Transcript
Page 1: Structure and properties of beryllium bronze microalloyed with phosphorus

STRUCTURE AND PROPERTIES OF BERYLLIUM

BRONZE MICROALLOYED WITH PHOSPHORUS

Kh. G. Tkhagapsoev, A. G. Rakhshtadt, and B. M. Zhilov

UDC 620.18:669.725.5:669.779

Beryl l ium bronzes, widely used as spring mater ia ls , frequently fail to meet the needs of modern in- s trument construct ion due to their e lec t r ica l res is t iv i ty (0.065-0.070 f2-mm2/m). The high res is t iv i ty of these alloys is due to elast ic deformation of the lattice result ing f rom the precipitat ion of d ispersed par t i - c les of 2/" and T' phases in the p roces s of aging [1]. The c~ solid solution also retains a high concentrat ion of beryl l ium after aging of the alloy, which also favors high resis t ivi ty .

The decomposi t ion of the solid solution during aging of beryl l ium bronzes B2 and B2.5 at 300-350°C for 2-4 h occurs by continuous and discontinuous mechanisms [2]. Discontinuous decomposition, resulting in a pear l i te - l ike s t ructure of noncoherent plates of 3/phase and a solid solution of equilibrium concent ra- tion (~ 0.3~c Be) leads to a sharp reduction of the res is t iv i ty and lower mechanical proper t ies (elastic limit, relaxation res is tance, hardness) [3, 4].

Discontinuous decomposi t ion occurs slowly in bronzes B2 and B2.5. For example, no more than 20~ (by votume) of the alloy undergoes discontinuous decomposit ion after aging under normal conditions. The rate of discontinuous decomposit ion can be increased by plastic deformation before aging [5]. The degree of deformation must be at least 30~c.

Microalloying with phosphorus (0.02-010~c) acce le ra tes discontinuous decomposit ion in beryll ium bronze B2 by a fac tor of 3-5 due to the reduction of the diffusion mobility of beryl l ium atoms, i.e., the decelerat ion of discontinuous precipi tat ion in the gra ins [6].

We investigated the possibil i ty of using microal loying with phosphorus to reduce the res is t ivi ty of beryl l ium bronze B2, with retention of a high elast ic limit.

Beryl l ium bronze B2 of standard composit ion and with additions of 0.02-0.10~ P was prepared (Table 1).

Phosphorus was added in the form of copper phosphide. Melting, homogenization of the ingots, and hot and cold rolling of bands to a thickness of 0.3 mm were conducted under commerc ia l conditions. The kinetics of precipi tat ion was determined f rom the change in the volume of the alloy undergoing discontinuous decomposit ion. The res is t iv i ty was measured by the double bridge method, the mic rohardness was mea- sured under a load of 10 g with the PMT-3 apparatus (the Vickers hardness under a load of 5 kg), and the

TABLE 1

Heat N o.

Composition, %

Be Ni

1,93 0,35 1,90 0,35 1,90 0,32 1,90 0,30

elastic limit was measured by the continuous bending method [7]. The mie ros t ruc tu re was examined in the MIM-7 light microscope and the EM-7 electron microscope (with use of negative carbon

0,02 0,05 0,i0

Note. The remainder Cu.

replicas) .

We investigated the effect of quenching t empera tu res f rom 720 to 820°C, aging at 280-380 ° for 0.1-15 h, and 10-30~c plastic deformation after aging at 340-360 ° for 4-6 h and repeated aging at 200-350 ° for 1-5 h on the s t ructure and proper t ies of the bronze s.

Kabardino-Balkarski i State Universi ty. N. E. Bauman Moscow Technical College. Translated f rom Metallovedenie i Te rmicheskaya Obrabotka Metallov, No. 10, pp. 55-59, October, 1975.

©19 76 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part o f this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, microfilming, recording or otherwise, without written permission o f the publisher. A copy o f this article is available from the publisher for $15.00.

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Page 2: Structure and properties of beryllium bronze microalloyed with phosphorus

%

8o

GO

~a

o

e 7fO 7t+O 760 780 80# ° C

[ ~ l l _ I 0 IfO 2z~O 560 mLu

Fig. 1. Effect of quenching tempera ture on the rate of discontinuous decomposit ion of the a, solid solution in beryl l ium bronze B2 with dif- ferent amounts of phosphorus after aging at 350°C for 2 h.

Fig. 2. Change in mic rohardness in p rocess of aging at 320°C. 1, 2) In zone of discontinuous decomposit ion; 3, 4) within grains; 1, 3) bronze B2 with 0.1% P; 2, 4) bronze B2.

F

/ 14 s8o i

a 4o~ % p

Fig. 3. Effect of phosphorus concentration and aging tem- perature on volume of the alloy undergoing discontinuous decomposition.

Quenching of beryl l ium bronze f rom tempera tures near the lower limit of homogeneity of the ~ solid solution ("720 °) ensures the highest rate of discontinuous decomposit ion of the alloy during subsequent aging at 350 ° . When the quenching tempera ture is raised to 820 ° the rate of discontinuous decomposit ion is lowered and the accelerat ing effect of phosphorus on discontinuous decomposit ion dec reases (Fig. 1).

The addition of phosphorus acce le ra tes grain growth of beryll ium bronze during heating to quenching tempera ture in the course of 50-90 min if the alloy has a single-phase s tructure of cold worked solid solu- tion before quenching° However, fine gra ins are obtained after quench- ing (10-15 ix) if the bronze is f i rs t aged at t empera tures suitable for discontinuous precipitation. It is probable that the nuclei of polyhedral c rys ta l s that grow in the course of heating to quenching tempera ture are plates of (~ phase in zones (cells) of discontinuous decomposit ion. Since the density of nuclei is high in this case, the grains do not grow to large sizes.

Discontinuous decomposition begins in the grain boundaries, the decomposition front advancing into the depth of the grain. Microhardness measurements showed that the maximum hardening in the process of aging after quenching from 760 ° occurs much earlier in zones of discontinuous decomposition than within the grains (Fig. 2). 9.~e addition of phosphorus accelerates hardening of boundary areas undergoing discon- tinuous decomposition. In bronze with 0.I~ P the maximum hardness in zones of discontinuous decom- position is obtained after aging at 320 ° for 15 rain (in 60 min for standard B2 bronze), and in 180 rain in grains hardened by continuous decomposition (Fig. 2). However, the maximum hardening of bronze B2 in regions of discontinuous decomposition (determined from the microhardness) is lower than in the grains. Softening due to overaging in zones of discontinuous decomposition begins sooner, which is explained by reerystallization of the solid solution and spheroidization of ~ platelets.

In beryllium bronze B2 of standard composition discontinuous decomposition occurs in a relatively narrow range of aging temperatures (300-380°). The addition of phosphorus, accelerating the migration of the decomposition front, has no noticeable effect on this temperature range. Raising the isothermal hold- ing temperature from 300 to 360 ° is accompanied by acceleration of discontinuous decomposition in bronzes quenched from 760 ° (Fig. 3). Thus, in beryllium bronze with 0.I~c P discontinuous decomposition at 320 ° reaches 25 vol. ~; only after holding for 10 h. When the temperature is raised to 340 °, 25~ of the alloy undergoes decomposition by the discontinuous mechanism in 15 rain. With heating above 360-370 ° com- plete discontinuous decomposition (1009 by volume) is not attained in these bronzes, evidently due to inten- sive discontinuous decomposition of 7' phase in the grains. The period of discontinuous decomposition is greatly affected by the phosphorus content (Fig. 3). Practically complete discontinuous decomposition

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Page 3: Structure and properties of beryllium bronze microalloyed with phosphorus

Hv

JO0

20g

700 . . , " .

O 2#0 .80 720 a

50O~ C

[ [ ! 2z~0 qsg 72o mm

b

Fig. 4. Change in hardness with aging of beryl l ium bronze B2 with no phosphorus (a) and with 0.05% P (b). The aging t empera tu re s are given on the curves .

Oo.oo2, k g / r a m 2 7~ ~ . . . . ~------

1 , / , !

I !

0 7a 20 % e a

Fig. 5.

i t ! /

2 3 q b

p, ~-mm2/m

o, o55

I ! d°,°*s . I I !

~o2 405 ~5~ o/~z

C

Properties of beryllium bronze after complete discontinuous decomposition in relation to cold working (a), aging time at 280°C (b), and phosphorus content (e). a, b) Bronze with 0.05~ P; a, c) aging at

280°C for 4 h.

(around 95~c) occu r s in beryl l ium bronze with 0.05~ P after aging at 360 ° for 4 h. The hardness of bery l - lium bronze microa l loyed with phosphorus af ter quenching f rom 760 ° and aging at 360 ° for 4 h with com- plete discontinuous decomposi t ion does not exceed HV 130-] 50 (Fig. 4), which affects the plasticity of the alloy.

After complete discontinuous decomposit ion sheets of the alloys 100 × 100 × 0.3 mm were pack rolled with 10, 20, and 309 reduction.

The deformed samples of beryl l ium bronze microal loyed with phosphorus and subjected to complete discontinuous decomposi t ion are hardened by low- tempera ture aging at 240-300 ° (Fig. 5). Since the solid solution is impoverished in beryl l ium (the beryl l ium concentrat ion cor responds to the equilibrium concen- t rat ion at the aging tempera ture for complete discontinuous decomposition), substantial decomposit ion as the resul t of repeated aging is possible only at low t empera tu res and only af ter plastic deformation. The hardening of bronze in the course of repeated aging inc reases with the degree of cold plastic deformation (Fig. 5).

After the hardening t rea tment of bronze B2 mieroal loyed with 0.05~ P (quenching from 760 ° in cold water, aging at 360 ° for 4 h, 30~c cold plastic deformation, aging at 280 ° for 4 h) the elastic limit (or0.002) is 70-75 k g / m m 2 and the e lec t r ica l res is t iv i ty does not exceed 0.043-0.045 9 -mm2/m, which is some 30-40~ lower than after the standard t reatment .

C ONC LU S I O N S

1. Microalloying with phosphorus (0.02-0.10~c) inc reases the tendency of beryl l ium bronze B2 to dis- continuous decomposit ion. The maximum rate of discontinuous decomposit ion is reached with 0.05~c P.

2. Discontinuous decomposi t ion in quenched bronze B2, including the bronze microal loyed with phos- phorus, occurs in a na r row range of aging t empera tu re s (300-380°). The lower the quenching tempera ture and the g r e a t e r the heterogeneity, the g rea t e r the degree of discontinuous decomposit ion during subsequent aging.

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Page 4: Structure and properties of beryllium bronze microalloyed with phosphorus

3. Microalloying with phosphorus and thermomechanical treatment under the conditions developed lower the electrical resistivity of beryllium bronze by 30-40% w4th retention of a high elastic limit (70-75 kg/mm2) .

L I T E R A T U R E C I T E D

1. Z. Henmi and T. Nagoi, Trans . Japan Inst. Metals, No. 10, 166 (1969). 2. A. So l ' n ' e , "S t ruc tu ra l changes during temper ing of Cu + 2~ Be, ~ in: Beryl l ium [in Russian], No. 4

(1956), p. 133. 3. A . G . Raldlshtadt, Spring Steels and Alloys [in Russian], Metallurgiya, Moscow (1971). 4. L. Murikami, H. Yashida, and S. Yamamoto, Trans . Japan Inst. Metals, No. 9, t l (1968). 5. H. Kreye, Z. Metallk., No. 7, 556 (1971). 6. Kh. G. Tkhagapsoev et al., "S t ruc ture and p roper t i e s of beryl l ium bronze microal loyed with magne-

sium, " Metal. i Te rm . Obrabotka Metal., No. 2, 19 (1970). 7. A.G. Rakhshtadt and M. A. Shtrcmel', ~New method of determining the elastic limit on thin flat

samples, " Zavod. Lab., No. 6, 744 (1960).

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