Conjugate heat transfer in a microchannel with a slip boundary condition imposed on the channel's walls by a uniform magnetic field is studied. The working fluid consists of a Water/Ag mixture nanofluid. A preconditioned lattice Boltzmann method (LBM), formed by combining the incompressible LBM with the regular LBM, is applied to the velocity field and temperature field, respectively. The microchannel's upper wall is thermally isolated when a constant heat flux is imposed on the basin of the microchannel. The simulations are carried out under a variety of different conditions, e.g., various Reynold numbers, Re = 50 and 150, nanoparticle concentrations (φ = 0, 3%), slip coefficients (0 ≤ B ≤ 0.03), and Hartmann numbers (0 ≤ Ha ≤ 30). Surface hydrophobicity results in a reduction of surface friction of up to 46% at B = 0.03 and Ha = 30. The surface friction reductions at Ha = 0, 10, and 20 are 15%, 27%, and 38%, respectively. These results indicate that as the surface slip increases, the drag resisting the fluid dynamics decreases. Moreover, adding the nanoparticles to the base flow can improve the heat transfer by 50%. Besides, using the magnetic field increase the shear stress and, consequently, the drag force dramatically (up 340%). On the other hand, the magnetic field enhances the heat transfer by improving the fluid velocity near the wall, while its effect on the Nu number improvement is not more than 20%. As a result, the magnetic power should be controlled to achieve the best heat transfer performance with the lowest pumping energy consumption.

In this study, fresh attempts have been made to identify and estimate the phase constituents of a high-silicon, medium carbon multiphase steel (DIN 1.5025 grade) subjected to austenitization at 900 °C for 5 min, followed by quenching and low-temperature bainitizing (Q&B) at 350 °C for 200 s. Several techniques were employed using different chemical etching reagents either individually (single-step) or in combination of two or more etchants in succession (multiple-step) for conducting color metallography. The results showed that the complex multiphase microstructures comprising a fine mixture of bainite, martensite and retained austenite phase constituents were selectivity stained/tinted with good contrasting resolution, as observed via conventional light optical microscopy observations. While the carbon-enriched martensite-retained austenite (M/RA) islands were revealed as cream-colored areas by using a double-step etching technique comprising etching with 10% ammonium persulfate followed by etching with Marble's reagent, the dark gray-colored bainite packets were easily distinguishable from the brown-colored martensite regions. However, the high-carbon martensite and retained austenite in M/RA islands could be differentiated only after resorting to a triple-step etching technique comprising etching in succession with 2% nital, 10% ammonium persulfate solution and then warm Marble’s reagent at 30 °C. This revealed orange-colored martensite in contrast to cream-colored retained austenite in M/RA constituents, besides the presence of brown-colored martensite laths in the dark gray-colored bainitic matrix. A quadruple-step technique involving successive etching with 2% nital, 10% ammonium persulfate solution, Marble’s reagent and finally Klemm’s Ι reagent at 40 °C revealed even better contrast in comparison to the triple-step etching technique, particularly in distinguishing the RA from martensite. Observations using advanced techniques like field emission scanning electron microscopy (FE-SEM) and electron back scatter diffraction (EBSD) failed to differentiate untempered, high-carbon martensite from retained austenite in the M/RA islands and martensite laths from bainitic matrix, respectively. Transmission electron microscopy (TEM) studies successfully distinguished the RA from high-carbon martensite, as noticed in M/RA islands. The volume fraction of retained austenite estimated by EBSD, XRD and a point counting method on color micrographs of quadruple-step etched samples showed good agreement.