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In this work, the effect of Cu on the phase transformation, microstructure, and properties of AlCrFeNiTiCux high-entropy alloys (HEAs) was investigated. Meanwhile, the formation mechanism of the FCC solid solution was discussed. The results showed that it was a beneficial effect on the formation of the FCC solid solution for the addition of Cu into HEAs from the perspective of valence electron concentration (VEC), enthalpy of mixing (ΔH mix) and difference in atom size (δ), which was confirmed by X-ray diffraction and Scanning Electron Microscope analysis. In addition, the increment of Cu content in the alloys promoting the segregation of Al, Ni and Ti in dendrite and Fe, Cr segregated into particles distributed in dendrite and inter-dendrite. The micro-hardness of the HEAs decreased with increasing Cu content and as the Cu content increased, oxidative wear intensified. However, the friction coefficient was significantly reduced, which was related to the lubricity of Cu itself. Besides, the segregation of Cu increased the corrosion current density of the passivation zone of the HEAs, thereby reducing the corrosion resistance of the HEAs in 0.5 mol/L H 2 SO 4 solution. • Added Cu promotes FCC phase forming. • Cu improves the wear resistance. • Cu element reduces the corrosion resistance. [ABSTRACT FROM AUTHOR]

Directionally solidified Mn–20Cu–5Ni–2Fe alloy under different pulling rates were investigated by means of dynamic thermal mechanical analysis, optical microscope, X-ray diffraction and scanning electron microscope. It has been found that the favorable microstructure can be tuned through adopting appropriate pulling rate during the process of directionally solidification to approach ultra-high damping performance by enhancing the mobility of twin and dislocation. It is disclosed that the microstructure characterized by regular arranged dendrites with fluctuated composition, single parent phase with favorable orientation, large low-angle misorientation grains, and micron-scaled twins with the low angle misorientation boundaries, can leads to a substantially increase of twin boundary IF up to 2 times higher than the present ordinary manufacturing process. The result has important guiding significance for the research and preparation of ultra-high damping Mn–Cu based alloy. • The parent-phase twinning has been induced by antiferromagnetic transition. • RSLM effect of directionally solidified Mn–Cu alloy significantly improves the damping capacity. • (111) orientation enhances the mobility of twins and dislocations. • Internal friction (IF) is a function of the pulling rate and the secondary dendrite spacing. [ABSTRACT FROM AUTHOR]

In the present study, a systematic investigation on the effect of Fe content on the Hall-Petch coefficient of (CoCrMnNi) 100- x Fe x (x = 20, 40, 50, and 60) medium- and high-entropy alloys (M/HEAs) was carried out. The cold-rolled alloys were annealed at 900 °C and 1000 °C between 3 min and 10 h for recrystallization. Scanning electron microscope with a backscattered detector was used to obtain micrographs of recrystallized specimens for grain size calculation. Tensile testing was used to evaluate the mechanical properties of the alloys. The microstructure showed that regardless of the alloy composition, the grain size was approximately similar when subjected to the same heat treatment condition. Moreover, all the alloys obeyed the classical Hall-Petch relationship. Friction stress (solid solution, SS strengthening) decreased with an increase of Fe content, which was attributed to weak lattice distortion caused by the reduction of the atomic size misfit. The Hall-Petch coefficient, which represents grain boundary (GB) strengthening, also decreases as the Fe content increases. A linear relationship between intrinsic stacking fault energy and Hall-Petch coefficient was found not to exist. However, it is proposed that the monotonic decrease of the Hall-Petch coefficient is directly related to the unstable stacking fault energy (γ USFE). As a result, an increase of Fe content in (CoCrMnNi) 100- x Fe x alloy system leads to an increase of γ USFE, which in turn weakens GB strengthening (Hall-Petch coefficient). Moreover, HEAs and MEAs with higher Fe content tend to have low yield strength due to weak contributions from both SS and GB strengthening. Therefore, to design superior MEAs and HEAs with enhanced strength, the choice of principal elements and their respective contents is imperative for an optimized contribution from both SS and GB strengthening mechanisms. • At a given heat treatment condition the grain size is fairly constant regardless of the alloy composition. • Friction stress and Hall-Petch coefficient decreases as the Fe content is increased. • Solid solution and grain boundary strengthening are significantly reduced at higher Fe content. • No linear relationship between intrinsic stacking fault energy and Hall-Petch coefficient was established. • Unstable stacking fault energy is directly proportional to the the Hall-Petch coefficient. [ABSTRACT FROM AUTHOR]

The corrosion properties of selective laser melted (SLM) and of additionally heat-treated (T6) Al-3.5Cu-1.5 Mg–1Si samples were assessed in 0.001 M and 0.1 M NaCl solutions (pH 7) by means of electrochemical polarization measurements and SEM surface analysis. In both solutions these new alloy types exhibited improved corrosion behavior compared to wrought and heat-treated AA 2024 reference states. This is attributable to more refined microstructural states and to the formation of particular phases. The SLM samples comprise finely dispersed Q-phase particles which act cathodic and thus, trigger adjacent matrix phase dissolution. Additional T6 heat treatment was efficient for attaining the lowest corrosion current density of ~0.04 μA/cm2 and a widening of the anodic passive range to ~400 mV in 0.1 M NaCl. Only this alloy state shows a repassivation ability upon reverse scanning. T6 treatment yields mainly the Mg 2 Si phase acting as local anode and a widely passive Al-based matrix phase. Altogether, SLM processing with final heat treatment of the Al-3.5Cu-1.5 Mg–1Si alloy is suitable for achieving not only superior mechanical performance but also improved corrosion stability. Image 1 • SLM and heat-treated (T6) Al-3.5Cu-1.5 Mg–1Si with refined microstructure, new phases. • New SLM alloys with better corrosion behavior than wrought AA2024 in NaCl solutions. • SLM samples with dispersed Q phase acting as cathode, triggering matrix dissolution. • T6 treated SLM alloy with best corrosion performance and repassivation ability. • T6 treated SLM alloy with Mg 2 Si phase as local anode, widely passive Al-rich matrix. [ABSTRACT FROM AUTHOR]

The processing route of the current TNM alloy (Ti-43.4Al–4Nb–1Mo-0.5B [in at %]) involves casting/HIP'ing plus forging and multiple heat treatments to achieve the required properties. This study seeks to design a modified TNM alloy with superior properties by utilising a simpler processing route, namely casting/HIP'ing plus a single-step heat treatment. The modified alloy composition was melted, cast, treated with HIP and given various heat treatments at selected temperatures near T G (gamma solvus). Images of the samples' microstructures were produced with a scanning electron microscope (SEM) and were evaluated with quantitative image analysis. Phase transformations as a function of temperature were determined using differential scanning calorimetry (DSC). These experimental results were assessed and integrated to identify a single-step heat treatment (1260 °C/1 h/30 K/min) that generates a two-phase (γ + α 2) nearly fully lamellar (NFL) microstructure in the cast/HIP-treated material. The resulting FL microstructure was characterized by refined colonies consisting of irregularly arranged laths with apparently low anisotropy, which may potentially yield higher temperature mechanical properties in comparison with TNM-NL. • Design of single-step heat treatment for a modified TNM alloy in cast/HIP state. • The minimum temperature for β-phase minimisation was set to 1260 °C. • A cooling rate of 30 K/min provided suitable microstructure for structural applications. • SEM image analysis in combination with DSC was used for phase control. [ABSTRACT FROM AUTHOR]

In this study, friction and wear behaviors of an amorphous Zr 20 Ti 20 Cu 20 Ni 20 Be 20 high-entropy alloy (a-HEA) were characterized using a nanoscratch method. Scratch tests were carried out under both ramping and constant loading conditions to measure the coefficient of friction (COF) and wear resistance of the amorphous alloy. The morphology of scratched surface and subsurface was further examined using scanning and transmission electron microscopes. It was found that friction behavior of the a-HEA can be divided into two regions: elastic and plastic. In the elastic region, the COF dramatically decreases with increasing normal force, however, it monotonically increases in the plastic region. Morphological observation indicated that brittle fracture dominated in the plastic region, which was consistent with the result that a series of load serrations were accompanied by the formation of brittle cracks along the scratched track. Wear performance of the a-HEA was also evaluated and compared with "conventional" metallic glasses and crystalline high-entropy alloys. As a result of high hardness and large elastic recovery, the current a-HEA exhibited good wear resistance and low COF, behaved more like brittle amorphous alloys rather than ductile high-entropy alloys. Image 1 • Friction and wear behaviors of an amorphous high-entropy alloy (a-HEA) were studied using a nanoscratch technique. • Friction of the a -HEA can generally be divided into two regions: elastic and plastic. • Brittle fracture dominates the plastic deformation, as manifested by load serrations and scratched surface morphology. • As a result of high hardness and large elastic recovery, the a-HEA exhibited good wear resistance and low COF. [ABSTRACT FROM AUTHOR]