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STRUCTURE AND PROPERTIES OF BERYLLIUM

BRONZE MICROALLOYED WITH MAGNESIUM

Kh. G. Tkhagapsoev, A. G. Rakhshtadt, Zh. P. Pastukhova, and A. G. Karpov

UDC 620.17:620.18:669.725

Spring beryll ium bronzes B2 and BNTI.9, used for elastic sensing elements, are often inadequate for present-day instruments. It is necessary to improve their strength character is t ics - the elastic limit, r e - laxation resistance, and cyclic strength.

Attempts to improve the strength character is t ics of beryll ium bronzes by increasing the amount of beryll ium (over 2.1%) have not succeeded [1, 2]. Apparently, with increasing solubility of beryll ium in copper the amount of fl-phase in the structure of the bronze increases rapidly, reaching 20 vol.% at 2.4% Be. At the same time, the volume per cent of regions undergoing two-phase (discontinuous) decomposition during aging increases (to 18% at 2.2% Be [2]). These phenomena reduce the strength.

The addition of 0.2-0.3% Ti inhibits grain boundary decomposition in beryll ium bronze during aging [3, 4] due to the fact that this element is horophilic [1]. It has also been assumed that in the beryll ium

Fig. 1. Structure of bronze BNT1.9 in Kc~ Ti radiation (a) and electron beam radiation (b). Intensity of Ko~ Ti (c) and Ko~ Nt radiation (d) in this section.

Bauman Moscow Higher Technical School. Translated from Metallovedenie i Termicheskaya Obra- botka Metallov, No. 2, pp. 19-24, February, 1970.

01970 Consultants Bureau, a division of Plenum Publishing Corporation, 227 ~'est 17th Street~ New York, IV. Y. 10011. All rights reserved. This article cannot be reproduced for any purpose whatsoever without permissior~ of the publisher. A cop), of this article is available from the publisher for $i5.00.

106

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3 o, o8

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o, oz: 720

Fig. 2.

i

/ i J

740 760 78# 800 ~ Quenching temperature

Fig. 2

.1'5,6%T;

~)

1 #,o% Ti

b)

Fig. 3

Variation of average grain size with quenching temperature (holding time 15 min). i) B2; 2) B2 + 0.1% Mg; 3) BNTI.9; 4) BNTI.9 + 0.1% Mg.

Fig. 3. Distribution of intensity of K s Ti radiation in BNT1.9. a) With- out Mg; b) with 0.1% Mg.

TAB LE 1

2=~2-

Composition, % Bronze

B2

Be

1,90 1,87 1,87 1,85 1,93 1,90 1,90

NI

0,26 0,28 0,30 0,26 0,35 0,35 0,32

TI Mg

m

0,05 0,10 0,15

0,02 0,05 0,10

7 1,97 0,20 0,17 I 0,05 -- 8 BNT1.9 1,97 0,20 10,15 0,10 -- 9 1,95 0,20 0,15 0,15 --

13 1,96 0,20 0,16 -- -- Note.: All alloys were copper-based. Heats 0 and 13 were of standard eompostion.

bronze BNT1.9 microal loyed with titanium the beryl l ium content dec reases somewhat (to 1.85-2.1%) with retention of the strength cha rac te r i s t i c s of B2 bronze. This is explained by the t i tanium- containing strengthening phase forming at the same aging temperature as the "/-phase (CuBe) [4]. The resul ts f rom qualitative mi- e ros t ruc tu ra l analysis of BNT1.9 bronze after homogenizing at 780~ for 10 h and quenching, obtained in the present work, in- dicate (Fig. 1) that in the presence of titanium a phase enriched in titanium and nickel is formed in the grain boundaries . It can be assumed that adsorption of titanium in the grain boundaries is the decisive factor in the formation of this phase. The c rys ta l s are large inclusions (Fig. 1) and therefore do not affect strengthening during aging. Also, with the format ion of a separate c o p p e r - t i - t an ium-n icke l phase (possibly containing beryl l ium as well) the a - so l id solution is impoverished in nickel and titanium, and therefore the proper t ies of BNT1.9 bronze which is more highly alloyed

are almost the same as those of B2 bronze . It was of in teres t to investigate the effect on the s t ructure and proper t i es of added elements more surface active than beryl l ium, titanium, and unavoidable impuri t ies in bery l l ium bronzes B2 and BNTI.9 [5, 6].

It was shown in [7] that the surface activity (horophilic tendency) of an element inc reases with de- c reas ing values of the general ized stat is t ical moment m c of the atom. Copper and alloying elements in bronze have the following values of the general ized stat is t ical moment, m e.10 -8 e l / cm:

C u - 0.769; N i - 0.740; M g - 0.282; B e - 0.533; T i - 0.460; P - 0.860.

Of the possible e lements for alloying beryl l ium bronze , magnesium is the most effective. Its s ta t is t i - cal moment is the lowest, indicating its high horophilic tendency.

Beryl l ium bronzes B2 and BNT1.9 of standard composit ion and with additions of Mg (0.05-0.15%) and P (0.02-0.1%) were melted (Table 1). The magnesium was added in the fo rm of a c o p p e r - m a g n e s i u m alloy (60.6% Mg). The ingots of the bronzes were homogenized, hot rolled, and then cold rolled to s t r ip 0.3 mm thick under commerc ia l conditions. The distribution of titanium, nickel, and magnesium was determined by mic ros t ruc tu ra l analysis . The kinetics of aging was determined f rom the change in the hardness and elast ic l imit measured by the longitudinal bending method [8] and the e lect r ical res is t iv i ty . The relaxation r e s i s - tance under prolonged static and cycl ic loads at room and elevated tempera ture (100~ was determined by the method given in [6]. The mic ros t ruc tu re was examined with the MIM-8 optical microscope and with an e lec t ron microscope , using carbon replicas.*

* These investigations were made by V. G. Kholodova.

107

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~ ~ N g g

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�9 o . ~

v ~ d < "~ �9 +

�9 +

We determined the effect of quenching t e m p e r a t u r e s from 720-820~ and also the aging conditions at 280-360~ (0.5-10 h) on the s t rength c h a r - a c t e r i s t i c s of b ronzes al loyed with magnes ium. Quenching f r o m 770 • 10~ resu l ted in the bes t combinat ion of s t rength and technological and working p rope r t i e s of the new composi t ions of be ry l l i um bronze a f te r aging.

The addition of magnes ium to b ronzes B2 and BNT1.9 substant ia l ly reduces the average g ra in s ize of the a - s o l i d solution and reduces the tendency to gra in growth during heating (Fig. 2). Magnesium i n c r e a s e s the d i spe r s i ty of C u - T i - N i inclusions, which is indicated by the change in the intensi ty dis tr ibut ion of Kc~-radiation of t i tanium and nickel in the be ry l l i um b ronzes mic roa l loyed with magnes ium (Fig. 3). The sharp but wide peaks of Kc~-radiation of Ti in b ronze BNTI .9 and of nickel in b ronze B2 without magnes ium a r e c h a r a c t e r i s t i c for l a rge inc lus ions of phases enr iched in these e l ement s , After the addition of 0.05-0.10% Mg these peaks a re more numerous but l ess intense, since the d i spers i ty of the pa r t i c les of C u - T i - N i phase i n c r e a s e s . The refining of the excess phase inclusions a f te r microal loying with magnes ium can a lso be observed in ord inary meta l lographic examinat ion of the s t ruc tu re .

The addition of su r f ace -ac t i ve magnes ium has the g r ea t e s t effect on the kinet ics and mechan i sm of d i spers ion hardening and also on the s t r u c - ture of the b ronzes a f te r aging. The re la t ionship between the e las t ic l imi t and aging t empe ra tu r e , shown in Fig. 4, indicates that with aging for 2 h the m a x i m u m strengthening of b ronze with magnes ium is at tained at a h igher t empera tu re , and for aging at the s ame t e m p e r a t u r e is at tained af ter a longer t ime, than for b ronze without magnes ium (Fig. 5). The op- t imal conditions for b ronzes B2 and BNT1.9 al loyed with magnes ium are aging at 320~ for 6 h, while for the s tandard b ronzes they are 2-4 h at the same t e m p e r a t u r e .

The re ta rd ing of the aging p r o c e s s in al loys with magnes ium is ac - companied by a substant ia l inc rease of s t rength . The inc rease in s t rength is highest with increas ing concentra t ions of Mg up to ~0.1%; no fu r the r improvemen t in the p r o p e r t i e s is noted at higher Mg concent ra t ions . The addition of 0.1% ~ g has the g r ea t e s t effect on the e las t ic l imi t (r = 73- 78 k g / m m 2 as compa red with 58-63 k g / m m 2 without Mg) and the re laxa t ion r e s i s t ance in stat ic and cyclic loading at e levated t e m p e r a t u r e s (Table 2) as well as in c o r r o s i v e media . The cyclic s t rength i n c r e a s e s 8-12%, while the ul t imate s t rength and ha rdness i nc rea se l i t t le.

When magnes ium is added to B2 bronze the discontinuous gra in boundary decomposi t ion that is cha r ac t e r i s t i c for aging of these al loys a f te r quenching is supp re s sed (Fig. 6). After 10 h at 320~ no signs of local gra in boundary decomposi t ion are obse rved in the B2 alloy with magnes ium, while without magnes ium there a re dist inct ly vis ible colonies of p rec ip i t a t e s . After aging at 340~ for 2.5 h one obse rves colonies with a pear l i t i c s t ruc tu re in the gra in boundary a r e a s of the c~-solid solution in B2 bronze ; they a re evidently a two-phase mixture of ~ - s o l i d solution of equi l ibr ium concentra t ion and "Y-phase; the addition of magnes ium prevents thei r fo rmat ion .

The c h a r a c t e r of the observed changes in the kinet ics of aging, the s t ruc tu re , and the p rope r t i e s of be ry l l i um b r o n z e s can be explained by the effect of the smal l magnes ium additions. The reduct ion of the average gra in s ize of the c~-solid solution and of the inclusions of other phases r e - sults f r o m the adsorpt ion of horophil ic magnes ium on the in te r faces and in the gra in boundar ies , which reduces the f r ee sur face energy .

108

O0.002, kg/mm 2

f"' i , ' 4.,4 0o'o02' dO[ --~-! ~ -~ t-4v

&_Z I i I i k~/mm 2 8,~ -I . . . . . T F - T q

' I < i i , 0 ~ " ~ . . . . . . . . . . . . . . !-- j :0 h I ' : ' ] , 0 80 120 /dO 24"0 300 ,380 4-Z0 rain

Z~O 300 3ZO d40 ~ k ging time Aging temperature b)

Fig. 4 Fig. 5 Fig. 4. Var ia t ion of e las t ic l imi t with aging t e m p e r a t u r e (2 h) for b ronze B2. 1) Without Mg; 2) with 0.1% Mg.

Fig. 5. Var ia t ion of the e las t ic l imi t with aging t ime (320~ fo r B2 and BNT1.9 b ronzes , a) B 2 ; b ) BNT1.9. 1) No addi- t ions; 2) with 0.05% Mg; 3) 0.10% Mg; 4) 0.05% P.

Thus, the s tabi l i ty of the d i spe r sed condition of the s t ruc tu re is i nc reased . The compet ing adsorpt ion of magnes ium preven ts en r i chment of the gra in boundar ies and other in ternal su r faces in be ry l l i um, and the re fo re the p r e s ence of m a g n e s i u m inhibits the nucleation of p rec ip i t a t e s (the occu r r ence of local decomposi t ion cen te r s on these su r faces ) and s u p p r e s s e s gra in boundary decomposi t ion in B2 bronze . A s i m i l a r ef fec t of s u r f a c e - a c t i v e e l emen t s on the s t ruc tu re of d i spers ion hardening al loys was obse rved in [9]. The supp re s s ion of g ra in boundary decomposi t ion in be ry l l i um bronze BNT1.9 with the addition of t i tanium, which is su r face act ive in re la t ion to be ry l l i um, also conf i rms this. On the addition of su r f ace - inac t ive phosphorus (m c = 0.860) to B2 bronze one obse rves intensive gra in boundary decomposi t ion (see Fig. 6) and acce l e r a t i on of the aging p r o c e s s (see Fig. 5a).

The i nc rea se in the d i spe r s i ty of B2 b ronze mic roa l loyed with magnes ium, which prevents gra in boundary decomposi t ion, leads to m o r e even decomposi t ion of the solid solution during aging. This s t r u c - tura l condition is respons ib le for the higher re laxat ion r e s i s t ance and e las t ic l imi t of the b ronze . However , the subs tant ia l i m p r o v e m e n t of the s t rength c h a r a c t e r i s t i c s of bronze BNT1.9 when alloyed with magnes ium makes it poss ib le to a s s um e that the degree of sur face act ivi ty of the alloying e lement de t e rmines the s ize of the p rec ip i t a t e s , in which case coheren t bonding with the ma t r ix is st i l l poss ib le .

The i nc rea se of the r e s i s t a n c e to mic rop la s t i c deformat ion at e r e m = 0.001-0.01% resu l t ing f r o m aging is inve r se ly propor t iona l to the size of the pa r t i c les of s t rengthening phase [10]. The inc reased d i s - pe r s i t y of the pa r t i c l e s of phase precip i ta t ing during aging of b ronzes B2 and BNT1.9 alloyed with magnes ium is p robably induced by the adsorpt ion of magnes ium a toms .

To de te rmine the change in a tomic mechan i sm of d i spe r s ion hardening p r o c e s s e s in B2 bronze , l ead- ing to the changes mentioned in the kinet ics and s t ruc tu ra l condition as the r e su l t of microa l loy ing with m a g - nes ium and phosphorus , we de te rmined the binding ene rgy of vacancies (Ev) with a toms of be ry l l ium, nickel , magnes ium, and phosphorus by the method desc r ibed in [11] a f te r quenching f r o m 720-820~ and aging at 100~

The binding energy of beryllium atoms with vacancies in B2 bronze is ~0.2 eV, which matches the re-

sults obtained for the binary alloy Cu + 2% Be [ii]. This makes it possible to assume that the binding en-

ergy of nickel atoms with vacancies is close to zero in the alloys investigated. For phosphorus, E v = 0.3 * 0.05 eV, while for magnesium E v = 0.45 • 0.05 eV.

Apparently because of the high binding energy of magnesium and phosphorus atoms with vacancies,

in their presence the beryllium bronze will be more supersaturated with vacancies than the bronze of standard composition.

109

Fig. 6. St ructure of B2 bronze a f t e r aging at 340~ for 150 min . a, b) Without Mg; e, d) with 0.1% Mg; e) with 0.05%P; a, c, e) • a., d) •

i

Calculation of the act ivat ion energy of aging of be ry l l i um bronze at 280-360~ f r o m the kinet ics of the var ia t ion in p rope r t i e s (hardness and e lec t r i ca l res i s t iv i ty) showed that for B2 bronze Q = 24-26 k c a l / m o l e . Phosphorus (0.1%) inc rea se s this value to 32-35 k c a l / m o l e , which is c lose to the diffusion act ivat ion energy of phosphorus in copper .

It can be a s sumed that phosphorus a toms combine with vacancies in be ry l l i um bronze and reduce the mobil i ty of the la t t e r . However , in b ronze alloyed with phosphorus , adsorpt ion enr ichment of the grain boundar ies with su r f ace -ac t ive be ry l l i um a toms occurs , which p romotes nucleation of p rec ip i t a tes in the boundar ies . Thus, in this b ronze there a re c rea ted conditions in which decomposi t ion of the supe r sa tu ra t ed a - s o l i d solution will occur mainly according to the type of discontinuous p rec ip i t a tes in the gra in bound- a r i e s and will usual ly develop at an acce l e r a t ed ra te [12, 13].

The addition of magnes ium, due to adsorpt ion of i ts a toms , apparent ly can weaken the ef fec t iveness of dis locat ion loops and the boundar ies of gra ins and subgra ins as vacancy sinks because of the reduction of e las t ic s t r e s s e s . This s u p p r e s s e s the initiating influence of these lat t ice defects on the nucleation of p r e - c ip i ta tes . Also, as the r e su l t of the e levated binding energy of magnes ium a toms with vacancies the solid solution becomes supe r sa tu ra t ed with vacancies , the p r e sence of which favors decomposi t ion during aging by the mechan i sm of overal l continuous precipi ta t ion, which usually occurs m o r e slowly than discontinuous precip i ta t ion .

It should be noted that alloying with magnes ium has such a s t rong effect on the s t ruc tu re of the al loys that they should be regarded as new mas with a be t t e r combinat ion of p rope r t i e s than b ronzes of s tandard composi t ion.

Fo r complete determinat ion of the p rope r t i e s of the new b ronzes we conducted c o m m e r c i a l t r i a l s . F r o m rol led s t r ip of BNT1.9 b ronze with 0.1% 1Vig quenched f r o m 770 • 10~ we p r epa red e las t ic sensing e l ement s . The e lements were aged at 320~ for 6 h. Tes t s of these e las t ic e l ements showed that with an

110

increase of elastic hysteresis the fatigue strength and relaxation resistance substantially exceed those of standard-composition beryllium bronzes.

In the process of commercial testing it was found that they are not inferior to the standard alloys in extrudability or weldability.

In conformity with the data obtained in the investigation and the commercial trials, these new bronze compositions have been accepted in TU specifications.

C O N C L U S I O N S

1. The addition of a surface-active element (magnesium) increases the dispersity and uniformity of the structure, reducing the average grain size of the s-sol id solution and the inclusions of excess phases in quenched beryllium bronzes.

2. Microalloying of beryllium bronzes B2 and BNT1.9 with magnesium substantially improves their strength characteristics (elastic limit, relaxation resistance, cyclic strength) as the result of suppression of the discontinuous decomposition mechanism and the uniform strengthening of both the bulk and grain boundary areas. The best strength characteristics of beryllium bronzes were attained with 0.1% Mg.

3~ The newly developed compositions, alloyed with Mg, are designated BNT1.9Mg and B2Mg. The op- timal heat treatment for these alloys is quenching from 770~ and aging at 320~ for 6 h.

4. Commercial trials of the new beryllium bronzes showed that elastic elements of these bronzes have better combinations of basic properties than those of the standard compositions.

LITERATURE CITED

1. A . G . Rakhshtadt, Spring Alloys [in Russian], Metallurgiya, Moscow (1965). 2. Hiroshi Itsu, Takashi Agatsuma, and Kimo Hashidzumi, Mitsubishi Denki Gaio, 4_! , No. 6 (1967). 3. A . I . Chipizhenko, Byull. TsIIN MTsM SSSR, 7 (84) (i957). 4. A. I . Chipizhenko, in: Promising Development-s in Elastic Sensing Elements [in Russian], TsIIN Elek-

trotekhnicheskoi Promyshlennosti i Priborostroeniya (1961). 5. A.G. Rakhshtadt and A. M Grishin, Stal', No. 9 (1969). 6. G.S. Ionychev, Zh. P. Pastukhova, A. G. Rakhshtadt, and N. F. Komissarova, Izv. Vuzov., Mashino-

stroenie, No. 1 (1968). 7. S.N. Zadumkin, Zh. Neorgan. Khim., No. 8 (1960). 8. A.G. Rakhshtadt and M. A. Shtremel', Zavod. Lab., No. 6 (1960). 9. V. I . Arkharov, Trudy Inst. Fiz. Metal., UFAN, No. 19 (1958).

10. W. Bonfeld, Trans. Met. Soc. AIME, 239, No. 1 (1967). 11. P. Wilkes, Aeta Met., 16 (1968). 12. G. Newkirk, Aging of Alloys [Russian translation], Metallurgizdat, Moscow (1962). 13. D. Turnbull, Aeta Met., 3 (1955).

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