DIFFUSIONAL PROPERTIES OF INTERPHASE BOUNDARIES IN TWO-PHASE FERRITIC-AUSTENITIC STEEL

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DIFFUSIONAL PROPERTIES OF INTERPHASE BOUNDARIES IN TWO-PHASE

FERRITIC-AUSTENITIC STEEL

W. Światnicki, J. Świderski, M. Grabski

To cite this version:

W. Światnicki, J. Świderski, M. Grabski. DIFFUSIONAL PROPERTIES OF INTERPHASE

BOUNDARIES IN TWO-PHASE FERRITIC-AUSTENITIC STEEL. Journal de Physique Colloques,

1990, 51 (C1), pp.C1-647-C1-651. �10.1051/jphyscol:19901102�. �jpa-00230009�

DIFFUSIONAL PROPERTIES OF INTERPHASE BOUNDARIES IN TWO-PHASE FERRITIC-AUSTENITIC STEEL

W.A. ~WIATNICKI, J. SWIDERSKI and M.W. GRABSKI

/

Institute of Materials Science and Engineering, Warsaw University of Technology, Narbutta

85, 02-524

Warsaw, Poland

Aesumt!

-

Nous avons etudit! en fonction de la temperature la stabilite des dislocations piegOs aux interfaces a/y pour determiner leurs proprietes diffusives dans un acier austeno-ferr itique. Naus avons trouv8, que les interfaces de f aible dif fusivith sont charact& isees par une structure ordonnee e t sem icoherente suivant les relations de Kurdjumov-Sachs ou Nishiyama-Wassermann. Le pourcentage de ces interfaces "spt!ciales"

var ie fortement avec le t r a itement thermo-mkcanique applique qui modif ie le processus de nucleation et croissance de la phase y dans la matrice a e t donc influence la formation des joints interphasks.

Abstract

-

Thermal stability o f trapped lattice dislocations in f e r r i t i c austenitic steel have been studied in order t o determine the diffusional properties o f internal interfaces.

It was found that interphase boundaries with low diffusivity are characterised by the ordered semicoherent structure, formed in Kurdjumov-Sachs or Nishiyama-Wasserrnann orientation relationship. The fraction o f these "special" interfaces varies considerably with the thermomechanical treatment employed as it acts on the processes o f nucleat ion and growth o f y phase in a matrix end by this manner influences the formation o f the

interphase boundaries.

I

-

INTRODUCTION

A number of processes occurring in materials, such as recrystall izat ion, grain growth, nucleation and growth o f new phases, plastic deformation, fracture, corrosion and the iike, depends on properties o f internal interfaces determined by their crystallographic structure and chemical composition C 1-31. It is possible therefore, by changing properties of boundaries, t o act on processes taking place in materials and in consequence t o influence formation o f microstructure and macroscopic properties o f polycrystals. This thesis has been confirmed in the course of investigation of grain boundaries' effect on properties and behaviour o f metals and single phase alloys 14-01.

In our recent paper t91, where a research on ferritic-austenitic steel was reported, it was found, that d i f fusional properties distribution of a/y interfaces differed significantly from properties distribution o f a/a and y/y grain boundaries in that a considerable fraction of a/y boundaries exhibited a very low diffusivity. They are interfaces between a and y grains exhibiting Kurdjumov-Sachs (K-S) or Nishiyama-Wassermann (WW) orientation relationship (OR). It has also been shown, that the structure of interphase boundaries depends on nucleation site o f y phase within f e r r i t e microstructure (in grains or on grain boundaries). It can be expected that by changing the a microstructure i.e. the kind o r density o f defects one can act on structure and properties o f

interphase boundaries formed during y precipitation.

The aim of the present paper is an attempt t o recognise the possibilities of controlling dif fus ional properties o f interphase boundaries. This is made by investigating changes in the share of boundaries with special structure and properties involved by thermomechanical treatments. An ability t o diffusional stress relaxat ion of lattice dislocations trapped in interphase boundaries was adopted as the ir diffusivity measure.

2

-

MATERIAL AND THERMAL TREATMENT

Tests were performed on ferritic-austenitic steel of composition: 0.007% C, 25.5% Cr, 6.6% Ni, 0.27% Mn, 0.22% Si and Fe balance. It was annealed a t 1350'C f o r 30 min. in argon and quenched in water. As a result, a coarse-grained ferritic microstructure was obtained with mean grain size o f about 1 mm, containing small precipitates of y phase.

Next, two variants o f thermal treatment were applied:

(A) - Annealing a t 980°C f o r 1 h, a i r cooling. The treatment resulted in a microstructure containing 50.7+6.0% o f phase y in the. form o f elongated precipitates 7.1t0.9 pm long and 1.020.2 pm wide, laying along definite crystallographic direct ions in large f e r r i t e grains (fig. 1 ).

(8)

-

A ca\d deformation o f 67% and annealing a t 9BO'C f o r 1 h, what enabled obtaining a fully recrystallized, microduplex microstructure, containing 49.626.1 % o f phase y in the form o f

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901102

Cl-648 COLLOQUE DE PHYSIQUE

equiaxial grains o f a mean size o f 2.060.3 pm. The shape and size o f y grains are similar t o those o f grains and subgrains o f a phase (fig. 2 ) .

The plastic deformation in B state was applied in order t o influence a y phase nucleatian and growth, which are modified h this case by dislocations, as well as by f e r r i t e grain boundaries and cell walls formed during recovery and recrystallitation. It can be expected that

,

as a consequence, the interphase boundary structure is changed, as compared with the A state.

Fig. l . Microstructure of the steel in the state A.

Fig.2. Microstructure of the steel in the state

B.

Invest lgat ing o f crystallographic structure o f interphase boundaries through determining mutual phase disorientation were carried out with trsnsmiss ion electron microscope (TEM).

A procedure proposed by Kozubowski based on diffraction pattern and Kikuchi line analysis was applied tJ.Kozubowski, internal report

IIM

Pbd 19883. A smallest deviation angles o f a/y disorientation f rm 'the two typical Kurdjumov-%chs (K-S) and Nishiyama-Wasserrnann (N-W) orientation relationships (OR) were determined. An electron microscope method f o r investigating thermal stability o f trapped lattice dislocations (TLDs) was used f o r characterize the dif fusional properties o f interfaces 110,111. The method consist in studying the TLDs stress relaxation as a effect o f temperature, which manifests itself by T U k disappearance (spreading) as observed in TEM. Samples from both A and B states were initially strained

up

t o E = 1% in order t o introduce dislocations into boundaries and t o form TLDs (Fig. 3).

Two variants o f TLDs thermal stability investigation were applied:

Ca3 in s i t u annealing (up t o

600-C)

o f thin foils directly in TEM,

Cbl annealing o f bulk material prior t o making thin foils, what permits t o use higher temperatures, up t o 700'C.

In both' cases the samples were isothermally annealed f o r 80 s and then the number o f interfaces on which TLDs disappeared was counted. It enabled the determination o f temperature-dependent distribution o f TLDs stability.

F ig.3. In terphase boundary containing TLDs a f t e r 1 X plastic deformation.

Fig.4. Interface a/y disoriented by 4.7O from K-S OR in B , s t a t e a f t e r annealing a t 550'C. TLDs has disappeared.

Microscopic observations of thin foils following annealing showed, that lattice dislocations trapped t o interphase boundaries become unstable a t elevated temperatures; this fact manifests itself by disappearance of their contrast in TEM (Fig. 4). It means, that interphase boundaries behave similarly as grain boundaries in single-phase metals and alloys tested so far.

Investigation results obtained during in situ heating tests are presented in fig. 5

I

^{I I I I I I I l I}

I

40 0 450 500 550 600

T E M P E R A T U R E ~ C ]

The diagram shows the existence o f a considerable difference in TL-Ds stability between both states of the material. TLOs annihilation in state A begins a t 550°C, in state B, on the other hand i t takes place a t 425'C. Up t o 600'C the TU3s disappeared in about 25% and 65% o f boundaries in states A and 8 respectively. The difference in TLDs stability reflects the difference in diffusional properties o f interfaces, as the time t, and temperature T, o f TLDs disappearance are related t o diffusivity of boundaries D, by equation t 1 1 7:

t, = K T,/D, where K is a constant dependent on the model's assumptions.

Analysis of the results obtained on the basis of the above equation shows, that the diffusivity o f interphase boundaries in state A is considerably lower than that in state E. It can be expected t o be a result o f differences between structural characteristics o f boundaries occurring in both states of material. The characteristics has been carried out by determining deviation angles o f experimental disorientat ions from two theoretical K-S and N - W or lentat ion relationships between a and y phases. The angles determined f o r each state of material are presented in table 1.

The results obtained show the existence o f a variation of 8 angles with no typical K-S or N-W orientation relationship dominating. As an experimental error o f disorientat ion determinat ion does not exceed 1 ^O, the distributions obtained result from real deviations o f experimental d isor ientat ions from the theoretical ones.

It was found that, in state A, the mean deviation angle B, is low, of about 3.7

-

^3.8O.

Almost 90% o f boundaries are characterised by 8 deviation angles smaller than So

.

These special disorientations o f a/y grains facilitate the growth y o f precipitates along most dense planes of a (1 1 l )y/ ( l 1 O)a type, with high atomic order and coherency in the boundary plane (Fig.6). Such boundaries are characterised by IOW energy and special properties which explains great stability of TLDs in this state.

In B state, on the other hand, which is characterised by low stability o f TLDs on interphase boundaries there occurs large fraction (about 50%) o f random a/y disor ientat ions, deviating largely (with 0>50 ) from exact N-W and ,K-S orientation relationships. The average deviation angle 0, is found t o be almost twice as high than in A state. Dispersion of B angles in B state can be explained as follows: during annealing, a recovery and recrystallization o f deformed matrix take place with resulting formation o f a grain boundaries and low-angle boundaries which limit the growth o f y precipitates. The growing y grain, which reaches an a/a boundary, looses i t s initial disorientation and aquires a new one in relation t o the encountered f e r r i t e grain. This effect explains also a different, as related t o A state, morphology of austenite precipitates, with their shape being similar to that o f f e r r i t e grains and subgrains

The study of TLDs stability in interphase boundaries with determined disorientation confirms the deoendence o f boundary oroperties on the B deviation angle. Boundaries on which TLDs are stable (up t o 600'C) display very low deviation angles (0<2@). The greater the deviation the lower the TLDs spreading temperature and the higher boundary diffusivity. It results from the fact that as the deviation angle increase the interface atomic order within the boundary decreases C31.

100 80 60 40

- -

^{Fig.5. Changes in the} percentage o f

interphase boundaries containing TLDs, as a

-

^function of temperature f o r the constant

time o f annealing ( t = 60 S).

-

-

Cl-650 COLLOQUE DE PHYSIQUE

TabLe 1. Tabulation o f deviation angles D o f experimental disorientat ions a / y from two theoretical K-S and N-W orientat ion relationships

S t a t e A S t a t e B

No

o f d e v i a t i o n d e v i a t i o n remarks concerning d e v i a t i o n d e v i a t i o n remarks concerning i n t e r - f r o m from TLDs s t a b i l i t y f r o m f r o m T L D s s t a b i l i t y

face N-W K-S N-W

K-

S

Q Cdegl Q I degl Q C degl Q I degl

1 4. 2 l. 4 s t a b l e up t o 625'C 5 . 0 0. 8 s t a b l e up t o 600'C

2 5. 1 1 . 4 1.3 4. 1 T,

<

575°C

3 2. 0 5. 8 t . 4 4. 7 s t a b l e up t o 600'C

4 3. 1 2. 5 T,

<

625'C 6. 0 1 . 8 s t a b l e up t o 600°C

5 3. 2 4. 3 2. l 4. 7 T,

<

550°C

6 3. 6 3. 7 3 . 0 5. 5

7 6. 0 3. 8 6. 4 3. 7

8 3. 8. 5. 1 4. 7 5. 9 T,

<

550°C

9 4. 1 4. 0 5. 1 4. 8

10 4 . 5 6. 1 5 . 0 6. 2

11 4. 8 5. 1 T,

<

625°C 7. 7 5. 5

12 5. 7 4. 9 7. 6 7. 3

13 14. 5 8. 9 T,

<

^{825°C 7. 7 8. 6}

14 10. 6 1 0 . 2

15 16. 2 13. 4

16 15. 1 14. 4

17 28. 2 29. l T,

<

525°C

mean :

0, 3.7 3. 6 6. 7 6. 9

The minimalisation o f the deviation angle is thus a necessary condition f o r the boundary t o exhibit low d i f f u s i v i t y and high s t a b i l i t y o f TLDs. Such condition, however is not a sufficient one as in the case o f boundary No 2 in B s t a t e (table l ) , f o r which deviation is small (Q=1.3') the TLDs disappeared already a t relatively low temperature (Td=575'C). It means t h a t there are some other f a c t o r s which control interphase boundary d i f f u s i v i t y and TLDs stability. Some TEM observations seem t o indicate, f o r example, t h a t TLDs s t a b i l i t y can be strongly influenced by the orientation o f boundary plane (fig.7), as well as the magnitude and orientation o f TLDs Burgers vector (fig.8.9). However the problem needs a more oetailed

investigations.

Fig.6. lnterfaces a/y in A s t a t e a f t e r 1 % plastic deformation and annealing a t 650'C f o r 60s. TLDs are stable. The plane o f interface

is : (1 1 l )y / / ( l 1 0 ) ~ .

Fig.7. Interface a / y in B state a f t e r annealing a t 550'C f o r 60s. TLDs has disappeared in p a r t (a) o f interface f o r which a/y disorientat ion is deviated by 2.1' from N-W OR and are stable in the other p a r t (b) f o r which the deviation is 1.4' and the boundary plane is different.

1% plastic 'deformation. The a/y disorientation is deviated by 1.3 from K-S OR.

set o f T U k has disappeared while the second has remain stable. It has disappeared only €fi ter annealing a t 575°C f o r 60s.

The TLOs thermal stability, being a measure o f diffusional properties of a/y interphase bounder ies in duplex steel depends on their crystallographic structure determined by deviatiun q k o f a/y disorientation from exact theoretical Nishiyama-Wassermann and Kurdjumov-Sachs orientation relationships.

The structure o f interphase boundaries being formed depends on thermomechanical treatment by affecting the microstructure o f f e r r i t e in which a process o f austenite nucleation and growth take place. It follows that it is possible t o change the structure and properties of interphase boundaries and influence in this way behaviour and macroscopic properties o f two- and multi-phase alloys.

ACKNOWLEDGEMENTS: This work was supported by the State Office o f Science, Technological Development and lmplementa t ion (Poland) under contract CFBR 2.4

REFEREKES

1. GRABSKI, M.W.: J. Fhysique 48 (1 985) C4-S67 2. WATANABE, T.: Res Mechanica l 1 (1984) 47

3. KOSEWITCH, V.M., YEVLIEV, W.M., PALATNIK, L.S., FEDDFEWO, A.J.: Strurture o f htercrystalline and fnterphese Boundaries (in Russian), 1980, Moscov, Metallurgia

4. GRABSKI, M.W.: J. Fhysique 49 (1 988) CS-497

S. WYRZYKOWSKI, J., GRABSKI, M.W.: Philos. Mag A 53 (1986) 505

6. SWI@TNICKI, W.A.. GRABSKI, M.W.: Metter. Sci. E~KJ 100 (1 988) 85 and 93 7.

SWI~TNICKI,

W.A., GRABSKI, M.W.: Acta. Metall. 37 (1 989) 1307

8. LARTIGUE, S. PRIESTER, L.: Acta Metall. 31 (1983) 1809

9. SWI~TNICKI, W.A., JEZIERSKA, E.,

GRABSKI,

M.W., accepted f o r publication in Materripk Science and Terhnology

1 l. SWI~TNICKI, W.A., LOJKOWSKI, W., GRABSKI, M.W.: Acta. Metall. 34 (1986) 599