Processing map for hot working of powder

The constitutive flow behavior of a metal matrix composite (MMC) with 2124 aluminum containing 20 vol pct silicon carbide particulates under hot-working conditions in the temperature range of 300 °C to 550 °C and strain-rate range of 0.001 to 1 s^-1 has been studied using hot compression testing. Processing maps depicting the variation of the efficiency of power dissipation given by [2m/(m + 1)] (wherem is the strain-rate sensitivity of flow stress) with temperature and strain rate have been established for the MMC as well as for the matrix material. The maps have been interpreted on the basis of the Dynamic Materials Model (DMM). [3] The MMC exhibited a domain of superplasticity in the temperature range of 450 °C to 550 °C and at strain rates less than 0.1 s^-1. At 500 °C and 1 s^-1 strain rate, the MMC undergoes dynamic recrystallization (DRX), resulting in a reconstitution of microstructure. In comparison with the map for the matrix material, the DRX domain occurred at a strain rate higher by three orders of magnitude. At temperatures lower than 400 °C, the MMC exhibited dynamic recovery, while at 550 °C and 1 s^-1, cracking occurred at the prior particle boundaries (representing surfaces of the initial powder particles). The optimum temperature and strain-rate combination for billet conditioning of the MMC is 500 °C and 1 s^-1, while secondary metalworking may be done in the super- plasticity domain. The MMC undergoes microstructural instability at temperatures lower than 400 °C and strain rates higher than 0.1 s^-1.     

Artificial neural-network-based diagnosis of CVD barrel reactor

This paper presents an artificial neural network (ANN) based diagnostic strategy applied to a chemical vapor deposition (CVD) barrel reactor of the type commonly used in silicon epitaxy. The strategy is based on the spatial variation of the rate of deposition of silicon on a facet of the reactor. Our hypothesis is that this spatial variation, quantified as a vector of variously measured standard deviations, encodes a pattern reflecting the state of the reactor. Therefore, a process fault (event) can be diagnosed by decoding the pattern by an ANN. We implemented this simple scheme by simulating different events by means of a regression model relating the rate of deposition to the process settings. Three different events were simulated and various ANNs were trained to detect and classify these events. It is shown that a single ANN or a combination of ANNs does an excellent job. We also demonstrate that the threshold rule for setting the threshold of a binary output neuron performing a classification task enhances the diagnostic performance. A novel multiple expert scheme that refers to several ANNs trained in the same classification task for decision-making in order to resolve ambiguities and improve the reliability of the final decision is presented and shown to be effective     

Effect of carbon deposition on porosity and reactivity of a lignite char

Carbon deposition from the cracking of methane into the pores of a lignite char occurs at a significant rate at temperatures between 815 and 855 °C. The maximum amount of carbon deposited is much less than the amount of open pore volume within the char which is potentially available to accommodate carbon. Removal of the inorganic impurities from the char by acid washing significantly reduces the extent of carbon deposition. Carbon deposition reduces surface area and open pore volume of the char and accessibility of methane into the pore structure. Deposition of carbon has a large and lasting adverse effect on the subsequent reactivity of the char to air. This has been attributed both to a decrease in active surface area, and deactivation of catalytic inorganic impurities due to coating with carbon. Deposited carbon is much less reactive to air than the lignite char. Results indicate that in order to maximize subsequent char reactivity to oxidizing gases, carbon deposition from volatiles during the conversion of coal to char should be kept to a minimum.     

A novel method for down-conversion of multiple bandpass signals

Simultaneous down-conversion of multiple band-pass signals is desirable for a number of wireless applications. Bandpass sampling technique can be used for this purpose, but it is difficult to implement and has several drawbacks. In this paper we propose a novel front-end technique to directly down-convert multiple frequency- division multiplexed (FDM) signals separated by certain minimum frequency. A special downconversion function is derived to achieve simultaneous downconversion of the received signals. The technique requires simpler bandpass filters and the ADC has a baseband input as compared to bandpass sampling, which imposes strict requirements on bandpass filters and requires an ADC which can handle RF inputs. The performance of the method has been evaluated by simulating a BPSK receiver employing this technique.     

Thermoelectric power of pure gallium. II. Size and impurity effects

The thermoelectric power as a function of specimen size for gallium single crystals and crystals doped with small percentages of indium has been measured along the three different directions of the orthorhombic lattice between 4.2 and 1.4 K by means of a superconducting potentiometer. From the observations of the temperature dependence of the size effects, evaluations were made of the derivative with respect to energy of the Fermi surface area and the mean free path along the three crystal axes. These data are tabulated. Along theA andB axes ( lnS _F /E) _E is about –0.65 eV^–1, the negative sign indicating conduction by a band of holes, whereas along theC axis this derivative is positive, indicating conduction by a band of electrons of smaller dimensions. Among other information derived from measurement of the indium-doped samples was that the intrinsic contribution to the diffusion thermopower of In along theC axis of gallium is+2.7×10^–8 T(V/K).Work supported by grants from the National Science Foundation and by contracts with the Office of Naval Research and DOD (Themis Program).     

Strehl ratio of a Gaussian beam

We discuss the Strehl ratio of systems with a Gaussian pupil and determine the range of validity of its approximate expression based on the aberration variance. The results given are equally applicable to propagation of Gaussian beams. The uniform and weakly truncated pupils are considered as limiting cases of a Gaussian pupil. We show that the approximate expression for Strehl ratio in terms of the aberration variance yields a good estimate of the true value for a strongly truncated pupil but a much smaller value for a weakly truncated pupil.     

Low cycle fatigue behavior of Ti-6AI-2Sn-4Zr-6Mo: Part II. Cyclic deformation behavior and low cycle fatigue

A series of strain-controlled low cycle fatigue tests have been carried out on Ti-6-2-4-6 at room temperature primarily for solution-treated and aged material. Tension-compression testing at R = 1 produced softening, irrespective of the morphology of the primary α. The amount of softening increased with increasing strain. Tension-tension testing for R = 0 produced strengthening. Unaged specimens showed behavior similar to aged material for both R = ?1 and R = 0 testing. Annealing at 621 °C produced hardening for R = ?1 and softening for R = 0 testing. Aging at 210 °C of R = ?1 specimens produced strengthening. Fatigue life data revealed longer fatigue life for equiaxed (E) structures than for Widmanstätten plus grain boundary (W +GB)α structures. ForEα, increasing α particle size tends to reduce fatigue life. For W +GBα alloys the situation is more complex, and both a decrease and increase in fatigue life may be seen for increasing particle sizes. There is also a prior β grain size dependency. As a result of softening, the slope of the log Δε _p/2 vs logN _f curve continuously decreases with increasingN. Explanations of the softening and hardening behavior in terms of dislocation rearrangement are offered. An explanation of the role of microstructure on fatigue life has been offered in the companion paper.