Superplasticity in A Zn-0.4% Al alloy

SUPERPLASTICITY

IN

A 2~0.4% Al ALLOY*

and R. PEARGEt

H. NAZIRIT

A fine-grained (No.6 pm) zinc-O.4 wt.% duminum alloy sheet was produced by rolling a cast slab to 90 per cent reduction et room temperature (0.42 T,). At 0.42 T,, m = 0.4 - 0.6 (m is the strain-rate sensitivity exponent) 8t an initial strain-rete of 6.67 x lo-* set-i and elongations of greater than 500 per cent could be obtsined. The structure of the as-rolled 8heet ~8s found to be unstclble at 0.42 T, and grein-growth readily occurred, resulting in 8 fin&l, stsble grain-size of shout 6 pm, but this gr8in-growth could be errested if the 8srolled sheet ~8s subjected to 8 tensile strain of 25 per cent immedietely after rolling. After strains >26 per cent, the microstructure ~8s virtuelly dislocetion-free. Surface observations showed that Fracture behaviour w&8 ductile in extensive gr8in movement accompenied superpla4tic deformation. nature 8nd reduction in ere8 w&s strain-rate dependent. SUPERPLASTICITE

DUN

ALLIAGE

Zn-0,4

% Al.

Une tale d’8lli8ge Zn-0,4% Al A gr8ins 6ns ( ~0,6 pm) 8 et& fabriquee en leminant une pleque moulee 8vec und reduction de 90% B 18 temperature smbisnte (0,42 T,). A 0,42 T,, nous 8VOn8 pu obtenir m = 0,4-0,6 (m eat l’ezposent de sensibilite B la vitesse de deformetion) pour une vitesse de deform8tion initiale de 6,67 x lo-‘ s-i, et des allongements sup&ieurs 8. 600%. On a trouve que la structure de 18 feuille brute de lammage &it insteble A 0,42 T, et qu’il se produiseit une croissance de8 grains jusqu’A une tttille finale steble d’environ 5 pm, meis que cette croissance des gr8ins pouvsit Btre err&e en soumettent 18 feuille brute de leminage A une deform8tion per traction de 25% immediatement 8pres le lamimbge. Pour de8 deformations superieures A 25%, 18 microstructure etait pratiquement s81l8 dislocations. De8 observations de surface ont montre que des mouvements importants des gmins socomp8gneient 18 deformation superplestique. L8 rupture et& de nature ductile et la reduction de section deppendeit de la vitesse de deformation. SUPERPLASTIZITAT

IN EINER

Zn-0,4%

Al-LEGIERUNG

Ein feinkijrniges (~0,6 ,um) Zink-0,4 Gew.% Aluminium-Blech wurde durch Welzen von GuDstiicken (90 Prozent Reduktion) bei Reumtemperatur (0,42 T,) erzeugt. Bei 0,42 T,,, konnten bei einer enfanglichen Abgk&geschwindigkeit von 6,67 x lo-* set-i Werte von m (m = Exponent der Dehnungsempfindlichkeit) im Bereich zwischen 0,4 und 0,5 und Dehmmgen von mehr als 500 Prozent erreicht werden. Die Struktur der so gewctlzten Proben ist bei 0,42 T, instebil und Kornwechstum fiihrt zu einer stabilen KorngrijBe von etw8 5 pm. Werden die Proben jedoch direkt nach dem Walzen um 25 y0 zugverformt, so erfolgt kein weiteres Kornwachstum. N8ch Abgleitungen > 26 prozent ist die Mikrostruktur prektisch versetzungsfrei. OberflitGhenbeobachtungen zeigen, de6 die superplestische Verformung von ausgedehnter Kornbewegung begleitet ist. Des Bruchverhalten ist von duktiler Natur und die Fliichenreduktion ist ebhiingig von der Abgleitgeschwindigkeit.

1. INTRODUCTION

The development of the Zn-0.4 wt. % Al alloy sheet was motivated by an earlier investigation by the present authors(i) on the extended plasticity observed at 0.42 T, in a commercial-purity zinc sheet. This followed from the observation that, on rolling a commercial-purity zinc slab at 0.42 T, to 90 per cent reduction, fine, stable grains of about 2,um in size were produced. Microstructure1 exclmination revealed the presence of fine particles in the grain boundaries which were thought to stabilize the grains and a strain-rate sensitivity (m) of 0.2 with accompanying elongations of -200 per cent was obtained. It was felt that if a smaller grain-size, in the region of 0.5 pm, could be obtained in a similar material, truly superplastic behaviour would result. A Zn-0.4 wt. % Al alloy was selected because firstly, l Received December 10, 1973. t Physical Metallurgy centre. Swinden Labor&tories, Special Steels Division, British Steel Corporation, Moorgate, Rotherham, Yorkshire, S60 SAR, England. $ Department of M8terisls. Cmnfield Institute of Technology, Cmnfleld, Bedford, MK 43 OAL, England.

ACTA 1

METALLURGICA,

VOL.

22, NOVEMBER

1974

the presence of aluminum in solid solution would raise the recrystallization temperature, and secondly that the precipitation of the aluminum-rich (8) phase, which occurs at room temperature would conceivably stabilize the small grains known to be produced by heavy rolling reductions. It is also known that the aluminum and zinc phases in this alloy system are superplastically compatible. Finally, it was hoped that this investigation would help to elucidate the mechtinism or mechanisms operating during superplastic deformation. 2. EXPERIMENTAL

The Zn-0.4 wt. % Al alloy was made from superpure materials and cast into steel moulds; the dimensions of the ingots were approx 30.5 x 12.7 x 1.105 cm. The castings were supplied by the Imperial Smelting Corporation Ltd., Avonmouth, Bristol. A spectrographic analysis of the alloy showed the following impurities : Cd o.004 1321

Cu -0.003

Pb 0.002

Fe

Mg

OX

0.001

Sn -wt.% 0.001

1322

ACTA

METALLU~G~CA,

The top and bottom of the slab were cropped in order to eliminate the high-porosity regions. The remaining surface was scarfed to a depth of 1.27 mm and the slab was homogenized at 325-350% for several hours. The material was then rolled in a Stan&-Mann two-high mill. The rolls were 14 cm dia and 15.25 cm wide. The rolling speed used was 15.25 cm/see, maint,ained constant throughout the rolling operation. The slab w&s first hot-rolled at 300°C and then rolled to a, 90 per cent thickness reduction (1.02 mm) at room temperature. This was achieved by cooling wit.h iced water where necesscbry between passes. Immediately after the last pass, the sheet was cut up and stored in a large flask containing dry ice, Tensile specimens were cut in a “Tensilkut” machine-an operation which took less than 5 min-and these were then stored in dry ice until testing. The tensile specimens were 12.7 mm wide with 31.8 or 62.5 mm parallel gauge-length. The mechanical property measurements were made on a 4540 kg “Instron” testing machine with available crosshead speeds from 0.0~837 to 8.37 mmjsec. mechanical polishing for met~llogr&phic examin&tion was difficult and unless extreme care was taken resulted in surface deformation which could obscure the true surface structure and thus the following method was used. The specimen was dipped in concentrated nitric acid which resulted in a vigorous reaction, when it was immediately removed and washed in water and methanol. The surface was well polished, though with slight pitting. No significant temperature rise was monitored during the chemical polishing procedure. The specimen was then etched in concentrated hydrochloric acid, immediately rinsed in water and methanol, and optically examined. Thin-foil preparation for transmission electron microscopy required a technique which was very quick, did not involve any mechanical cutting or thinning and which at the same time had a high percentage of success. The method found to meet these requirements, which had been previously tried for certain .steels,(2*3)involved the chemical thinning of the specimens, followed by electropolishing. Specimens were chemically thinned in a W-70 per cent nitric acid solution t.ill peroration, followed by the electropolishing of the perforated edges in a solution of 67.5 per cent ethanol~orthophosphoric acid at a voltage of 8-10 for an average specimen area of 6.25 mm square. These polished areas were cut out and examined in a Siemens Elmiskop I electron microscope. ‘Using the above technique, thin foils could easily be prepared from a specimen of 1.02 mm thickness and then examined in less than 16 min.

VOL.

22,

1974 3. RESULTS

3.1 Material In order to adopt a, rolling procedure which would produce a sheet exhibiting the highest strain-rate sensitivity (m), some processing variations were tried. The cast ingot was hot-rolled at 300°C to 40, 50 and 60 per cent reduction, followed by room-temperature rolling to 90 per cent reduction, finally to give 8 sheet of 1.02 mm thickness. m vs strain-rate (li) behaviour was determined on the di~e~ntly processed sheets by the Backofen et al(*) sap-meth~, and the results in Fig. 1 show that the material which received the greatest room temperature rolling reduction exhibited the highest m-value. Therefore, all subsequent material was reduced 40 per cent at 3OO”C,then rolled to 90 per cent at 4.42 T,. Figure 2 shows the microstructural changes which took place during 0.42 T, rolling after 40 per cent hot rolling. On hot rolling, a heavily deformed and directional structure is observed. Slip and twinning were evident, and gt a higher ma~ification, substructure formation w&s observed. On room-temperature rolling, severe fra~entation of the grains was seen and up to 50 per cent reduction extensive slip and twinning was still evident. After ‘75 per cent reduction, directionality still existed, with grains becoming too tine for optical resolution. After 90 per cent reduction, however, a consistent fine-grrtined structure was obtained, with no indication of grain directionality, though the rolling direction w&s indicated by the presence of stringers. Examination of the internal structure (Fig. 3), showed the grains to be equiaxed and to contain a high was density of dislocations ; heavy precipi~tion evident in the grain boundaries and the matrix. The

( Set-J) 1~66x10”

I 66 x10-”

l-66.10-

l

Hot-rolled

60%

+ r t rolled

to t 02mm

‘

Hot-rolled

40%

+ f.t. rolled

to

IO2 mm

FIG. 1. The effect of initial hot rolling reduction of the ingot on the m vs d relationship.

X’AZIRI

AND

PEARCE:

SUPERPLASTICITY

IN

A Zn-0.4?&

Al

ALLOY

1323

Hot rolled + 75 % r.t red.’ . Hot rolled+90% r,t. red. FIU.2. Shows the microstructures produced during increasing room temperature cr.&) rolling reduction (red) of the as cast, hot rolled ingot. Surface x 225.

grain size, measured by the linear intercept method, gave an average value of No.6 pm on the as-rolled sheets when examined within 15 min of the final rolhng pass. Also, the observation of a high dislocation density and the lack of substructure formation, suggested that the thin-foil preparation technique had not introduced any significant effects due to an increase in temperature.

IO 0 X

reduction

Do

Tim*,

hr

FIG. 4. (a) The effect of percentage reduction at room temperature on the hardness (H,). (b) The effect of room temperature ageing of the as-rolled sheet on the hardness.

3.2 65&n

growth effects

Though the rolling procedure was successful in producing sheets with No.6 ,um grain size, which gave -500 per cent elongation in tension, after ageing for 24 hr at 0.42 T,, this fell to -100 per cent. ~icrost~ctural examination (Fig. 5) of a strained specimen, aged for 3 hr at 0.42 T,, revealed that grain

Fm. 3. Shows the microstructure of the sheet rolled to 90 per cent reduction at room temperature. The sheet was examined within 16 min after the final pass.

Hardness measurements (II,,) were carried out at room temperature after various rolling reduct,ions. Figure 4 shows the effect of increasing rolling reduction on sheet hardness. The hardness increased to a maximum value of 70 H,* at 12 per cent reduction, but with further inoreases in rolling reduction it fell, until finally after 90 per cent reduction, a value of 30 II, was obtained. * H, = Viekers hardness number.

Fm. 5. Shows the microstructure of the as-rolled sheet strained to an elongation of 200 per cent at the crosshead velocity of 0.~25mm~~c. The specimen was strained after a room temperature ageing of 3 hr. Surface x 2250.

1324

45 min

2.5 hr

Bhr

FIG. 6. Shows the grain growth that takes place during room temperature ageing of the as-rolled sheet. Surface .

growth had occurred with certain grains having grown more rapidly than others. The microstructures of as-rolled specimens aged for increasing time at 0.42 T, are ehown in Fig. 6. Grain-growth took place until a final, stable grain-size in the region of ~5 ,um was reached. A structural change was also indicated by the change in hardness (Fig. 4), where the hardness of the as-rolled sheet increased from 30 to 43.6 H, after being left at. 0.42 T, for 7 hr. Based on these observations, it was not clear why this material behaved superplastically if rapid graingrowth was taking place during deformation. For example, at a crosshead speed of 0.0425 mm/set, it would take about 100 min for a specimen, having a gauge length of 62.5 mm, to reach an elongation of 500 per cent, and in this time substantial grain growth should have occurred with a consequent marked decrease in ductility. 3.3 Grain growth inhibition due to deformation Tensile specimens were now strained to various points on the load-extension curve (Fig. 7) and then Figure 8 shows the effect of inaged at 0.42 T,. creasing amounts of prestrain on grain growth.

%

Elongotlon

II

The specimen strained to 25 per cent elongation showed no signs of grain growth even after 28 hr, while specimens strained to less amounts exhibited some grain growth, though the greater the strain the slower the growth-rate (Figs. 8a-c). The 0.2 per cent proof strength and strain-rate sensitivity (m) were also measured as a function of prior tensile deformation. Figure 9 shows that after 25 per cent elongation, the proof strength remains unaltered even after 24 hr at room temperature, while the marked increase in proof strength with ageing of the unstrained material is quite evident. The material strained to 3.4 per cent elongation, though exhibiting a change in proof strength with time, showed much lower stress values than those obtained from the unstrained material. Figure 10 shows a similar trend with the m-value. Thus a strain of 25 per cent, while sufficient to inhibit any grain growth in the gauge length of a tensile specimen, should have no effect upon the grip ends, which should show grain coarsening. The difference in structure along a single tensile specimen is shown by X-ray back-reflection photographs from the gaugelength and from the grip-ends (Fig. 11). The gauge length shows a continuous ring pattern, while the coarsening of the structure in the grip ends is indicated by the breaking up of ring pattern into a series of spots. Based on the above observations, the following manufacturing procedure was followed. Immediately after the final rolling pass, the sheets were cut to the appropriate size and stored in dry ice. Specimens subsequently t,ested were first strained to 25 per cent, within 10-15 min after removal from the dry ice. This ensured the inhibition of room temperature grain-growth.

l-0.4

2- 3.4 :I

265

Extension

FIG. i. A tvoical load-extension curve obtained at room

temperaturein straining at a crosshead velocity of 0.0425 mm/see. The curve exhibits points to which specimens were strained followed by room temperature ageing.

3.4 Superplastic deformation Typical flow stress (Us) vs strain-rate (8) and m vs 6 relationships were determined at increasing strain intervals up to 450 per cent elongation at 0.0425 mm/set, and the effect of rotating the tensile axis to 0, 45 and 90” to the rolling direction was examined.

NAZIRI

PEARCE:

AND

SUPERPLASTICITY

IN

A Zn-0.40/,

Al

ALLOT

(b)

8hr

FIU.

8.

the effect of (a) 0.4 per cent, (b) 6 per cent and (cf 25 per cent elongations on grain growth at room temperature. Surface. x 225.

Shows

I l

aStrained

Straliud

0%

34%

+ ag*

+ a**

r.t. .

r.t. *

l

.-.

.-.

40

I0

I 5

I 10 Tlme.

Strained

I 15

25%

+ aga r.t.

.

I 20

1

25

hr

FIN. 9 Shows the effect of per cent elongation, as indieated in Fig. 7, on the 0.2 per cent proof stress determined after increasing room temperature ageing.

0.6-

Iwrlo-’ I *As rolled OAs rolled XAs rolled

1.66 I IS ,.66X 10‘2 1,66x10-1 I shaet+ag+ rt. for24hr ’ sheat + strained 3.4% fpeok loocfl+ age r.t. for 24hr snect +stromad 25% +aga r.t. for 24hr

i

(rnltT’1

Fm. 10. Shows the effect of per cent elongation, as indicated in Fig. 7, on the m vs L relationship determined after increasing room temperature ageing.

FIG. 11. Shows X-ray back reflection patterns. (a) Gauge length strained to 300 per cent elongation and aged at room temperature for 1 week. (b) Grip end of the same specimen as (a) and aged at room temperature for 1 week.

ACTA

1326

METALLURGICA,

VOL.

Figures 12 and 13 show that both the peak onvalue and the cr, values increase with strain and the curves move towards lower strain-rates, the sheet remains anisotropic, but a noticeable effect is the decrease in the anisotropy in the m value with increasing tensile strain.

22,

1974 ( ssc-1)

1.66X 10-J

1.66~ IO-’

1.66x10-5

k66rlo-*

csec-‘1 1.66 *lo"

IcbXlO-4 ,

I.66"10‘3 i

0.6~-

166X13“

Sr:“do&d

!“’

6.6gL

x 50% 0 300%

i

(min-‘1

13(a)

m

(set-‘1

0 *--

1.66X 10-s 669

tax

10-4

1~66111 -2

1.66x IO-b

I

I

I

(bf

o,i --

IO% 50% x 300%

l

0

6.891 10-a

i

(mit-4‘9

13(b)

I.664 10-5 669

I.66X1O‘a

1$6x10-J

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1.66x1

‘1

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(c)

*IO!&

050% x300x k6l 0

6F

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S;r$i,d X50% n 300%

0 5

0

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(C

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IO-

1

1 1 1

I*(*1

1

1 11

10-z

11-11 la’

1

1 11

ff(11 100

i (mln-‘1

13(c)

4-t

FIU. 13. The effect of increasing per cent elongation on the a, vs 12 relationship determined in the (a) rolling direction, (b) 45” and (c) 90” to the rolling direction.

0

0

0

i

(inin-‘)

12(c) The effect of increasing per cent elongation on the m vs E relationship determined in the (a) rolling direction, (b) 45’ and (c) 90” to the rolling direction.

FIU.

12.

M~cr~t~ctural observations both on the surface and internally, were made on a specimen strained up to 400 per cent elongation at 0.0425 mmjsec (Fig. 14), which shows that with inoreasing amounts of strain, the structure becomes coarser, Examination of the surface by the scanning electron microscope (Fig. 15) shows that extensive grain movement occurs, increasing with increasing strain, and that the grain

1327

FIG. 14. Shows the increase in grain size with increase in elongation. was used. Surfaoe. x 200.

firm

velocity

-

200%

of 0.0425 mm/see

ozrn

0.5pm

L.A

50 %

A crosshead

400 %

FIG. 15. Shows the effect of increasing per cent elongation on the surface behaviour. The specimen length was chemically polished before straining at the crosshead velocity of 0.0425 mm/see.

gauge

boundaries normal to the stress axis cavitate first. Furthermore, with increasing deformation, the directionality of int~granular cavitiation is lost and cavities are seen to form at all the boundaries. Figure 16 is a section of the specimen and gives an idea of the depth of the intragranular cavitation. Transmission electron microscopy shows that a marked decrease in dislocation density occurs with strain (Fig. 17, see Fig. 3). Injection between precipitates, dislocations and grain boundaries are Also grains remain equiaxed though noticeable. grain coarsening is evident. Finally the fracture behaviour of this material was studied in the scanning electron microscope. The FIG. 17. Shows the microstructure of a specimen strained to 400 per cent elongation at the crosshead velocity of 0.0425 mm/see.

of nz-value (or strain-rate) on the fracture mode was observed from specimens strained at the initial strain-rates of 6.67 x lW, 6.67 x lOmaand 1.33 x m-values at these 10-l set-’ ; the ~o~~ponding strain-rates were 0.34, 0.17 and 0.05 respectively. Figure 18 shows the three types of fracture surface. The difference in the fracture mode is indicated in the two extreme m-value results, i.e. 0.05 and 0.34. The lower m-value specimen exhibits a ductile ‘Lhoneycomb” surface, while in the higher m-value effect

FIG. 16. Shows the longitudinal section of a specimen strained to 400 per cent elongation at the crosshead velocity of 0.0425 mm/see. x 1000.

1228

ACTA

i = 6.67x FIQ.

L~4sec-’

METALLURGICA,

VOL.

4: S*67Xlcr3

SW+-’

22,

1974

25 [.33X16’

18. Shows the effect of different crosshead veloeit.ies on the fracture

specimen an “‘ideal” ductile failure is obtained, where the two halves have separated to knife-edges. At the intermediate m-value (0.17), there is a mixture of the two types of fracture surface, with the ‘&honeyoomb’ surface on a finer scale. 4. DISCUSSION It was shown by Lutts and We~a(6) that by the addition of 0.35 wt. % aluminum to zinc, the room temperature myself-annealing”behaviour of the alloy is retarded and a and-grain-~ze material was obtained. Cook and Riseboraugh@n produced a Zn-O.2 wt. % AI alloy by extrusion, which resulted in a material with a gram size of about 3.5 pm. These authors obtained a peak m-value of 0.8 at the strain-rate of about 1.66 x IO-5 see-l, while in the present case, a peak 4n-value of 0.4 was obtained at the strain-rate of 3.3 x Xl+ see-l. Whilst the m-value of 0.8 is rather high, these results are in general accord with the superplastic results, where the effect of the grain size increase is to raise the peak m-value aud move it to lower strain-rates.(7i Figures 2 and 4, which show grain refinement and work softening respectively, are typical of zinc and dilute zinc alloy~.(~“&-~~~ The work of Cay el:z.&(rdfon the rim-~mperat~ roiling of electrolytic zinc using an X-ray micro-beam technique, showed that after X1-30 per cent deformation sub-cells were formed within the grains, and after about 40 per cent reduction, a limiting cell-size was formed followed by spontaneous recrystallization. This recrystallization does not appear to take place in the present work, and it so seems that the presence of the alum~um-ph~e must retard the process- Deighton and ParkinG’f explanation of a similar softening ef&&, is that, during the early stages of deformation, sub-grains are formed and then with increasing deformation coalesce, with increasing misorientation so that the subgrain boundaries finally become high-angle boundaries and so a fine grain-size is obtained. The decrease in grain size is supported by the

set -’ behaviour.

decrease in hardness (Fig. 4), an observation also made previously.(r*s*12) It has also been shown, in a similar aIloy(rg) and Zn-ZnCY1r’ alloys, that there is an optimum grain size above which the Hall-Peteh ~latio~hip is observed. While below this size the proof strength increases with increasing grain size, a behaviour exhibited by the range of grain sizes showing su~~pl~ieity.(‘) 4.2 @ruin-growth effects It was observed by Chad~ck(l~) that the physical properties of rolled zinc are subject to considerable variation aeeording to the impurity eontent, especially immediately after rolting. In electrolytic zinc rolled to 80 per cent reduction, complete recrystallization took plltce at room temperature in a month, while the addition of 0.02 per cent iron or magnesium was found to retard or prevent this process. On the other hand, Lee and Niessen(r*t obtained a fine grained (l-8 pm) dispersion st~ngthened zinc alloy in which recrystallization was arrested for temperatures in excess of 150ac?. In the present investigation, the as-rolled sheet structure is unstable at room temperature and graingrowth occurs readily (Figs. 4 and 6). While the increase in hardness with increasing grain size is supported by the strength vs grain size behaviour observed at superplastic grain sizes.@ It has been established that grain-boundary migration, either during recrystallization or grain-growth, is dependent upon grain-Sunday structure, the driving force deriving from the increase in grain-boundary area, proportional to the grain-boundary free-energy and inversely proportional to the grain size. An increase in dislocation density will also add to this driving force. Thus in the present ease, the presence of aluminum, both in precipitate form and in solution, will produce a drag which should inhibit dislocation and grain-boundary mobility. The drag is not capable of completely suppressing boundary mobility, and a fin&i, stable grain-size of about 5 pm is obtained at 0.42 T,.

NAZIRI

SUPERPLASTICITT

PEARCE:

ASD

4.3 Grain-growth inhibition due to deformation The

way

in which

completely

inhibited

grain

growth

by tensile

appear to have been reported that the mechanism superplasticity

in this

associated

with

Structural

observations

that

tensile

the

this

density

can

of dislocations

or if the dislocations

slip plane,

climb

or cross-slip

possibility

zinc bicrystals

as

with those already

sources

and

sinks

case surface

place

deformation

through

the adjoining

that,

associated

in

matrix. with

Thus it seems

the

deformation

conditions

in this

dislocation

annihilation

both

grain boundaries fine

the

of sliding, was the climb and glide of

along the boundaries.‘22-26’

could facilitate observed

for

observa-

sliding is taking of

the

are emitted by the sliding boundary

suggested

basic mechanism that

occur.(1B~20)

have shown that during grain boundary

was further

clear

first

the role which

Studies

and these glide freely

dislocations

the

and also to generate

In the present

sliding, dislocations It

is by

will allow the existing

concerns

play,

deformation.

it by

are not on the same must

tions show that grain boundary during

place

which will interact

dislocations.(21)

and

reduced,

One way in which

to glide and interact

boundaries

is

of opposite sign on the same

The imposed tensile deformation new dislocations

is a

migration

density,

take

slip plane,

place

there

and the final grain

in this alloy.

interaction

grain

takes

that

dislocation

annihilation

present. The other

effect.

can be substantially

deformation

dislocations

inhibition

suggest

activity

of a high

that

dislocation

way be

force for grain-boundary

presence

tensile

in some

3 and 17). which show

in dislocation

dislocat,ion

size. One driving

It is also evident,

must

(Figs.

or

does not

which gives rise t)o

grain-growth

deformation,

link between

appears

alloy,

the

a reduction

during

before.

or mechanisms

and in the matrix, grain

size

(~0.6

work at the

and preserve

pm)

found

in

the this

alloy.

of ~0.4

at a strain-rate

(Fig. 12b). While aft,er a prestrain rn,-value increases

of 3.3 X lo-*

see-l

of 450 per cent the

to a value

of 4.5

at the

growth

ALLOY

is

ing which takes place on further to a strain-rate Another towards

deformation

deformation

isotropy

is also found

tended

in this alloy is the tendency

in the strain-ratio

m-values

showed

that

Microstructural

extensive

sliding

lack of cavitation the

interior

during

grain movement/

is t,aking place.

but that

the

near the surface? coupled with the c0nstraint.s

of the

measurements

sheet’

of the surface

show that

in t,he interior

trates the different

as-rolled

300 per cent elongation.

observat,ions

cavitation

that

measurements

text,ured,

after

deformation

grain-boundary

results

the strain-ratio

Texture

highly

to randomness

superplastic

This

and texture

alloy,(2s.2g) which showed

(isotropy).

the

in t’he

is strained.

400 per cent elongation.

t.o unity

tended

must be

and occurs due

of the anisotropic

in a similar

about

tensile

of strain effect.(27)

observation

obtained

by

the grain coarsen-

plane of the sheet when the material

after

1329

inhibited

to 25 per cent elongation,

test

of the specimen

illus-

between the surface and

piece.

Thus

used to determine

tion from grain-boundary

any

surface

the strain contribu-

sliding must be viewed with

some suspicion. However, cavitation terior, at,

the

discrepancy

indicates

for

between

the

extensive

on the surface and no cavitation

operating

that

some

in the in-

ot,her process

must

be

in order to relieve the stress concent,ration example,

maintain

grain

boundary

compatability.

polycrystals,

In

the

triple

point’s

creep

of

there is ample evidence

grain-boundary

migration

intragranular

cavitation

occurs,

to

coarse

bo show that

the propensity

decreases.f30)

Packer

if for

et aZ.(ai)

have also suggested that when grains of the same kind slide,

local

hardening

at

the

boundaries

relieved

by migration,

material

in which sliding can cont’inue.

Thin-foil shows

retention

duringsuperplastic

t,he

electron

deformation,

of recovery

is the

maintenance

curved

be

Another

of equiaxcd

obser-

grains

boundaries,

in contrast

boundaries

found

in

with to the

alloys.

of the change in fracture

sensitivity

which

free-structure

suggests the operation

process.

superplastic

The observation with strain-rate

grain

interphase

eutectic/eutectoid

microscopy,

of a dislocation

form

straight

can

and this creates new st.rain-free

transmission

the

relatively

This alloy, after 10 per cent prest’rain, shows a peak

grain

Al

the result of superplastic

vation

4.4 Superplastic deformation

peak

straining

of some

m-value

A Zn-0.4O;,

temperature

is partially

deformation

IX

is in qualitative

behaviour agreement,

strain-rate of 5 x 10e5 see-l. This increase in mvalue is brought about, by the coarsening of the structure which occurs during straining and is typical

with the other results. The fracture mode at the highest, strain-rate (m = 0.05) is typical of that observed in conventional

of superplasticity, that is. the peak m-value increases and moves to lower st,rain-rates with increasing grain

large-grained materials(32) containing second-phase particles or inclusions. In these materials a honey-

size.“)

comb

Also,

since

it

has

been

shown

that

room

ductile

network

is observed,

similar

to

that

ACTA

1330

found where there are carbides the centre

or similar

the particles

of each dimple,

sible for the void nucleation. aluminum-rich

METALLURGICA,

particles

In the present

precipitates

in

being responboundaries

would appear to be the most likely fracture

nucleation

as some grain-boundary

work

at

an

intermediate

that

the

retarded

with

increasing

is further

reinforced

network

ductile

to the

Imperial

Bristol

for supplying

(m = O.l’i), is

This

idea

(m = 0.34),

operate

rapidly

where

enough

to

by coalescence.

5. CONCLUSIONS

1. A fine grained sheet

(4.6

pm), Zn-AlO.

can be produced

sensitivity

which

grain size results

the aluminum, precipitates,

both

a strain rate

from the influence

in solution

and in the

on the 0.42 T, recrystallization

of the material. 3. The 0.6 ,um grain size material temperature occur

wt. “i, alloy

exhibits

(m) of 0.4-0.5.

2. The tie

(0.42

resulting

is unstable

observations

grain size stabilization recovery

about

that. by

spectrum.

of up to 500 per cent to the lower strain-rate

These

the observed

observations

increase

6. Surface

elongation

formation

show

movement,

while internal

maintained

consistent

with

the

during

occurrence

superplastic of extensive

structural

low-dislocation

grain

observations density

deshe\\

structure

is

during deformation.

7. Though maintained

some during

planar

anisotropy

deformation,

with increasing

8. Fracture 9. The

are

of 0.5,

end of the

in grain-size.

observations

a void-free,

the

a dis-

the U, values and the peak m-values

but moves them

marked

suggest

is brought

mechanism.

5. Deformation

that

5 ,um,

by a pre-

25 per cent.

4. Microstructural

increases

at room

in a final grain size of about

tensile strain of about

of of

can readily

but this grain growth effect can be inhibited

location

form

behaviour

and grain growth

T,)

behariour

superplasticity

comments

it

in w-value becomes

is less

strain. is

strain-rate

in

Zn-0.4

dependent,. %

Al

can

boundary

be

are grateful to Professor P. Siessen, Waterloo. Ontario. Canada for his

on this manuscript. Smelting the

Thanks

Cranfield

Institute

are expressed

Corporation.

Avonmouth,

alloy.

investigation

was done when one of the authors

of the honeycomb

strain-rate

void growth

net-

coalescence

superplasticity.

mechanisms

extensive

and

recovery/grain

ACKNOWLEDGEMENTS

sliding takes

strain-rate growth

by the absence

at the lowest

superplastic prevent

void

by a dislocation

The authors University of

of a finer honeycomb

suggests

lQi4

sliding mechanism.

place in this allo_v. The observation

explained

24,

case the

at the grain

sites, particularly

VOL.

This

(H. S.)

was at the

of Technology. REFERENCES

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