Microstructures Of Zinc And Its Alloys

Metallography and Microstructures of Zinc and Its Alloys

Page 1 of 14 Metallography and Microstructures of Zinc and Its Alloys, Metallography and Microstructures, Vol 9, ASM Handbook, ASM International, 2004, p. 933–941

Metallography and Microstructures of Zinc and Its Alloys

Microstructures of Zinc and Zinc Alloys The natural impurities, contaminants, and alloying additions present in commercial zinc materials have extremely limited solid solubility. They readily produce alterations in cast or wrought microstructures and changes in one or more properties. High-purity zinc, UNS Z13002, for example, is 99.99% Zn with maximum limits of 0.003% each on lead, iron, and cadmium and is almost free of mircosegregation (Fig. 1, 2). Nominal compositions of the alloys depicted in this article are noted in the captions.

Fig. 1 Special high-grade zinc, UNS Z13002 [99.99% Zn (min), 0.003% Pb (max), 0.003% Fe (max), 0.003% Cd (max)], as-cast. Almost free of microsegregation. Etchant 1, Table 1. 100×

Fig. 2 Same alloy as Fig. 1 under polarized light illumination to show the extent of grain growth from original etched grain boundaries within large grains. Etchant 1, Table 1. 100×

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Metallography and Microstructures of Zinc and Its Alloys

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The elements commonly found in zinc are lead, cadmium, iron, copper, aluminum, titanium, and tin. Lead, cadmium, tin, and iron are natural impurities in zinc and are also added to zinc to develop desired properties. Zinc casting alloys are primarily zinc-aluminum with small additions of other elements, such as copper and magnesium. Wrought zinc alloys for rolled products generally contain lead, iron, cadmium, copper, or titanium alone or in combination and usually in concentrations under 1%. The effects on microstructure produced by these elements are described as follows. Zinc has a familiar role as a protective coating for steel in galvanizing processes. Pure zinc and zinc-aluminum alloys are used in continuous hot dip processes. The galvanneal process uses zinc-iron alloys (Ref 1). Batch process hot dip galvanizing uses high-grade zinc (UNS Z15001, with impurities less than 0.10%; UNS Z13001, with impurities less than 0.010%; and prime western zinc, UNS Z19001) (Ref 2). The interaction between base materials and coatings results in interesting profiles of microstructures (Ref 1, 3, 4, 5). Lead. The solubility of lead in solid zinc is extremely limited. A monotectic is formed at 418 °C (784 °F) and a lead content of 0.9%, and zinc crystals and liquid exist in equilibrium down to the eutectic temperature of 318 °C (604 °F). As a result, lead appears in cast zinc and zinc alloys at the dendrite boundaries in the form of small, spherical droplets or surface films (Fig. 3). Because of their softness, the droplets can be easily pulled out during polishing, leaving holes that appear black in the microstructure. Special care in polishing is required to retain the lead particles.

Fig. 3 Prime western zinc, UNS Z19001 [98% Zn (min), 1.4% Pb (max), 0.05% Fe (max), 0.20% Cd (max)], as cast. The dark spots are lead particles at the grain boundaries. Etchant 1, Table 1. 100×

When rolled, the particles of lead are elongated in the rolling direction and are not located preferentially at the recrystallized grain boundaries. In zinc-aluminum alloys, lead induces intergranular corrosion; concentrations must be maintained below 0.004%. Lead was added to UNS Z33520 to illustrate this effect in Fig. 4 and 5.

Fig. 4 Fracture surface of the 10 mm (0.375 in.) diameter end of a tension test bar die cast from alloy 3 (UNS Z33520) to which 0.018% Pb was added (0.005% Pb is allowed). Exposed 10 days to wet steam at 95 °C (205 °F). Dark ring is intergranular corrosion. See also Fig. 5 Not polished, not etched. 6×

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Fig. 5 Micrograph of edge of fracture surface in Fig. 4 Subsurface intergranular corrosion (top) causes swelling and decreases mechanical properties. Deliberate addition of 0.018% Pb to the alloy approximates the contamination that might occur from the use of remelted scrap. As-polished. 100×

The cadmium present in most commercial zinc products is in solid solution and produces no change in microstructure, except coring in the cast structure. In rolled zinc, the cadmium remains in solid solution, increasing strength, hardness, and creep resistance and raising the recrystallization temperature (Fig. 6, Fig. 7). In zinc-aluminum alloys, because cadmium lowers resistance to intergranular corrosion, concentrations must remain below 0.003%.

Fig. 6 Hot-rolled special zinc [99% Zn (min), 0.6% Pb (max), 0.03% Fe (max), 0.50% Cd (max)], under polarized light; grains are clearly defined. Etchant 1, Table 1. 250×

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Metallography and Microstructures of Zinc and Its Alloys

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Fig. 7 Same alloy as Fig. 6 except cold rolled and photographed under polarized light. Note distortion of the grains caused by cold working. Etchant 1, Table 1. 250×

Iron, when present in zinc in amounts exceeding approximately 0.001%, appears in the microstructure as an intermetallic compound containing approximately 6% Fe. The particle size is controlled by the amount of iron present and the thermal history of the part. Fine particles in a casting can be coalesced to a coarser form by prolonged heating at 370 °C (700 °F).

Cast specimens with fast cooling (Fig. 8) and very slow cooling (Fig. 9) show the zeta-phase intermetallics. Four distinct phases are seen in the profiles of galvanized coating on steel (Ref 5, Fig. 1) and galvannealed coating (Ref 1, Fig. 7).

Fig. 8 Zn-0.025Fe alloy, permanent mold, rapid cooling, annealed 40 h at 380 °C (716 °F). Zeta-phase intermetallic compounds appear as fine precipitate through annealing. Electrolytic polish. Etchant: similar to etchant 1, Table 1 (but only 10 g Na2SO4). 500×

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Metallography and Microstructures of Zinc and Its Alloys

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Fig. 9 Zn-0.025Fe alloy, hot graphite mold, slow cooling. Zeta-phase intermetallic compounds. Electrolytic polish. Electrolytic etch: etchant 6, Table 1 (short time). 200×

The iron-zinc compound, like lead, precipitates at dendrite boundaries. When a zinc casting is rolled, the iron-zinc particles are elongated in the rolling direction, along with any lead particles present. The presence of iron particles in the proper concentration and distribution in rolled zinc assists in control of grain size. Iron in zinc-aluminum alloys is present as FeAl3 particles, which can significantly lower ductility. Alloy ZA-27, UNS Z35841, with 0.05% Fe added (Fig. 10) and with 0.013% Fe added (Fig. 11) results in an FeAl3 intermetallic.

Fig. 10 ZA-27 alloy, UNS Z35841, with 0.05% Fe (0.075% Fe max allowed), as sand cast. Structure consists of intermetallic FeAl 3 particles (dark gray) in a matrix of α particles (light) and ε particles (light gray). As-polished. 100×

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Fig. 11 ZA-27 alloy, UNS Z35841, with 0.13% Fe (excess, 0.075% Fe max allowed), as sand cast. Structure is intermetallic FeAl 3 particles (dark gray) in a matrix of α phase and ε phase. Structure is much coarser than in Fig. 10 As-polished. 100×

Copper, when present in zinc in amounts to approximately 1%, is in solid solution and results in a cored structure. During hot rolling at approximately 205 °C (400 °F), the

copper is retained in supersaturated solid solution. On cooling, some of the zinc-copper ε phase precipitates at the final recrystallized grain boundaries (Fig. 12). During long exposures near room temperature, ε phase will continue to precipitate at grain boundaries and finally in the interior of the grains, ultimately forming T′ phase (ternary eutectic). When cold rolled, ε phase precipitates rapidly and abundantly in the cold-worked structure (Fig. 13). In concentrations beyond 1% in zinc-aluminum alloys, ε phase precipitates as an interdendritic phase.

Fig. 12 Zinc containing 1% Cu, UNS Z44330, hot rolled. Polarized light illumination clearly defines the zinc-copper ε phase at grain boundaries. Etchant 1, Table 1. 250×

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Fig. 13 Cold-rolled Zn-1Cu alloy, UNS Z44330, photographed under polarized light. Note the severe distortion of grains caused by cold working (compare with Fig 12). Etchant 1, Table 1. 250×

Die cast alloy UNS Z35531, alloy 5, containing 1% Cu is seen in Fig. 14. Aging for 10 days at 95 °C (205 °F) increases the amount of precipitation in the zinc solid solution (Fig. 15). An alloy with 0.9% Cu, UNS Z35841, ZA-27, shows primary cored aluminum-rich dendrites with peritectic α + η and white ε-phase particles (Fig. 16). The same alloy in a sand cast specimen that was treated 3 h at 360 °C (680 °F) shows course ε-phase particles at old dendritic boundaries (Fig. 17). With longer lower-temperature exposure, the ε phase is converted to a fine T′ phase (ternary eutectic) (Fig. 18). Continuously cast, the same alloy has a finer microstucture. The presence of the ε-phase particles varies with the size of the bar being cast (Fig. 19, 20).

Fig. 14 Alloy 5 (UNS Z35531, ASTM AC41A, Zn-4.1Al-0.055Mg-1.0Cu), as die cast. Alloy 5 has more copper and magnesium and higher strength and hardness than alloy 3. Etchant 2, Table 1. 1000×

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Fig. 15 Same as Fig. 14 except aged 10 days at 95 °C (205 °F). Aging had the same effect on the die cast structure as for alloy 3 (UNS Z33520) (Fig. 24b). Etchant 2, Table 1. 1000×

Fig. 16 ZA-27 alloy (UNS Z23841, Zn-11Al-0.9Cu-0.02Mg), as sand cast. Primary, cored aluminum-rich dendrites surrounded by peritectic α + η. White particles are ε phase. Less eutectic is apparent than in Fig. 1, 2, 23, 24 Etchant 1, Table 1. (a) 100×. (b) 500×

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Fig. 17 Same alloy as Fig. 16 sand cast, homogenized 3 h at 360 °C (680 °F), and furnace cooled. Structure is fully stabilized β phase decomposed into α + η lamellar eutectoid. Coarse ε-phase particles are present at old dendritic boundaries. Etchant 1, Table 1. 500×

Fig. 18 Same alloy as Fig. 16 sand cast, heat treated 12 h at 250 °C (480 °F), and furnace cooled. Structure consists of coarse eutectoid α + η phase and eutectic. The metastable ε phase has been converted to fine T′′ (ternary eutectic). Etchant 1, Table 1. 500×

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Fig. 19 Same alloy as Fig. 16 except continuous cast in a 25 mm (1 in.) diameter bar. There is a much finer microstructure, with more eutectic and no coarse ε-phase particles. Compare also with Fig. 20. Etchant 1, Table 1. (a) 100×. (b) 500×

Fig. 20 Same alloy as Fig. 16 except continuously cast in a 150 mm (6 in.) diameter bar. Same constituents as Fig. 16, but a finer structure. Some coarse ε-phase particles (white) are evident. Compare with Fig. 19(a) and (b), which show the same alloy cast in a smaller section. Etchant 1, Table 1. (a) 100×. (b) 500×

Titanium. The solid solubility of titanium in zinc is very limited. At approximately 0.12% Ti, a lamellar eutectic of zinc and TiZn15 (4.66% Ti) forms (Fig. 21). The

eutectic forms at dendrite boundaries in a casting. The TiZn15 compound decreases the cast grain size of zinc and restricts grain growth in hot-rolled zinc. In rolled strip, particles of the compound are elongated in the rolling direction. Zinc grain growth is limited to the spacing between the stringers of compound.

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Fig. 21 Cast zinc with 0.6% Cu and 0.14% Ti. Eutectic (zinc and titanium-zinc phases) at grain boundaries. Both etched in etchant 1, Table 1. (a) 100×. (b) Showing the lamellar eutectic. Coarse needles of titanium-zinc phase are parallel to the polishing plane; fine needles, perpendicular. 250×

Aluminum. A lamellar eutectic forms at 5% Al between aluminum (α) and zinc (η) solid solution at 382 °C (720 °F). The α constituent of the eutectic is stable only above 275 °C (527 °F); at lower temperatures, it transforms eutectoidally into α and η phases (Fig. 22, 23). Although the solid solubility of aluminum in zinc at the eutectic temperature is approximately 1%, castings containing as little as 0.10% Al display the eutectic structure in interdendritic areas. At the normal aluminum concentration in standard zinc die-casting alloys (4.0% Al), the rate of attack by the melt on iron is sufficiently low to permit die casting in hot-chamber machines in which the operating mechanism is immersed continuously in the molten alloy.

Fig. 22 ZA-8 alloy (UNS Z35636, Zn-8Al-1Cu-0.02Mg), as sand cast. Both etched in etchant 1, Table 1. (a) Dendrite structure at a lower magnification, 100×. (b) Coarse, zinc-rich dendrites in a matrix of α + η eutectic phase. 500×

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Fig. 23 Same alloy as Fig. 22 except as pressure die cast. Same constituents as Fig. 22, but pressure die casting has yielded a much finer microstructure. Etchant 2, Table 1. 500×

During solidification of hypoeutectic zinc die-casting alloys containing approximately 4% Al, such as alloy 3 in Fig. 24(a), the first material to freeze appears as primary particles of zinc-rich solid solution (η phase). Later, the remaining liquid solidifies as eutectic composed of η phase and the unstable high-temperature constituent α. Aluminum acts as a grain refiner in cast zinc; together with the high solidification rates of the die casting process, this results in a fairly fine equiaxed grain structure, which is primarily responsible for the strength, ductility, and toughness of zinc die castings.

Fig. 24 Alloy 3 (ASTM AG40A, Zn-4.1Al-0.035Mg). (a) As die cast. Structure is zinc solid solution surrounded by eutectic. (b) Same as (a) except aged 10 days at 95 °C (205 °F). Aging increased the amount of precipitation in the zinc solid solution. Both etched in etchant 2, Table 1. 1000×

When die castings are aged at room temperature or a slightly elevated temperature, a precipitation reaction occurs in the zinc-rich η phase. In a freshly made die casting, η phase may contain approximately 0.35% Al in solution. During five weeks at room temperature, this will decrease to approximately 0.05%, the difference appearing as minute particles of α within the η-phase structure. Most of the aluminum can be precipitated in much less time by aging at a slightly elevated temperature (Fig. 24b). Similar aging effects are observed in alloys containing copper, which is added to increase tensile strength and hardness. Copper additions to 1.5% yield die castings with reasonably stable properties and dimensions. Low-temperature stabilization annealing is occasionally used to achieve maximum stability. The zinc-aluminum alloys extend the amount of primary phase; in the ZA-8 (Fig. 22) and ZA-12 (Fig. 25) alloys, this is η phase but is replaced by α in the ZA-27 alloy (Fig. 11).

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Fig. 25 ZA-12 alloy (UNS Z35631, Zn-11Al-0.9Cu-0.02Mg), as sand cast. Coarse, zinc-rich dendrites in a matrix of eutectic α + η phase. This alloy contains less of the eutectic than ZA-8 (Fig. 22, Fig. 23). Etchant 1, Table 1. (a) 200×. (b) 500×

The casting method will influence the cooling rate and the microstructure that results. Figure 26 shows permanent mold, pressure die cast, and continuous cast specimens of the same alloy that was sand cast in Fig. 25.

Fig. 26 Same alloy as Fig. 25 (a) As-cast in a permanent mold. Same constituents as Fig. 25, but permanent mold casting results in a finer microstructure. Etchant 1, Table 1. (b) Same constituents, but pressure die casting results in a much finer microstructure. Etchant 4, Table 1. (c) Continuous cast in an 80 mm (3.25 in.) diameter ingot results in finer structure than the sand casting. Etchant 1, Table 1. All: 500×

Wrought alloys based on the eutectoid composition of 78% Zn and 22% Al are of commercial interest because of their superplastic properties. Microstructures developed in this alloy depend on the heat treatment used (Fig. 27).

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Fig. 27 Zn-22Al alloy (eutectoid composition). (a) Superplastic, fine-grained structure obtained by annealing at 350 °C (660 °F) and water quenching. (b) Same alloy held 1 h at 350 °C (660 °F) and air cooled. Structure consists of lamellar and granular α and η, both products of eutectoid transformation. Etchant 2, Table 1. 2500×

Magnesium additions in concentrations of 0.01 to 0.03% to the ZA-8, ZA-12, and ZA-27 alloys increase strength and hardness but decrease ductility. Tin forms a low-melting eutectic with zinc at 91% Sn at 198 °C (388 °F). The solubility of tin in solid zinc is extremely restricted, and the zinc-tin eutectic appears in alloys

containing as little as 0.001% Sn. Figure 28 shows the dendritic distribution of tin.

Fig. 28 Zn-1Sn permanent mold with rapid cooling. There is a dendritic distribution of tin. Mechanically polished. Etchant: similar to etchant 1, Table 1 (but only 10 g Na2SO4). 500×

The only deliberate use of tin additions to zinc is in certain hot dip galvanizing operations. In some galvanizing operations, tin additions are widely used to regulate the formation of bright, smooth, large white spangles in the coating. Tin present in hot-rolled zinc can cause hot shortness, the tendency of an alloy to separate along grain boundaries under stress near its melting point. In zinc-aluminum alloys, tin induces intergranular corrosion; concentrations must be maintained below 0.002%.

References cited in this section 1.

R.W. Leonard, Continuous Hot Dip Coatings, Corrosion: Fundamentals, Testing, and Protection, Vol 13A, ASM Handbook, ASM International, 2003, p 786–793

2.

T.J. Langgill, Batch Process Hot Dip Galvanizings, Corrosion: Fundamentals, Testing, and Protection, Vol 13A, ASM Handbook, ASM International, 2003, p 794–802

3.

H.E. Townsend, Continuous Hot Dip Coatings, Surface Engineering, Vol 5, ASM Handbook, ASM International, 1994, p 339–348

4.

S.G. Fountoulakis, Continuous Electrodeposited Coatings for Steel Strip, Surface Engineering, Vol 5, ASM Handbook, ASM International, 1994, p 349–359

5.

D. Wetzel, Batch Hot Dip Galvanized Coatings, Surface Engineering, Vol 5, ASM Handbook, ASM International, 1994, p 360–371

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