energy dispersive X-ray analysis, it was shown that there was a significant increase in the Ag concentration in the light bands.

Hardening of GaAs by solute‐vacancy pairs

The effect of various thermal and thermomechanical treatments on the yield stress and the dislocation substructure of heavily Si‐doped GaAs is presented. It is shown that the yield stress at 400 °C of samples annealed at 1100 °C, quenched and subsequently aged at an intermediate temperature, increases with increasing aging time, and for constant aging time, decreases with increasing temperature. A defect model based on the principles of chemical thermodynamics of imperfections is proposed to calculate the concentrations of the various possible defect species in Si‐doped GaAs as a function of temperature. The correlation between the concentrations of the various defects and the mechanical behavior observed in this investigation along with the optical and electrical measurements and the microstructural observations of previous investigators demonstrates the important role of solute‐vacancy pairs in determining a variety of properties. The solute‐vacancy pair model may be applied to explain the mechanical be...

Laser Assisted Machining

The availability of high power cw carbon dioxide lasers with sufficient ruggedness, reliability and simplicity of operation for use in manufacturing facilities has led to the development of new machining methods. These methods currently involve the localized vaporization or melting of the material due to beam heating. (1) In the case of metallic materials beam heating is supplemented with burning enhanced by a flow of oxidizing gases at the point of impingement of the laser beam. In our research, we have developed a new and dif-ferent method of cutting with a laser, laser assisted hot spot machining (LAM), in which the laser is used to heat the volume of material directly in front of a single point cutting tool to a temperature less than its melting point.(2,3)The application of gas torch and induction heating to assist, in the turning of metals was first studied in the United States by Tour and Fletche(1949). Concurrently, Schmidt investigated the use of gas torch heating in milling. Although many advantages were reported such as reduction in power consumption tool life improvement and improvement in surface finish, hot-machining has not been perceived as a practical and economically viable method by industry. Recently, however, with the development of more intense heat sources such as the plasma-arc(5) and the laser, hot machining has become more attractive in specific applications as a metal removal technique.

Curvature of laser‐machined grooves in Si3N4

In laser machining, the article to be machined is moved with respect to the laser beam resulting in the formation of a groove in its surface by vaporization. Grooves formed in hot‐pressed Si3N4 with a CO2 laser beam at normal incidence are observed to have cross sections that curve out of the plane defined by the velocity direction of the article and the direction of the laser beam. The curved cross sections change from one side of this plane to the other if the velocity direction is reversed. The effect is most pronounced at the lowest velocities and highest incident powers. The cross‐section shape is shown to vary systematically with the angle between the velocity direction and the direction of the electric vector of the incident beam, which was found to be partially polarized. A mechanism for the curved cross‐section effect is proposed based on the existence of a significant difference in reflectance between the transverse electric (TE) and transverse magnetic (TM) reflections from groove walls in Si3N...

A new model for precipitation at moving interphase boundaries

A new model has been developed to describe precipitation in sheets at a moving, planar, interphase boundary. The model assumes that the diffusion profile developed by the growing sheet of inter-phase precipitates is similar to that resulting from the growth of a “pseudophase” of constant compo-sition equal to the average solute concentration in the sheet. It assumes that the solute lost by the depleted layer during growth is equal to the excess solute found in the sheet. Nucleation of a new sheet is determined by the local solute concentration at migrating ledges at the interphase boundary. The model provides a theoretical rationale for correlations among parameters important in interphase precipitation and gives a coherent description of this phenomenon, upon which further understand-ing can be based.

Phase Equilibria and Interstitial Effects in the Ti-V-Mo Alloy System

The stability of theβ phase in the Ti-V, Ti-Mo, and Ti-V-Mo alloy systems was investi-gated, and theβ/α + β phase boundaries in these systems were determined in the range 300 to 600° C. The results indicate that Mo is more potent than V in stabilizing theβ phase with respect to α phase formation and in retarding the β → α reaction kinetics. It is shown that increasing the oxygen concentration in the alloys tends to enhance α phase formation in Mo-lean alloys (Mo contents < 15 wt pct), whereas it leads to the formation of an oxide phase in Mo-rich alloys (Mo contents ≥15 wt pct).

Mass transport during laser chemical vapor deposition

Two analyses of steady‐state mass transport during laser chemical vapor deposition (LCVD) are compared for a hemispherical source/sink geometry. The first considers only diffusion while the second includes diffusion and also convection due to the mole number change of vapor species in the reaction, which is modeled as the radial outflow of an incompressible gas from a point source. The results of the two analyses differ markedly for high deposition fluxes and large radii. For example, in the LCVD of Ni from Ni(CO4), reactant partial pressures at the hemispherical surface predicted by the diffusion plus convection analysis are less than those predicted by the diffusion analysis because the inward diffusion of the reactant species is hindered by the convective outflow of the product species. The diffusion analysis predicts an increase in total pressure at the hemispherical surface, which normally is relieved by the neglected convective flow. Bounds for transport controlled deposition and for deposition wher...

The effect of interstitials on phase equilibria in P[Ti, Zr, Hf]-X[β-stabilizer (s)] alloy systems

Disagreements existing in the literature concerning phase equilibria inP [Ti,Zr,Hf]-X [β-stabilizer(s)] alloy systems are explained on the basis of three features believed to be characteristic ofP [Ti,Zr,Hf]-X [β-stabilizer(s)]-I [P,N,C] systems.

CONSTITUTION AND MICROSTRUCTURE OF Ag-Cu ALLOYS PRODUCED BY CONTINUOUS LASER MELT QUENCHING

A high power CW CO 2 laser was used to scan Ag-Cu alloys in order to obtain metastable phases. The laser employed to irradiate the fully annealed alloys of 25 at pct Ag, 50 at pct Ag, 75 at pct Ag (the balance being Cu) had an incident power of 1050 W, was operated in the TEM oo mode, and was focussed to an average power density of 13.4 × 10 6 W cm –2 . Scan rates, ranging from 10 to 207 cm s –1 , were obtained using a precision lathe with arrangements made to keep the samples in an inert atmosphere to prevent oxidation. Initially the alloys were β phase (copper-rich phase) in a matrix of α phase (silver-rich phase). The surface was coated with a trichloroethane suspension of graphite to enhance absorption. x-ray examination of the trails indicated that they consist primarily of the extended metastable solid solution. Scanning electron microscopy of the regions occupied by the metastable phase showed the microstructure to be either cellular dendritic or featureless with alternate light and dark bands perpendicular to the local growth direction.