Materials Science Forum Vol. 941

Abstract: Laser beam welding and friction stir welding of high entropy alloys (HEA) of the CoCrFeNiMn system were studied. The HEAs were produced by self-propagating high-temperature synthesis (SHS). Along with the principal elements, Al, C, S, and Si impurities were detected in the composition of the alloys. The as-cast alloys consisted of columnar fcc grains with coarse precipitates of MnS and fine Cr-rich M₂₃C₆ carbides. Laser beam welding resulted in the formation of a defect-free weld joint. Precipitation of nanoscale B2 phase particles in the weld zone leaded to a pronounced increase in microhardness from ~150 HV of the base material to ~220 HV in the fusion zone. Friction stir welding (FSW) of a recrystallized state of the HEA with the average grain size of 3-5 μm resulted in the formation of a fine microstructure with a grain size of ~1.5 μm in the most strained area. Noticeable rise in strength and some decrease in ductility of the processed alloy in comparison with the initial condition can be associated with the formation of nanosized M₂₃C₆ carbides.

Abstract: Structural modification of ternary aluminium – zinc – coper alloys influences their wear properties. In a series of studies, Al – 30 wt. % Zn – 3 wt. % Cu alloy (Al-30Zn-3Cu) has been doped with 1 wt.% Mn introduced with AlMn-based master alloys. The alloy microstructure and mechanical properties have been studied using light and SEM/EBSD microscopy, and measurements of wear resistance and dimensional changes. Reducing Cu content to 3 wt.% and doping with 1 wt.% Mn allows obtaining alloy of significantly refined grains, improved wear resistance and preserved high dimensional stability.

Abstract: The present work investigates the influence of shear deformation on microstructural-texture and mechanical behavior of ZEW200 Mg alloy sheets. For the introduction of extra shear deformation during thermomechanical processing, the separate effect of differential speed rolling (DSR) and equal channel angular pressing (ECAP) were analyzed. The results were compared with the microstructure and mechanical behavior of equal speed rolled (ESR) ZEW200 sheets. No significant texture changes were observed after the utilization of DSR, while ECAP processing was effective in changing the character of the texture and reducing the texture intensity. The large yield stress asymmetry observed in the rolled sheet is strongly reduced in the ECAP processed sheets. Results showed the potential to use shear deformation to modify the crystallographic texture via the profuse activation of {10-12}<10-11> extension twins. The presence of a large twin fraction of the microstructure modified the work hardening behavior of the processed sheets due to the further activation of basal <a> slip. The application of extra shear deformation to tailor the texture during processing is, therefore, an alternative to optimize the deformation behavior of already formable Mg alloys.

Abstract: In this work, Al-Fe-(Cu) alloys for aluminum cables were designed and the related annealing behaviors were discussed in detail to help understand the influence of processing and heat treatment on the electrical conductivity and mechanical properties of the studied alloys. The interaction between different solute elements was tracked by using hardness and electrical conductivity testing. The microstructure was investigated by using Electron Backscattered Diffraction (EBSD) technique, along with Scanning Electron Microscopy with Backscattered Electron Detector (BSE). The results show that the conductivities of Al-Fe-Cu alloy increased with the elongated annealing time, and reaches its maximum at 6 h, when annealed at temperatures from 275 °C to 375 °C. The addition of Fe to Al can strengthen the alloy and decrease its conductivity slightly, while the addition of Cu will influence the alloy conductivity significantly. The morphologies of precipitates will change with different amount of alloying elements as well.

Abstract: TiAl alloys are increasingly used as a lightweight material, for example in aero engines, which also leads to the requirement for suitable repair techniques. Transient liquid phase bonding is a promising method for the closure of cracks (in non-critical or non-highly loaded areas). The brazing solder Ti-24Ni was investigated for brazing the alloy Ti-45Al-5Nb-0.2B-0.2C (in at. %). After brazing, the joint exhibits different microstructures and phase compositions. The transient liquid phase bonding process was investigated in the middle of the joint region in situ to acquire time resolved information of the phases, their development, and thus the brazing process. These investigations were performed using high-energy X-ray diffraction at the “High-Energy Materials Science” beamline HEMS, located at the synchrotron radiation facility PETRA III at DESY in Hamburg, Germany. For this, we used an induction furnace, which is briefly described here. During the analysis of the diffraction data with Rietveld refinement, the amount of liquid was refined with Gaussian peaks and thus could be quantified. Furthermore, while brazing four different phases occurred in the middle of the joint region over time. Additionally, the degree of ordering of the β_o phase was determined with two ideal stoichiometric phases (completely ordered and disordered). Altogether, the phase composition changed clearly over the first six hours of the brazing process.

Abstract: Aging treatments of an Al-9.0Zn-2.0Mg-2.0Cu alloy, which belongs to high strength aluminum alloy widely used in aerospace industry, are investigated by various techniques, including hardness, electrical conductivity, mechanical properties, transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). The result shows that hardness and conductivity for one-step aging treatment increase with aging time prolongs while those for two-step aging treatment exhibit increment and decrement, respectively. Besides, the ultimate tensile strength (UTS) and yield strength (YS) for one-step and two-step aging treatments show slow increase and obvious decrease, respectively. Based on these, typical T6 and T76 aging regimes are extracted for microstructure observation. The matrix precipitates for the T6 alloy have small size and dispersive distribution while that for the T76 alloy has big size and sparse distribution. The grain boundary precipitates for both exhibit discontinuous distribution and the T76 alloy has larger size and broader precipitate free zones. The selected area diffraction patterns and HREM observations reveal that main precipitates for the T6 alloy are GPI zone, GPII zone and η' phase while for the T76 alloy are η' phase and η phase.

Abstract: In this study, the effect of Mg/Si ratio, Cu content and/or pre-ageing treatment (e.g. 100 °C for 2 h and/or 200 °C for 20 s) on the stretch formability of 6xxx alloys was investigated through their influence on the work hardening and strain-rate hardening behaviour using tensile testing and forming limit diagram tests. The results showed that a high Mg/Si ratio, a low Cu content and/or the employment of pre-ageing could deteriorate the stretch formability due to the decrease in work hardening and/or strain rate hardening capabilities. Moreover, the stretch formability was observed to have an opposite correlation with the paint-bake response of the alloys studied.

Abstract: A Zn-added Al-Mg-Si-Cu alloy during aging at 170 °C up to 34 h exhibits an interesting age-hardening effect. Small clusters, enriched in Mg and Si, are present in the sample after 0.25 h aging. The β′′ phase is dominant with the peak hardness of 135 HV after aging of 8 h. A decrease in hardness of the alloy occurs with the aging time increasing to 34 h, due to the coarsening of β′′ phase. It is also found that the Cu-containing L phase co-exists with the β′′ phase at this aging condition. The quantitative solute concentrations of the matrix show that the formation of clusters is consistent with the slight lower contents of Mg, Si and Cu compared with the alloy chemical composition, and the present of β′′ and L phase is associated with the further partitioning of Mg, Si and Cu from the Al matrix into the precipitates. No Zn-rich clusters and precipitates are observed and the Zn concentration in matrix has no significant change during aging for up to 34 h. This result means that the major of Zn remains in the matrix as aging continues.

Abstract: The degradation of creep resistance in Nickel-based single crystal superalloys is essentially ascribed to their microstructure evolution. Yet there is a lack of work that manages to simulate the effect of alloying element concentrations on microstructure degradation. In this research, a computational model is developed to connect the rafting kinetics of Ni superalloys with their chemical composition, by combining thermodynamics calculation and an energy-based microstructure model. The isotropic coarsening rate and γ/γ misfit stresses have been selected as composition related parameter, and the effect of service temperature, time and applied stress are also taken into consideration to simulate the evolutions of microstructure parameters during creep process. The different generations of commercial Ni superalloys are selected and their chemical compositions are calculated based on this model. The simulated microstructure parameters are validated by the results from experimental results and the existing analytical model. The capability of the model in predicting the microstructure characteristics may provide instructional thought in developing a novel computational guided design approach in Ni superalloys.

Abstract: High and medium entropy alloys are an intriguing new material class, revered for possessing an exceptional combination of primarily mechanical properties. In this scientific contribution, the CoCrNi medium entropy alloy has been produced by a combination of mechanical alloying (MA) and spark plasma sintering (SPS). The properties have been characterized by means of X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and tensile testing. The powder after the MA process exhibited nano-grained microstructures with fine chemical homogeneity. After the SPS densification, a full density of the bulk products has been achieved. The microstructure was composed of major FCC phase and minor secondary precipitates. The materials possessed high strength values coupled by reasonable ductility levels, therefore overall satisfactory results.