Structure and properties of beryllium bronze microalloyed with phosphorus

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<ul><li><p>STRUCTURE AND PROPERT IES OF BERYLL IUM </p><p>BRONZE MICROALLOYED WITH PHOSPHORUS </p><p>Kh . G . Tkhagapsoev , A . G . Rakhshtadt , and B . M. Zh i lov </p><p>UDC 620.18:669.725.5:669.779 </p><p>Beryll ium bronzes, widely used as spring materials, frequently fail to meet the needs of modern in- strument construction due to their electr ical resistivity (0.065-0.070 f2-mm2/m). The high resistivity of these alloys is due to elastic deformation of the lattice resulting from the precipitation of dispersed parti- cles of 2/" and T' phases in the process of aging [1]. The c~ solid solution also retains a high concentration of beryl l ium after aging of the alloy, which also favors high resistivity. </p><p>The decomposition of the solid solution during aging of beryll ium bronzes B2 and B2.5 at 300-350C for 2-4 h occurs by continuous and discontinuous mechanisms [2]. Discontinuous decomposition, resulting in a pearl ite-l ike structure of noncoherent plates of 3/phase and a solid solution of equilibrium concentra- tion (~ 0.3~c Be) leads to a sharp reduction of the resistivity and lower mechanical properties (elastic limit, relaxation resistance, hardness) [3, 4]. </p><p>Discontinuous decomposition occurs slowly in bronzes B2 and B2.5. For example, no more than 20~ (by votume) of the alloy undergoes discontinuous decomposition after aging under normal conditions. The rate of discontinuous decomposition can be increased by plastic deformation before aging [5]. The degree of deformation must be at least 30~c. </p><p>Microalloying with phosphorus (0.02-010~c) accelerates discontinuous decomposition in beryll ium bronze B2 by a factor of 3-5 due to the reduction of the diffusion mobility of beryll ium atoms, i.e., the deceleration of discontinuous precipitation in the grains [6]. </p><p>We investigated the possibility of using microalloying with phosphorus to reduce the resistivity of beryll ium bronze B2, with retention of a high elastic limit. </p><p>Beryll ium bronze B2 of standard composition and with additions of 0.02-0.10~ P was prepared (Table 1). </p><p>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 commercial conditions. The kinetics of precipitation was determined from the change in the volume of the alloy undergoing discontinuous decomposition. The resistivity was measured by the double bridge method, the microhardness 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 </p><p>TABLE 1 </p><p>Heat N o. </p><p>Composition, % </p><p>Be Ni </p><p>1,93 0,35 1,90 0,35 1,90 0,32 1,90 0,30 </p><p>elastic limit was measured by the continuous bending method [7]. The mierostructure was examined in the MIM-7 light microscope and the EM-7 electron microscope (with use of negative carbon </p><p>0,02 0,05 0,i0 </p><p>Note. The remainder Cu. </p><p>replicas). </p><p>We investigated the effect of quenching temperatures from 720 to 820C, 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 structure and properties of the bronze s. </p><p>Kabardino-Balkarski i State University. N. E. Bauman Moscow Technical College. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 55-59, October, 1975. </p><p>19 76 Plenum Publishing Corporation, 22 7 West 17th Street, New York, N. Y. 10011. No part of 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 of the publisher. A copy of this article is available from the publisher for $15.00. </p><p>872 </p></li><li><p>% </p><p>8o </p><p>GO </p><p>~a </p><p>o </p><p>e 7fO 7t+O 760 780 80# C </p><p>[ ~ l l _ I 0 IfO 2z~O 560 mLu </p><p>Fig. 1. Effect of quenching temperature on the rate of discontinuous decomposition of the a, solid solution in beryll ium bronze B2 with dif- ferent amounts of phosphorus after aging at 350C for 2 h. </p><p>Fig. 2. Change in microhardness in process of aging at 320C. 1, 2) In zone of discontinuous decomposition; 3, 4) within grains; 1, 3) bronze B2 with 0.1% P; 2, 4) bronze B2. </p><p>F </p><p>/ 14 s8o i </p><p>a 4o~ % p </p><p>Fig. 3. Effect of phosphorus concentrat ion and aging tem- perature on vo lume of the alloy undergo ing discont inuous decomposi t ion. </p><p>Quenching of beryl l ium bronze from temperatures near the lower limit of homogeneity of the ~ solid solution ("720 ) ensures the highest rate of discontinuous decomposition of the alloy during subsequent aging at 350 . When the quenching temperature is raised to 820 the rate of discontinuous decomposition is lowered and the accelerating effect of phosphorus on discontinuous decomposition decreases (Fig. 1). </p><p>The addition of phosphorus accelerates grain growth of beryllium bronze during heating to quenching temperature in the course of 50-90 min if the alloy has a single-phase structure of cold worked solid solu- tion before quenching However, fine grains are obtained after quench- ing (10-15 ix) if the bronze is f irst aged at temperatures suitable for discontinuous precipitation. It is probable that the nuclei of polyhedral crystals that grow in the course of heating to quenching temperature are plates of (~ phase in zones (cells) of discontinuous decomposition. Since the density of nuclei is high in this case, the grains do not grow to large sizes. </p><p>D iscont inuous decompos i t ion begins in the grain boundaries, the decompos i t ion front advanc ing into the depth of the grain. M ic rohardness measurements showed that the max imum hardening in the process of aging after quenching f rom 760 occurs much earl ier in zones of discontinuous decompos i t ion than within the grains (Fig. 2). 9.~e addition of phosphorus accelerates harden ing of boundary areas undergo ing discon- t inuous decomposi t ion. In bronze with 0. I~ P the max imum 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 gra ins hardened by cont inuous decompos i t ion (Fig. 2). However , the max imum hardening of b ronze B2 in regions of discont inuous decompos i t ion (determined f rom the mic rohardness ) is lower than in the grains. Softening due to overag ing in zones of d iscont inuous decompos i t ion begins sooner, which is explained by reerystall ization of the solid solution and spheroidization of ~ platelets. </p><p>In bery l l ium bronze B2 of standard compos i t ion discont inuous decompos i t ion occurs in a relatively nar row range of aging temperatures (300-380). The addition of phosphorus , accelerating the migrat ion of the decompos i t ion front, has no noticeable effect on this temperature range. Rais ing the i sothermal hold- ing temperature f rom 300 to 360 is accompan ied by accelerat ion of discontinuous decompos i t ion in b ronzes quenched f rom 760 (Fig. 3). Thus, in bery l l ium bronze with 0.I~c P discont inuous decompos i t ion at 320 reaches 25 vol. ~; only after holding for 10 h. When the temperature is raised to 340 , 25~ of the alloy undergoes decompos i t ion by the discont inuous mechan ism in 15 rain. With heating above 360-370 com- plete discont inuous decompos i t ion (1009 by vo lume) is not attained in these bronzes, evidently due to inten- sive discontinuous decompos i t ion of 7' phase in the grains. The per iod of discont inuous decompos i t ion is greatly affected by the phosphorus content (Fig. 3). Practical ly complete discontinuous decompos i t ion </p><p>873 </p></li><li><p>Hv </p><p>JO0 </p><p>20g </p><p>700 . . , " . </p><p>O 2#0 .80 720 a </p><p>50O~ C </p><p>[ [ ! 2z~0 qsg 72o mm </p><p>b </p><p>Fig. 4. Change in hardness with aging of beryll ium bronze B2 with no phosphorus (a) and with 0.05% P (b). The aging temperatures are given on the curves. </p><p>Oo.oo2, kg/ram2 7~ ~ . . . . ~-- - - - - </p><p>1 , / , ! I ! </p><p>0 7a 20 % e a </p><p>Fig. 5. </p><p>i t ! / </p><p>2 3 q b </p><p>p, ~-mm2/m </p><p>o, o55 </p><p>I ! d,*s . I I ! </p><p>~o2 405 ~5~ o/~z C </p><p>Propert ies of bery l l ium bronze after complete discontinuous decompos i t ion in relation to cold work ing (a), aging t ime at 280C (b), and phosphorus content (e). a, b) Bronze with 0.05~ P; a, c) aging at 280C for 4 h. </p><p>(around 95~c) occurs in beryll ium bronze with 0.05~ P after aging at 360 for 4 h. The hardness of beryl- lium bronze microal loyed with phosphorus after quenching from 760 and aging at 360 for 4 h with com- plete discontinuous decomposition does not exceed HV 130-] 50 (Fig. 4), which affects the plasticity of the alloy. </p><p>After complete discontinuous decomposition sheets of the alloys 100 100 0.3 mm were pack rolled with 10, 20, and 309 reduction. </p><p>The deformed samples of beryl l ium bronze microal loyed with phosphorus and subjected to complete discontinuous decomposition are hardened by low-temperature aging at 240-300 (Fig. 5). Since the solid solution is impoverished in beryl l ium (the beryll ium concentration corresponds to the equilibrium concen- tration at the aging temperature for complete discontinuous decomposition), substantial decomposition as the result of repeated aging is possible only at low temperatures and only after plastic deformation. The hardening of bronze in the course of repeated aging increases with the degree of cold plastic deformation (Fig. 5). </p><p>After the hardening treatment of bronze B2 mieroalloyed 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 kg /mm 2 and the electr ical resistivity does not exceed 0.043-0.045 9-mm2/m, which is some 30-40~ lower than after the standard treatment. </p><p>C ONC LU S ION S </p><p>1. Microalloying with phosphorus (0.02-0.10~c) increases the tendency of beryll ium bronze B2 to dis- continuous decomposition. The maximum rate of discontinuous decomposition is reached with 0.05~c P. </p><p>2. Discontinuous decomposition in quenched bronze B2, including the bronze microalloyed with phos- phorus, occurs in a narrow range of aging temperatures (300-380). The lower the quenching temperature and the greater the heterogeneity, the greater the degree of discontinuous decomposition during subsequent aging. </p><p>874 </p></li><li><p>3. Microal loy ing with phosphorus and thermomechan ica l t reatment under the conditions developed lower the electrical resistivity of bery l l ium bronze by 30-40% w4th retention of a high elastic limit (70-75 kg/mm2). </p><p>L ITERATURE C ITED </p><p>1. Z. Henmi and T. Nagoi, Trans. Japan Inst. Metals, No. 10, 166 (1969). 2. A. Sol 'n 'e,"Structural changes during tempering of Cu + 2~ Be, ~ in: Beryl l ium [in Russian], No. 4 </p><p>(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., "Structure and propert ies of beryl l ium bronze microal loyed with magne- </p><p>sium, " Metal. i Term. Obrabotka Metal., No. 2, 19 (1970). 7. A .G . Rakhshtadt and M. A. Shtrcmel' , ~New method of determin ing the elastic limit on thin flat </p><p>samples, " Zavod. Lab., No. 6, 744 (1960). </p><p>875 </p></li></ul>