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High-throughput structural and functional characterization of the thin film materials system Ni-Co-Al Peer Decker, Dennis Naujoks, Dennis Langenkaemper, Christoph Somsen, and Alfred Ludwig ACS Comb. Sci., Just Accepted Manuscript • DOI: 10.1021/acscombsci.6b00176 • Publication Date (Web): 24 Jul 2017 Downloaded from http://pubs.acs.org on July 28, 2017

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High-throughput structural and functional characterization of the thin film materials system Ni-CoAl Peer Decker, Dennis Naujoks, Dennis Langenkämper, Christoph Somsen, Alfred Ludwig Institute for Materials, Ruhr-Universität Bochum, 44780 Bochum, Germany Keywords: x-ray diffraction, sputtering, materials library, combinatorial materials science, magnetooptical Kerr effect, transmission electron microscopy

Abstract High-throughput methods were used to investigate a Ni-Co-Al thin film materials library, which is of interest for structural and functional applications (superalloys, shape memory alloys). X-ray diffraction (XRD) measurements were performed to identify the phase regions of the Ni-Co-Al system in its state after annealing at 600°C. Optical, electrical and magneto-optical measurements were performed to map functional properties and confirm XRD results. All results and literature data were used to propose a ternary thin film phase diagram of the Ni-Co-Al thin film system. Introduction A Ni-Co-Al materials library is investigated to derive a thin film phase diagram based on combinatorial methods and mappings of optical, electrical and magneto-optical properties. The materials system Ni-Co-Al is the basis for applications like Ni- and Co-based superalloys 1,2 or potential high temperature shape memory alloys 3,4. The Ni3Al-based phase is the most important precipitate in Ni-based superalloys 5 since it contributes significantly to creep resistance 6. Co is a prominent alloy element in Ni-based superalloys up to 20% 7, providing a better solubility of the Ni3Al precipitates in the Ni-based matrix. Another interesting phase is (Ni,Co)Al, showing a ferromagnetic shape memory behavior in the composition range of NiCo30–45Al27–32 8. Understanding in which composition ranges these phases appear is a requirement for using and optimizing their properties. Combinatorial materials science allows the analysis of material properties in a wide composition range on thin film materials libraries using automatic measurements methods 9. Automated X-ray diffraction (XRD) is often applied to acquire structural information of the materials libraries. Using cluster analysis methods, the XRD data of materials libraries can be sorted into groups of similar properties 10. To verify and extend the structural analysis, functional properties can be additionally measured. If the structural analysis is correct, then the functional properties should correlate to the identified phase regions. The magneto-optical Kerr effect (MOKE) is used to identify ferro- or ferrimagnetic areas in materials libraries 11. Experimental Two Ni-Co-Al materials libraries were deposited at room temperature on a 100 mm diameter c-plane sapphire wafer and a 100 mm diameter thermally oxidized Si wafer using a multilayer approach with a moving shutter and a rotating substrate holder 9. The wedge-type multilayer approach is necessary for covering complete ternary systems. While sputtering one material at a time the shutter covers ACS Paragon Plus Environment

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the substrate at first and is then retracted with a defined speed. In this way, a thin film wedge from 0 nm to 12 nm of the respective material is deposited. Then the substrate is covered again by the shutter and rotated by 120°. The system is changed to the cathode with the second material and the procedure is repeated followed by the third element. The process is repeated until 60 layers are sputtered and a nominal total film thickness of 600 nm of the multilayered precursor structure is reached. The thin film materials library deposited on the Si wafer was annealed for 1 h at 600°C, whereas the materials library on sapphire was annealed for 20 h at 600°C in vacuum (2.6∙10-7 mbar) to completely mix the precursor layers and get a homogenous and well-crystallized thin film. Both materials libraries were cooled down to room temperature with an average cooling rate of 50°C/min. Most measurements were performed in an array of rectangular measurement areas, with a distance of 4.5 mm between the measurement areas, resulting in 342 investigated measurement areas. The composition of the materials library was determined in a scanning electron microscope (SEM, Jeol JSM5800LV) using energy dispersive x-ray spectroscopy (EDX, Oxford Inca X Act SiLi detector). The acceleration voltage was 20 kV. Since the Al-signal from the sapphire substrate would falsify the measured Al concentration, the chemical composition was determined on the library deposited on the Si wafer. To verify the mixture of the precursor single layers, a TEM cross-sectional sample from each annealed materials library was prepared by FIB-lift-out (FEI Helios G4 CX) and investigated in a transmission electron microscope operated with 200 kV acceleration voltage (TEM; FEI Tecnai F20 G2). The sample was taken from the center of the wafer (Ni34Co29Al37) since here all precursor layers have a similar thickness and the complete mixture at this position can be identified. X-ray diffraction (XRD) measurements were performed with a Bruker D8 Discover with a Vantec500 area detector. An IµS source with a Cu cathode was used. A Montel optic filtered Cu Kβ radiation leaving only Cu Kα for diffraction. The illuminated area had a diameter of 1 mm. The acquired patterns were integrated into one-dimensional data sets (diffracted intensity in dependence of the diffraction angle 2θ) and were then used for phase identification. The magneto-optical Kerr effect (MOKE) was measured with a modified setup of a high-throughput test stand 12 using a red laser diode and an electro magnet with a maximal flux density of 0.3 T. MOKE can determine magneto-optical anisotropy of magnetic materials, using two different laser-tomaterials library orientations; here it was used to identify ferro- or ferrimagnetic regions in the materials library. The acquired hysteresis loops are fitted using a dose response fit function. The coercivity HC and the normalized maximum intensity were determined. MOKE is not able to measure the absolute magnetization since surface effects like oxidation and roughness affect the MOKE signal as well. Therefore, the normalized maximum intensity of the hysteresis loops was mapped as a screening parameter for magnetization. The maximum intensities were normalized for the data in each setup separately, because of the different magnitudes of the overall intensities. In correlation to the phase analysis based on EDX and XRD, the acquired magnetic data can help to verify and extend a thin film phase diagram of Ni-Co-Al. The same high-throughput test stand was used to acquire the sheet resistance of the materials library by automated four-point probe measurements. A rectangular array of 3084 measurement areas with 1.5 mm spacing was defined. The measurement current was 100 mA. The intermediate chemical compositions were interpolated using 342 measurement areas at 4.5 mm distances. Finally, ACS Paragon Plus Environment

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based on optical data acquired by a RGB photo and the EDX data a ternary luminosity plot was established 13. Prior work has shown that metallic films mainly show color variations in form of luminosity changes, which can be a hint to phase formation and surface oxidation 14.

Results and discussion Verification of phase formation from the precursor multilayer by TEM Fig. 1 shows EDX line scans together with the respective STEM image for cross-sections of the Ni34Co29Al37 thin films after different annealing times at 600°C. The TEM-EDX results for the film annealed 1 h at 600°C (fig. 1a) show that the precursor layers did not completely mix and are visible in STEM image (fig. 1b). Fig. 1c shows TEM-EDX for the film after 20 h annealing at 600°C. The compositions of the sapphire substrate (fig. 1c, I), the thin film (fig 1c, II) and the Pt protection layer (fig. 1c, III) are indicated. The STEM image of the film shows a two phase microstructure (fig. 1D) corresponding to the two phases found for this composition in XRD: (Ni,Co)Al and (Ni,Co)3Al (see “Cluster analysis and Phase identification”), additionally confirmed by electron diffraction. It is therefore concluded that an annealing time of 20 h is sufficient to dissolve the precursor layers and to induce phase formation. Furthermore, no diffusion between substrate and thin film is identified.

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Figure 1: a) Cross-sectional EDX line scan in TEM of the thin film in the center of the materials library after 1 h annealing. b) STEM image of this sample. c) Cross-sectional EDX line scan in TEM of the thin film in the center of the materials library after 20 h annealing. d) STEM image of this sample. The sapphire substrate (I), the thin film (II) and the Pt protection layer (III) are tagged.

Chemical composition Automated EDX measurements on all measurement areas of the materials library confirmed an almost complete ternary composition spread (fig. 2). Only the corners (> 95 at.% of one element) of the composition triangle are not covered. The compositional distances between the measurement areas of the materials library lie between 2 to 5 at.%.

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Cluster analysis and phase identification A cluster analysis was performed on the 342 one-dimensional XRD data sets using the CombiView software [2], resulting in 69 groups. After refining, 20 groups were identified consisting of 13 different phases. A high initial number of clusters was chosen. Previous experiences showed that XRD patterns with a high number of low intensity peaks were grouped in one cluster, even when they included different peaks per pattern. As a consequence, phase regions showing few peaks were separated into more groups in the initial clustering, e.g. the (Ni,Co)Al single phase region was found in 7 initial clusters while the NiAl3 and Al binary phase region only appeared in one cluster. Combining cluster groups after the initial clustering was found to efficient. Fig. 2 shows the results of this phase analysis in the ternary triangle. Table 1 lists all identified phases. The trivial name is used when relating to a specific phase.

Trivial name

Prototype

Space group

(Ni,Co) Co (Ni,Co)3Al (Ni,Co)Al Ni2Al3 NiAl3 Co2Al9 Al Co4Al12.13 Co2Al5 Ni3Al4 (Ni,Co)Al martensite Ni5Al3

Cu Mg Cu3Au CsCl Ni2Al3 Fe3C Co2Al9 Cu Co4Al12.13 Co2Al5 Ni3Ga4 VRh2Sn Pt5Ga3

Fm-3m P63/mmc Pm-3m Pm-3m P-3m1 Pnma P121/a1 Fm-3m C1m1 P63/mmc Ia-3d I4/mmm Cmmm

Space number 225 194 221 221 164 62 14 225 8 194 230 139 65

group Reference 15 16 17 18 19 20 21 22 23 23 24 25 26

Table 1: Overview of the phases identified in the Ni-Co-Al materials library. The space group and the respective phase prototype are shown for all phases. The diffractograms were taken from the Pearson Crystal Data Base. Original references are cited.

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Figure 2: Assessment of the Ni-Co-Al thin film phase diagram in its state after annealing at 600°C for 20 h. The phase regions are color-coded: 20 different groups consisting of 13 phases in total are identified. Single phase regions are marked with squares, two phase regions with diamonds and three-phase regions with hexagons.

Measurement areas near the binary Ni-Co compositions with < 20 at.% Al show the (Ni,Co) solid solution phase. In the Co-rich corner with > 75 at.% Co a two phase region is found comprising the phases (Ni,Co) and Co. The (Ni,Co)Al phase is found in a wide range from 20 to 70 at.% Al, forming four two phase regions and one three phase region. (Ni,Co)Al forms two phase regions with: a) (Ni,Co) from 20-50 at.% Al and > 25 at.% Co, b) with (Ni,Co)3Al from 30-45 at.% Al and < 40 at.% Co, c) with Co2Al5 for Co-Al binary compositions from 30-45 at.% Co and d) with Ni5Al3 at 40 at.% Al in the Ni-Al binary compositions. A small three phase region containing (Ni,Co)Al, (Ni,Co)Al martensite and (Ni,Co)3Al is found at 40 at.% Al and 20 at.% Co. (Ni,Co)Al is a one phase region around 50 at.% Al The (Ni,Co)3Al phase occurs from 20-30 at.% Al and 55 at.% Al. Ni3Al4 forms a small one phase region around 60 at.% Al up to 10 at.% Co. NiAl3 is found in a wide composition range in the Al-rich part of the Ni-Co-Al system. It forms a one phase region in the Ni-Al binary compositions from 55 to 70 at.% Al. A two phase region of NiAl3 and Ni2Al3 exists around 65 at.% Al. NiAl3 forms another two phase region with Co4Al12.13 at 70 to 80 at.% Al up to 15 at.% Co and a third one with Al around 80 to 87 at.% Al. A small three phase region (one measurement area) of NiAl3, Al and Co2Al9 is found at 90 at.% Al. Co2Al9 forms a two phase region with Al in the Al-rich corner and a one phase region near the Al-Co binary compositions at 10-20 at.% Co. Another two phase region of Co2Al9 together with Co4Al12.13 is found around 20 at.% Co. Co4Al12.13 forms a two phase region with Co2Al5 between 25 and 30 at.% Co ACS Paragon Plus Environment

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and < 10 at.% Ni. It also appears in a small two phase region together with Ni2Al3 at 70 at.% Ni and 5 at.% Co.

Figure 3: Close-up of the measurement areas in the Al-rich region (> 55 at.%). The same color-code as in fig. 1 was applied. One phase regions are marked with squares, two phase regions with diamonds and three-phase regions with hexagons.

XRD results for the 1 h annealed materials library are not discussed as the TEM results showed that this annealing time was not sufficient for complete phase formation.

Magneto-optical properties MOKE measurement results are discussed to investigate magneto-optical properties of the Ni-Co-Al system and to confirm the results achieved from XRD analysis regarding the existence of ferromagnetic phases. The measured magneto-optical hysteresis loops for the parallel laser-towafer-flat setup are plotted in a ternary diagram (fig. 4). The respective phase regions from XRD are shown for measurement areas showing a hysteresis loop. The normalized intensity and the coercivity HC of both setups are plotted in fig. 5.

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Figure 4: Plot of the hysteresis loops (fitted with a dose response function) in a ternary composition diagram for the parallel laser-to-wafer-flat setup. The hysteresis loops are centered at the composition of the respective measurement area. In the Ni-Co binary composition area only every third plot was plotted for a better overview. Phase regions for the measurement areas showing a hysteresis loops are included. The known 8 composition area for ferromagnetic shape-memory alloys (FSMA) is marked in green .

Hysteresis loops were mainly found for compositions with 40 at.% Co showed a local maximum. Co is known to show a high magnetic anisotropy 30,31 and increases the saturation magnetization in Ni-based magnetic materials 32. The overall intensity decreases and vanishes with increasing Al concentration.

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Figure 5: Results of high-throughput MOKE. a-b) Ternary visualizations of the coercivity values; c-d) ternary visualizations of the normalized maximum Kerr signal intensity. Grey squares indicate regions showing no hysteresis.

Sheet resistance mapping and optical properties To gain additional information on the materials library the luminosity of the library as well as the sheet resistance were examined by high-throughput measurements. These measurements have a supportive function by comparing the optical appearance and the sheet resistance of the materials library with the results of the other measurements. The luminosity map (fig. 6a) shows a region of similar optical appearance (purple in fig. 6a) fitting to the single phase region (Ni,Co)Al evaluated in XRD. Another region (red in fig. 6a) fits to the phase regions showing Al in XRD. An overall change of luminosity with increasing Al is furthermore discovered. This change is eventually related to the increased formation of Al-rich surface oxides with increasing Al concentration. Fig. 6b shows a contour map of 3084 measurement areas. In case of Ni−Co−Al, phase formaRon and oxidaRon are assumed to cause the main influences on the resistance values. While multiphase regions and thick surface oxides typically cause a resistance maximum, one-phase regions and easily penetrable thin oxide scales could show local resistance minima 14,33. While the binary Ni-Co region as well as the Alrich area of the ternary system show a relatively low resistance there are three regions where the resistance shows a maximum. In a broad region between 30 to 40 at.% Al the resistance is increased with a maximum in the binary Co-Al composition range with 45 at.% Al. This region correlates to the two-phase regions of (Ni,Co)/(Ni,Co)Al and (Ni,Co)Al/(Ni,Co)3Al. Additionally, there is another local maximum at 20 to 25 at.% Co and 60 to 70 at.% Al indicating a beginning oxidation. In addition, the Al-rich (< 80 at.%) region and the Al-poor (< 10 at. %) region show a relatively low resistance. This can be interpreted as an indication of one-phase regions.

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Figure 6: a) Luminosity map of the Ni-Co-Al materials library. b) Contour map of the sheet resistance across the library with a fine grid of 3084 measurement areas. Intermediate chemical compositions were derived from the 342 measurement areas.

Assessment of the thin film phase diagram Investigations of the Ni-Co-Al materials system resulting in ternary phase diagrams were carried out since the 1940s but covered mostly isothermals at elevated temperatures of 600 °C and higher 34–36. Only few isothermals for lower temperatures exist 37. The phase diagrams in literature are either calculated or are the sum of several experiments on bulk material performed by different authors. The present work is different since it describes a full ternary materials library fabricated and analyzed in one process. In the following the above-described findings are compared to literature data. Hubert Protopopescu et al. showed a partial ternary isothermal (< 60 at.% Al) at 25°C including the phases (Ni,Co)Al, (Ni,Co)3Al and (Ni,Co) 37. The single phase region of (Ni,Co)Al is in accordance with the previous work of Hubert Protopopescu et al. The present work shows a two phase region of (Ni,Co) and Co not appearing at Hubert Protopopescu et al. The (Ni,Co) single phase region determined in this work is larger with up to 20 at.% Al compared to 8 at.% Al in Hubert Protopopescu et al. A three phase region of (Ni,Co)3Al, (Ni,Co)Al and (Ni,Co) was not identified in this work. (Ni,Co) and (Ni,Co)3Al share a similar crystal structure. Therefore, (Ni,Co)3Al was only verified in measurement areas showing XRD patterns including peaks which are unique for (Ni,Co)3Al. Comparing the XRD results with binary phase diagrams showed differences for the existence range of Co. This work verified its existence up to 20 at.% Ni near the Ni-free side of the composition triangle while the APDIC-approved calculated phase diagram by Nishizawa et al. verifies Co up to 35 at.% Ni at room temperature 38. The transformation temperature from fcc to hcp Co in literature is given with 422°C 39. It is therefore possible that the combined influences of elemental Al and Ni favor (Ni,Co) and not Co, which was reported for Al in Co before 40. Furthermore, McAlister reports on the occurrence of the phase CoAl3. However, this phase was not given a specific crystal structure by McAlister and did not have one in a non-APDIC approved binary Co-Al phase diagram 41 making it impossible to identify this phase in XRD. However, all peaks from all compositions of the materials library could be assigned to known phases, which means that the existence of this phase is not confirmed.

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An APDIC Ni-Al binary by Singleton et al. is mostly consistent with the results of this work phases mentioned are found in this work in similar composition ranges.

42

. All

Conclusion XRD was performed on a Ni-Co-Al thin film materials library covering a full ternary composition spread. TEM investigations were performed to confirm the mixture of the precursor layers. 20 h annealing time was sufficient to achieve mixture and phase formation. Cluster analysis was used as a basis for the phase analysis. 13 different phases were identified, consistent with literature data. The results are plotted as a thin film Ni-Co-Al phase diagram. To support the diffraction results, MOKE, sheet resistance and photo luminosity measurements were performed to verify if these properties fit to the structural data. Only where (Ni,Co), Co or (Ni,Co)Al were identified, magnetic hysteresis loops could be measured. (Ni,Co)Al showed an increased HC in the composition range known for ferromagnetic shape memory alloys in the Ni-Co-Al materials system. All other phases identified are not known to be ferromagnetic at room temperature, which was verified by the MOKE measurements. Sheet resistance and photo luminosity showed areas of distinct properties fitting to the structural data. Finally, the thin film phase diagram was compared to published phase diagrams. The phase distribution in the thin film phase diagram is mostly consistent with literature. The distinction between (Ni,Co) and (Ni,Co)3Al differs from the literature. Additional research will be undertaken to understand this difference. The thin film phase diagram shows a detailed overview of the Ni-Co-Al materials system due to the high measurement point density applied. It is a basis for the further development of Ni-Co-Al-based alloys using combinatorial materials science methods or classic metallurgical approaches.

Acknowledgements The authors would like to thank the DFG for funding of this work within the research group FOR 1766 and the research project B5 in the SFB/Transregio 103. Aleksander Kostka and Axel Marquardt are acknowledged for preparation of the TEM sample.

Supplemental material

Raw data of EDX, XRD and MOKE was made publicly available in Citrination under the following link: https://citrination.com/datasets/153138/show_files References (1) Pollock, T. M.; Dibbern, J.; Tsunekane, M.; Zhu, J.; Suzuki, A. New Co-based γ-γ′ high-temperature alloys. JOM 2010, 62, 58–63. (2) Pollock, T. M.; Tin, S. Nickel-Based Superalloys for Advanced Turbine Engines: Chemistry, Microstructure and Properties. Journal of Propulsion and Power 2006, 22, 361–374. (3) Oikawa, K.; Ota, T.; Gejima, F.; Ohmori, T.; Kainuma, R.; Ishida, K. Phase Equilibria and Phase Transformations in New B2-type Ferromagnetic Shape Memory Alloys of Co-Ni-Ga and Co-Ni-Al Systems. Mater. Trans. 2001, 42, 2472–2475. ACS Paragon Plus Environment

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