Rapid in situ alloying of CoCrFeMnNi high-entropy alloy from elemental feedstock toward high-throughput synthesis via laser powder bed fusion

High-entropy alloys(HEAs)are considered alternatives to traditional structural materials because of their superior mechanical,physical,and chemical properties.However,alloy composition combinations are too numerous to explore.Finding a rapid synthesis method to accelerate the development of HEA bulks is imperative.Existing in situ synthesis methods based on additive manufacturing are insufficient for efficiently controlling the uniformity and accuracy of components.In this work,laser powder bed fusion(L-PBF)is adopted for the in situ synthesis of equiatomic CoCrFeMnNi HEA from elemental powder mixtures.High composition accuracy is achieved in parallel with ensuring internal density.The L-PBF-based process parameters are optimized;and two different methods,namely,a multi-melting process and homogenization heat treatment,are adopted to address the problem of incompletely melted Cr particles in the single-melted samples.X-ray diffraction indicates that HEA microstructure can be obtained from elemental powders via L-PBF.In the triple-melted samples,a strong crystallographic texture can be observed through electron backscatter diffraction,with a maximum polar density of 9.92 and a high ultimate tensile strength(UTS)of(735.3±14.1)MPa.The homogenization heat-treated samples appear more like coarse equiaxed grains,with a UTS of(650.8±16.1)MPa and an elongation of(40.2%±1.3%).Cellular substructures are also observed in the triple-melted samples,but not in the homogenization heat-treated samples.The differences in mechanical properties primarily originate from the changes in strengthening mechanism.The even and flat fractographic morphologies of the homogenization heat-treated samples represent a more uniform internal microstructure that is different from the complex morphologies of the triple-melted samples.Relative to the multi-melted samples,the homogenization heat-treated samples exhibit better processability,with a smaller composition deviation,i.e.,≤0.32 at.%.The two methods presented in this study are expected to have considerable potential for developing HEAs with high composition accuracy and composition flexibility.

Studies on Acid Leaching Methods for Fine Metal Powder Production

Two different processes (i) alloying followed by selective leaching of alloying constituent and (ii) controlled chemical attack of oversize powder stock were studied in some detail to assess their suitability for metal powder production. In a typical series of experiments on the alloying process, titanium, zirconium and nickel were alloyed with aluminium. The sample was then leached out with acid to yield the metals in powder form. The metal powders generally had a particle size spread in the range of <5 μm. The acid concentration and contact time were varied and both have influences on particle size of the final product. In the second process involving chemical attack of coarse powder, it is shown by taking the example of nickel that a range of particle size could be generated through close control of acid concentration and contact time.

Microstructure Evolution and Mechanical Properties of a SMATed Mg Alloy under In Situ SEM Tensile Testing

Surface mechanical attrition treatment（SMAT） has been recently applied to bulk polycrystalline magnesium（Mg） alloys with gradient grain size distribution from the impact surface to inside matrix, hence effectively improving the alloys＇ mechanical performances. However, in-depth understanding of their mechanical property enhancement and grain size-dependent fracture mechanism remains unclear. Here,we demonstrated the use of in situ micro-tensile testing inside a high resolution scanning electron microscope（SEM） to characterize the microstructure evolution, in real time, of SMATed Mg alloy AZ31 samples with different grain sizes of ~10 μm（＇coarse-grain sample＇） and ~5 μm（＇fine-grain sample＇）, respectively, and compared the results with those of a raw Mg alloy AZ31. The quantitative tensile tests with in situ SEM imaging clearly showed that fracture of ‘fine-grain sample＇ was dominated by intergranular cracks,while both trans-granular and intergranular cracks led to the final failure of the ‘coarse-grain samples＇.It is expected that this in situ SEM characterization technique, coupled with quantitative tensile testing method, could be applicable for studying other grain-refined metals/alloys, allowing to optimize their mechanical performances by controlling the grain sizes and their gradient distribution.