Search

A method for compensation of frequency-selective (FS) in-phase/quadrature (IQ) imbalance of a wideband transceiver is proposed in the paper. It is dedicated for implementation in software defined radio (SDR) cellular base stations. Both transmitter (TX) and receiver (RX) IQ impairments are corrected by complex valued finite impulse response (FIR) filters which are designed based on previously found imbalance correction models. The compensation performance is assessed after the method was implemented in the SDR platform capable of transmitting signals at different central frequencies. At frequencies higher than 3 GHz measured IQ gain and phase error functions exhibit asymmetrical characteristic. In order to reduce the level of asymmetry, adopted IQ gain correction model incorporates odd polynomial elements while the phase correction model includes even polynomial parts. Regardless of utilized central frequency IQ impairments are efficiently compensated. The advantage of the proposed method is low complexity. The method doesn't require specialized hardware for calibration, instead, it uses the RF loopback. At central frequency of 3.5 GHz, transmitter image rejection ratio (IRR) is increased from 20 dBc to 45-50 dBc by applying the proposed method. After receiver imbalance is compensated, the improvement in IRR of more than 25 dBc is achieved.

High Entropy Alloys (HEAs) are an emerging alternative to other structural alloys that are used nowadays such as austenitic stainless steels or Ni-base superalloys, due to their favourable combination of properties such as thermal stability, corrosion and oxidation resistance and notable mechanical properties among others. Most of the FCC-HEAs contain Cobalt since induces microstructural stability and good mechanical properties. Nevertheless, Cobalt is a scarce element which is expensive (used in electric vehicles) and the HEAs community is currently investigating towards the development of Co-free HEAs to allow the industrial application of structural HEAs. In this work, the design and characterisation of a Co-free FCC HEA of the FeNiCrMn family is presented. This alloy has been computationally designed to maximize the FCC stability, avoiding the formation of other phases such as sigma and without excluding other strengthening intermetallic phases that would improve the mechanical strength. The addition of Al in small amounts is investigated in order to enhance the mechanical properties, but also with the aim to obtain an alumina protective layer. This could relevant for the application in the energy industry as a structural alloy. In this work, two different processing routes are compared to get knowledge about the production of this HEAs family: additive manufacturing by laser powder bed fusion (LPBF) and conventional routes (hot forging). This will allow to highlighting the differences found in the microstructure, mechanical behaviour and thermal stability.

High Entropy Alloys (HEAs) have gained significant attention in the scientific community over the last two decades. They can display a favourable combination of properties such as thermal stability, corrosion and oxidation resistance, and notable l mechanicaproperties. Therefore, HEAs have been identified as a promising alternative for industrial applications to some traditional structural alloys, like austenitic stainless steels and Ni-base superalloys, especially for high temperatures conditions and harsh environments. A large amount of FCC-HEAs containing cobalt has been studied and reported in the literature, but Co is considered a critical metal. As a result, part of the research in the HEAs community is nowadays the development of Co-free HEAs, trying to obtain similar microstructural stability while maintaining their beneficial properties. In this study, a Co-free FCC HEA of the Fe2Ni2CrMn family processed through Laser Powder Bed Fusion (LPBF) has been designed to maximise its FCC stability at high temperatures and to avoid the formation of other detrimental phases for mechanical properties, such as the sigma phase. Besides, the influence of small amounts of copper and molybdenum have also been investigated to enhance its mechanical properties and to study their influence on the FCC microstructural stability. The alloys have been characterised initially with Electron probe micro analysis (EPMA) and scanning electron microscopy (SEM) to study the elemental distribution. The results revealed different segregation patterns for the Cu and Mn elements depending on the printing parameters. For some conditions, segregation occurs at the grain boundaries (Figure 1(a)) and cell boundaries (Figure 2(a)) while, in others, such segregation is associate to the melt pools (Figure 1(b)) and not clearly at cell boundaries (Figure 2(b)). In order to understand these results, Transmission Electron Microscopy (TEM) measurements were performed upon the cellular structure to hav

This research aims to study the laser powder bed fusion (L-PBF) processability, the microstructure and the tensile mechanical properties of an AlFeCr alloy at a wide range of temperatures. With this goal, Al-5Fe-6Cr (wt%) pre-alloyed powder was produced by casting and gas atomization. The microstructure of the as-atomized powders and the as-built specimens was characterized via scanning electron microscopy, x-ray diffraction, and transmission electron microscopy. Following a parameter optimization study, dense as-built specimens with a high relative density of 99.8% and a Vickers hardness at room temperature of 192 HV were fabricated. In addition, the newly developed AlFeCr alloy exhibits yield strength values of 273 ± 5 MPa and 179 ± 3 MPa at 300ºC and 400ºC, respectively. The exceptional strength of this alloy at high temperature is attributed to the homogeneous precipitation during processing of large density of nanoscale icosahedral (i-phase) and intermetallic phases, which are endowed with high thermal stability. These values are significantly higher than those corresponding to the conventional wrought Al7075 aluminum alloy in the T6 condition and to all Al alloys manufactured by L-PBF to date. The current research sheds guidelines for the design of high strength aluminum alloys containing transition alloying elements that are suitable to be processed by L-PBF for high temperature structural applications in a cost effective way.

We investigated the Fe-Fe2Ti eutectic microstructure obtained by Directed Energy Deposition (DED) with a hypereutectic composition of Fe-17.6 at.% Ti. Ultrafine lamellar spacings as low as 200¿nm were achieved, features which otherwise can only be obtained in thin specimens, e.g. by suction casting. However, at interlayer boundaries (ILBs) a globular morphology of the primary Fe2Ti phase is observed with halos of the Fe phase. For the given DED conditions the crystalline structure is thus discontinuous across the ILBs. Both 2D and 3D analysis methods were used to quantify the microstructure, including high resolution synchrotron holographic X-ray computed tomography (HXCT). The generic behaviour of eutectic systems under conditions that qualitatively correspond to those of laser additive manufacturing was explored by phase-field modelling for selected nucleation scenarios and alloy compositions spanning from eutectic to hyper-eutectic. While providing valuable insights into microstructure formation, the simulations point out the need to further deepen our understanding about melting under additive manufacturing conditions in order to implement suitable nucleation and / or free growth models. The simulations also show that globular ILBs can be prevented when using exactly eutectic alloy compositions., Peer Reviewed, Postprint (author's final draft)

Rapid alloy prototyping is quickly emerging as a manufacturing approach to design novel structural alloys since allows producing low volume of high quality castings quickly at low cost. Thus, a large number of alloys can be produced and tested for finding the optimal composition in an alloy system. In this work, a new high throughput method for rapid alloy manufacturing has been developed and employed to cast AISI 304 stainless steel samples modified with an addition of Cu in the range of 1-4 wt%. The homogeneity of the microstructure and chemical composition of as-cast materials has been characterized at the macro- and micro-scale, using optical and scanning electron microscopy, x-ray fluorescence, x-ray diffraction and electron probe microanalysis. At the macro-scale, no compositional variations have been found, while variations in the distribution of phases in the microstructure were related to variations in the cooling rate and thermal path undergone across the solidifying ingot. At the micro-scale, it has been found that segregation of austenite and ferrite stabilizing elements occurs during solidification, as expected from the phase diagram. Additive Alloy Melting (ADAM) has been demonstrated to be a suitable, rapid and versatile casting method for manufacturing metallic samples on a laboratory scale with good chemical homogeneity at the macro-scale.