The Occurence, Effect, and Control Of Twinned Columnar Growth

In Aluminum Alloys

I I

500j-tm Figure 1 a. Appearance of the TCG struc­ture in a transverse slice from a 406-mm­thick semicontinuously DC cast ingot of 2219 alloy. Etch: Flick's reagent.

Figure 1 b. Photomicrograph of the TCG structure in 2219 alloy. Electropolished and photographed under polarized light to re­veal the highly oriented twin planes bound­aries. Etch: Electropolished.

54

Douglas A. Granger and John Liu

SUMMARY

The cast structure known as twinned columnar growth (TCG) is character­ized by large colonies of twinned columnar grains which are most likely to occur when conditions are unfavorable for the nucleation and growth of new grains during solidification. Casting methods and alloy systems that are most susceptible to TCG formation are identified, and the effects of the structure on mechanical properties are quantified for sections where rem­nants of the defect exist after fabrication processing. Observed loss in ductility is associated with planes of weakness which can be traced back to TCG structure in the ingot. Methods available to control ingot grain structure, and arrest the formation of TCG, through an understanding of the solidifica­tion process are discussed.

INTRODUCTION

Twinned columnar growth (TGC), sometimes referred to as feathery crystals, is a well-known phenomenon in commercial aluminum ingots. Numerous articles have described the characteristics of this structurel -8

which is usually observed in the form of large colonies of twinned den­drites (Figures la and Ib).

Effects of TCG on mechanical properties have been much less intensively studied:3,5,9,lO the most detailed work has been that of Eady et a1.,l0 who investigated the effect of the structure on the tensile properties of direct­chill (DC) semi continuously cast commercially pure aluminum. Using as­cast ingots, they showed that the TCG structure may result in either higher or lower tensile strengths and elongations depending on the orienta­tion of the twin planes with respect to the tensile axis, thus confirming the extremely anisotropic characteristics of TCG material found in the earlier investigations.

In spite of all the work on the characteristics and effects on mechanical properties there appears to be no published work describing the influence of TCG on the properties of heat-treatable wrought aluminum alloys. In view of the highly anisotropic nature of TCG in as-cast material, it seemed not improbable that remnants of the structure, which can be found in thick plate, could have a detrimental effect on mechanical properties. This possibility has been investigated in production lots of alloys 7050 and 2219-two alloys that are particularly susceptible to TCG formation when direct-chill cast into ingot. Alloys referred to in the text are listed with their nominal composition in Table I.

This paper will survey the circumstances under which TCG may be expected to arise, the impact on tensile properties in wrought products, and methods available to suppress its formation.

Aluminum Association

Alloy Designation

1145

2219

7050

Table I: Composition of Selected Wrought Aluminum and Aluminum Alloys (wt. %)

Si Fe Cu Mn Mg Cr Ni Zn Ti V

0.30 0.45 0.10 0.05 0.05 0.05 0.03

0.20 0.30 5.S- 0.20- 0.02 0.10 0.02- 0.05-6.S 0.40 0.10 0.15

0.12 0.15 2.0- 0.10 1.9- 0.04 5.7- 0.06 2.6 2.6 6.7

Others Zr Each Total

0.03

0.10- 0.05 0.15 0.25

O.OS- 0.05 0.15 0.15

JOURNAL OF METALS· June 1983

OCCURRENCE OF TWINNED COLUMNAR GROWTH

While TCG may be a well-known heterogeneity in aluminum alloy ingots, it is fortunate that there are remedies to prevent its occurrence. The most effective method is to use one of the common grain-refining master alloys containing titanium and boron. However, under some circumstances this approach alone is not likely to succeed:

• At cooling rates > 100C/s such as encountered in strip casting and close to the surface of a DC cast ingot.

• With alloys containing elements such as Cr and Zr (e.g. 2219 and 7050). • In high purity base, low-solute-containing alloys (e.g. 1145).

In order to appreciate why some alloys, and casting processes, are more likely to generate TCG than others, it is essential to recognize that grain refining a casting comprises two equally important steps: nucleation of primary aluminum grains and growth of grains.

When this is realized, the occasional occurrence of TCG can be explained in terms of inadequate inoculation or failure to provide suitable growth conditions ahead of the solidification front. Both nucleation and growth can be a difficulty in controlling the grain structure of DC cast 2219 and 7050 alloy ingot-a fact that made the investigation of TCG in these alloys a necessity.

INFLUENCE OF TCG ON PROPERTIES

The effect of the TCG structure on the properties of heat treatable products is complicated by the introduction of mechanical anisotropy, the amount of which depends upon the type and extent of the deformation process. In order to identify the effects of the TCG structure on mechanical properties in these products, the following general structural categories may be conveniently classified:

(1) Unrecrystallized or partly recrystallized and segregated structures in "lightly" worked products.

(2) Recrystallized and segregated structures in moderately worked products. (3) Recrystallized structures in extensively worked products in which gross

segregation resulting from TCG is completely redistributed.

Only categories (1) and (2), in which the TCG structure clearly influ­ences mechanical properties, will be discussed and illustrated using hot rolled plate. The effects of the structure in category (3) are very subtle, and will not be addressed here.

Partly Recrystallized and Segregated Structures

Alloy 7050-T73651, 144-mm-thick plate of nominal chemical composition given in Table I, was selected for study. This product is typically hot rolled to gauge from 406-mm-thick DC cast ingot.

Although the TCG structure appears as colonies of elongated feathery grains emanating from the chill zone in a wide range of orientations, with respect to the direction of the heat flow, these grains are realigned during fabrication so that they are more or less parallel to the rolling plane. The microstructure of the TCG-bearing plate is illustrated in Figures 2a and 2b, and for comparison the microstructure of the TCG-free material is shown in Figure 2c. Intergranular segregation in the TCG-bearing plate is illustrated in Figures 3a and 3b, and that in the TCG-free plate in Figure 3c.

In this product, the segregation comprises predominantly AhCu2Fe intermetallics, which are sites for crack initiation when the material is subjected to tensile stresses. Cracks· are observed in particles oriented

JOURNAL OF METALS· June 1983

c Figure 2a. TCG structure in the short trans­verse plane, parallel to the rolling direction, at 25 mm below the surface of 144-mm­thick 7050-T73651 plate. Note the relatively straight traces of {111} twin planes and interposing grain boundaries. Etch: Electropolished.

Figure 2b. TCG structure in the long trans­verse plane, parallel to the rolling direction, at 25 mm below the plate surface of same material as in Figure 2a. Etch: Electropolished.

Figure 2c. Grain structure in the short transverse plane, parallel to the rolling direction, at 25 mm below the plate sur­face of TCG-free 144-mm-thick 7050-T73651 plate. Etch: Electropolished.

.c , , . "

" ~ '~ . 'r,. :> '.

c Figure 3. Photomicrographs of TCG-bearing 7050 plate showing: a) traces of {111} twin planes and intergranular segrega­tion, b) higher magnification of Figure 3a, and c) more randomly oriented segrega­tion compared with TCG-bearing plate. Etch: Keller's reagent.

55

FLOng transverse

Longitudinal (Rolling direction)

Short transverse

" .L ____________ _

~-------------- -

r ~7''b:-V<~ . 25 15 30· 45.

144 mm mm O· l ________ _

Figure 4. Sketch showing orientation of the tensile specimens.

Tensile Properties as a Function of Orientation

480

:. 470

== iii 460

>= .. 450 CD III

~ 440

~ 430

420

8 c :8 6 fII

~4 0 iii 2

#0

TCG-free ----0---

TCG-bearing

0 15 30 45 Angle in deg. between tensile axis

and short transverse direction

Figure 5. Tensile properties as a function of orientation In 144-mm-thick 7050-T73651 plate.

56

perpendicular to the tensile axis in a sample stressed to a level just below the onset of plastic instability. Since the morphology of the cast;-grain structure dictates the intergranular segregation pattern, it follows qualitatively that the fracture sensitive properties of the TCG-bearing plate are expected to be more adversely affected. In addition, the TCG­associated crystallographic characteristics are also expected to play an important role due to (1) the presence of more or less parallel twin planes, and (2) associated {111} slip planes, and (3) grain boundaries as much as 50 mm long.

To study these effects, mechanical tests were made on specimens with the TCG structure variously oriented to the tensile axis and the results compared with similarly fabricated TCG-free plate. All tensile specimens were 4.1 mm in diameter, 25.4 mm in gauge length, and sectioned as illustrated in Figure 4. The midpoint of the gauge length of the specimens was always 25 mm below the plate surface so as to include the TCG structure.

Figure 5 is a plot of 0.2% offset yield stress (Y.S.) and elongation at fracture as a function of orientation. The deformation texture in the prod­uct results in some mechanical anisotropy for the normal, TCG-free material, but the degree of anisotropy in Y.S. and elongation is clearly accentuated for the TCG-bearing material.

In qualitative agreement with considerations of plastic deformation by slip, crystallographic planes parallel to twin planes provide preferred frac­ture paths when favorably oriented, as shown in Figure 6. This is further illustrated by the scanning electron microscope (SEM) fractograph of the tensile specimen from the TCG-bearing plate where the tensile axis was oriented at 30° with respect to the short-transverse direction (elongation = 1.6%). In this case, traces of the twin planes were coincident with the maximum shear stress, i.e., nearly 45° with respect to the tensile axis. Fractographs in Figures 7a, 7b, and 7c reveal that nearly 50% of the surface comprises planes parallel to the twin planes exhibiting brittle fracture mode. The remaining portion of the fractured surface failed in a ductile mode. Figures 8a and 8b are fractographs of a similarly oriented tensile specimen representing the TCG-free plate (elongation = 4.7%) and clearly illustrate a ductile mode of failure.

Electron microprobe analysis was used to determine and compare the chemical composition of fracture surfaces with the bulk material in tensile specimens, as described by Vruggink.ll Specimen elongations as a function % Fe enrichment (predominantly AhCu2Fe intermetallics) of fractures are plotted in Figure 9. The fractures in TCG-bearing material are more enriched than those of the TCG-free material despite the fact that a portion of the fracture surface in the TCG-bearing material comprises planes close to twin planes that are free from intermetallic particles. This shows that significantly enhanced solute segregation is associated with the TCG structure.

Recrystallized and Segregated Structures

The example selected to illustrate the effects of the TCG structure in this case was 76-mm-thick alloy 2219-T851 plate, whose nominal chemical composition is given in Table I. The recrystallized structure in the final product masks the TCG structure that was present in the parent ingot. Nevertheless, certain characteristics of the structure are found to persist through recrystallization, i.e.: (1) TCG-induced segregation (in the form of CuAh particles); and (2) traces of twin planes (being of low energy configuration, they often persist through fabrication). Figure 10 shows both conditions.

Ductility in the long transverse direction is a crucial property for this product. To examine the effects of segregation associated with TCG, three successive 6.4-mm-long transverse slices parallel to the rolling surface were obtained from a TCG-bearing plate as well as from a TCG-free plate. Tensile elongation data are presented in Figure 11.

High concentrations of CuAh particles are found in fracture surfaces and lead to the conclusion that TCG-induced segregation is the cause of the loss in ductility.

SUPPRESSION OF TCG

In the production of commercial aluminum alloy ingots, the occurrence of TCG is clearly associated, as mentioned earlier, with a problem in the nucleation and growth of new grains early in the solidification event. The difficulty may depend upon grain-refiner, alloy, or solidification conditions so no one grain-refining practice will be universally applicable. Nevertheless, the essential principles of grain refining are readily enunciated.

JOURNAL OF METALS· June 1983

Nucleation

Two inoculants (or grain-refining master alloys) are commonly used in the production of wrought alloy ingot: AI-6% Ti, and AI-5% Ti-X%B where X is usually in the range 0.2-1%. For most practical purposes only the ternary alloys are used, since they generally give a finer as-cast grain size, by a factor of 3 to 5, when used at the same titanium addition rate.

The ternary grain refiners contain two intermetallic constituents (Figure 12): TiAla, which is soluble in molten aluminum, and a TiB2-type constitu­ent which is insoluble .. When held in an aluminum melt, the TiB2 constitu­ent particles react to form a surface layer of (TiAl)B2. 12,13 It is the insolu­ble constituent particles that determine the grain-refining effectiveness of the master alloy. Two principal problems can affect the efficiency of the grain refiner: the loss of nuclei due to "settling" when the addition is held for extended times in the melt (such as when used as a furnace addition), and as a result of "poisoning" of the nuclei surface by substitution of the titanium by elements such as Cr and Zr.l4 Both difficulties are overcome by minimizing the residence time of the grain-refining nuclei in the melt by, for instance, adding the master alloy in rod form to the molten metal stream as it flows from the holding furnace to the casting unit.

Growth

Conditions for the growth of freshly nucleated grains are enhanced by maximizing the region of constitutional supercooling ahead of the solidifica­tion front. Constitutional supercooling15 can be illustrated schematically (Figure 13). The zone of supercooling is eliminated if the slope of the actual temperature distribution is equal to, or greater than, the gradient of the equilibrium liquidus-temperature curve at the interface. This occurs when:

G/R~m ~ D (1)

where G = the temperature gradient in the liquid, R = the rate of solidification, m = the liquidus slope, Co = the original solute concentra­tion in the melt, ko = the equilibrium distribution coefficient, and D = the diffusion coefficient of the solute in the liquid.

From this expression it follows that the zone of constitutional supercooling is increased by minimizing G and ko and maximizing m and Co. This is in accord with practical experience. The expression also suggests that raising the rate of solidification (R) also increases the zone of supercooling. In practice, however, there is a rate beyond which it appears that the zone is effectively minimized (e.g., in strip casting where rate is ~ 10 mm/s). It is postulated that at these high rates new grains are overwhelmed by the rapidly advancing interface.

SUMMARY

Some aluminum alloys, such as those containing zirconium and chromium, are particularly susceptible to TCG formation when cast as DC ingot. It has been shown that remnants of the structure in the form of segregation, twin planes or both in recrystallized, and partly recrystallized, thick plate have a detrimental effect on tensile properties. Anisotropy in mechani­cal properties is accentuated, and a serious reduction in ductility is found, at ~ 45° to the orientation of the TCG structure. Therefore, it is essential that TCG be avoided in the commercial production of aluminum ingot, especially when it is to be used in thick plate, forging, or extrusion applications.

Use of commercially available grain-refining master alloys allows TCG to be completely eliminated. However, in order to suppress this undesirable structure, strict attention must be paid to both the nucleation and growth conditions.

a b

JOURNAL OF METALS· June 1983

Figure 6. Photomicrograph showing that the plane parallel to the twin plane is the preferred fracture path. Etch: Keller's reagent.

Figure 7a. SEM fractograph (secondary electrons) of TCG-bearing specimen oriented at 30° with respect to the short­transverse direction. Note the brittle frac­ture in the lower half. Compare with Fig­ure 8a. Accelerating Voltage: 30Kv.

Figure 7b. A magnified view of Figure 7a illustrating features of the ductile (top) and brittle (bottom) fracture. Compare with Fig­ure 8b. Accelerating Voltage: 30Kv.

Figure 7c. Backscattered electron image to illustrate the topography of the fracture in Figure 7b. Accelerating Voltage: 30Kv.

c

57

Figure 8a. SEM fractograph (secondary electrons) oriented at 30° with respect to the short-transverse direction. Note the ductile failure mode over· the entire frac­ture surface. Accelerating Voltage: 30Kv.

Figure 8b. A magnified view of Figure 8a. Accelerating Voltage: 30Kv.

GI :; U ~ -co c: 0

:; 01 c: 0 iii #-

14

12

10

8

6

4

2

OL---L-__ ~ __ ~ __ -L __ -L __ ~

200 250 300 350 400 450 500 % Fe enrichment of fracture

Figure 9. Percentage elongation (%£) as a function of iron enrichment in fractured surfaces.

.. I-----i ;,/ . - .. :: _.- . 50#-,m

_'-_-0.. ____ .. • . __

Figure 10. Photomicrograph of 2219-T851 plate showing remnants of TCG structure: the faint twin plane and the segrega­tion roughly parallel with it. Etch: Keller's.

Figure 11. Long Transverse ductility in 76-mm 2219-T851 plate.

58

a

ACKNOWLEDGMENTS

The authors are indebted to their colleagues Messrs. R. S. James, R. J. Stokwisz, and R. W. Westerlund (Alcoa, Davenport Works) for the supply of samples. Thanks are due to the Alcoa Editorial Committee for permis­sion to publish the work.

References

1. J . Herenguel , "Analysis of a Solidification Structure of the Basaltic Type." Rev. Met., 46 11949). pp. 309·314.

2. L. R. Morris. J. R. Carruthers. A. Plumtree, and W. C. Winegard, "Growth Twinning in Aluminum Alloys," Trans. TMS.AIME, 236 (1966), pp. 1286·1291.

3. W. D. Walther. C. M. Adams, and H. F. Taylor. "Effect of Casting "Fiber" on Mechanical Properties of Aluminum·4 Percent Copper Alloys," Trans. Found. Soc .• 61,119531. pp. 664·673 .

4. H. Fredriksson and M. Hillert , "On the Mechanism of Feathery Crystallization of Aluminium ," J . Mat. Sci., 6 (1971). pp. 1350·1354.

5. S. Watanabe, U. Honma. and S. Oya. "Growth Twin Crystals of Aluminum and Aluminum-Base Alloys­Introduction to Studies on Growth Twin Crystals in Aluminum and Aluminum·Base Alloys I1st Report!." J. Japan Inst. Light Metals, 19 (1969), pp. 279-286.

6. S. Miyazawa. U. Honma, and S. Oya, "Growth of Growth Twin Crystals in Aluminum and Aluminum-Base Alloys 13rd Report)." ibid. 22 (2) (1972), "p. 143·150.

7. J. A., Eady and L. M. Hogan, "Some Crystallographic Observations of Growth·Twinned Dendrites in Aluminum." Journal of Crystal Growth, 23 (1974). pp. 129·136.

8. L. R. Morris and M. Ryvola. "Growth Twins in Aluminum Alloys," Microstructural Science , 9 (19811. pp. 241·248.

9. E. Benn and W. W. Walker, "The Properties and Microstructure of Directionally Cast Alloys," Met. Trans. 2 (1971), pp. 2735·2736. 10. J . A. Eady, I. O. Smith, and C. McL.Adam. "The Effect of Grain Structure on t he Tensile Properties of Cast Aluminum," J. Inst. Metals, 101 11973), pp. 162·166. 11. J. E. Vruggink. "Use of the Microprobe in Fracture Analysis." Aluminium, 49 119731. pp. 601-605 . 12. L. Backerud, "On the Grain Refining Mechanism in AI·Ti·B Alloys." Jernkont. Ann., 155 119711, pp. 422·424. 13. L. Arnberg, L. Backerud, and H. Klang, "Grain Refinement of Aluminium 2: Intermetall ic Particl es in AI·Ti-B·Type Master Alloys for Grain Refinement and Aluminium." Metals Technology, 9 119821. pp. 7-13 . 14. G. P . Jones and J. Pearson, "Factors Affecting the Grain-Refinement of Aluminum Using Tita nium and Boron Additives," Met. Trans. 7B (1976), pp. 223·234. 15. J. W. Rutter and B. Chalmers, "Prismatic Substructure Formed During Solidification of Metals," Canadian J, Physics, 31 11953), pp. 15·39 .

1 st slice (surface to 6.4 mm)

2nd slice (6.4 mm to 12.8 mm)

3rd slice (12.8 mm to 19.2 mm)

3rd slice

Long transverse % elongation at fracture

TCG-free

9.3

9.3

9.3

TCG-bearing

4 .0, 5.0, 6.0, 6 .3

9.3

8.8

JOURNAL OF METALS· June 1983

~' . ' . • , .. ~! .: J. ,', . ; . ~ ..

"

" .' .. ~ ~ ,. ~:.: .~ .. ~ :::'-.", ... ... .. ..

" . ' .. :.. :

.... ," . . : ' . " : ,

:'" '; ~ .. ' .. :... ... .... ,..

. ,. .,~

. ". ; ",

, , , , ,

' If ...... oil ' . .

~. . .. , I

.. ~ .. :;; Co E .. ...

Actual

Liquidus temperature

Zone of constitutional supercooling

Interface position

Distance --------

Figure 12. Longitudinal section through an Figure 13. Schematic illustration of consti­AI-5% Ti-O.2% B alloy 9.5-mm-diameter rod tutional supercooling in alloy solidification. showing large, soluble Ab Ti particles and smaller, insoluble TIB2 particles. As polished.

Electronic, Magnetic, and Optical Proper­ties of Materials Committee of ASM. Em­phasis will be on recent and novel results, both theoretical and experimental, in the following areas: flux pinning, technologically important superconduct­ing materials, high -Tc materials, and radia­tion effects.

Submit abstracts to Amit DasGupta, Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box X, Oak Ridge, Tennessee 37830, or C. S. Pande, Code 6325, Naval Research Laboratory, Washington, D.C. 20375.

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Submit 500-word abstracts (including purpose, approach, results, and signifi­cance of results) to Maurice Gell, MIS 162-04, Pratt & Whitney Aircraft, East Hartford, Connecticut 06108.

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Abstracts are due by August 1, 1983 for a session on "Modeling of Superalloy Materials and Processes," sponsored by the TMS High Temperature Alloys Com­mittee at the 113th AIME Annual Meeting. Papers dealing with, but not limited to, modeling of superalloy production, use, and properties (mechan­ical, physical, environmental, etc.) are sought for any alloy systems used at sig­nificant fractions of their melting points.

Submit abstracts up to 200 words on the appropriate TMS abstract forms to Greg K. Bouse, TRW Inc., Mail Stop 37, 23555 Euclid . Avenue, Cleveland, Ohio 44117; telephone (216) 692-6668.

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Abstracts are due by August 1, 1983 for the Symposium on Rapidly Solidified Powder Aluminum Alloys, sponsored by American Society for Testing and Materials, to be held April 4-6, 1984, Philadelphia, Pennsylvania. Papers are sought on the following topics: high­performance RSP aluminum alloys; cur­rent status of alloy development, powder manufacture, and processing; characteri­zation of powders; characterization of con­solidation and forming; characterization

ABOUT THE AUTHORS

Douglas A. Granger, senior scientific associ­ate, Alcoa Technical Center, Alcoa Center, Pennsylvania 15069.

Mr. Granger received his BSc (Hans) and MSc in metallurgy from the University of Manchester

(U.K.). Since 1972 he has worked in the In­got Casting Division at Alcoa Technical Cen­ter specializing in cast structure analysis, struc­ture control, and casting process development. From August 1979 to September 1980 he was guest professor in the Department of Metallurgy, Delft University of Technology, Netherlands. He is a member of The Metallurgical Society of AIME.

John Liu, senior engi­neer, Alcoa Technical Center, Alcoa Center, Pennsylvania 15069.

Mr. Liu received his BS in metallurgical engi­neering from the Univer­sity of Texas at EI Paso, and is currently pursuing

an advanced degree in metallurgical engineer­ing at the University of Pittsburgh. Since 1980 he has worked in the Alloy Technology Divi­sion at Alcoa Technical Center in structural analysis, aerospace alloy development, precipi­tatioR strengthening, and recrystallization. He is a member of The Metallurgical Society of AIME.

of microstructures; measurement of properties; and powder-micro structure­property relations.

Submit 300- to 500-word abstracts to M.E. Fine, Northwestern University, De­partment of Materials Science and Engineering, Evanston, Illinois 60201; telephone (312) 492-5579, or E.A. Starke, Jr., University of Virginia, Department of Materials Science, School of Engineer­ing and Applied Science, Charlottesville, Virginia 22903; telephone (804) 924-3462.

Composite Materials

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(Continued on page 76)

59