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HEAs represent a relatively new class of materials. The the alloy concept is fundamentally different from the most conventional materials and alloys that are used today. Recently, the focus of HEA designs is more application-based. For that purpose, the elements of interest are carefully selected and multiple phases as well as micro-structures are deliberately adjusted. Currently, only limited attention has been paid to weldability of HEA. This encompasses possible effects on metallurgy and its influence on the desired properties. It remains open if welding causes e.g. considerable number of intermetallic phases or segregations and their effect on weld joint properties. For that reason, the scope of this study is to summarize already available studies on welding of HEAs with respect to the HEA-type, the applied welding process and its influence on the weld joint properties.
Hochentropie-Legierungen (HEAs) zeichnen sich durch einen Mischkristall-System aus mindestens fünf und Mittelentropie-Legierungen (MEAs) durch mindestens drei Hauptlegierungselemente aus, in äquiatomarer Zusammensetzung. Sie zeigen außergewöhnliche Anwendungseigenschaften, wie z.B. hohe Festigkeit, Duktilität oder Korrosionsbeständigkeit. Zukünftige HEA/MEA-Komponenten aufgrund ihrer Eigenschaften für wasserstoffhaltige Umgebungen (wie Behälter für kryogene oder Hochdruckspeicherung) von Interesse. Daher ist die Bewertung der Wasserstoffabsorption und die Diffusion in diesen Materialien von großer Bedeutung. Dazu wurden in unserer Studie eine CoCrFeMnNi-HEA und eine CoCrNi-MEA untersucht. Die Proben wurden elektrochemisch mit Wasserstoff beladen. Für die Ermittlung des Wasserstoffdiffusionsverhaltens wurde die thermische Desorptionsanalyse (TDA) mit unterschiedlichen Heizraten bis zu 0,250 K/s angewandt. Die nachfolgende Peakentfaltung der Signale führte zu Hochtemperatur-Desorptionsspitzen und Wasserstofftrapping auch über 280°C. Eine resultierende Gesamtwasserstoffkonzentration > 40 ppm wurde für den MEA ermittelt und > 100 ppm für den HEA. Dies deutet auf zwei wichtige Effekte hin:
(1) verzögerte Wasserstoffdiffusion und (2) eine beträchtliche Menge an getrapptem Wasserstoff auch bei hoher Temperatur. Beide Effekte können hinsichtlich einer wasserstoffunterstützten Rissbildung kritisch werden, dies erfordert jedoch weitere Untersuchungen. Zusätzlich erfolgte die Bestimmung des lokalen Volta-Potentials mittels hochauflösender Kelvin-Sonden-Kraft-Mikroskopie (SKPFM). Die ermittelten Scans zeigen einen bestimmten Einfluss der Wasserstoffbeladung auf die Potentiale.
This presentation summarizes the latest results on the BAM-Themenfeld project SURDIA on processing of high-entropy alloys (HEAs) at BAM. At first, the influence of machining by ultrasonic-assisted milling on the surface integrity is presented. Second, the weld processing by Tungsten Inert Gas (TIG) welding is presented and the results of the Friction Stir Welding (FSW), which is conducted at BAM for the first time.
Multiple principal element alloys (MPEA) encompass the well-known high entropy alloys (HEAs). MPEA/HEA represent a new class of materials consisting of at least three alloying elements, each containing 5 to 35 at.-%. This alloying concept thus differs fundamentally from conventional materials such as steel or nickel alloys. For this purpose, the alloying elements are specifically selected, and the microstructures are adjusted in a single-phase and, in some cases, multi-phase manner. In particular, conflicting goals, such as the trade-off between strength and ductility in conventional steels, are overcome. In the last 20 years, however, the focus has been on material synthesis. With the increase in available material quantities, the focus is now on pro-cessing issues such as joining and welding processes. The weldability of MPEA has received very little atten-tion so far. Experience with dissimilar metal welds (DMWs) is completely lacking but is essential for the appli-cation of these materials in combination with conventional materials. The present study presents, comprehen-sive experimental results on the weldability of MPEA-DMWs. For that purpose, a Co20Cr20Fe20Mn20Ni20 HEA in cold-rolled and heat-treated condition was joined by means of tungsten inert gas welding (TIG) with the austenitic Cr-Ni steel AISI 304. The DMWs resulted in interesting mechanical properties. They were obtained by instrumented tensile tests as well as the local deformation in the weld area by using digital image correlation (DIC) technique. A significant softening in the heat-affected zone (HAZ) of the MPEAs as well as a slightly reduced tensile strength with a significant decrease of the elongation at fracture were found. The experiments provided proof in principle of the weldability of the MPEAs for DMWs with conventional materials that ensure a corresponding capability for mechanical loading. This allows further considerations on the application of these innovative materials.
Multi-principal element alloys (MPEAs) are innovative materials that have attracted extensive research attention within the last decade. MPEAs are characterized by a solid solution of equiatomic metallic elements. Depending on the number of elements, they are also referred as high entropy alloys (HEAs with n ≥ 4 elements like CoCrFeMnNi) and medium-entropy alloys (MEA with n = 3 elements CoCrNi). Depending on the alloy concept, MPEAs show exceptional properties in terms of mechanical performance or corrosion resistance at extreme environments. In that connection, hydrogen and its challenges for the most metallic materials gets more and more important. MPEAs are candidate materials for the substitution of conventional materials like austenitic stainless steels e.g., at very high-pressure up to 1000 bar. Those pressures are typically reached in valves or compressors for refueling of tanks with operational pressure of 700 bar. So far, the susceptibility of HEA/MEAs to hydrogen assisted cracking (if any) and the especially the underlying hydrogen uptake and diffusion was not within the scientific scope and not investigated in detail yet. For that reason, we focused on the hydrogen absorption the characterization of the hydrogen diffusion and trapping at elevated temperatures in a CoCrFeMnNi-HEA (each element with 20 at.-%) and CoCrNi-MEA, each element with 33.3 at.-%). As reference grade, the commercially available austenitic stainless steel AISI 316L was investigated. High-pressure hydrogen charging was conducted at different pressures in autoclave environment with maximum value of 1,000 bar. Thermal desorption analysis (TDA) via carrier gas hot extraction with coupled mass spectrometry was used with a max. heating rate of 0.5 K/s up to 650 °C. The measured desorption spectra of the different samples were deconvoluted into a defined number of individual peaks. The individually calculated peak temperatures allowed the definition of activation energies for predominant trap sites in the respective materials as well as the percentage share of the totally absorbed hydrogen concentration. The results present for the first time the complex interaction of both MPEAs and high-pressure hydrogen charging. A deconvolution of four peaks was selected and a main desorption peak was identified the dominant hydrogen trap containing the biggest share of the absorbed hydrogen concentration. The chemical composition an austenitic phase of both MPEAs is responsible for delayed hydrogen diffusion and strong, but mostly reversible, trapping. The comparison with the 316L samples showed significantly higher activation energies in the MPEAs, whereas hydrogen was also trapped at very high extraction temperatures. The absorbed maximum hydrogen concentration at 1,000 bar was 130 ppm for the CoCrFeMnNi-HEA, 50 ppm for the CoCrNi-MEA and 80 ppm for the 316L. It is interesting that the CoCrFeMnNi-HEA has obviously a way higher trapping capability compared to the conventional austenitic 316L, which could be a major advantage in terms of resistance to hydrogen assisted cracking.
The presentation gives an overview of BAM's activities on processing influences and application properties of MPEAs in the form of joined and machined high and medium entropy alloys (CoCrFeMnNi and CoCrNi). In the case of welding, the focus is on defect-free welded joints with sufficient mechanical properties. In the case of machining, the focus is on the possible influence on the surface quality of the materials through adequate milling parameters. In addition, the hydrogen absorption and diffusion properties as well as the electrochemical corrosion behavior are fundamentally examined.