Nanocrystalline structure and Mechanical Properties of Vapor Quenched Al-Zr-Fe Alloy Sheets Prepared by Electron-Beam Deposition

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1 Materials Transactions, Vol. 44, No. 10 (2003) pp to 1954 Special Issue on Nano-Hetero Structures in Advanced Metallic Materials #2003 The Japan Institute of Metals Nanocrystalline structure and Mechanical Properties of Vapor Quenched Al-Zr-Fe Alloy Sheets Prepared by Electron-Beam Deposition Hiroyuki Sasaki 1, Naoko Kobayashi 1, Kazuhiko Kita 1, Junichi Nagahora 1 and Akihisa Inoue 2 1 Sendai Institute of Material Science and Technology, YKK Corporation, Kurokawa , Japan 2 Institute for Material Research, Tohoku University, Sendai , Japan Nanocrystalline Al-based ternary alloys with additions of Zr and Fe have been prepared by electron-beam deposition. The composition dependence of microstructures and mechanical properties has been investigated. Fine -Al grains are observed and the grain size significantly decreases with the increase of Zr content. Al 95:3 -Zr 4:0 -Fe 0:7 and Al 96:5 -Zr 2:8 -Fe 0:7 alloys consist of -Al containing the coherent meta-stable precipitate. The addition of Fe, effectively refined the grain structure, increases hardness and improves the thermal stability of the meta-stable -Al 3 Zr phase. Al 95:3 -Zr 4:0 -Fe 0:7 alloy exhibits remarkably high tensile strength both at room temperature and above 523 K, 817 MPa at room temperature, 536 MPa at 523 K and 434 MPa at 573 K, respectively. The microstructure of Al 95:3 -Zr 4:0 -Fe 0:7 alloy exhibits excellent thermal stability under prolonged annealing at 523 K. (Received April 25, 2003; Accepted July 8, 2003) Keywords: electron-beam evaporation, aluminum alloy, high strength, high temperature strength, -Al 3 Zr precipitate, thermal stability 1. Introduction Refinement of grain is one of the most effective strengthening methods. It can be achieved by increasing the rate of solidification. It is also possible to increase the solubility limit and form non-equilibrium phases and amorphous phases. As compared with the conventional casting process, physical vapor deposition (PVD) and liquid quenching techniques have much higher cooling rates. 1) The liquidquenched Al-based alloys produced by gas-atomization or melt-spinning have amorphous phases or nanocrystalline microstructures and exhibit high strengths of MPa. 2) On the other hand, PVD is the method that condenses metallic vapor on a substrate, namely vapor quenching method, and has higher cooling rate in comparing with liquid quenching. 1) Especially, electron beam evaporation has very high deposition rate and enables the production of bulky alloys. For example of applying 250 kw electron-beam gun, deposition rate of 72 mm/h is achieved. 3) Moreover, the highly productive air-to-air coating line for long time deposition cycles of up to 120 production hours without interruption is realized. 4) Therefore, the vapor quenching method has excellent potential to the industrial application of the bulky sheet materials. Recently, Sasaki et al. reported that the vapor quenched binary aluminum alloy sheets with additions of Ni, 5) Ti 6) or Fe, 7,8) Zr or Cr 9) have ultra fine grain and excellent mechanical properties. In the case of Al-Fe binary alloy, the grain size decreases drastically with addition of Fe and Al-2.5 at%fe exhibits excellent tensile strength of 1000 MPa with grain size of 40 nm. Moreover, vapor quenched Al-Fe alloy forms supersaturated solid solution of Fe in Al. This non-equilibrium microstructure, however, has not enough thermal stability and the tensile strength decreases after heat treatment at above 523 K. On the other hand, Zr has low diffusivity in Al and metastable -Al 3 Zr phase induces high thermal stability. 9) This paper is intended to present the microstructure and evaluate the mechanical properties at room and high temperature of the bulk Al-Zr-Fe alloy prepared by vapor quenching. 2. Experimental Procedure Al-based alloys with varying content of Zr and Fe have been prepared by PVD using two electron-beam evaporation sources. A schematic diagram of the equipment is shown in Fig. 1. The equipment contained two independent evaporation sources heated by two 7 kw electron guns, using a 10 kv high-voltage supply. In this work, Al and Zr-14 at%fe alloy rods were continuously supplied as evaporation sources. The composition of the vapor from the Zr-Fe alloy rod is varied with the temperature of molten pool due to their different vapor pressure at a given temperature. The composition of each deposit was controlled by means of the automatic adjustment of electron-beam power with sensing the vapor concentration of Al and Zr by Sentinel 1 III electron impact emission spectroscopy system. After reaching steady evaporation state (approximately 3.6 ks running), the shutter between evaporation sources and substrate was opened and then deposition was started. The equipment was operated in a vacuum chamber that was maintained at a pressure higher than Pa. Substrate temperature was controlled at 523 K and the substrate was rotated at 20 min 1. Fig. 1 Vacuum Chamber Heater Rotation Electron beam Rod Rod feed feed Al Zr 86 Fe 14 Heater Substrate Deposit Electron beam Schematic diagram of continuous electron-beam evaporation setup.

2 Nanocrystalline structure and Mechanical Properties of Vapor Quenched Al-Zr-Fe Alloy Sheets Prepared by Electron-Beam Deposition 1949 For this study, the Al-Zr-Fe deposited alloys prepared by the evaporator were approximately 340 mm in thickness after deposition runs of 7.2 ks. The structures of the deposits were examined by X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The composition was analyzed by energy-dispersive X-ray spectroscopy (EDX). Some of these deposits were heat treated at 523 to 723 K for 3.6 ks in air. Microhardness measurements were carried out under N at room temperature. Tensile tests were carried out at an initial strain rate of 6: s 1 and at various temperatures (room temperature, 423, 473, 523 and 573 K). The tensile specimen had a gauge length of 8 mm, width of 3 mm and thickness of 0.15 to 0.2 mm. Before the tensile test at elevated temperature, the tensile specimen was kept at the testing temperature for 3.6 ks to stabilize the testing system. 3. Results and Discussion 3.1 Microstructures The effect of Zr and Fe content on phase formation was investigated. Figure 2 shows the XRD patterns of the ternary Al 99:3 x -Zr x -Fe 0:7 (x ¼ 2:8, 4.0 and 6.7) and binary Al 100 x - Zr x (x ¼ 3:3 and 5.0) vapor quenched alloys. In the ternary Al-Zr-Fe alloys, strong (111) texture is observed but this texture weakens slightly with increasing the Zr content. On the other hand, the binary Al-Zr alloys exhibit (200) texture. In the high Zr content alloy (Al 95 Zr 5 ), the (200) texture weakens and the other diffraction peaks from -Al are observed. Therefore, it can be considered that the addition of Fe changes the texture of Al-Zr alloys from (200) to (111). In binary Al 95 Zr 5 alloy, the diffraction peaks from -Al 3 Zr phases are also identified. Intensity (arb.units) Fig. 2 (100) (110) Al (111) Al (200) (200) (211) Al (220) Cu-Kα (221) (310) Al (311) Diffraction Angle, 2θ / (π /180) rad Al 92.6 Zr 6.7 Fe 0.7 Al 95 Zr 5 Al 96.7 Zr 3.3 XRD patterns of Al-Zr-Fe and Al-Zr vapor quenched alloys. The TEM bright-field images and selected-area diffraction pattern (SAED) of the ternary Al 99:3 x -Zr x -Fe 0:7 (x ¼ 2:8, 4.0 and 6.7) and binary Al 100 x -Zr x (x ¼ 3:3 and 5.0) alloys are shown in Fig. 3. Fine -Al grains are observed in ternary Al- Zr-Fe alloys and the grain size of this -Al phase significantly decreases with the increase of Zr content. The grain size is approximately 370 nm for Al 96:5 -Zr 2:8 -Fe 0:7, 260 nm for Al 95:3 -Zr 4:0 -Fe 0:7 and 70 nm for Al 92:6 -Zr 6:7 -Fe 0:7, respectively. Binary Al-Zr alloys also have fine -Al grains of approximately 1000 nm for Al 96:7 Zr 3:3 and 800 nm for Al 95 Zr 5, respectively. The decrease of the grain size is caused by both the suppression of grain growth due to an ultrahigh cooling rate during vapor quenching and the addition of Zr and/or Fe that has a low diffusion coefficient in Al. 10) In Al-Zr-Fe ternary alloy, the grain refinement with the increase of Zr content is significant as compared with Al- Zr binary alloys. Therefore, it is noted that the addition of Fe plays an important role to the grain refinement. The SAED pattern of Al 95:3 -Zr 4:0 -Fe 0:7 alloy (Fig. 3(b)) was taken from the region with a diameter of 0.24 mm and the others were with a diameter of 0.6 mm. Except for Al 92:6 -Zr 6:7 -Fe 0:7 alloy (Fig. 3(c)), the presence of phase was distinguished in the SAED pattern. Figure 4 shows the enlarged image of Al 95:3 - Zr 4:0 -Fe 0:7 alloy. The dark field image was obtained using the (011) superlattice spot. The bright regions were weakly observed within -grain corresponding to the coherent Al 3 Zr phase. Moreover, no dispersoids are observed on grain boundaries. The reason why no diffraction peaks from phase were observed in XRD of ternary Al-Zr-Fe alloys can be explained by the formation of strong (111) textured microstructure. On the other hand, in the SAED pattern of Al 92:6 -Zr 6:7 -Fe 0:7 alloy (Fig. 3(c)), some weak super-lattice diffraction spots are observed. However, it was difficult to find particles within the -Al grain and/or grain boundaries from any TEM images. On the basis of XRD results, Al 92:6 -Zr 6:7 -Fe 0:7 alloy predominantly consists of - Al containing supersaturated Zr and Fe solutes. Figure 5 shows the DSC curves of the ternary Al 99:3 x -Zr x - Fe 0:7 (x ¼ 2:8, 4.0 and 6.7) and binary Al 95 Zr 5 alloys. Ternary Al-Zr-Fe alloys exhibit exothermic peak around K. The exothermic calorific value increases as the Zr content increases. On the result of the XRD pattern obtained from the specimens after DSC tests, this exothermic peak is considered to indicate the decomposition of phase. On the other hand, binary Al 95 Zr 5 alloy exhibits a small and broad exothermic peak corresponding to the precipitation of a stable phase of D0 23 -Al 3 Zr phase around 650 K. This result suggests that the thermal stability of the meta-stable -Al 3 Zr phase is improved by the addition of Fe. Additionally, in Al-Zr-Fe ternary alloys, Al 92:6 -Zr 6:7 - Fe 0:7 alloy starts exothermic reaction at approximately 50 K lower than Al 95:3 -Zr 4:0 -Fe 0:7 alloy. Furthermore, in Al 92:6 - Zr 6:7 -Fe 0:7 alloy, a change in slope of DSC curve is observed at around 650 K. On the basis of the TEM results, this exothermic reaction consists of two peaks. The first exothermic peak corresponds to the precipitation of the meta-stable phase from supersaturated solid solution and the second one is considered to the transformation of the phase into the D0 23 phase. Similar results were reported in vapor quenched Al-Ti binary alloy, i.e. the formation of the

3 1950 H. Sasaki, N. Kobayashi, K. Kita, J. Nagahora and A. Inoue (a) (d) 100 nm 400 nm (b) (e) 100 nm 400 nm (c) 100 nm Fig. 3 TEM bright-field images and diffraction patterns of Al-Zr-Fe ternary and Al-Zr binary alloys as quenched state. (a) Al 96:5 -Zr 2:8 -Fe 0:7, (b) Al 95:3 -Zr 4:0 -Fe 0:7, (c) Al 92:6 -Zr 6:7 -Fe 0:7, (d) Al 96:7 Zr 3:3, (e) Al 95 Zr 5. phase from supersaturated solid solution and the decomposition of the meta-stable -Al 3 Ti phase into the stable D0 22 -Al 3 Ti phase. 6) Furthermore, in vapor quenched Al-Fe binary alloys, an exothermic peak of the decomposition from the supersaturated solid solution of Fe in Al was observed around 580 K 7) and this decomposition temperature was much lower than that of these Al-Zr-Fe alloys. Therefore, it is concluded that the addition of Fe stabilizes the meta-stable phase. 3.2 Mechanical properties at room temperature Al-Zr-Fe ternary alloys exhibit much finer grain size and higher thermal stability of the meta-stable -Al 3 Zr phase as compared with Al-Zr binary alloys. Figure 6 shows the Vickers hardness of the Al 99:3 x -Zr x -Fe 0:7 (x ¼ 2:8, 4.0 and 6.7) ternary alloys and Al-Zr binary alloys plotted as a function of Zr content. The Vickers hardness increases with increasing Zr content. Al-Zr-Fe ternary alloys exhibit higher hardness as compared with Al-Zr binary alloy with the same Zr content. The addition of Fe is effective to increase hardness. The grain refinement by the addition of Fe is one of the reasons why Al-Zr-Fe ternary alloys exhibit superior hardness property to Al-Zr binary alloys. This figure also shows the tensile strength of Al 99:3 x -Zr x -Fe 0:7 (x ¼ 2:8, 4.0 and 6.7) ternary alloys. The tensile strength increases with increasing Zr content and reaches 817 MPa in Al 95:3 -Zr 4:0 - Fe 0:7. This trend is similar to that of the composition dependence of hardness. In the high Zr content alloy of Al 92:6 -Zr 6:7 -Fe 0:7, the tensile strength is lower than those of other Al-Zr-Fe alloys owing to the fracture during elastic deformation. These excellent mechanical properties were obtained by both the grain refinement strengthening and the

4 Nanocrystalline structure and Mechanical Properties of Vapor Quenched Al-Zr-Fe Alloy Sheets Prepared by Electron-Beam Deposition 1951 (a) (b) (c) 50 nm Fig. 4 Enlarge TEM bight and dark field images of Al 95:3 -Zr 4:0 -Fe 0:7 alloy as quenched state. (a) B. F. image, (b) D. F. image of phase using the (011) super lattice spot, (c) diffraction pattern from (a). Exothermic (arb. units) Al 92.6 Zr 6.7 Fe 0.7 Al 95 Zr K/s Vickers Hardness, Hv 350 Solid mark Open mark Al 99.3-x Zr x Fe 0.7 (TS) 150 Al 99.3-x Zr x Fe 0.7 (Hv) Al 100-x Zr x (Hv) Zr Content, x (at%) Tensile Strength, / MPa σ UTS Fig Temperature, T / K DSC curves of Al-Zr-Fe and Al-Zr vapor quenched alloys. Fig. 6 Zr content dependence on Vickers hardness and Tensile strength in vapor quenched Al alloys. Annealed 3.6ks 350 precipitation hardening of the coherent phase. Moreover, these alloys have no secondary phase on grain boundaries. Takehisa et al. reported similar trend in the vapor quenched Al-Fe; 11) the alloys exhibited both high tensile strength and high fracture toughness and they were strengthened by the grain refinement and solid solution hardening of Fe. There are no secondary phases which act as an initiation of fracture by stress concentration. Therefore, it is expected that these high-strengthened Al-Zr-Fe alloys also exhibit excellent fracture toughness. 3.3 Mechanical properties and microstructure at elevated temperature Figure 7 shows the changes in hardness for vapor Vickers Hardness, Hv As deposited Al 92.6 Zr 6.7 Fe Annealing Temperature, T / K Fig. 7 Vickers hardness of vapor quenched Al-Zr-Fe alloys isochronally annealed at temperatures ranging from 523 to 723 K for 3.6 ks.

5 1952 H. Sasaki, N. Kobayashi, K. Kita, J. Nagahora and A. Inoue (a-1) (a-2) (b-1) 200 nm (b-2) (c-1) 200 nm (c-2) (d) 200 nm 1 µm Fig. 8 TEM bright (a-1, b-1, c-1, d) and dark (a-2, b-2, c-2) field images of Al 95:3 -Zr 4:0 -Fe 0:7 and Al 96 Zr 4 alloy isochronal annealed for 3.6 ks. (a) Al 95:3 -Zr 4:0 -Fe 0:7 at 573 K, (b) Al 95:3 -Zr 4:0 -Fe 0:7 at 623 K, (c) Al 95:3 - Zr 4:0 -Fe 0:7 at 723 K, (d) Al 96 Zr 4 at 523 K. quenched Al-Zr-Fe ternary alloys isochronally annealed at temperatures ranging from 523 to 723 K for 3.6 ks. Al 92:6 - Zr 6:7 -Fe 0:7 alloy shows significant age hardening behavior at K. This behavior is due to the precipitation of the phase from supersaturated solid solution. The first exothermic peak of DSC curve disappeared after these heat treatments. On the other hand, Al 96:5 -Zr 2:8 -Fe 0:7 and Al 95:3 - Zr 4:0 -Fe 0:7 alloys, in which the precipitation of the phase was observed in the as-deposited, show almost the same hardness. Moreover, the excellent hardness properties are maintained up to around 673 K. However, at 723 K, the hardness drops by aging. To clarify the thermal stability of Al-Zr-Fe ternary alloy, the change of microstructure was investigated. Figure 8 shows the microstructures of Al 95:3 -Zr 4:0 -Fe 0:7 ternary alloy and Al 96 Zr 4 binary alloy isochronal annealed for 3.6 ks at

6 Nanocrystalline structure and Mechanical Properties of Vapor Quenched Al-Zr-Fe Alloy Sheets Prepared by Electron-Beam Deposition Tensile Strength, σ UTS / MPa T6 (ESD) Test Temperature, T / K Fig. 9 High temperature strength of vapor quenched Al-Zr-Fe alloys. Fig. 10 TEM bright-field images of Al 95:3 -Zr 4:0 -Fe 0:7 alloy prolonged annealed for 360 ks. (a) at 523 K, (b) at 573 K. varying temperatures. In the microstructure of Al 95:3 -Zr 4:0 - Fe 0:7 alloy annealed at 723 K, grain coarsening and precipitation on grain boundaries are observed. The chemical composition of annealed Al grain measured by EDX was Zr: 3.2 at% and Fe: 0.2 at%. The decrease of Zr and Fe content in Al grain suggests the decrease of volume fraction of the phase in Al grain at high temperatures. Therefore, the decrease of the volume fraction of the coherent particles and the grain coarsening are attributed to the rapid decrease of hardness on annealing at 723 K. In the sample, which exhibits no decrease of hardness by annealing at 623 K, no grain growth and little change in composition of Al grain are confirmed, although some precipitates on grain boundaries are observed. The crystal structures of this grain boundary precipitates is not determined, but they do not contain Zr. Interior of Al grain, the particles are retained. On the other hand, in Al-Zr binary alloy annealed at 523 K for 3.6 ks, D0 23 needle-shaped inter-granular particles are observed as well as the coherent precipitate. This suggests that the addition of Fe improves the thermal stability of the metastable Al 3 Zr phase. Note that little change is observed in Al-Zr-Fe ternary alloy annealed at 573 K for 3.6 ks. Figure 9 shows the elevated temperature tensile test results of Al-Zr-Fe alloys up to 573 K. The elevated temperature strength of vapor quenched Al-Zr-Fe is significantly higher than that of conventional aluminum alloys. 12) Especially, Al 95:3 -Zr 4:0 -Fe 0:7 alloy achieves remarkably high strength of 536 MPa at 523 K and 434 MPa at 573 K, respectively. It should be noted that the alloy exhibits high strength above 523 K, in which the decomposition from the supersaturated solid solution occurs in Al-Fe binary alloys. 13) The tensile strength moderately decreases with increasing test temperature without sharp decrease as compared with conventional precipitated hardening Al based alloys. This suggests that thermally stable precipitate, i.e. the phase, is attributed to the excellent elevated temperature strength. Figure 10 shows the microstructure of Al 95:3 -Zr 4:0 -Fe 0:7 alloy annealed for 360 ks at 523 K and 573 K, respectively. Microstructural evolution was observed after annealing at 573 K; the grains grew from 260 to 680 nm with the intergranular precipitation. On the basis of the microstructural evolution, prolonged anneal at 573 K may degrade mechanical properties. On the other hand, no microstructural change is observed after annealing at 523 K. Therefore, the excellent mechanical properties are sustained for a prolonged period below 523 K. 4. Conclusions The microstructures and mechanical properties of the vapor quenched Al-Zr-Fe ternary alloys prepared by electron-beam evaporation have been investigated. (1) Fine -Al grains are obtained by vapor quenching process. The grain size significantly decreases with the increase of Zr content. The grain sizes are approximately 370 nm for Al 96:5 -Zr 2:8 -Fe 0:7, 260 nm for Al 95:3 -Zr 4:0 -Fe 0:7, and 70 nm for Al 92:6 -Zr 6:7 -Fe 0:7, respectively. In Al-Zr-Fe ternary alloys, the grain refinement with the increase of Zr content is significant as compared with Al-Zr binary alloys. The addition of Fe is effective to stabilize the fine-grained microstructure. (2) Al 95:3 -Zr 4:0 -Fe 0:7 and Al 96:5 -Zr 2:8 -Fe 0:7 alloys consist of -Al containing the coherent meta-stable precipitates. The thermal stability of the meta-stable Al 3 Zr phase is improved by the addition of Fe. (3) The Vickers hardness increases with increasing the Zr content and reaches values of 200 Hv for Al 96:5 -Zr 2:8 -Fe 0:7, 245 Hv for Al 95:3 -Zr 4:0 -Fe 0:7 and 300 Hv for Al 92:6 -Zr 6:7 - Fe 0:7, respectively. Al-Zr-Fe ternary alloys exhibit higher

7 1954 H. Sasaki, N. Kobayashi, K. Kita, J. Nagahora and A. Inoue hardness than Al-Zr binary alloy with the same Zr content. The addition of Fe affects the increase of hardness. (4) Al 95:3 -Zr 4:0 -Fe 0:7 alloy exhibits high tensile strength of 817 MPa at room temperature. This high tensile strength is explained by both of the grain refinement strengthening and the precipitation hardening of the coherent phase without secondary phase on grain boundaries. (5) Al 95:3 -Zr 4:0 -Fe 0:7 alloy achieves excellent tensile strength above 523 K, namely, 536 MPa at 523 K and 434 MPa at 573 K, respectively. The microstructure of Al 95:3 -Zr 4:0 -Fe 0:7 alloy exhibits excellent thermal stability annealed at 523 K for 360 ks and annealed at 573 K for 3.6 ks. REFERENCES 1) S. B. Dodd and S. Morris: Society of Vacuum Coaters 40th Annual Technical Conference Proceedings (1997) pp ) A. Inoue, K. Ohtera and T. Masumoto: Jpn. J. Appl. Phys. 27 (1988) L736-L739. 3) S. Schiller, U. Heisig and Goedicke: Vakuum-Technik 27 Jahrgang Heft 3. 4) E. Reinhold, J. Richter, H. Waydbrink, E. Zschieschang: Thin Solid films (2000) ) K. Kita, H. Sasaki, J. Nagahora and A. Inoue: J. Jpn. Soc. Powder Metall. 47 (1999) ) N. Kumagai, H. Sasaki, K. Kita, J. Nagahora and A. Inoue: J. Japan Inst. Metals 65 (2001) ) H. Sasaki, K. Kita, J. Nagahora and A. Inoue: Mater. Trans. 42 (2001) ) T. Mukai, S. Suresh, K. Kita, H. Sasaki, N. Kobayashi, K. Higashi, and A. Inoue: Acta Mater. (2003) in press. 9) J. Nagahora: Proceedings of The Fourth Symposium on SUPER METAL, RIMCOF&JRCM, Japan (2001) ) S. Fujikawa: J.JILM 46 (1996) ) H. Takehisa and N. Ando: Proceedings of The Fourth Symposium on SUPER METAL, RIMCOF&JRCM, Japan (2001) ) Aluminum handbook; ed. by JILM (1990) ) T. Takahashi, T. Yamada and T. Tsuzuku: Proceedings of The Third Symposium on SUPER METAL, RIMCOF&JRCM, Japan (2001)