The In x Ga 1-x N system has electronic band gaps extending from under 1eV to 3.4 eV, and as such they are used as the active layer in commercially available visible-light emitting devices. There are many interesting features that make these nitride semiconductor alloys especially useful for efficient light emitters. It has been conjectured that the combination of piezoelectric fields and local composition inhomogeneities may be responsible for the observed high emission efficiencies, in spite of their characteristic high dislocation densities. But it is very difficult to grow In x Ga 1-x N layers with high indium composition. This paper presents an overview of the properties of In x Ga 1-x N epilayers based on a systematic study of thick layers and of quantum well structures. We find that the microstructure of thick films varies significantly with indium composition. For x < 0.08, the composition is uniform and unperturbed by dislocations. For 0.10 < x < 0.20, secondary phases nucleate at threading dislocations. For x > 0.20, spontaneous phase separation occurs resulting in a polycrystalline, inhomogeneous layer. A correlation between optical properties and microstructure is presented. It is observed that the misfit strain is affected by threading dislocations. Mechanisms of misfit strain relaxation are presented for In x Ga 1-x N layers grown on standard GaN on sapphire and on epitaxial-lateral-overgrowth GaN layers. In addition, we have studied the properties of quantum well structures using several novel techniques. The electrostatic fields across the wells have been profiled using electron holography in the TEM. The effect of well thickness on the strength of the fields is reported. The effects of localization by compositional fluctuations and of internal field screening have been studied using time-resolved cathodoluminescence spectroscopy. In spite of significant progress that has been made in the last ten years, much work remains ahead in order to master the science and technology of these alloys.
SynopsisBecause of improved strength-ductility combination over HSLA steels, dual phase steels have recently become of commercial importance to both the sheet users and producers. These steels possess good properties by virtue of their microstructure which consists, typically, of about 15'20 % martensite uniformly distributed in a soft matrix of ferrite. Although, the desired microstructural features of a dual phase steel can be obtained by various process routes, the most economical method is the production of this steel in as hot rolled condition. The sucessful production of dual phase steels in the hot strip mill, however, requires a careful control of process parameters particularly the finishing temperature, the cooling of the sheet on the runout table, the coiling temperature and the subsequent cooling of the coils.As a development effort some commercial heats of dual phase steel in C-Mn-Si-Cr-Mo chemistry have been produced in as hot rolled condition at Rourkela Steel Plant. The effect of coiling temperature and cooling rate on the final structure and properties of the steel has been discussed in detail. All the coils coiled at lower temperatures of about 470° C or less showed dual phase structure with uniform properties where as coiling at about 500 °C or above did not yield the desired microstructure and properties. Possible reasons have been given to explain the effect of coiling temperature on microstructure. Acceleration of the cooling rate of the coil, after coiling, has been found to improve the tensile strength without significantly affecting the ductility of the material.
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