Microwave-Assisted Preparation of High Entropy Alloys

Microwaves at the ISM (Industrial, Scientific and Medical, reserved internationally) frequency of 2450 or 5800 MHz have been used to prepare FeCoNiCuAl, FeCrNiTiAl and FeCoCrNiAl2.5 high entropy alloys by direct heating of pressed mixtures of metal powders. The aim of this work is to explore a new microwave-assisted near-net-shape technology, using a powder metallurgy approach for the preparation of high entropy alloys, able to overcome the limits of current melting technologies (defects formation) or solid state ones (time demanding). High entropy alloy compositions have been selected so as to comprise at least one ferromagnetic element and one highly reactive couple, like Ni-Al, Ti-Al, Co-Al or Fe-Al. Results show that direct microwave heating of the powder precursors occurs, and further heating generation is favored by the ignition of exothermal reactions in the load. Microwaves have been applied both for the ignition and sustaining of such reactions, showing that by the proposed technique, it is possible to control the cooling rate of the newly-synthesized high entropy alloys. Results showed also that microwave heating in predominant magnetic field regions of the microwave applicator is more effective at controlling the cooling rate. The herein proposed microwave-assisted powder metallurgy approach is suitable to retain the shape of the load imparted during forming by uniaxial pressing. The homogeneity of the prepared high entropy alloys in all cases was good, without the dendritic segregation typical of arc melting, even if some partially-unreacted powders were detected in the samples

Microwave processing of high entropy alloys: A powder metallurgy approach

Microwaves at the ISM frequency of 2450 and 5800 MHz have been exploited to prepare FeCoNiCrAl-family high entropy alloys by direct heating of pressed mixtures of metal powders. The aim of this work is to explore a new microwave assisted near-net-shape technology, using powder metallurgy approach for the preparation of high entropy alloys, able to overcome the limits of current melting technologies (defects formation) or solid state ones (time demanding). Results show that direct microwave heating of the powder precursors occurs, and further heating generation is favored by the ignition of exothermal reactions in the compound. Microwave processing, exploited both for the ignition and sustaining of such reactions, has been compared to reactive sintering in laboratory furnace and mechanical alloying in a planetary ball milling. Results demonstrate that microwave required the shortest time and lowest energy consumption, thus it is promising time- and cost-saving synthetic route

Al, cu and zr addition to high entropy alloys: The effect on recrystallization temperature

The equimolar Cr, Mn, Fe, Co and Ni alloy, first produced in 2004, was unexpectedly found to be single-phase. Consequently, a new concept of materials was developed: high entropy alloys (HEA) forming a single solid-solution with a near equiatomic composition of the constituting elements. In this study, an equimolar CoCrFeMnNi HEA was modified by the addition of 5 at% of either Al, Cu or Zr. The cold-rolled alloys were annealed for 30 minutes at high temperature to investigate the recrystallization kinetics. The evolution of the grain boundary and the grain size were investigated, from the as-cast to the recrystallized state. Results show that the recrystallized single phase FCC structures exhibits different twin grains density, grain size and recrystallization temperatures as a function of the at.% of modifier alloying elements added. In comparison to the equimolar CoCrFeMnNi, the addition of modifier elements increases significantly the recrystallization temperature after cold deformation. The sluggish diffusion (typical of HEA alloys), the presence of a solute in solid solution as well as the low twin boundary energy are responsible for the lower driving force for recrystallization

Procedure to generate a selection chart for microwave sol-gel synthesis of nanoparticles

Numerical simulation is used to compare the temperature distribution existing during microwave or conven-tional heating by hot oil bath of a reaction volume. The simulation is applied to the sol-gel synthesis of TiO2 nanopowders, considering temperature dependent materials parameters. A WR340-based single mode applicator operating at 2.45 GHz is considered, with a cylindrical load positioned in regions of predominant electric field. A temperature homogeneity index, defined as the ratio between the average temperature and its standard deviation at a certain time, is used to compare homogeneity of conventional and microwave heating at different power densities, from 370 to 7400 W/L. Based on that comparison, a selection chart of the more homogenous heating process as a function of power density and average temperature of the reaction volume is defined and can be used to select the experimental conditions expected to lead to a more or less homogeneous particle size of the products. A dedicated instrumented model, including three optical fibres, is developed as well, for validation purposes. Experimental validation showed a very good predictive capability of the model, with errors on estimated tem-perature lower than 3 degrees C at temperatures lower than 80 degrees C

Nanoporous micro-element arrays for particle interception in microfluidic cell separation

The ability to control cell-surface interactions in order to achieve binding of specific cell types is a major challenge for microfluidic immunoaffinity cell capture systems. In the majority of existing systems, the functionalized capture surface is constructed of solid materials, where flow stagnation at the solid-liquid interface is detrimental to the convection of cells to the surface. We study the use of ultra-high porosity (99%) nanoporous micro-posts in microfluidic channels for enhancing interception efficiency of particles in flow. We show using both modelling and experiment that nanoporous posts improve particle interception compared to solid posts through two distinct mechanisms: the increase of direct interception, and the reduction of near-surface hydrodynamic resistance. We provide initial validation that the improvement of interception efficiency also results in an increase in capture efficiency when comparing nanoporous vertically aligned carbon nanotube (VACNT) post arrays with solid PDMS post arrays of the same geometry. Using both bacteria (∼1 μm) and cancer cell lines (∼15 μm) as model systems, we found capture efficiency increases by 6-fold and 4-fold respectively. The combined model and experimental platform presents a new generation of nanoporous microfluidic devices for cell isolation

Influence of process parameters and alloying type on properties of laser quenched PM-steels

Different alloyed PM steels have been laser quenched in industrial equipment laser diodes (4 kW, controlled by material surface temperature). The aim of this work is to investigate their responses to different process condition and different alloying metals, i.e. Cu, Ni, Mo, Cr and C. Furthermore the microstructure of hardened layer, heat affected zone (HAZ) and bulk zone Pre-alloyed, diffusion bonded and hybrid raw materials have been used. Design of Experiments has been the approach for evaluating the effect of treatment parameters (i.e temperature, spot size and speed) and to develop predictive models, correlating such parameters to hardening depth and scratch hardness number. Results demonstrated which valuable properties could be achieved, even through relatively low alloying. The promising results are encouraging since they allow to forecast a possible positive combination of high local hardness and wear resistance of high precision PM part

A methodology to reduce the computational effort in 3D-CFD simulations of plate-fin heat exchangers

The analysis of a plate-fin heat exchanger performance requires the evaluation of key parameters such as heat transfer and pressure drop. In this regard, computational Fluid Dynamics (CFD) can be proficiently adopted, at the design stage, to predict the performance of plate-fin heat exchangers. However, these last are often characterized by a complex geometry, such as in the case of plate exchangers with turbulators, leading to a huge computational effort, which often exceeds the available resources. In this study, a numerical methodology for the simulation of plate heat exchangers is proposed, to bypass the limits imposed by the computational cost. The methodology relies on the simulation of a minimal portion of the exchanger (two plates, one per fluid) characterized by periodic boundary conditions (that mimic the presence of several layers). The total heat exchanged is obtained simply multiplying the calculated heat transfer by the number of plate couples composing the device. Moreover, the two plates allow to calibrate porous media which are adopted to rebuild (in a simplified version) the two fluid circuits of the whole exchanger and obtain the overall pressure drop across the device for both the hot and cold fluids. The proposed approach is validated against experimental data of an oil cooler for automotive application, that is a plate-fin heat exchanger characterized by the presence of turbulators. The numerical outcomes are compared to the experiments in terms of pressure drop and heat transfer for a wide range of volumetric flow rates. Particular attention is devoted to the mesh sensitivity and the adopted computational grid minimizes the number of cells (and, thus, the computational cost), without compromising the accuracy. Moreover, the Reynolds-Stress-Transport turbulence model is accurately selected among the most diffused ones, in order to properly match the test bench data. The proposed methodology allows to reduce of nearly one order of magnitude the total number of cells required for the simulation of the heat exchanger performance. The heat transfer is predicted with high accuracy, i.e. error is always lower than 4%. As for the pressure loss, the deviation compared to the experiments increases up to nearly 15% (for one of the simulated conditions) but it is considered still acceptable