INFLUENCE OF PROCESSING CONDITIONS ON THE MONOTONIC PROPERTIES OF 8090 ALLOY

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INFLUENCE OF PROCESSING CONDITIONS ON THE MONOTONIC PROPERTIES OF 8090 ALLOY

C. Damerval, J. Raviart, G. Lapasset

To cite this version:

C. Damerval, J. Raviart, G. Lapasset. INFLUENCE OF PROCESSING CONDITIONS ON THE

MONOTONIC PROPERTIES OF 8090 ALLOY. Journal de Physique Colloques, 1987, 48 (C3),

pp.C3-661-C3-667. �10.1051/jphyscol:1987376�. �jpa-00226607�

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INFLUENCE OF PROCESSING CONDITIONS ON THE MONOTONIC PROPERTIES OF 8090 ALLOY

C. D A M E R V A L , J.L. R A V I A R T a n d G. L A P A S S E T

O N E R A , 29, A v e n u e d e la Division Leclerc, B.P. 7 2 , F-92322 C h g t i l l o n C e d e x , France

SUMMARY

Three processing conditions, namely quench rate after solution heat treatment, cold work prior to aging and aging conditions, were varied and their effect on the mechanical properties of 8090 extruded flat bars was evaluated. The influence of cold work and aging conditions was examined on the underaged material for the same proof strength level.

A slow quench is unfavourable both for tensile properties and SL fracture toughness due to a heavy precipitation of stable phases within grain (and subgrain) boundaries. For a same proof strength level, raising the aging temperature from 170 to 210°C improves ductility. This effect mainly results from an increased resistance to shear rupture, which is related to changes in precipitation characteristics.

INTRODUCTION

The development of low density Al-Li-Cu-(Mg) alloys for aircraft structural applications requires different combinations of engineering properties [1,2]. A common goal is to achieve adequate ductility and fracture resistance. As emphasized by several authors [3-51, these properties strongly depend on processing variables.

Main work has been devoted to the influence of stretch and aging conditions [3-51.

The effects of homogenization practice and solution heat treatment were also investigated [3]. Furthermore, the influence of grain structure and crystallographic texture developed during processing has been highlighted [6].

The objective of this paper is to examine some effects of quench rate after solution heat treatment, cold work and aging practice on the tensile properties and the short transverse fracture toughness of 8090 extruded flat bars.

EXPERIMENTAL

The 8090 alloy was supplied by PECHINEY in the form of extruded flat bars (section: 100 x 13 mm). Its composition is A1-2.5Li-1.3Cu-1Mg-0.09Zr (weight percents). Ti, Fe and Si are the main impurities (less than 0.03% for each of them).

Heat treatments were conducted in salt baths.

All specimens were machined at mid-thickness and their location in the bars was carefully controlled. Aging kinetics were determined by means o.f pyramidal Vickers hardness measurements. Tensile properties were measured using cylindrical test bars with a 4 mm diameter and a 20 mm gauge length. Fracture toughness tests in SL orientation were carried out with chevron-notched short bar specimens (B = 13 mm, w = 24 mm, 2H = 12 mm) [7,8]. R o , P , R, E and KB are symbols used respectively for 0.2% proof strength, tensile strength, elongation to rupture and SL fracture toughness value.

Microstructure was studied both by optical and transmission electron microscopy (TEM). Thin foils for TEM investigations were prepared by a conventional electrothinning technique. Failed tensile or fracture toughness specimens were

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987376

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C3-662 JOURNAL DE PHYSIQUE

observed by scanning electron microscopy (SEM) or optical microscopy on polished cross sections.

PRELIMINARY MICROSTRUCTURAL EXAMINATIONS

The structure of the bars is unrecrystallized and consists of pancake grains with a very high aspect ratio: the average grain size is approximately 50 pm in the short transverse direction (S), 400 pm in the long transverse direction (T) and 5 mm in the extrusion direction (L). Most of the grains are polygonized with a subgrain size of about 5 urn. Due to the high purity of this alloy, the volume fraction of insoluble phases is very low.

In artificially aged conditions, matrix precipitation consists mainiy of 6' (A13Li) and S' (AlzCuMg). A few TI (AlzCuLi) plates can also be seen in overaged structures. The 6' phase nucleates homogeneously within the matrix or heterogeneous- ly on AlgZr dispersoids. Dislocations and subgrain boundaries are preferred sites for the nucleation of S' particles but, in some areas, this phase precipitates also homogeneously. Two phases can form in grain boundaries during the aging treatment:

6 (AlLi) and an icosahedral phase. In the following, this phase is called Tz but its composition may be different from AlsCuLi3.

INFLUENCE OF QUENCH RATE Procedure

Flat bars were solution treated at 540°C during 1 h and then quenched either in cold water (CWQ) or in boiling water (BWQ). They were subsequently stretched 2%.

Three typical aging conditions were selected: an underaged temper (UA) of 6 h at 170°C, a peak aged temper (PA) of 12 h at 190°C and an overaged temper (OA) of 24 h at 210°C. During the quench, the cooling rate was monitored directly on the specimens. Average cooling rates between 540 and 250°C were approximately 100°C/s for CWQ and 3.7OC/s for BWQ.

Microstructure

Microstructural investigations of the effect of quench rate can be summarized as follows :

- no significant difference was observed regarding the size of 6' and S ' hardening precipitates or the width of precipitate free zones (PFZ);

-

examination of as-quenched samples showed a heavy intergranular precipitation of Tz and 6 (but mainly Tz) which occured during the BWQ itself. These Tz particles often exhibit a flower-like shape (see for instance fig. 3 ) . Tz precipitates were also observed in subgrain boundaries and more rarely within the matrix. On the contrary, no precipitation of stable phases was detected in CWQ specimens;

-

in aged CWQ specimens, the mean size of TZ particles (which have nucleated exclu- sively at grain boundaries during the aging treatment) can be estimated: respectively 0.05, 0.15 and 0.2 gm for UA, PA and OA tempers. In aged BWQ specimens, the measurements of the size of intergranular Tz particles are widely scattered, since these precipitates could have formed either during the quench (hence in a large range of temperatures) or during the aging treatment. Some of them have size in excess of 5 Dm:

-

for PA and OA conditions, the size of 6 particles depends also on the quench rate:

this size can reach 3 pm in the case of BWQ, whereas the maximum size is about 0.1 pm in the case of CWQ. Moreover, 6 particles can grow i.n subgrain boundaries and inside the matrix. In fact, 6 seems to nucleate preferentially at T~/matrix interfaces. Thus T2 and 6 precipitates form aggregates around which PFZ can form in the same way as along grain boundaries (fig. l)..

Mechanical ropert ties

Tensile properties in L and T dsrections and KB values are given in table 1.

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Table 1

-

Influence of quench rate on the mechanical properties

In PA and OA conditions, strength is lower for BWQ than for CWQ. This quench sensi- tivity was confirmed by the examination of the age-hardening response at 210°C for BWQ and CWQ specimens (fig. 2). A reasonable explanation is that in the case of BWQ, the heavy precipitation of Tz and 6 phases during the quench reduces the supersaturation of the as-quenched matrix.

But the most striking point is the drop in elongation and in fracture toughness which accompanies the change from CWQ to BWQ.

Tensile rupture modes

The examination of failed tensile specimens revealed that two rupture modes can be distinguished. The first one is a combination of transgranular shear rupture and splitting of grain boundaries which are parallel to the LxT plane, and thus to the stress axis (delamination). The second mode is failure of subgrain boundaries;

in this mode, no delamination was observed. Of course, whatever the,rupture mode, decohesion of the grain boundaries which are inclined to the stress axis is also involved, especially for PA and OA tempers: but, due to the high aspect ratio of the grain structure, this contribution can be neglected at least for L tensile bars.

The first rupture mode is dominant in all CWQ specimens (UA, PA and OA), but also in BWQ specimens (UA only). Shear rupture is flat, featureless in the case of UA temper (CWQ or BWQ); on the contrary, in the case of OA temper (CWQ only), serrated appearance, evidence of local necking between delaminated areas, curvature of grain boundaries exactly under the shear rupture surface strongly suggest that shear rupture needs a higher strain to occur (fig. 4). This is also evidenced by the presence of macroscopic necking for OA specimens, whereas for UA specimens failure precedes geometrical instability. Shear rupture is obviously related to the characteristics of strain localisation in the matrix. For UA condition, 6' shearing is well-known to be responsible for strain localisation. For OA condition, it can be said that this localisation is more difficult although the exact 'role played by different possible mechanisms (slip dispersal due to S' precipitation, transition from 6' shearing to by-passing) has not yet been well established.

The-second rupture mode is dominant in BWQ for PA and OA tempers (fig. 5). The coarse precipitation of Tz and 6 phases in subgrain boundaries (instead of S' for CWQ specimens), which initiated during the slow quench, clearly weakens these boundaries. In addition to elongation data, the comparison of strains to rupture of CWQ and BWQ specimens 0 0.13 for CWQ and < 0.10 for BWQ in PA and OA conditions) allows to emphasize how catastrophic this intersubgranular rupture is.

It should be noted that no valuable explanation was found concerning UA results: indeed, elongation is lower for BWQ than for CWQ. But in both cases, the first rupture mode is operating. On the basis of microstructural examinations, it is questionable whether shear rupture is easier in the case of BWQ.

Quench

CWQ

BWQ

Aging

6h 170°C 12h 190°C 24h210°C 6h 170°C 12h 190°C 24h 210°C

L tensile properties

SL fracture toughness

KB (MPaJm)

13.4 10.9 8.3 10.3

9.3 6.8 E

(%I 5.9 7.4 6.8 4.5 4.8 5.1

-

R 0 , z (MPa)

415 482 456 407 468 394

T tensile properties R

(MPa) 491 543 520 479 533 475

R o , z (MPa)

358 443 431 353 412 388

R (MPa)

472 519 494 459 499 463

E

(%)

6.5 5.9 6.8 5.2 5.1 4.7

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Fig, 1

-

la) Bright field TEM micrograph showing two aggregates (Tz + 6) within the matrix

lb) Dark field TEM micrograph imaging 6'. Same area as la. Note the PFZ around the aggregates

.

^CWQ

BWQ

.Sh lh 5h 10h

Aging time

.

^hours

Fig. 3

-

SEM micrograph of' SL fracture Fig. 2 - Aging kinetics at 210°C toughness specimen (BWQ, UA)

showing T2 marks on intergranular rupture surface

Fig. 4

-

Shear rupture. Optical micro- Fig. 5

-

Intersubgranular rupture.

graph of a cross section of a Optical micrograph of a cross failed L tensile specimen section of a failed L tensile

(CWQ, OA) specimen (BWQ, OA)

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The KB values obtained in these tests can be considered as indicative of crack propagation resistance, in other words of the resistance to delamination. As shown by SEM examinations, failed grain boundaries of CWQ and BWQ specimens exhibit common features:

- in UA condition, surface appearance is quasi-brittle. Dimples seem to be rare and shallow. In the case of BWQ, large Tz precipitates can be evidenced (fig. 3 ) ;

-

in PA and OA conditions, the intergranular surface is uniformly covered with well- formed deep dimples: it is the classical aspect of intergranular rupture when induced by strain localisation in soft PFZ and void formation around intergranular precipitates.

Whatever the aging conditions, the heavy precipitation of stable phases which takes place within grain boundaries during BWQ has a detrimental effect on the resistance to delamination. Nevertheless, this conclusion does not imply that this precipitation is basically responsible for intergranular rupture in the case of UA temper. Indeed, the poor resistance to delamination exhibited by CWQ specimens, although superior to that of BWQ specimens, can hardly be explained by the low density of tiny Tz particles formed during the UA treatment.

INFLUENCE OF COLD WORK AND AGING TEMPERATURE Procedure

A second series of flat bars were cold rolled 0. 5 or 10% after a conventional solution heat treatment (1 h 535OC, cold water quench). An underaged temper was chosen as it gives higher fracture resistance than peak aged or overaged ones. Aging temperatures were 170, 190 and 210°C. Once the aging kinetics determined at these different temperatures, aging times were selected (see table 2) in order to ensure an approximately constant yield stress (Ro,z = 400 MPa) in T direction. This condition does not imply equivalent yield stress level in L direction: in fact, R o , ~ depends only on the cold work percentage (Ro,z is respectively 410, 440 and 470 MPa for 0 , 5 and 10% cold work).

Mechanical properties

Elongation (in L and T directions) is given as a function of aging temperature in fig. 6 and 7. Elongation is clearly improved (or in some cases unchanged) by raising the aging temperature from 170 to 210°C. On the other hand, the behaviour of 0 and 5% cold rolled bars is quite similar. The 10% cold worked material has the lowest ductility in L direction and the highest one in T direction.

In all cases, tensile specimens failed by the first rupture mode, i.e. by a combination of shear rupture and delamination. Concerning the effects of cold work and aging temperature, fractographic features did not show any significant variation. So, in order to evaluate the evolution of the propensity for shear rupture with the aging temperature, it appeared necessary to carry out TEM investigations of hardening precipitation.

Table 2

-

^6'size fmm) as a function of cold work and aging temperature for iso-strength underaged temper (aging times are indicated within brackets)

Aging temperature

170°C

210°C

% cold work 0

19 (70h)

2 3 (4h)

5 11 (10h)

16 (lh)

10 9 (6h)

9 (25mn)

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+ 0Z "old n ~ r k

-

0 51 001d xork

- - - -

m 10Xs.ld work

- - -

I I I

I75 190 210

A G I N G T E M P E R A T U R E .OC

Fig. 6 - Elongation in L direction as a function of aging temperature

BYoold work

-

c9 o 5X o d d rsrk

- - - -

I 10X o d d work

- - -

i I I

I76 190 210

A G I N G T E M P E R A T U R E .OC

Fig. 7

-

Elongation in T direction as a function of aging temperature

Microstructure

TEM investigations were performed only for the two extreme aging temperatures (170 and 210°C). It must be kept in mind that following comparisons concern different aging conditions, all of them giving the same yield strength i n , T direction.

Although measurements were not numerous enough to allow for statistical analysis, several striking results emerge:

-

S' volume fraction increases with the aging temperature. It decreases when the amount of cold work is increased;

-

^6'size increases when aging temperature is raised and decreases when the amount of cold work is increased (table 2). At each temperature, the variation of 6' size with aging time is consistent with the LSW coarsening law. Therefore, the 6' volume fraction is the equilibrium one and then it decreases when the aging temperature is raised.

Shear rupture

Taking into account the TEM results, it can be shown that raising the aging temperature is beneficial for resistance to strain localisation and thus to shear rupture: indeed, the increase in S' volume fraction and the changes in 6' precipitation characteristics (decrease in volume fraction, increase in size) are favourable.

This last point needs some remarks. In Al-Li-Cu-Mg alloys, the critical 6' radius for Orowan by-passing is still unknown. As a consequence, the worst case has to be assumed, i.e. 6' shearing is operative whatever the amount of cold work and the aging temperature: this is a realistic assumption since, in all cases, slip bands steps were observed on delaminated areas of ruptured tensile bars. Referring now to the model of Hornbogen and Lutjering [9], both the decrease in 6' volume fraction and the increase in 6' size lead to a decrease in the propensity for slip copla- narity.

Increasing the resistance to shear rupture by raising the aging temperature is probably the main cause of the improvement of elongation at least for L tensile specimens, where this rupture mechanism is predominant.

TEM results actually do not allow to assess the influence of cold work on shear rupture: indeed, the efficiency of Slip dispersal by S' precipitation cannot

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the preceding analysis.

CONCLUSION

The influence of three processing variables on mechanical properties of 8090 extruded flat bars has been studied.

A slow quench is detrimental as well for tensile properties as for SL fracture toughness. This effect is mainly caused by a heavy precipitation of stable phases (Tz and 6) in grain and subgrain boundaries.

Cold work and aging practice were varied in order to obtain an underaged temper with a constant yield stress level in T direction. Raising the aging temperature (from 170 to 210°C) is beneficial for ductility and it is suggested that this effect results primarily from an increased resistance to shear rupture, which 1s related to changes in hardening precipitation characteristics. Properties of 0 and 5% cold work specimens are quite similar.

ACKNOWLEDGEMENTS

The DRET is thanked for its financial support. PECHINEY Voreppe Research Center is acknowledged for supplying the alloy. The authors are grateful to J.C.

Daux and B. Kohn for technical help and to M. Thomas and J.F. Stohr for useful discussions.

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-

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