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Six Co-substituted magnetites Co"Fe3 "0&, with x ~0.04 have been studied by Fe Mossbauer spectroscopy at temperatures T between their respective Verwey transition and =900 K. The Curie temperature T& decreases from 868 K for x =0 to 859 K for x =0.03. All spectra were decomposed into tetrahedral (A-site) and octahedral (B-site) components. The obtained results are consistent with the fact that at low temperatures above the Verwey transition, the spins lie along a domain s [100] axis, but fiip to a [111]axis at a temperature strongly increasing with increasing x. The temperature variations of the hyperfine parameters (center shift 6, quadrupole shift c. &, magnetic hyperfine field Hhf and dipolar field Hd;~) have been determined. The 5 and Hhf values for the octahedral Fe are consistent with the well-known fast electron exchange between the involved Fe + and Fe + species. From the temperature dependence of the B-site linewidth it is concluded that the nature of the exchange process at relatively low temperatures, depending on x, is different from that at higher temperatures. The c& vs T and Hd'p vs T curves for the B sites of the x =0, 0.005, and 0.01 samples (both mentioned parameters for the A sites are zero) were interpreted on the basis of a common crystalline-field model, yielding, as argued, unrealistic values for certain involved physical quantities. The behavior of the center shifts implies a slight temperature variation for the intrinsic isomer shift. Unlike the A-site hyperfine fields, the temperature dependence of the B-site fields could not be reproduced adequately by the usual formulas based on Heisenberg-type exchange. Instead, a non-localized-electron model led to an excellent description of the experimental curves. The bandwidth, which appears in that model, turned out to be high (0.82 eV for x =0 at 300 K) and is suggested to be responsible for the failure of the crystalline-field model to explain the temperature variation of e qg/2 and H~;~. For all compositions e& and H~;~a t selected temperatures have also been determined from spectra recorded in external magnetic fields. These spectra further indicate that a field-induced contribution to the total hyperfine field on the B sites (not on A sites) is to be taken into account. Its origin is discussed briefly.

Laser cladding is used to coat and repair the surface of various components. A significant issue relating to optimisation of the process is the generation of residual stresses. These are affected by the high thermal gradients inherent in the process, and associated differential thermal contraction. These stresses can lead to various types of distortion. A customised 3‐D finite element model has been developed, incorporating these effects, based on simulation of conductive, convective and radiative heat transfer, and assuming elastic‐perfectly plastic deformation behaviour. Creep effects have been neglected and the cladding (particulate metal matrix composite) has been treated as a continuum. Comparisons are presented between measured and simulated thermal fields and specimen deflection histories. The results indicate that the main features of residual stress generation in this type of system have been captured in the model. Implications for process optimization and control are briefly discussed.

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