Radiotracer diffusion in refractory high-entropy alloys S. V. Divinski1, L. Rogal2, M. Deicher3, B. Grabowski4 1 Institute of Material Physics, University of Münster, Germany 2 Institute of Metallurgy and Materials Science of the Polish Academy of Sciences, Krakow, Poland 3 Experimentalphysik, Universität des Saarlandes, 66123 Saarbrücken, Germany 4 Max-Planck-Institut für Eisenforschung GmbH D-40237 Düsseldorf, Germany I will present this proposal on behalf of Sergiy Divinski, who is now attending a conference in the USA to give an invited talk. He asked me to convey his best wishes to the INTC-committee and all people in the audience. Sergiy is an associate professor at our Institute in Münster . He has an impressive record of about 160 publications mainly on diffusion in metals. He is also supervising scientist of the certified radiotracer lab at the Institute of Material Physics.

High Entropie Alloys (HEAs) Examples: CoCrFeMnNi & AlHfScTiZr > 5 metallic components ~ equal proportions disordered solid solutions single phase, no precipitates High Entropie Alloys consist of 4 or more different metallic elements, in about equal proportions. In the simplest case, the atoms are disordered and do not contain crystalline precipitates. HEAs have remarkable thermal and mechanical properties which are promising for practical applications. HEAs is a hot topic and the subject of new scientific programs all over the world. Some basic questions concern the role of the following features: B.S. Murty, J.W. Yeh, S. Ranganathan, High Entropy Alloys, Elsevier (2014)

Core effects of High Entropie Alloys (HEAs) Examples: CoCrFeMnNi & AlHfScTiZr high entropy severe lattice distortion "cocktail" effect sluggish diffusion High Entropie Alloys consist of 4 or more different metallic elements, in about equal proportions. In the simplest case, the atoms are disordered and do not contain crystalline precipitates. HEAs have remarkable thermal and mechanical properties which are promising for practical applications. HEAs is a hot topic and the subject of new scientific programs all over the world. Some basic questions concern the role of the following features: ??? B.S. Murty, J.W. Yeh, S. Ranganathan, High Entropy Alloys, Elsevier (2014)

Preliminary work: Tracer diffusion in FeCoCrMnNi 700 mm Recent work of the Divinski group was focused on this special alloy, which crystallizes in the fcc structure. Indeed, it is demonstrated by this X-ray diffractogram that the alloy of interest – with 5 constituting elements - only contains peaks indicative of a single fcc phase. The random alloy character is supported by the homogeneous distribution of all elements, as shown by EDX analysis. There is also information about the grain structure by Electron Back Scattering Diffraction. The picture here shows the sample geometry and the sectioning treatment used for the measurement of diffusion profiles.

Preliminary work: 63Ni profiles CoCrFeNi CoCrFeMnNi The Divinski group has investigated self-diffusion in HAEs. Here you see diffusion profiles of Ni-63 in 4- and 5-element fcc high entropy alloys arising from annealing at different temperatures. These profiles are of Gaussian type as demonstrated by the straight lines in logC-vs.-x-squared plots, where x denotes penetration depth. Maybe I should emphasize that Ni is a constituting element in these alloys. JALCOM 688 (2016) 994-1001. 5

Preliminary work: Diffusion of different elements in CoCrFeMnNi Tracer Q (kJ/mol) D0 (m2/s) Co 276 4.52E-5 Cr 312 2.46E-3 Fe 255 1.05E-5 Mn 267 1.25E-4 Ni 304 6.24E-4 This is an overview of self-diffusion in this 5-element alloy. It can be concluded that the diffusion coefficients are in the normal range for fcc metals. This also true for the activation energies and pre-exponential factors. So diffusion is not slow or sluggish in this particular alloy. This contrasts with reports in the literature which find indications for slow diffusivity in other high entropy alloys. Conclusion: diffusivities in the normal range for fcc metals ! 6

System to study: Al-Hf-Sc-Ti-Zr HEA Phase stability / ordering (a  a2) Rogal, Bobrowski, Körmann, Stein, Grabowski, submitted Goal of present project: understand phase stability, ordering and atom diffusion in Al-Hf-Sc-Ti-Zr HEA Diffusion of 46Sc / 48Sc and 173Hf in AlHfScTiZr alloy: Measurements at ISOLDE using ODC (available) 48Sc (Ta target, implantation time about 1 hour, 43.7 h half time) 173Hf (Ta target, implantation time ~30 min, 23.6 h half time) 46Sc (Ti target, ~30 min, 83.8 d half time) – ideal for off-site measurements in Münster These measurements will be combined with tracer experiments on 44Ti & 95Zr in Münster The Divinski group plans to do experiments on this special HAE. In particular it is intended to characterize the diffusivity of all alloy components. Titanium and zirkonium will be investigated in Münster. However, for Sc and Hf, the proposal relies on the ISOLDE /ODC facilities. In particular, this concerns Sc-48 and Hf-173, having half-lifes of 44 h and 24 h, respectively. An alternative isotope is Sc-46, which seems to be ideal for off-site measurements in Münster after implantation at ISOLDE. The main goal is to understand phase stability, ordering and atomic diffusion in this alloy. For this purpose, a full diffusion database will be required. Interpretation of the data further relies on theoretical calculations of point defect properties and diffusion barriers. This will be done in cooperation with the Max-Planck-Institute in Düsseldorf. Full diffusion database for the HEA will be produced! In parallel, finite temperature DFT calculations (B. Grabowski, MPIE Düsseldorf) – point defects, jump barriers

Refractory Al-Hf-Sc-Ti-Zr HEA Effect of Cu on phase stability and decomposition in Al-Hf-Sc-Ti-Zr HEA Diffusion of 67Cu in AlHfScTiZr-Cu0.5 alloy: Measurements at ISOLDE using ODC (available) 67Cu (UCx target, implantation time ~30 min, 2.6 d half time) Challenge: 29Al (6.6 min half time) – still it is worth to try! It is also interesting to study the effect of a common impurity element on phase stability and decomposition in this alloy. This goal can be achieved in cooperation with my own project on Cu diffusion in thin-film solar cells. An absolute challenge is the measurement of Al, which is also a component of the HAE of interest. This is due to the fact that the required Al-29 isotope has a half-life of only 6.6 minutes. In summary, this proposal requests 11 shifts over 3 years. Summary of requested shifts: 11 shifts (3 years) share beamlines for ions we are not limited to the usage of the mentioned isotopes of Sc, Hf or Cu main requirements: acceptable half-life (at least 10 hours) and good yields for g-rays