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\u2013941


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 altera 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% eac 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\u00d7

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\u00d7

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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 a added to zinc to develop desired properties. Zinc casting alloys are primarily zinc-aluminum with small  additions of other elements, such as copper and magnes Wrought zinc alloys for rolled products generally contain lead, iron, cadmium, copper, or titanium alone or in combination and usually in concentrations under 1 effects on microstructure produced by these elements are described as follows.

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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. galvanneal process uses zinc-iron alloys (Ref 1). first Batch process hot dip galvanizing uses high-grade zinc (UNS Z15001, with impurities less than 0.10%; UNS Z1 need to download it. impurities less than 0.010%; and prime western zinc, UNS Z19001) ( Ref 2). The interaction between base materials and coatings results in interesting profiles microstructures (Ref 1, 3, 4, 5).

Lead. The solubility of lead in solid zinc is extremely limited. A monotectic formed at 418 \u00b0C (784 \u00b0F) and a lead content of 0 Cancel DownloadisAnd Print equilibrium down to the eutectic temperature of 318 \u00b0C (604 \u00b0F). As a result, lead appears in cast zinc and zinc alloys at the dendrite boundaries in spherical droplets or surface films (Fig. ). Because 3 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\u00d7

When rolled, the particles of lead are elongated in the rolling direction and are not located preferentially at the recrystallized grain boundaries. In zinc-aluminu 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 \u00b0C (205 \u00b0F). Dark ring is intergranular corrosion. See also Fig. 5 Not polished, not etched. 6\u00d7



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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\u00d7

The cadmium present in most commercial zinc products is in solid solution and produces no change in microstructure, except coring in the ca the cadmium remains in solid solution, increasing strength, hardness, and creep resistance and raising the recrystallization temperature ( Fig. 6, Fig. 7 ). In zinc-a 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\u00d7



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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 contain 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 pro 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 galvani 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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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 directio 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-a 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 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 appro 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 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 ( ). In concentrations beyond 1% in zinc-aluminum alloys, ε eutectic). When cold rolled, ε phase precipitates rapidly and abundantly in the cold-worked structure ( Fig. 13 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 solut (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 sam 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-temperatu 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-pha 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×


microstructures of zinc and its alloys

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