Overview Aluminum Composites The Properties and Application of Scandium-Reinforced Aluminum

Zaki Ahmad

Scandium-reinforced aluminum industries. The information on scandium- ADVANTAGES OF alloys represent a new generation of reinforced alloys is scanty; very little SCANDIUM ADDITION high-performance alloys that display has been published on the mechanical, numerous advantages over high-strength microstructural, and corrosion behavior Scandium-reinforced aluminum aluminum alloys. Scandium-reinforced of these alloys. The following fills alloys offer design engineers several alloys are much stronger than other this gap. significant advantages over other high- high-strength alloys, exhibit significant strength aluminum alloys, including: INTRODUCTION grain refinement, strengthen welds, and • Inhibition of recrystallization. eliminate hot cracking in welds. These Scandium, a novel alloying element Transition such as zirco- alloys also exhibit a good resistance to for aluminum, is mined and processed in nium, chromium, manganese, corrosion as shown by recent studies. Zhovti Vody, , the only primary , or are not very A review of their mechanical, micro- scandium mine in operation in the world. effective at inhibiting recrystalliza- structural, and corrosion characteristics The mine has a proven minable scandium tion because most of the high- shows that scandium-reinforced alloys reserve of 7.58 million t of ore grading strength and precipitation- can be usefully employed in aerospace, at 1.5 g/t of scandium, likely to yield hardenable aluminum alloys are sports, transportation, and process 907,185 t of final Al-Sc product. Initial solution-heat-treated at tempera- mining and processing of the complex ore and refining of scandium concentrates 670 —Initial Condition have resulted in the Al-Sc master alloy.

) —Cold-Rolled 84% and Heat Treated 25,000 a Ashurst Technology Ltd. developed

650 e

ur 20,000 (MP il

, a fully integrated program aimed at a h F 630 the worldwide commercialization of 15,000

trengt 1 es to 10,000 scandium alloys. The development of l S

610 yc ld C e Al-Sc alloys first flourished in the Soviet 5,000 Yi 590 Union, where military demand was the 0 Conventional Scandium Modified High- Medium- Medium- main driving force. At the time of the Strength Strength Strength Figure 1. The yield strength of conven- break-up, scandium alloys Al-Mg-Zn/ Al-Mg-Zn/ Al-Mg-Zn/Sc tional and scandium-modified experi- 5356 5356 5XXX were on the verge of major application Filler Filler Filler mental 7XXX alloy in the initial and in MIG 29 fighters because of their cold worked 84% and heat-treated Figure 3. The fatigue life of a high-strength conditions. advantages over the low and aluminum--zinc alloy almost high strength of Al-Mg, Al-Li, and triples when it is welded by a scandium- 370 —5356 Filler other recent alloys. Scandium, with modified 5000 series filler . 365 —Sc-Modified 5XXX Filler 360 ) its unique strength and weight-saving 355 80 350 characteristics, has been introduced as 345 70 —Conventional 5356 Filler a potent alloying element in several —Sc-Modified 5XXX Filler UTS (MPa 340 60 335 aluminum alloys (e.g., Al5052 and 50 330 Al7075) in recent years, bringing 40 (mm) 30 Conventional 7XXX Sc-Modified 7XXX about dramatic improvements in their 20 mechanical and physical characteristics. 10 It has been possible to achieve an ideal 0 Figure 2. The ultimate tensile strength of Conventional 7XXX Sc-Modified 7XXX a 7000 series weldment is considerably combination of strength, density, and higher when a scandium-modified 5000 thermal stability because of the unique Figure 4. As shown by crack length in mil- aluminum filler metal is used compared limeters, the Houldcroft test greatly favors to a conventional 5356 filler material. precipitation-hardening characteristics of the scandium-modified vs. conventional The difference is even more pronounced scandium. These alloys are gaining a wide filler metal in the welding of a 7000 series when the base metal is also modified popularity in aeronautical, automotive, aluminum alloy. by scandium. and transportation industries.

2003 February • JOM 35 1,100 filler was replaced with scandium- 1,050 modified filler. Welding studies on L L + ScAl Al 7XXX by conventional filler ) 1,000 3 alloys and scandium-modified 950 933 K 0.52 932 K fillers have shown the capability of 900 (Al) 0.38

mperature (K scandium to convert non-weldable Te 850 (Al) + ScAl 3 alloys to alloys with a promising 800 degree of weldability. In addition, 750 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 the weldment tensile strength of Al Scandium (wt.%) 7XXX is significantly improved Figure 5. A binary diagram of when welded with modified 5XXX aluminum and scandium. alloys versus conventional 5XXX tures well above their recrystalliza- filler (Figure 2).6 Strength is further tion temperatures. Surprisingly, improved when scandium is added Figure 8. A photomicrograph showing scandium increases the recrystal- to Al 7XXX. the pinning of grain boundaries by lization temperature of aluminum Because of these advantages, the dislocations. alloys to above 600°C, well above scandium-modified aluminum alloys the temperature range of heat- have a promising potential for military treatable aluminum alloys. It has and commercial aerospace applications been observed that the addition of including bulk heads, heat shields, sheet scandium with zirconium is more for upper skin, fuel and exhaust systems, effective than the addition of and in automotive and transport systems. scandium alone.2 The addition of scandium also • Small additions of scandium in the improves fatigue life.6 Medium- to range of 0.2–0.6 wt.% bring about high-strength Zr-Mg-Al alloys failed at a high specific-strengthening 16,000 cycles, indicating their limitation 3 effect. An Al-5.25Mg alloy con- for intended applications. Surprisingly, Figure 9. A TEM micrograph of Al-2.5Mg- taining 2.5% magnesium showed a the fatigue life was increased to 25,000 0.5Sc alloy showing a high dislocation yield strength of 365 MPa, more cycles with a scandium-modified filler density at the sub-grain boundaries. than double the strength of the scandium-free alloy. Scandium metal (Figure 3). In a Houldcroft welding provides the highest increment of test, cracking was significantly reduced strengthening per atom percent of by a scandium-modified filler metal. any alloying element when added When both the filler and base metal are to aluminum.4 Al-Mg-Sc alloys are modified by scandium, cracking is capable of developing strength and eliminated (Figure 4).6 fracture toughness similar to that OPTIMUM AMOUNT OF of Al 2024-T3 alloy. The effect of SCANDIUM scandium addition on the yield strength of conventional and The Al-Sc binary phase diagram is scandium-modified experimental shown in Figure 5.7 As observed from 7XXX alloys is shown in Figure 1.2 the figure, Al-Sc is slightly hyper- • Scandium has the ability to refine 10 µm eutectic and hence, very small amounts grain size. It is a strong modifier of Figure 6. A SEM micrograph showing of Al Sc precipitate could form prior to 3 Mg2Si (dark) precipitates. Black (Fe) and cast structure, and the addition of white particles contain Mg, Ti, and Sc. the solidification of the aluminum phase. scandium makes it possible to The aluminum-rich side of the Al-Sc obtain continuously cast billets with phase diagram shows a eutectic point at a non-dendritic structure.5 655°C. The system has also a very narrow • Reduction and elimination of hot freezing range. The maximum equilib- cracking in welds. The scandium rium solubility of scandium in aluminum modification of welding filler alloys is 0.35–0.4 wt.%. With cooling rates in as well as base alloys are capable of solidification corresponding to the preventing hot cracking. Aluminum continuous casting of ingots, a super- alloy 2618 is known to be hot-crack saturated solution of scandium (up to sensitive, and, when welded with 0.6%) with aluminum is obtained.5 An conventional filler, it develops a 10 µm equilibrium phase Al Sc with Li 3 2 high level of cracking.2 However, Figure 7. A SEM micrograph showing structure precipitates in cast alloys. The rectangular precipitates enriched in Ti, its cracking susceptibility was Al, and Sc. supersaturated solution of scandium in reduced when the conventional aluminum is unstable compared with

36 JOM • February 2003 the solution of other transition zirconium is effective in inhibiting metals. The rate of incubation preceding recrystallization through the formation Table IV. Corrosion Rates of Al-Mg-Sc Alloys in 3.5 wt.% NaCl17 decomposition is three to four orders of of extremely fine Al (Zr Sc ) particles. 3 x 1-x Corrosion Rate magnitude shorter compared to Al-Mn These particles are less prone to coagulation compared to Al Sc particles. Alloy (mm/year) (mdd) and Al-Zr alloys and the rate of 3 decomposition is five to six times higher Al-2.5Sc 0.023 1.477 MICROSTRUCTURE 8 Al-2.5Mg-0.10Sc 0.031 1.980 compared to Al-Mn and Al-Zr alloys. Al-2.5Mg-0.15Sc 0.046 2.914 The stability of a solid solution of A microstructural examination by Al-2.5Mg-0.30Sc 0.038 2.460 scandium in aluminum drops with an optical microscopy reveals little in alloys containing less than 0.6 wt% of scan- increase in scandium content under Table V. Pitting Data conditions of long, high-temperature dium. Al-Mg alloys containing 6% on Al-Mg-Sc Alloys17 heating such as those as encountered in magnesium show β-Al-Mg (Al Mg ) E E E 8 5 p pp corr production of billets. Under such precipitate.10 The combination of Mg Alloy (V) (V) (V) conditions, decomposition and coagula- (0.6–1.2%) and Si (0.4–1.1%) results in Al-2.5Sc –0.521 –0.697 –0.643 tion of Al Sc secondary particles occurs. the formation of a Mg Si metastable Al-2.5Mg-0.10Sc –0.578 –0.713 –0.659 3 2 phase.11 The dark Mg Si particles are Al-2.5Mg-0.15Sc –0.518 –0.694 –0.628 In view of these findings, the optimum 2 limit for scandium is 0.6%. The desirable shown in Figure 6. A scanning electron Al-2.5Mg-0.30Sc –0.670 –0.729 –0.742 limit is, however, 0.1–0.5%.5 microscopy examination of the Al- Experience accumulated over a few 2.5Mg-0.15Sc alloy revealed rectangular size of 1 µm, however, it was difficult to years has shown that a lower content of precipitates containing , aluminum, resolve the structure. It was only after scandium can have a very strong and scandium particles as confirmed by overaging for 5,500 min. at 320°C that electron dispersive spectroscopy (EDS) coherent precipitates of Al Sc (15 nm modifying effect when zirconium is 3 present.9 In the presence of zirconium, a analysis (Figure 7).8 Transmission diameter) were observed. The poor aging non-dendritic structure is formed with a electron microscopy (TEM) studies on response of deformed samples was scandium content as low as 0.2%, the microstructure of the alloys contain- attributed to the discontinuous precipita- ing higher scandium contents (1%) were tion of Al Sc.11 It was also found that whereas, without zirconium, more than 3 0.5% scandium is required to obtain a also conducted. After cold rolling to coherent precipitation is possible only non-dendritic structure. The synergistic 85% and aging to peak hardness at under high super saturation in alloys interaction between scandium and 320° C, Al-4Mg-1Sc showed a sub-grain with 1% of scandium content. In another study, microstructural studies on an Al-1.0Sc chill-cast alloy showed the Table I. Mechanical Properties of Al-Sc and Al-Mg-Sc Alloys (WR/A)10 presence of Al Sc. The particles of Al Sc 3 3 Strain Unit were found to be highly faceted with a Yield Tensile Hardening Propagation cubic morphology.10 By x-ray diffrac- Strength Strength Elongation Exponent Tear Strength* Energy** tion, the structure of Al Sc was found to Alloy (MPa) (MPa) (%) (n) Yield Strength (kJ/m)2 3 be face-centered cubic with a lattice Al-0.5Sc 286 297 14.5 0.026 1.74 319 parameter of 0.4105 nm. It had a Li -type Al-2.5Mg-0.5Sc 341 370 13.5 0.051 1.95 317 2 Al-4Mg-0.5Sc 381 443 14.5 0.072 1.93 270 ordered structure. In samples aged for 3 Al-6Mg-0.5Sc 381 467 10.5 0.101 1.22 84 h and 100 h, a large number of rod-shaped * Tear strength = maximum load divided by net cross-sectional area + bending stress. dendritic or spherical precipitates were ** Unit propagation energy = integral of load deformation curve after crack propagation divided by the total area of the specimen. observed.11 Efforts have been made to study the Table II. Mechanical Properties of Al-Sc and Al-Mg-Sc Alloys (WR/A) precipitation sequence in Al-Sc alloys Strain Unit by lifetime measurements.12 It Yield Tensile Hardening Propagation was observed that coherent Al Sc Strength Strength Elongation Exponent Tear Strength Energy 3 Alloy (MPa) (MPa) (%) (n) Yield Strength (kJ/m)2 precipitates were formed from solution- treated alloys at around 300°C in alloys Al-0.5Sc 298 319 10.5 0.008 1.59 193 Al-2.5Mg-0.5Sc 376 401 8.5 0.052 1.74 113 containing 0.006 wt.% scandium and Al-4Mg-0.5Sc 414 460 9.5 0.064 1.68 114 0.03 wt.% scandium. The technique is Al-6Mg-0.5Sc 433 503 10.5 0.074 1.33 100 very cumbersome and it has not been widely used. The precipitation process Table III. Mechanical Properties of Al-Mg-Sc Alloys has also been investigated by resistivity Ultimate Tensile and hardness measurements.13 Investiga- Yield Strength Strength Elongation tion into alloys containing 0.090 wt.% No. Alloy MPa MPa % scandium and 0.15 wt.% scandium 1 Al-0.5Sc 158.63 213.80 14 showed that the equilibrium Al Sc phases 2 Al-2.5Mg-0.5Sc 202.08 265.35 10 3 with Li structure precipitates homoge- 3 Al-4Mg-0.5Sc 215.87 272.43 9.5 2 4 Al-6Mg-0.5Sc 255.19 286.22 9 neously without the formation of any metastable phase and the Al Sc phase 3

2003 February • JOM 37 ture. One disadvantage of using Al-Mg for the Al-0.5Sc alloy.16 The increase in alloys is the loss in strength they exhibit strength was due to the precipitation of Al Sc particles both within the grains during the process of recovery of 3 ductility and toughness. Scandium and at grain boundaries. Mechanical addition minimizes these losses. Despite properties of Al5052 containing 0.1% the low solubility of scandium and, scandium and 0.15% scandium were hence, a limited volume fraction of Al Sc evaluated by the authors. These are 3 phase, it produces a significant strength shown in Table III. increment. The other advantage of An outstanding contribution of scandium is its capability to provide scandium to the modification of alumi- thermal stability by a substantial increase num alloys is its novel effect in 10 µm in the re-crystallization temperature. The preventing strength loss after thermal Al Sc precipitate is also highly resistant processing and its resistance to crystal- Figure 10. A SEM micrograph showing 3 layer mudcracking. to coarsening, unlike the traditional lization even after 75–85% cold reduc- precipitate phases. Data on mechanical tion, in which range conventional alloys properties of Al-Mg-Sc alloys is not are completely recrystallized. The data abundant.10 reported only limits to few alloys and the The mechanical properties of five effect of age hardening of alloying Al-Sc and Al-Mg-Sc alloys (warm rolled elements, and secondary particles such as Al Sc on the mechanical properties is and aged [WR/A] and warm rolled/cold 3 rolled and aged [WR/CR/A]) are shown not fully understood. Several areas need in Tables I and II. to be fully explored, such as the superplastic forming nature of Al Sc Both the tear and tensile strength 3 increase with increasing magnesium precipitate, temperature dependence of content, as does the difference between mechanical properties, and the effect of 100 µm the yield strength and tensile strength. alloying elements like titanium, chro- Figure 11. The formation of gelatinous The strain-hardening coefficient has a mium, vanadium, and hafnium. Al(OH) in the form of mothballs and 3 lower value for Wr/Cr/A alloys with 4% its cracking. CORROSION BEHAVIOR and 6% magnesium because of the hardening that occurs during cold-rolling The existing interest in Al-Mg-Sc maintained coherency with the matrix operation. One point of interest is a very alloys is centered around their excellent for a long aging time. low value of n (0.01) for the magnesium- combination of physical and mechanical It is a common observation that Al Sc properties, but not their service perfor- 3 free Al-0.5Sc alloy that is suggestive of precipitates are very effective in pinning strengthening by coherent particles, mance in a corrosive environment. It is grain boundaries and thus maintaining a which may shear during the deformation therefore of prime interest to evaluate fine grain size. The dislocations that process.10 A similar behavior is observed the corrosion performance of Al-Mg-Sc emerge from deformation are the in Al-Li alloys.15 Crack propagation alloys. The author has recently reported favorable sites for the nucleation of Al Sc the only information on the corrosion 3 toughness as shown by the tear- precipitate. The coherent small Al Sc behavior of Al-Mg-Sc alloys.17 The 3 strength/yield-strength ratio increases precipitates are highly effective in with magnesium content and then drops weight-loss studies conducted in 3.5 impeding the grain boundary motion. suddenly above 4% for WR/A treatment. wt.% NaCl showed that the corrosion The effect of grain size and pinning It may be attributed to the presence of rate decreased as the exposure down of grain boundaries on the microvoids at a higher magnesium level. increased. For instance, the corrosion strengthening mechanisms of Al-2.5Mg- When interpreting the data and compar- rate of the alloy Al-2.5Mg-0.1Sc 3Sc alloys9 has also been observed by ing it with conventional aerospace alloys, decreased from 2.4 mdd to 1.44 mdd the authors.14 The pinning down of the it is fair to say that Al-4Mg-0.5Sc alloys (0.038 mm/y to 0.023 mm/y) after an grain boundaries by Al Sc precipitate is exposure period of 800 h. This has been 3 compete favorably with the conventional shown in Figure 8 and the high disloca- aerospace alloys like 2024 and 7075. attributed to the build-up of a protective film of either Bayerite β( -Al O .3H O) tion density at the sub-grain boundaries Beyond the scope of this review, it may 2 3 2 or Boehmite (γ-Al O .H O). The surface is shown in Figure 9. also be mentioned that the Al-4Mg-0.5Sc 2 3 2 alloys are capable of producing super- of Al-Mg-0.1Sc was covered with a film STRENGTHENING EFFECT plasticity. of boehmite as identified by EDS studies. As stated previously, scandium is said The nature of artificial aging response Table IV, which summarizes the to produce the highest increment of by the alloys Al-0.5Sc and Al-4Mg-0.5Sc corrosion rates the alloy, shows it strengthening per atom percent when was investigated. An aging temperature provides good resistance to corrosion added to aluminum, increasing strength of 561K was considered adequate,16 and typical of other Al-Mg alloys such as both by solid solution and precipitation peak hardness was produced by aging 5456 and 6061, which are well known hardening of Al Sc particles. These for their performance in seawater service. 3 times in the range of 10–20 h. The yield particles pin the dislocation cell struc- strength was raised by at least 100 MPa Pitting studies were conducted by

38 JOM • February 2003 ASTM cyclic-polarization technique. use these alloys in a wide spectrum APPLICATIONS Age hardening did not seriously affect of applications. the pitting potential. The pitting data With the passage of time and new References obtained from polarization studies is developments, scandium has found a reported in Table V. wide range of applications. A leading 1. Ashurst Tech., “Ashurst Makes Progress with Scandium Alloys Development,” Aluminum Today (18 The pitting potential of Al-Mg-Sc sports company successfully employed December 1997). alloys meets the criteria that E be scandium alloys in the production of 2. Lawrence S. Kramer and William T. Tack, “Scandium corr more negative to E for good pitting softball and baseball bats. This success in Aluminum Alloys” (Paper presented at the Alumtech P Cong., Atlanta, GA, 19–23 May 1997). performance. On comparing the pitting was followed by the manufacture of 3. L.A. Willey, U.S. patent, 3,619,181 (1971). potentials of well-known commercial mountain and road bicycle frames 4. M.D. Drits, L.S. Toropova, and Yu G. Bykov, aluminum alloys such as Al6061, and components. The bicycle frames “Metalloved. Term.,” Obbrab. Met., 7 (1983), pp. 60–63. Al6013, Al2024, and Al5456 with showed a 12% reduction in weight, a 5. V.I. Elagin, V.V. Zakharov, and T.D. Rostova, Tsvet. Al-2.5Mg-0.1-0.3Sc alloys, it is 50% increase in yield strength, and Met. (12) (1982), pp. 96–99. observed that pitting potentials of a 24% improvement in fatigue life 6. Bob Irvine, “Scandium Places Aluminum Welding on a New Plateau,” Welding Journal, 7 (July 1995), Al-Mg-Sc alloys are more positive, over the best-selling aluminum bicycle. p. 53. suggesting a good pitting resistance of Potential aerospace applications include 7. L.A. Willey, ALCOA Laboratories Physical Metallurgy these alloys. bulk heads, heat shields, forgings and Division Report #13-62, EH3 (1962). 8. V.I. Dobatkin et al., The Metallurgy of Light Alloys The pitting morphology on the extrusions for seat tracks, wheels, (Russian) (Moscow: Nanka, 1983), pp. 214–219. Al-2.5Mg-0.1-0.3Sc alloys was studied running gear, and fuel and exhaust 9. V.I. Elagin et al., F12, Met. Metalloved., 60 (1) (1985), pp. 97–100. by scanning electron microscopy.17 The systems. 10. Ralph R. Sawtell and Craig L. Jensen, “Mechanical maximum pit depth on these alloys Scandium alloys are promising for Properties and Microstructure of Al-Mg-Sc Alloys,” compared favorably with the pitting automotive and air transportation Metallurgical Transactions A, 21A (1992), p. 421. depths observed in Al6061 and Al6013. applications because of their capability 11. B.D. Parker, Z.F. Zhou, and S.P. Nolle, “The Effect of Small Addition of Scandium on the Properties of One reason for good resistance to of weight reduction on critical moving the Aluminum Alloys,” J. of Mater. Sci., 30 (1995), localized corrosion is that the Al Sc parts. Scandium alloys could also be used pp. 452–458. 3 precipitates are of a very small size (8–10 in wheels, bumpers, frames, pistons, 12. K.A. Jackson, J. Crystal Growth, 1 (1967). 13. S. Krause et al., Phy. Stat., vol (a), 191 (1994), nm) and they are coherent with the and air bag canisters. The aluminum p. 311. matrix. One would expect fewer flaws scandium welding wire provides a 14. Hyung Ho Jo and Shin Hiroh Fujikawa, Mater. Sci. and lesser discontinuities at the very strong bonding while welding and Eng., A171 (1993), pp. 151–161. 15. T.H. Sanders, E.A. Ludwiczak, and K. Sawtell, Mater. precipitate/oxide interface aluminum. This alloy could also be Sci. Eng., 43 (1980), pp. 247–260. compared to Al2024, Al6061, and used in the cylinders of diesel engines for 16. L.E. Drits et al., Sov. Phys. Dokl., 26 (1981), Al6013, which contain much larger power boats. Because of the good corro- p. 344. 17. Zaki Ahmad, Anwar Hameed, and Abdul Aleem, “The precipitates of ClAl and CuMgAl. It sion resistance shown by recent studies, 2 Corrosion Behavior of Scandium Alloyed Aluminum has been shown that an increase in the scandium alloyed with aluminum could Alloys” (accepted for publication in Corr. Sci.). precipitate size decreases the pitting also be used in a saltwater environment Zaki Ahmad is with the Department of Mechanical potential and increases the susceptibility (e.g., heat exchanger tubes in desalina- Engineering at King Fahd University of Petroleum to pitting. Pits on the alloy surface are tion plants). The potential of scandium & . covered by Al(OH) . The gelatinous alloys is highly promising, but, like new 3 For more information, contact Zaki Ahmad, King Al(OH) , which is pumped by pits, dries technologies, it is going to take time and Fahd University of Petroleum & Minerals, Depart- 3 ment of Mechanical Engineering, KFUPM Box 1748, up eventually in the form of mud cracking painstaking research and development Dhahran – 31261, Saudi Arabia; fax 966-38-602-949; (Figures 10 and 11). efforts to convince the designers to e-mail [email protected]. What’s new at TMS On-Line? The TMS web site brings you continual updates of the latest society information. What’s new this month? Visit the TMS web site to find:

The Personal Conference Scheduler is back! Visit the site regularly and Coordinate your 2003 TMS Annual Meeting activities pcs.tms.org click on “What’s New” to The March issues of JOM, Journal of Electronic Materials, and Metallurgical find out the most up-to-date and Materials Transactions A and B will be available on-line at the end information on meetings, of this month doc.tms.org publications, membership The March issue of TMS e-News will be available on-line at the beginning activities, and more. of next month www.tms.org/newsletter/newsletter.html www.tms.org

2003 February • JOM 39