A study of fatigue saturation range of 5N polycrystalline aluminium

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A study of fatigue saturation range of 5N polycrystalline aluminium

R. Fougeres, J. Chicois, A. Hamel, A. Vincent

To cite this version:

R. Fougeres, J. Chicois, A. Hamel, A. Vincent. A study of fatigue saturation range of 5N polycrystalline aluminium. Revue de Physique Appliquée, Société française de physique / EDP, 1988, 23 (4), pp.689- 689. �10.1051/rphysap:01988002304068900�. �jpa-00245838�

689

A STUDY OF FATIGUE SATURATION RANGE OF 5N POLYCRYSTALLINE ALUMINIUM by

R.FOUGERES1, J.CHICOIS1,

1 - Groupe d’Etudes de Métallurgie Physique ^et Physique ^des Matériaux, UA CNRS 341.

2 - Laboratoire d’Ultrason et de traitement du signal

INSA-LYON, 69621 VILLEURBANNE Cedex, ^FRANCE.

Revue

Phys. Appl.

(1988)

Recently, ^several data have been obtained concerning microscopic mechanisms which control the cyclic deformation of pure pol ycrystai 1 i ne aluminium at raom temperature ^{ll/. Twa} types ^of interactions between dislocations and crystal defects have been identified : at low fatigue ^strain amplitude (03B5t ~ 3xl0 - 5) the dislocation point-defect interaction plays ^an important role, ^{whereas at} high amplitude, ^where

di sl ocat i on-ce11 s are observed, ^the fatigue déformation i s controlled by an athermal interaction between dislocations. The aim of this paper is to analyse the fatigue ^{behaviour by means}

of a compsi te type model. As a matter of fact, composi te ^model

i s assumed to be canuen i ent for the fatigue behaviour of 5N

pol ycrystal 1 i ne aluminium, ^{because a} Baushinger effect has been observed along ^the fatigue loop (Fig-1). ^{After an} arrest of the

fatigue process at ^a given position ^{in the} fatigue loop, and unloading ^{of the} sample ^{at zero} stress, ^an unsymmetrical behaviour is generally ^observed according ^{as the} reloading ^{i s}

carried out in traction or in compression. The degree of ^the unsymmetrical behaviour is dependent ^{on the} stopping position.

However, ^{for a} given position, ^{as well in traction as in} compression, ^{defined by a} stress value approximatively equal to 0.5 Dm «(lm being the maximal stress ^on the fatigue loop), ^a symmetrical behaviour is observed (F and H points in ^traction

and compression respectively (fig.1». ^In addition, ^{from one}

side to the other si de of th i s especial position the direction of the unsymmetrical behaviour i s reversed.

hfareouer, recent data concerning ^lattice misorientation between cell 1 wal 1 s and the i nsi de of cells /2/, suggest ^{i n fact}

that cell walls can be considered as "hard phases" inserted in a

softer matrix. Thus, ⁱⁿ sp i te of ^the po1 ycrystal 1 i ne ^{nature of} cycled specimens, ^{it t} is ascribed that internal stresses which

assume the strain compatibility between the cell wall and the cell interior is. probably ^a significant component af ^the fatigue

stress. Th i s hypothesis is ⁱⁿ good concordance with early results /3/ showing that similar Baushinger ^{effect occurs in}

both pal ycrystal 1 i ne ^and si ngl e-crystai spécimens af pure aluminium.

According ^{to these} hypothesis ^and taking into account the attractive tree interaction, fatigue loops ^{can be analysed} u’sing

a madel derived fram the »composite model",

The fatigue ^stress corresponding ^{to the} symmetrical behaviour described above (F point ^of fig ¹⁾ is ascribed to the interaction between dislocations and attractive trees of the forest in the wall. iompatibility stresses between ’hard and soft phases" ^are assumed equal to ^{zero for this} particular point

of the fatigue 1 oap, ^because dislocations 1 oops ^are probabl y just being emitted from cell walls ^as it seems to be shown by MET in situ observations /1/.

At other points ^{af the} fatigue loop, between F and A (fig 1), ^the fat i gue stress i s ascr i bed to be the sum of the forest stress and of the compatibility stress, Baushinger ^effect being

assumed to be due to the 1 ast stress component. Compatibility

stresses can be cal cul ated by considering : ^{i) A} network of infinité parallel dislocations at both interfaces of the cell wall. i i 1 An infinité number af équivalent parallel ^ceil 1 wal l s.

iii) An interface dislocation density-039B-i given ^byAi= 4~03B3/Lb.

This expression is derived from Ashby-s formul at i on for two

phases materia1s,o i s the shear strain in the slip plane, ^{L the} distance between cell walls and b the Burgers ^{vector of} dislocations. The 03C8 parameter ^{i s} introduced i n order to take into account the difference of plastic behaviour between cell walls and the interior of the cell. ’P parameter ^{is such} as 03C8=

03C8( 03B3) ^and 0=03C8=1, when 03C8=0 cell walls are plastically ^defarmed

1 i ke the interior of the cel 1. When 03C8 ^{=1 cell 1 walls are} plastically undeformed 1 ike in Ashby"s formulation. It has been shown /4/ that the value of the r parame ter ^can be deduced from

experimental ^{data of} fatigue loop between F and A points.

(f i g ^1). Figure 2 shows the 03C8 parameter uariatian with the

fatigue strain between F and A points. ^{In the same strain}

range, the ultrasonic attenuation variationAcK, ^recorded dur i ng ^the fatigue test, ^{i s al so} reported ^{i n} figure ^2.039403B1

variations have been interpreted ^{as the} uariatianQfthemobii

dislocation density ^inside the cell /1/. It can be shown in

f i gure ^{2 a} qu i te similar variation of the two parameters 03C8 and

039403B1 what ^{can be} interpreted ^{as follow : in} every case, ’f =0 ^{at F} point, ^{as a} conséquence ^of di sl ocat i on 1 oop emi ssi on from the walls. As the fat i gue stra i n i s i ncreased, di sl ocat i on loops

are deue 1 oped i nsi de the cell. The 039403B1 uariation i s i ntreased as a result t of the i ncrease af mobile dislocation 1 i nea. The dislocation 1 oop development ^{1 eads to an} i ncrease of compat i bi 1 i ty stresses and consequently ^{to an i ncrease} of the 03C8 parameter ualue. When dislocations reach opposite ^{cell wall} they ^can penetrate i n th i s wall. Two po in ts ^can be deduced from this : i) The difference of plastic ^{behaviour between cell} wal 1 s and the interiors of cells is reduced what correspond ^{to a}

decrease in the’f parameter value. ii) The dislocation density inside the cel 1 is then sl i gthl y dimi n i shed what 1 eads to ^a smal 1 decrease i n the atténuation.

In conclusion, ^a good concordance is observed between resul ts concern i ng ^{the dynami c} behau i our of dislocations along

the fatigue 1 oap ^{deduced from} the attenuat i on uar i at i on /1D( or f rom the anal ysi s of the fatigue loop ^curue usi ng ^a composite type ^model.

Références :

/1/ J.CHICOIS, R.FOUGERES, ^{G.GUICHON and} A.VINCENT - Acta

Metall., vol.34, n°11, pp.2157-2170 ^(1986).

/2/ A.HAMEL, G.THOLLET, ^{C.ESNOUF and} R.FOUGERES - Submitted to

Scripta ^Met.

/3/ S.N.BUCKLEY and K.M.ENTWISTLE - Acta Metall., vol.4, july, pp.352-361 ^(1956).

/4/ J.CHICOIS - Thèse doctorat d’Etat. INSA et UER LYON, ^Feb.

(1987).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01988002304068900