Structure and properties of beryllium bronze microalloyed with magnesium

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<ul><li><p>STRUCTURE AND PROPERTIES OF BERYLL IUM </p><p>BRONZE MICROALLOYED WITH MAGNESIUM </p><p>Kh. G. Tkhagapsoev , A . G. Rakhshtadt , Zh . P . Pas tukhova , and A . G. Karpov </p><p>UDC 620.17:620.18:669.725 </p><p>Spring beryllium bronzes B2 and BNTI.9, used for elastic sensing elements, are often inadequate for present-day instruments. It is necessary to improve their strength characteristics - the elastic limit, re- laxation resistance, and cyclic strength. </p><p>Attempts to improve the strength characteristics of beryllium bronzes by increasing the amount of beryllium (over 2.1%) have not succeeded [1, 2]. Apparently, with increasing solubility of beryllium in cop- per 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 dur- ing aging increases (to 18% at 2.2% Be [2]). These phenomena reduce the strength. </p><p>The addition of 0.2-0.3% Ti inhibits grain boundary decomposition in beryllium bronze during aging [3, 4] due to the fact that this element is horophilic [1]. It has also been assumed that in the beryllium </p><p>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. </p><p>Bauman Moscow Higher Technical School. Translated from Metallovedenie i Termicheskaya Obra- botka Metallov, No. 2, pp. 19-24, February, 1970. </p><p>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. </p><p>106 </p></li><li><p>mm </p><p>3 o, o8 </p><p>o, os </p><p>E a,a# </p><p>o, oz: 720 </p><p>Fig. 2. </p><p>i </p><p>/ i J </p><p>740 760 78# 800 ~ Quenching temperature </p><p>Fig. 2 </p><p>.1'5,6%T; </p><p>~) </p><p>1 #,o% Ti </p><p>b) Fig. 3 </p><p>Variation of average grain size with quenching temperature (holding t ime 15 min). i) B2; 2) B2 + 0.1% Mg; 3) BNT I .9 ; 4) BNT I .9 + 0.1% Mg. </p><p>Fig. 3. Distribution of intensity of Ks Ti radiation in BNT1.9. a) With- out Mg; b) with 0.1% Mg. </p><p>TAB LE 1 </p><p>2=~2- </p><p>Composition, % Bronze </p><p>B2 </p><p>Be </p><p>1,90 1,87 1,87 1,85 1,93 1,90 1,90 </p><p>NI </p><p>0,26 0,28 0,30 0,26 0,35 0,35 0,32 </p><p>TI Mg </p><p>m </p><p>0,05 0,10 0,15 </p><p>0,02 0,05 0,10 </p><p>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 -- </p><p>13 1,96 0,20 0,16 -- -- Note.: All alloys were copper-based. Heats 0 and 13 were of standard eompostion. </p><p>bronze BNT1.9 microalloyed with titanium the beryl l ium content decreases somewhat (to 1.85-2.1%) with retention of the strength character ist ics of B2 bronze. This is explained by the titanium- containing strengthening phase forming at the same aging tempera- ture as the "/-phase (CuBe) [4]. The results from qualitative mi- erostructural 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 crystals are large inclusions (Fig. 1) and therefore do not affect strengthening during aging. Also, with the formation of a separate copper - t i - tanium-nickel phase (possibly containing beryl l ium as well) the a-sol id solution is impoverished in nickel and titanium, and there- fore the properties of BNT1.9 bronze which is more highly alloyed </p><p>are almost the same as those of B2 bronze. It was of interest to investigate the effect on the structure and propert ies of added elements more surface active than beryll ium, titanium, and unavoidable impurities in beryl l ium bronzes B2 and BNTI.9 [5, 6]. </p><p>It was shown in [7] that the surface activity (horophilic tendency) of an element increases with de- creasing values of the generalized statistical moment m c of the atom. Copper and alloying elements in bronze have the following values of the generalized statistical moment, m e.10 -8 e l /cm: </p><p>Cu- 0.769; N i - 0.740; Mg- 0.282; Be- 0.533; T i - 0.460; P - 0.860. </p><p>Of the possible elements for alloying beryl l ium bronze, magnesium is the most effective. Its statisti- cal moment is the lowest, indicating its high horophilic tendency. </p><p>Beryl l ium bronzes B2 and BNT1.9 of standard composition 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 form of a copper -magnes ium alloy (60.6% Mg). The ingots of the bronzes were homogenized, hot rolled, and then cold rolled to strip 0.3 mm thick under commercia l conditions. The distribution of titanium, nickel, and magnesium was determined by microstructural analysis. The kinetics of aging was determined from the change in the hardness and elastic l imit measured by the longitudinal bending method [8] and the electrical resistivity. The relaxation res is - tance under prolonged static and cyclic loads at room and elevated temperature (100~ was determined by the method given in [6]. The microstructure was examined with the MIM-8 optical microscope and with an electron microscope, using carbon replicas.* </p><p>* These investigations were made by V. G. Kholodova. </p><p>107 </p></li><li><p>&lt; </p><p>o o </p><p>, ,77 </p><p>l U l l </p><p>~ dddd i i i i </p><p>o o ~ </p><p>g </p><p>~ ~Ngg </p><p>9 o . . </p><p>9 o . ~ </p><p>v ~ d &lt; "~ 9 + </p><p>9 + </p><p>We determined the effect of quenching temperatures f rom 720-820~ and also the aging conditions at 280-360~ (0.5-10 h) on the strength char- acter is t ics of bronzes alloyed with magnesium. Quenching f rom 770 10~ resulted in the best combination of strength and technological and working propert ies of the new composit ions of beryl l ium bronze after aging. </p><p>The addition of magnesium to bronzes B2 and BNT1.9 substantially reduces the average grain size of the a-so l id solution and reduces the tendency to grain growth during heating (Fig. 2). Magnesium increases the dispers i ty of Cu-T i -N i inclusions, which is indicated by the change in the intensity distribution of Kc~-radiation of titanium and nickel in the beryl l ium bronzes microal loyed with magnesium (Fig. 3). The sharp but wide peaks of Kc~-radiation of Ti in bronze BNTI .9 and of nickel in bronze B2 without magnesium are character ist ic for large inclusions of phases enriched in these elements, After the addition of 0.05-0.10% Mg these peaks are more numerous but less intense, since the dispersity of the part ic les of Cu-T i -N i phase increases. The refining of the excess phase inclusions after microal loying with magnesium can also be observed in ordinary metal lographic examination of the structure. </p><p>The addition of surface-act ive magnesium has the greatest effect on the kinetics and mechanism of dispersion hardening and also on the struc- ture of the bronzes after aging. The relationship between the elastic l imit and aging temperature, shown in Fig. 4, indicates that with aging for 2 h the maximum strengthening of bronze with magnesium is attained at a higher temperature, and for aging at the same temperature is attained after a longer time, than for bronze without magnesium (Fig. 5). The op- t imal conditions for bronzes B2 and BNT1.9 alloyed with magnesium are aging at 320~ for 6 h, while for the standard bronzes they are 2-4 h at the same temperature. </p><p>The retarding of the aging process in alloys with magnesium is ac- companied by a substantial increase of strength. The increase in strength is highest with increasing concentrations of Mg up to ~0.1%; no further improvement in the propert ies is noted at higher Mg concentrations. The addition of 0.1% ~g has the greatest effect on the elastic l imit (r = 73- 78 kg /mm 2 as compared with 58-63 kg /mm 2 without Mg) and the relaxation res istance in static and cyclic loading at elevated temperatures (Table 2) as well as in corros ive media. The cyclic strength increases 8-12%, while the ultimate strength and hardness increase little. </p><p>When magnesium is added to B2 bronze the discontinuous grain boundary decomposit ion that is character ist ic for aging of these alloys after quenching is suppressed (Fig. 6). After 10 h at 320~ no signs of local grain boundary decomposit ion are observed in the B2 alloy with mag- nesium, while without magnesium there are distinctly visible colonies of precipitates. After aging at 340~ for 2.5 h one observes colonies with a pearl i t ic structure in the grain boundary areas of the c~-solid solution in B2 bronze; they are evidently a two-phase mixture of ~-so l id solution of equil ibrium concentration and "Y-phase; the addition of magnesium prevents their formation. </p><p>The character of the observed changes in the kinetics of aging, the structure, and the propert ies of beryl l ium bronzes can be explained by the effect of the small magnesium additions. The reduction of the average grain size of the c~-solid solution and of the inclusions of other phases re - sults f rom the adsorption of horophilic magnesium on the interfaces and in the grain boundaries, which reduces the free surface energy. </p><p>108 </p></li><li><p>O0.002, kg/mm 2 </p><p>f " ' i , ' 4.,4 0o'o02' dO[ - -~- ! ~ -~ t-4v </p><p>&amp;_Z I i I i k~/mm 2 8,~ -I . . . . . T F -Tq </p><p>' I &lt; i i , 0 ~ " ~ .. . . . . . . . . . . . . !-- j :0 h I ' : ' ] , 0 80 120 /dO 24"0 300 ,380 4-Z0 rain </p><p>Z~O 300 3ZO d40 ~ k ging time Aging temperature b) </p><p>Fig. 4 Fig. 5 Fig. 4. Variation of elastic l imit with aging temperature (2 h) for bronze B2. 1) Without Mg; 2) with 0.1% Mg. </p><p>Fig. 5. Variation of the elastic l imit with aging time (320~ for B2 and BNT1.9 bronzes, a) B2;b) BNT1.9. 1) No addi- tions; 2) with 0.05% Mg; 3) 0.10% Mg; 4) 0.05% P. </p><p>Thus, the stabil ity of the d ispersed condition of the structure is increased. The competing adsorption of magnesium prevents enr ichment of the grain boundaries and other internal surfaces in beryl l ium, and therefore the presence of magnesium inhibits the nucleation of precipitates (the occurrence of local decom- position centers on these surfaces) and suppresses grain boundary decomposit ion in B2 bronze. A s imi lar effect of sur face-act ive elements on the structure of dispersion hardening alloys was observed in [9]. The suppress ion of grain boundary decomposit ion in bery l l ium bronze BNT1.9 with the addition of t itanium, which is surface active in relation to beryl l ium, also conf i rms this. On the addition of surface- inact ive phosphorus (m c = 0.860) to B2 bronze one observes intensive grain boundary decomposit ion (see Fig. 6) and accelerat ion of the aging process (see Fig. 5a). </p><p>The increase in the dispers i ty of B2 bronze microal loyed with magnesium, which prevents grain boundary decomposit ion, leads to more even decomposit ion of the solid solution during aging. This s t ruc- tural condition is responsible for the higher relaxation res istance and elastic l imit of the bronze. However, the substantial improvement of the strength character is t ics of bronze BNT1.9 when alloyed with magnesium makes it possible to assume that the degree of surface activity of the alloying element determines the size of the precip i tates, in which case coherent bonding with the matr ix is stil l possible. </p><p>The increase of the res is tance to microplast ic deformation at e rem = 0.001-0.01% result ing f rom aging is inversely proportional to the size of the part ic les of strengthening phase [10]. The increased dis- pers i ty of the part ic les of phase precipitating during aging of bronzes B2 and BNT1.9 alloyed with magnesium is probably induced by the adsorption of magnesium atoms. </p><p>To determine the change in atomic mechanism of dispersion hardening processes in B2 bronze, lead- ing to the changes mentioned in the kinetics and structural condition as the result of microal loying with mag- nesium and phosphorus, we determined the binding energy of vacancies (Ev) with atoms of beryl l ium, nickel, magnesium, and phosphorus by the method descr ibed in [11] after quenching f rom 720-820~ and aging at 100~ </p><p>The binding energy of bery l l ium atoms with vacanc ies in B2 bronze is ~0.2 eV, wh ich matches the re- sults obtained for the b inary alloy Cu + 2% Be [ii]. This makes it possible to assume that the binding en- e rgy of nickel a toms with vacanc ies is close to zero in the alloys investigated. For phosphorus , E v = 0.3 * 0.05 eV, whi le for magnes ium E v = 0.45 0.05 eV. </p><p>Apparent ly because of the high binding energy of magnes ium and phosphorus a toms with vacancies, in their p resence the bery l l ium bronze will be more supersaturated with vacanc ies than the bronze of stan- dard composi t ion. </p><p>109 </p></li><li><p>Fig. 6. Structure of B2 bronze after 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) </p><p>i </p><p>Calculation of the activation energy of aging of beryl l ium bronze at 280-360~ f rom the kinetics of the variat ion in propert ies (hardness and electr ical resist ivity) showed that for B2 bronze Q = 24-26 kcal /mole. Phosphorus (0.1%) increases this value to 32-35 kcal /mole, which is close to the diffusion activation energy of phosphorus in copper. </p><p>It can be assumed that phosphorus atoms combine with vacancies in bery l l ium bronze and reduce the mobil ity of the latter. However, in bronze alloyed with phosphorus, adsorption enrichment of the grain boundaries with surface-act ive beryl l ium atoms occurs, which promotes nucleation of precipitates in the boundaries. Thus, in this bronze there are created conditions in which decomposit ion of the supersaturated a -so l id solution will occur mainly according to the type of discontinuous precipitates in the grain bound- ar ies and will usually develop at an accelerated rate [12, 13]. </p><p>The addition of magnesium, due to adsorption of its atoms, apparently can weaken the effectiveness of dislocation loops and the boundaries of grains and subgrains as vacancy sinks because of the reduction of elastic s t resses . This suppresses the initiating influence of these lattice defects on the nucleation of pre- cipitates. Also, as the result of the elevated binding energy of magnesium atoms with vacancies the solid solution becomes supersaturated with vacancies, the presence of which favors decomposit ion during aging by the mechanism of overall continuous precipitation, which usually occurs more slowly than discontinuous precipitation. </p><p>It should be noted that alloying with magnesium has such a strong effect on the structure of the alloys that they should be regarded as new mas with a better combination of propert ies than bronzes of stan- dard composition. </p><p>For complete determination of the propert ies of the new bronzes we conducted commerc ia l tr ia ls . F rom rolled str ip of BNT1.9 bronze with 0.1% 1Vig quenched f rom 770 10~ we prepared elastic sensing elements. The elem...</p></li></ul>


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