The influence of quenching rate on precipitate-free-zones in an AlZnMg alloy

THE

INFLUENCE

OF

QUENCHING RATE IN AN Al-Zn-Mg S.

CHANGt

and

J.

ON PRECIPITATE-FREE-ZONES ALLOY* E.

MORRALt

A high-purit.y aluminum alloy cont8ining 6 :10 Zn and 2.2 yi Ug was quenched 8t various rates betn-een 3 x 10-l and 2.5 X 10’ Kjsec and w&s studied focusing on the relationship between precipitate structure near grain boundaries and quench rate. -4 theoret,ical prediction was made that, the precipitate-free-zone width should vary linearly with respect to the inverse square root of quenching rate, a relationship which is consistent with a vacancy depletion model, and it w8S found to be near the observed behavior. IPI’FLUENCE

DE

LA VITESSE DE TREMPE DANS UT\’ ALLIAGE

SUR LES ZONES Al-Zn-Mg

SAKS

PRECIPITE

On a 6tudiB un alliage d‘aluminium de haute purete contenant 6 “/dZn et 2,2 7,; Mg. trempe B des vitesses comprises entre 3 x 10-i et 2,5 X 10’ K/set, en s’interessant surtout 8 18 relation entre la structure des precipites au ooisinage des joints de grains et la vitesse de trempe. On avait prevu une variation line&e de la largeur de la zone s8ns p&cipit.P en fonction de l‘inverse de la racine car&e de la vitesse de tmmpe. relation compatible avec un modele d’epuisement des lacunes, et c’est a peu prea ce que I’on a observe esperimentalement. EINFLUB

DER

XBSCHRECKGESCHWIh’DIGhEIT AUF AUSSCHEIDCSGSFREIE IN EINER ,41-Zn-Mg-Legierung

BEREICHE

Eine hoohreine ~4luminiumlegierung mit ST/, Zn und 2,2 “/dMg wurde naeh Abschrecken mit verschiedenen Abschreckgeschwindig eiten zaischen 3 x 10-i und 2,5 x 104 B/see mit besonderer Beriicksiohtigung des Zusammenhangs zwischen Ausscheidungsstrukturen in der N&he von Korngrenzen und Abschreckgeschwindigkeit untersucht. Es wurde theoretisch vorhergesegt, deD die Breite des ausscheidungsfreien Bereiches umgekehrt proportional zur Wurzel aus der Abschreckgeschwindigkeit ist, Dieser Zusammenhang ist konsistent mit einem Leerstellenverarmungs-Model1 und ist in guter Ubereinstimmung mit den Beobachtungen.

1. INTRODUCTION

Research on quenched and aged Al-Zn-Mg alloys over the past, three decades has served to characterize phases involved in these alloys and to describe qualitatively how the microstructural features, especially precipitate-free-zones, are influenced by processing. However, in the absence of sufficient, quantitative informat.ion it’ has been impossible to vary the microst,ructure systematically, a necessary requirement for relating microstructure bo properties.(i) Accordingly, t,he objective of this work was to establish the relationship between precipitate-free-zone width and initial quenching rate for the specific case of vacancy deplet,ed zones. Precipit#at.e-freezones (PFZ) form adjacent t,o grain boundaries, dislocations, or precipitates and they are caused by eit,her solute or vacancy depletion. Xicholson et a1.(2*3)found that’ precipitate-free-zones which form because of vacancy depletion tend to shrink with ageing t,ime while Smit,h and Grantc4) found t,hat(those controlled by solute depletion(5) tend to expand with ageing time. Because of these t,endencies and the relative mobility of vacancies and solute atoms? vacancy depletion t’ends to dominate during short ageing times. while solute depletion dominates after long ageing times as is illustrated in Fig. 1. It should be * Received September li, 19i4. i Department of Metallurgv, Institute of Materials Science, U-136 Unn-ersity of Connect&t, Storrs, Connecticut 06268, U.S.A. _4c’T_4 METXLLURGICA, 3

VOL.

23, JUNE

1975

685

mentioned that there are two cases when only solut,e depleted zones would be observed: one is when fa*, t’he time required for observable precipitation to form, occurs when the solute depletion curve is above the vacancy depletion one ; and the other, a relat’ed case, is ahen excess thermal vacancies anneal out during slow cooling. On ageing Al-GZn-2Mg alloys that have been quenched to a temperature below the G.P. zone solvus, several different phases precipitate out of the supersaturated a matrix: they form in the well knrnvn sequence(6) G.P. Zones -+ 7’ -+ 11 in which G.P. Zones are a coherent phase, vf is a hexagonal semicoherent phase and 17 is the hexa.. gonal, incoherent, equilibrium phase MgZn?. In the meantime q precipitates at the grain boundary. This system has the special property mentioned above that. precipitates first appear during the period of ageing bime when solute depletion dominates t,he PFZ. In order to study the early stages of PFZ forma.tion when vacancy depletion is important it is necessary to up quench the sample to above the G.P. zone solvus where 7’ precipitates heterogeneously on incipient G.P. zones. Near grain boundaries where G.P. zone format.ion is retarded b-y vacancy depletion no ,q’ is found, hence a PFZ is observed. In this study, which was on vacancy deplet,ed PFZ, it was necessary, t,herefore. to perform a two-step heat t,reatment : first

ACTh

636

t’,-

3

I

Time required precipltatlon

JlETALLURGIC.4,

for observable to form

1 ;\

\

-Precipitate

free

.H

/

/Solute depleted

‘Tyacancy zone

depleted

---z_one --__

--__

---__

Fra. 1. General relationship between vacancy end solute depleted precipitate-he-zones (PFZ). The dashed lines

extrapolate to the PFZ width that would be found if only one type of depletion occurred.

ageing below the G.P. zone solvus temperature to develop incipient G.P. zones and then ageing above it to reveal the PFZ. With regard to vacancy depleted PFZ, Embury and Nicholson”) proposed that there is a critical concentration of vacancies required for precipitation to occur. From t.his assumption one can show that the PFZ width should be inversely proportional to the squareroot of initial quenching rate according to the equation w = 4roT,(RD,/AH,!i’8)1f2

0)

in which r, is a constant that is related to the critical concentration of vacancies; R is the gas constant; D, is the diffusion coefllcient for vacancies at the solutionizing temperature, T,; AH,,, is the activation enthalpy for vacancy movement and !I”‘,is the initial quenching rate from the solutionizing temperature. Equat.ion (1) is a simple extension of Embury and Nicholson’s treatment.c2) Previous work on how quenching rate influences PFZ widt.h, notably by Nicholson et uZ.(~*~Jand Shastry and Judd,“) given in Table 1 has not confirmed this inverse, square-root relationship. There are various reasons that could be given for their observations. Shastry and Juddi suggested that vacancies generated by grain boundary precipitates were influencing t,he vacancy profiles in a way which decreased the effect of quenching, while Nicholson et aZ.f2z3)gave no explanation. Another reason could be a statistical one based on the scatter of PFZ width observed for a given quench rate and the limited range of quenching rates st,udied. The large scatt.er arises primarily from the variat,ion of PFZ widt,h with grain boundary

VOL.

23,

1975

orientation.(2*3*7JIn t,his work only high angle boundaries, as evidenced by a large grain boundary precipitate density, were considered. In addition, the quenching rate was varied over five orders of magnitude in contrast to prior work which was apparently limited to several orders of magnitude. With these changes, it was intended either to confirm the theoretical relationship or to establish another empirical law between initial quench rate and PFZ width. Still another reason why the inverse, square-root relationship might not be observed is related t,o the assumpt.ion that there is a critical vacancy concentration required for precipitation which is independent of thermal cycle below the G.P. zone solvus. Since the observat,ion has been made that the PFZ width and hence the critical vacancy concentration depends on ageing time,(2*3) it is also possible that the critical vacancy concentration and, therefore, r, depends on quench rate. If such is the case, one would not expect the general form of equation (1) to be followed unless r, is fairly independent of ps or varies with respect to the inverse square-root of it. 2. EXPERIMENTAL

PROCEDURE

X&e&al preparation The Kaiser Aluminum Company supplied 4-9’s Al, &9’s Zn and 3-9’s Mg for preparing the Al-6Zn-2.2 Mg alloys. A heat of 250 g was melted in a carbon crucible and cast into a book mold to obtain a slab with dimensions of (125 x 78 x 6) x 1O-3 m. The east slab was then homogenized at 686 K for 2 hr. The results shown in Table 2 of a chemical analysis on the homogenized slab was very close to the expected composition. After soaking the slab at 686 K for an additional hour it was rolled, with intermediate anneals, to a thickness of low3 m. The sheet was finally cold rolled down to 0.25 x 1O-3 m. Quench& rate ,measurement and heat treatment Samples were cut into a rectangular shape with dimension (20 x 7 x 0.25) x 10e3 m and were annealed at 793 K for 30 min. The_result was a uniform grain size which had a mean intercept length of 0.132 x low3 m. Chromel-alumel thermocouple wires 5 x lo-’ m dia. were spot welded onto the specimen surface and were connected to a millivolt potentiometer for measuring the specimen temperature. The thermocouple was also attached to a Tektronix oscilloscope with a c-27 camera that was triggered as the sample left the furnace and entered the quenching bath. The resulting cooling curve was analyzed by measuring the average quenching rate within the first 100 K of cooling. Various quenching curves obtained from different media are shown in Fig. 2.

CHASG

MORRAL:

AXD

TABLE 1. Comparison

PRECIPITSTE-FREE-ZOSES

XS

Al-Zn-Mg

of previous and current work relating quenching rat,e to precipitate-free-zone Al-6Zn-2Mg alloys Sampl;,;?b:;ness

ALLOY (wt. 5)

1:;YESTIGATOR

J. D. Embury an+ R. B. Nicholson (1965)

IX

Al - 5.9Zn - 2.9 Mg

Quench bath

width in nominally

T, = 738'K

‘3

T,,= ambient T, = 2 T = a2

P. N. T. Unwin, G. W. Lorimer, 3 R. B. Nicholson (1969)

Al - 5.97-n- 2.9 Mg

C. K. Shastry and G. Judd' (1971)

Al - 6.9Zn - 2.34Mg

oil

2,000

1.28

water

30,000

.50

150

air oil water

2,000 10,000 (est.)

brine S. Chang and J. E. Korral

Al - 5.9Zn - 2.2 Mg

250

.21

T

T, = 748'K

4.99

T

3.UO 25.DOO

1.00 .55

= 29B°K, 10 set a1 T = 4530 K, 3 hrs a2

2::

.;9 -13

T = 748'K Ti,= 298'K, 10 set

3,340 25,000

.23 .17

T

rw

.3

oil brine

TABLE 2. Chemical composition CU

Fe

Si

Mn

Cr

Ti

Zn

Mq

Al

wt.:,

0.007

0.03

0.01

0.002

0.002

co.002

5.99

2.16

bal.

,,,‘,

I

10-Z

I

II”‘,

I

I

I

,,,,a,

IO-

,

loo Time,

I

= 44l'K: 3 hrs. a2

of the alloy sample

element

,

= 473'K, 4 hrs a2

200

the G.P. Zone Solvus

,

T = 753'K Ti,= ambient

liquid N2

*Temperature is below

,

.34 .31 .29

3 hrs

.39 8.15

furnace cool air liquid N2

;

= 453'K. a2

.3 42

oil

IO-3

T,,= 295'K

furnace cool air

hri

0

453'K. 3 hrs (oil quench) 453'K, 12 hrs (water quench)

Ts = 738'K

T not reported

667

PFZ Width (lo-6m)

Reported quench rate (OK/set)

oil water

100

ALLOT

1,111,

,

IO’

,

,

I,,,,

I

:02

I

lllllr,

,

-3 I”

set

FIG. 2. Quenching curves, obt,ained from various quenching media. plot,ted on a log time scale in order to show how the different thermal cycles compared with each other.

BCT.4

688

MET_4LLURGICS,

&lost quenched samples were aged for 10 set at room temperature and then were placed in liquid nitrogen to await further heat treatment. However, the sample quenched in liquid nitrogen received no room temperature treatment and was cooled directly to t’he storage bath temperature. The quenched samples were next separated into two groups : one group was aged below the G.P. zone solvus at 441 K for 3 hr and the other above it at 453 K for 3 hr. The samples were aged in a constant temperature circulator containing glycerol for 441 K ageing or glycerine for 453 K ageing. Microstructure study Thin foils were prepared using standard technique@ and were viewed on a goniostage-equipped microscope, 100 KV Philips EM 300. On average, eight photographs were t.aken for each sample receiving a given treatment, from which the average PFZ width was determined by using the method given in Ref. 7. 3. RESULTS

Quenching rate and PFZ width when age&g below the G.P. zone sokus In t,he group of samples that were aged below the G.P. zones solvus at 441 K, the PFZ width was relatively insensitive to quenching rate. Although some deviations in the PFZ width were measured, they were negligibly small and could have resulted from the measuring uncertainty. In addition, the PFZ are not clearly defined and precipitates appear all the way up to the grain boundary interface. Measured values of t,he PFZ width for this case are listed in Table 1.

Ouenching FIG.

rate,

VOL.

23,

1975

Quenching rate and PFZ width whelaageitlg abore the G.P. zone solvus When ageing at 153 K, above the G.P. zone solrus, the PFZ width was found to decrease as the quenching rate increased. As is illustrated in Fig. 3, the line relating log (PFZ width) with log p,, det.ermined by a least square 6t of the data, has a slope of a -0.44 which compares with t’he value of -0.5 expected from equation (1). Selected pictures of PFZ from the eight) t’aken for each quenching rate, are shown in Fig. 4. Only one sample quenched at the slowest rate of 3 /: 10-l K/set, did not follow the expected behavior. It had a much smaller precipitate free zone than would have been predicted. 4. DISCUSSION

The observation that PFZ widths are ‘independent of quenching rate in samples aged below the G.P. zone solvus is an observation that has been made before”) and was found again in this work. The reason is that t,hey are solute denuded zones which form primarily during the three hour ageing treatment at 441 K. In samples aged only a short time below the G.P. zone solvus and then aged for three hours above it, at 453 K, the PFZ widt,h varied more that an order of magnitude with quench rate: these are vacancy denuded zones. The plot given in Fig. 3 of log w vs log l’8 has a slope of -0.44 which is near the theoretical slope of -0.5. The deviation from -0.5 could have a theoretical basis if the vacancy concent.ration required for precipitation depends on quenching rate. If, as one would expect, the critical vacancy concentration is smaller for slower quenching rates, t.hen the

K/s+

3. Plot of log (PFZ width) vs log (initial quenching rate). The slope is -0.44

predicted value of -0.50.

which compares with the

FIG. 4. Microstruct,ure of precipitate-free-zones in samples aged above the G. P. zone sohus at 453 K. The quenching rates corresponding t,o the micrographs we (a) 4.2 Y 10’ Ksnec. (b) 2.0 x 162 K/see, (c) 3.3 x 10s K/see, and (cl) 2.5 i: li)l Klsec. These are all vacancy depleted zones.

PFZ

width will be smaller

and a more gradual

slope

in a plot of log ~(7vs log rts will result. Table 1 compares the measured widths of this work with those of previous investigations. Similarities exist, between Nicholson

the results

et uZ.‘~*~)but not, with those of Sha&ry

Judd ~(~1 their of other that

of this work and those

PFZ

were much

investigations.

by ageing

narrower

One reason

at a higher

than

and t.hose

for this may

temperature

of

be

for a longer

time (473 Ii. for 4 hr) t.here may have been additional precipitation

which

partially

garding the Nicholson variat,ions

width

approximate

with cooling

ment s may have been responsible

grain

boundary

rat,e measure-

for t,heir not obtain-

ing the predict’ed behavior. The sample that was furnace the 1)rharior likely

that

expect’ed of vacancy excess

t.hermal

cooled did not follow depleted

vacancies

during cooling and that t,he PFZ resulted deplet,ion.

PFZ.

annealed

In fact, the a-idbh of these PFZ

formed

in samples

aged below the G.P.

It is out

from solute was of t’be

same order of magnit8ude as the solut’e denuded

width

zones

zone solvus.

of precipitate-free-zones

because of vacancy

the initial quench

and aged alloys.

A simple

width should vary linearly square-root somewhat

of initial supported

2.2 Mg system be proportional

which

form

deplet.ion can be varied systemati-

cally by changing

Re-

ef al. data, one could argue that

in t.he PFZ

orientat ion and

filled in the PFZ.

5. CONCLUSIONS

The

theory

rate of quenched predicts

with respect

quenching

rate.

by observations

in which the PFZ to the -0.44

that

the

to the inverse. This theory

is

of an Al-6Zn-

width was found to

power of quenching

rate.

6. ACKNOWLEDGEMENTS

The authors

would like to acknowledge

of the Connecticut In addition

Research

Foundation

S. Chang was supported

of Mat,erials Science

the support for this work.

by an Institute

Fellouship.

REFERENCES 1. A. J. DEARDO, JR. and R. D. TOWXSESD, ~311~2. !?'ra,zs. 1, 2573 (1970). 2. J. D. E&IBVRYand R. B. ~ICHOLSOS, Acfn Mer. 13, 463 (1965). 3. P. S. T. Ux-nrs, G. IV. LORIMER and R. B. Xrcao~sor. _&%a Met. 17, 1363 (1969). 4. W. F. SMITH and X. J. GRANT, Trans.dSN 62. i21 (1969). 5. P. R. SPERRT. Mel. Tra~te. 1,2650 (1970). . 6. G. THOMAS and J. STTTIXG. J. Inst. Met. 88,81 ( 1966). 5. C.R. SHASTRY anclG.Jr~)r).Traj1s._41IIIE. 62,iZi (1969). 8. A. R. DAVIU. J. Zust. Mef. 96, 61 (196x).