Microstructural evaluation and nanohardness of an AlCoCuCrFeNiTi high-entropy alloy

An AlCoCuCrFeNiTi high-entropy alloy(HEA) was prepared by mechanical alloying and sintering to study the effect of Ti addition to the widely studied AlCoCuCrFeNi system. The structural and microstructural characteristics were investigated by X-ray diffraction(XRD), scanning electron microscopy(SEM), and transmission electron microscopy(TEM). The formation of four micrometric phases was detected: a Cu-rich phase with a face-centered cubic(fcc) structure, a body-centered cubic(bcc) solid solution with Cu-rich plate-like precipitates(fcc), an ordered bcc phase, and a tetragonal structure. The XRD patterns corroborate the presence of a mixture of bcc-, fcc-, and tetragonal-structured phases. The Vickers hardness of the alloy under study was more than twice that of the AlCoCuCrFeNi alloy. Nanoindentation tests were performed to evaluate the mechanical response of the individual phases to elucidate the relationship between chemical composition, crystal structure, and mechanical performance of the multiphase microstructure of the AlCoCuCrFeNiTi HEA.

Influence of ZnO nanoparticles on the microstructure of a CoCrFeMoNi matrix via powder metallurgy

In the last decade,extensive research has been carried out on the microstructural behavior of high-entropy alloys(HEA),for which the in-situ formation of nanoparticles has been reported.However,studies of the incorporation of nanoparticles in HEA have been rarely reported.In this work,the addition of zinc oxide nanoparticles(ZnO NP)as reinforcement in a CoCrFeMoNi high-entropy alloy matrix,as well as the morphological,structural,and microstructural evolution of composites synthesized via powder metallurgy,were studied.Scanning electron microscopy and X-ray diffraction analysis were performed in order to study the microstructural and phase characterization of the composites.After sintering,it was found that the ZnO NP addition(0.5 wt%,1 wt%and 2 wt%)had a significant influence on the microstructure and hardness of the CoCrFeMoNi high-entropy alloy.Stronger bonding among metal particles was promoted with the additions of Zn O NP.A reduction in porosity as a function of ZnO NP content was also observed.The microhardness results showed that the composite reached its highest reinforcement in bulk samples with 1 wt%ZnO NP(HV 870),which represented a 20%improvement over the unreinforced HEA matrix.

Effect of brazing parameters on fillet size and microstructure of cladded fin-microchannel tube joints

This work concerns the optimization of furnace brazing conditions for joining micro-multiport aluminum tubes and fins made with AA4343/AA3003/AA4343 brazing sheet in mini-assemblies mimicking the core of an automotive heat exchanger.Taguchi method was used for design of experiment,considering five process parameters with two levels of values.The aim was to maximize the fillet size of the brazed joints,which has an important influence on the thermal integrity and mechanical properties.Fillet length measurements of brazed joints were performed with a metallographic microscope.The statistical analysis allowed to obtain the optimum values of process parameters(peak temperature,residence time,heating rate,microchannel tube type and flux).At a 95%confidence level,the variability of fillet length is most significantly affected by the peak brazing temperature(77%),residence time(15%)and heating rate(7%).The predicted maximum fillet length was(152±11)μm,which was corroborated by confirmation trials.The microstructural analysis of tube−fin joints showed that variations in peak temperature and residence time affect only the size of the eutectic zone of fillet formed,but not the nature or composition of the constituent phases.