The nature and magnitude of the residual stresses within laser-deposited titanium carbide (TiC) coatings on 2024 and 6061 aluminum (Al) alloys were investigated. Macro-and micro-stresses within the coatings were determined using an X-ray diffraction method. Owing to increased debonding between the coating and the substrate, the macro-stresses were found to be compressive and to decrease in magnitude with increasing processing speed. The origin of the macro-and micro-stresses is discussed. To that end, transmission electron microscopy observations showed no evidence of plastic deformation within selected coatings. The micro-stresses in the TiC particulate and aluminum matrix phases within the coatings were found to be independent of the amount of debonding. This study could assist in optimizing the processing parameters to fabricate composite coatings for specific applications.

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Refractory materials such as ceramics and refractory metals/alloys are widely used for structural applications in severe environments where more efficient use of energy is a prime need. Owing to properties such as high strength and refractoriness, low thermal expansion and electric conductivity these materials may be used as turbocharger rotors, valves, valve seats and turbocharger turbine wheels in gas turbine engines, tubes in industrial heat exchangers, fuel elements in nuclear reactors, fusion reactor wall material, tool materials in machining and casting and as electronic packaging. In these applications, however, these materials often experience extreme mechanical, thermal and chemical loading conditions and are expected to perform effectively beyond existing limits. The existing limits of performance for these materials can be extended only via coating technology, which presently appears to be the most suitable technical and economic approach. Thus there remains a critical need to develop a suitable and reliable refractory coating system and coating technique.In view of the above requirements, the present paper embarks on a brief review of refractory coatings. The review begins with a set of criteria required for an effective refractory coating. The criteria include general thermophysical and mechanical properties for the refractory coating only and properties of the coating-substrate as a system. The review continues with a section on methods of coating. The classification of methods of coating is based on categories such as gaseous, solution, molten/semi-molten and solid state processes. Each category is explained with conventional and novel techniques and coatings that can be synthesized using these techniques. These categories are supplemented also with examples of applications. A major emphasis has been put on refractory materials in the following section on coating material systems. The most important materials systems have been grouped as oxides and nitrides, carbides, borides, silicides and aluminides. The general properties that make them useful as refractory coatings, along with specific materials and applications, have been reviewed. Wettability and adhesion between coating and substrate are very important for the coating-substrate system to perform effectively and reliably. The last section discusses the fundamentals of wettability and work of adhesion, based on which wetting in metal-metal, metal oxide, metal-carbon/carbide systems has been discussed. Finally, the review sets the motion of thoughts with a description on future developments.

The precipitation behavior of AA2618 was studied by a multitude of characterization techniques: microhardness testing, lattice parameter measurement through X-ray diffraction (XRD), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), and atom probe field ion microscopy (APFIM). The matrix lattice parameter increased during the first 20 hours of natural aging, due to the formation of Cu clusters and decreased over the next 24 hours, due to the formation of Mg-rich clusters. Prior natural aging weakened subsequent artificial aging hardening at 180°C, 200°C, and 230°C, due to the cluster reversion that delayed the precipitation of strengthening phases. The matrix lattice parameter exhibited erratic changes during artificial aging that corresponded to the formation and partial dissolution of Guinier-Preston-Bagaryatsky (GPB) zones, the transformation of GPB zones to GPB2 zones, and the precipitation of S¢. The structural changes during the artificial aging of AA2618 occur in this sequence: supersaturated solid solution fi clusters + GPB fi GPB + GPB2 fi GPB2 + S¢ fi S¢+ S fi S.