Revista Latinoamericana de Metalurgia y Materiales, Vol. 8 Nos. 1 & 2 (1988) 63

Influence of Phase Transformations on the Structure and Propertiesof en S~ ZnljMn-Casting Bronce

Mieckzyslaw Tokarski and Stanislaw Krol

Institute of Maehine Building, Teehnieal University, ZSP 5. 45-233 Opole, Poland.

Se presentan los resultados de las investigaciones realizadas en relación a la influencia de las transformaciones de fases en la estruc-tura y propiedades del bronce Cu-3Si-5Zn-1Mn. Se han determinado los parámetros de sobresaturación que aseguran la presencia de lafase K durante el enfriamiento y la sobresaturación de la solución sólida a .Durante el envejecimiento tiene lugar la descomposición de lafase y sobreenfriada (K-+a + o) y la precipitación de las fases o, MIlóSisa partir de la solución sólida sobresaturada e. Se ha determi-nado que la máxima dureza y resistencia a la tracción se obtienen después del envejecimiento a 670 K durante 6 horas, y las mejores pro-piedades plásticas se obtienen después del envejecimiento a 570 K durante 6 horas.

The results of investigations concerning influence of phase transformations on structure and properties of CuSiaZI16Mnlbronze havebeen presented. The supersaturation parameters, ensuring x-phase overcooling and supersaturation ofthe solid solution a have beendetermined. During ageing decomposition of the overcooled x-phase (K -+ a + o) and precipitation of o, MI16Siaphases from the super-saturated solid solution a take place. It has been determined that the maximum hardness and tensile strength are obtained afterageing at 670 K for 6 hours and the best plastic properties after ageing at 570 K for 6 hours .

.. 1. INTRODUCTION

Casting bronzes Cu-Si-Zn-Mn have satisfaetoryantifriction properties and corrosion resistance at tem-peratures to ea.600 K. Thus are used for machine ele-ments working under changing loads both at elevatedtemperatures and in corroding media as well. However,their larger application is limited by relative low me-chanical properties, e.g. tensile strength equal to ca.300MPa [1, 2].

Mechanical properties can be, however, increasedby application of precipitation hardening [3-8]. K. Dre-yer has already found in paper [7]. that in those bronzesprecipitation hardening is a result of preeipitation ofMflnSis-phase from a supersaturated solid solution a,(Fig. 1).

.! m3r--h~~-+--~~:::J1973 t----+-t------+--+---+------lE o( -tMns Si.3

{E. 873

773~--~~---+---r--~

10 20 30 40 50

Mn5Si3' %

Fig. 1. Quasi-binary system Cu-Mll¡¡Sis[7).

The influence of manganesa and zinc, consisting inlimitation of the range of the solid solution 0:, probablymakes the precipitation processes easy (Fig. 2).

Since the zinc content in the discussed bronzes isseveral times greater than the manganese content, it canbe assumed that zinc exerts an essential influence ondescomposition of e-phase. namely on ageing stages.

Basing on the above data research has been carriedout of preeipitation hardening process as well as itsinfluence on structure and mechanical properties ofa chosen CuSigZn5Mnlcasting bronze has been deter-mined.

1373~~--~~--.---r-~

Q)L-::J..•....OL-Q)o.E

~

973

2 3 4Si,

5 6%

Fig. 2. Zinc influence on existence range of solid solution a in Cu-Sialloys [2).

64

ceeds. It is more intensive within the grains than in theformer interdendritic spaces (Fig. 6).

EXPERIMENTAL PROCEDURE

LatinAmeriean Journal o/ Metallurgy and Materials, Vol. 8 Nos. 1 & 2 (1988)

The research of commercial casting bronze hasbeen carried out. The examined bronze had the follow-ing composition: 3.1%Si, 5.5%Zn, 1.21%Mn, 1.46%Fe,0.21%Pb, 0.18%AI, 0.01%Sb, 0.02%P, balance-Cu.

The supersaturation conditions have been selectedbasing on the results of the structure and hardnessresearch of the examined bronze specimens.

Heat treatment of the ahove specimens included thefollowing operations:

- solutionizing at 1120 K, 1070 K and 1020 K for 15, 30and 45 minutes as well as for 1, 2, 3 and 6 hours,

- the following cooling into cold water at temperature280 K.

Temperature of 1070 K and annealing time of 6-8min./1 mm of thickness have been assumed as the mostfavourable conditions of supersaturation. 'I'he supersa-turated specimens were subjected to ageirrg at 520, 670and 770 K up to 12 hours. Next metallographic and X-ray examinations, researches of mechanical properties,hardness and specific electric resistance of the heat trea-ted specimens have been carried out.

RESULTS OF INVESTIGATIONS

After casting into sand moulds the CUSiaZn5Mnl-bronze has a structure consisting of a heterogeneoussolid solution a and an overcooled x-phase, situated ininterdendritíc spaces (Fig. 3).

Fig. 3. Cast structure of CuSiaZl15Mnl-bronze.

In o-phase single precipitates of x-phase occur,however in ínterdentritic space some traces of K -+ a + otransformation, in the shape of fine plates typícal foreutectoidal decomposition and chainlet, single, irregularprecipitates of FeSi can be observed. The structure ischaracterized by the following parameters: hardnessca.125 HB, tensile strength ca.240 MPa, yield strengthca.175 MPa, elongation ca.12% and reduction ca.22%.

After supersaturation the supersaturated a-phase,- the overcooled x-phase and primary undissolved precio

pitates oí FeSi can be observed (Fig. 4 and 1)).During ageing at 520 K. first of al! precipitation of

,o-phase from the supersaturated solid solution a pro-

Fig. 4. Structure of supersaturated CuSiaZll6Mn¡-bronze.

>-+-"o¡¡;eQ)-e

60 58 56 54 52 50 48 4628 angle

Fig. 5. Phase composition heat-treated CuSisZl15Mnl-bronze: a) su-persaturation, b) ageing at 520 K for 12 h, e) ageing at 670 Kfor 3 h, d) ageinz at 670 K for 6 h, e) ageing at770 K for 6 h.

From changes of mechanical and physical proper-ties during ageing oí the examined alloy it results, thatthe process is the most intensive up to 3 hours (Fig. 7).After a longer ageing, for example after 12 hours, alsoM!l¡;Sia-;:hase precipitate (Fig. 5b). But the amount ofthephase, formed during ageing at 520 K, is small and it can-not have an important influence on hardening the exami-ned alloy,

Decomposition of metastable phases during ageingat 670 K takes place at time up to 6 hours (Fig. 8). It canbe assumed that during the fírst stages oí ageing decorn-position oí the supersaturated o-phase is the main pro-cess. Such supposition results from rate of decreasing alattice parameter of the solid solution a.

Revista Latinoamericana de Metalurgia y Materiales. Vol. 8 Nos. 1 & 2 (1988) 65

\'iIL ti. Structure of supersaturated and aged at 520 K for 3 hCu Si3ZnóMn¡ bronze.

3 6 9 12

Ageing time, hFiz. 7. The rnechanicul and phvsical propert.ies of supersaturnted

and aged al 520 K Cu SÍ3ZnsMn¡ bronze.

300

~.,..--- UTS 11

~,-¡ --xx..,/" -- ..xs.- 1

f- ,,/ __

-/ l-J--.,

K -..---- RA

r- -- EL

~

'-., - --- a-

o o;;.;; ~ 150- -~...Ja::W

25

20

15

10

5

30 CD:I:Ifl-

20 ~"E On

10 ~ (1)

J19 g E

O (J1B Ul c.;17'~ ~

a::

13635 -

363) i'bE~

3525 ~ Éo;

X-ray phase analysis and results of examiningmechanicalproperties and structure allow to state. thatat first nucleation of o-phase and next precipitation ofMn5Si3 and FeSi phases can be observed (Figs. 5, 8, 9).The precipitates of o-phase from in the neighbourhood ofFeSi-particles, undissolved duríng supersaturation, andon the grain boundaries of o-phase, causing their partialfragmentation (Fig. 9).

Precipitation processes from the supersaturatedsolid solution a proceed at time up to 3 hours. After 2-3hours of ageing at temperature 670 K a distinct decorn-position ofthe overcooled phase K .•.• a + 8 can be obser-ved. A renewed, small in crease of the lattice parameter ofo-phase and a higher intensity of diffraction lines of a-and o-phases are the evidence of it (Fig. 8). The occuringtransformations cause the alloy hardening and its maxi-mum takes place after ca.G hours of ageing. Then hard-ness is equal to 150 HB, tensile strength ca.400 MPa,yield strength ca.260 MPa, at the same time the values of

4°°rl~F::::::F==prI8 E&=,<:>-,<lIoe

~(/)

~23 ()

'C-O21~

<lI2') ()

19 '§18 ~

Ul

~ ez 2:vi

Ul1-::> >-

~ ~o o -« ..Ja:: llJ

53 G 9 12

Ageing time, h

Fig. 8. The meehanical an physical properties of supersaturated andaged at 670 K Cu SisZn6Mn¡ bronze.

Vig.~. Structure of supersaturuteri and azed at 670 KCu Si3ZnóM nI bronze,

66 LatinAmerican Journai o/ Metallurgy and Mtueriole, Vol. 8 NOB. 1 & 2 (1988)

elongation and contraction of area are similar to thevalues as supersaturated. Further lengthening ageingtime causes a smalI decrease of hardness and strength ofthe alloy,

Ageing at 770 K, i.e. slightly below the eutectoidaltransformation temperatura does not cause the changesequence. but it causes their occurrence at shorter time(Fig. 2). During ageing up to 1 hour Mn5Si3- and o-phasesprecipitate from the solid solution (}'at the same time. Itcauses a visible decrease of the lattice parameter of (}'-phase and increase of mechanical and plastic propertiesof the examined bronze (Fig. 10).

RA, %

EL, %

UTS, MPaYS, MPa

:;;t.

~ ~~-+--~~~--~----~~-------*~---

Hardness, HB~~'~-+'~~~~~~~~~-7~Ui ~ ~ o ~

Parameter o , nm· 10-4

~ ~ ~ ~ ~Resistance f, fSl.cm

Fig. lU. Thc mechanicn l and ph:;sicul pl'OI'l'I'ti,,, of supersaturntedand aued ut 7701< Cu SisZn5\ln¡ 1)I'f'I1Z,'o

In case-of long times of ageing x-phase decomposeand this processis characterized by a higher intensity ofdiffraction lines of (\'- and o-phases (Fig. 5), The procesadoes not cause the alloy hardening in such a degree as incase of lower temperatures. The cause ofthat is simulta-neous increase of precipitates of Mn5Sis and FeSiphases.

On the base of changes ofhardness and specific elec-tric resistance during ageing at 770 K it can be assumed,that at this temperature grain growth and coagulatíonprecipitates are particularly intensive after 6 hours oíageing (Fig. 10).

CONCLUSIONS

On the base of carried investigations the followingconclusions can be formulated:

1. Precipitation hardening of CuSi3Zn5Mn¡-bronzé ispossible as a result of precipitation o-phase from thesupersaturuted solid solution «, decomposition ofthe overcooled o-phase and precipitation of MnsSi3-

phase.

20 Influence of particular nrocesses on the alloy harde-ning depends on temperature and ageing time.

3, At lower temperatures the precipitation procesa ofo-phase with a low intensitv and long course time isdominatinz. As a result hurdening effect is small butplastic properties are the best,

4. Hizher temperatures cause increasinz an amount ofprecipitated o-phase and they are favourable tonucleation ofthe Mn5Si3-phase. It causes increasinzthe alloy hardening after short time of ageing.

5, The máximum hardness and tensile strengthobtained after ageing at 670 K for 6 hours,

HEFERENn;s

are

1. Sorikowski K, ,,1 1110: Atlas of structuros uf stnndun! cnst.inua!lo,os (in Polish). WNT, Wa!"sal\' !(j,:¿o

2, Adarnsk i C. el '11,: Microstructures of copper-und zine-castingo

allovs (in Polish). Slusk: Kutowice 1872,

:) Hansen:\L Anderko K,: Constitution 01'Hinarv Alloys. Md;l'a\\OHill. Nel\' YOl'k isss.

~o Non,' Moo l ireyer K Metal!. Bo 1Bfí5, 8n-W'iH,

"o Die, K.. Kupferund Kupferlegiet'unrren in del'Teehnil" Spl'inol'l'l'Verluu. HerIin 1!)(i7,

(;0 \laILsev:\1.: Metullournphv of Inrlusuiul Non-Fvrrous \1l'lal"anrl Allovs (in Russian). Metallurahizdut. Mosco«: HJ7110

7, ])!"l'~¡e!"K,: Metal!. 70 1'l5:~0 lÍ'i(i-1H(Jo

Xo Kolachev v 1'1 filo: Metallugl'aph~o .mrl Heut Trentment of Non-F01'l'OUS Metals and Alloys (in Russiun r. !\1dallul'ghiao Mosco\\'1!i72,