Embrittlement of beta-brass alloys by liquid metals and aqueous ammonia

Materials Science and Enyineerin9, 12 (1973)245-253 © ElsevierSequoia S.A., Lausanne - Printed in the Netherlands

245

Embrittlement of Beta-Brass Alloys by Liquid Metals and Aqueous Ammonia M. M. SHEA* and N. S. S T O L O F F

Rensselaer PolytechnicInstitute, Troy, New York 12181 (U.S.A.) (Receivedin revised form December20, 1972)

Summary** The tensile deformation of several binary and ternary beta-brass alloys has been studied in the temperature range of 25° C-300 ° C in several liquid metal environments. Additional tests were conducted in aqueous ammoniacal solutions. The deoree of susceptibility of each alloy to test environments and the observed crack paths correlated with slip character and stress relaxation phenomena at grain boundaries, as revealed by optical metallography. Manganese additions restricted cross slip and 9rain boundary joqoin9 phenomena, and increased the susceptibility o f beta-brass to embrittlement in gallium solutions. Nickel additions had the opposite effect on each property. Single crystals of beta-brass also revealed severe embrittlement in 9allium, with cleavage observed along {100}. Mercury caused less severe embrittlement of single crystals, consistent with intergranular crack paths observed in polycrystalline beta-brass wetted with mercury.

species to strained bonds at a crack tip--there has been no general agreement that the two phenomena are related. Nevertheless, it appears that the degree of susceptibility to both LME 6 and SCC 7 is increased when cross slip is suppressed in the solid metal, whether through alloying to induce either short range order or a lowered stacking fault energy or through changes in test temperature. It is the purpose of this paper to show that the ductility and fracture modes of ternary beta-brass alloys in air, several liquid metals, and in aqueous ammonia can be related directly to the slip processes and grain boundary relaxation phenomena that are operative in the various alloys. Nickel and manganese were chosen as ternary solid solution alloying elements for beta-brass since, on the basis of their effects on T c, the critical temperature for long range order, they were expected to respectively enhance and suppress cross slip 8.

EXPERIMENTAL PROCEDURE INTRODUCTION The degree to which solid metals are embrittled by specific liquid metals depends upon such diverse factors as the composition and microstructure of the solid, and the test temperature, strain rate and composition of the liquid metal 1'2. Similarly, both microstructural and external test variables may influence the degree to which alloys undergo stress corrosion cracking a. Although there have been claims 4'5 that both liquid metal embrittlement (LME) and stress corrosion cracking (SCC) arise from a common factor--adsorption o fan embrittling * Now at General Motors TechnicalCenter, Warren, Michigan. ** R6sum6 en franqais&la fin de l'article. Deutsche Zusammenfassung am SchluB des Artikels.

Alloy preparation Both single crystals and polycrystalline specimens were used although the main emphasis was on polycrystals. Table 1 shows the compositions of all alloys used in this study, together with measured values of T~8. To obtain polycrystalline material, two-pound ingots of each composition shown in Table 1 were cast in vycor capsules in an atmosphere of ultra-high purity argon. The starting materials were 99.999 % copper and zinc and 99.98 % nickel and manganese. To ensure sound, homogeneous ingots, solidification was done directionally with a hot-top arrangement. Immediately after solidification, the alloys were homogenized in argon at 700°C for 12 hours and air-cooled to room temperature. Metallographic examination showed each alloy to be single phase, except the ternary

246

M. M. SHEA, N. S. STOLOFF

TABLE 1

Alloy Polycrystals Cu-45 ~Zn Cu-48 ~Zn Cu~5 ~Zn-5 ~Mn Cu-48~Zn-2~Mn Cu~8 ~Zn-2 ~Ni Cu-44 ~Zn-7 ~Mn Single crystals Cu-48 ~Zn Cu-48~Zn-2~oMn Cu-50 ~oZn-2 ~Ni

Alloy composition Zn, Mn, Ni, wt. % wt. % wt. %

Critical N, temp., Tc p.p.m. ° C

45.49 48.41 44.60 5.49 47.61 2.02 47.51 1.95 Not analyzed

60

48.52 48.02 1.81 50.69

64 61

457 468 429 451 488

468 454 1.97

containing 5 ~o nickel. This alloy was obtained in the single phase condition by quenching into 10~ NaOH after 30 minutes at 650°C. Specimens for metallographic examination were mounted in epoxy, mechanically polished through Linde B abrasive and swab-etched with the following etchant: 2g K E C r 2 0 7 , 8cc H2SO4, 4 cc saturated NaC1 and 100cc H20. Initial ingot breakdown consisted of forging at 600-650°C to a rectangular cross-section 0.4 in. thick. After surface finishing, each alloy was reduced 82 ~ at 200°C. Recrystallization to 120 #m grain size was carried out for each alloy at temperatures in the range of 480-630°C in a salt bath. Each alloy was then quenched into water. Samples to be tested as single crystals were remelted in a Bridgeman apparatus under an argon atmosphere, with a temperature gradient of about 100 deg C/inch to produce 1/4 in. diam. rods. Orientations were determined by the Laue back reflection technique.

and its temperature monitored by thermocouples wired to its gage section. After tensile testing, surface and fracture replicas of carbon were made on selected specimens by the two-stage preshadowed technique first developed by Bradley9. Tests in liquid metal environments were made by applying the liquid to only one surface of the polished polycrystalline specimens in order to facilitate examination of the fracture path. The liquid metals were applied completely around the circumference of the single crystals. For tests with indium (m.p. 156°C), a soldering iron was used to bond the metal to the specimens before testing. The cleavage planes were determined by etching or polishing away the liquid metal from the fracture surfaces of the single crystals and taking a Laue back-reflection pattern of the fracture surface. Tensile tests also were conducted in 10M NH4OH + 0.12M Cu + at a strain rate of 0.02 min- ~ to evaluate resistance to environments known to induce stress corrosion cracking in copper-base alloys.

RESULTS

Mechanical behavior of polycrystals in air Both binary and ternary beta-brass alloys exhibited a peak in yield stress with increasing test temperature, as shown in Fig. 1. This behavior, which has previously been noted in beta-brass single crystals and polycrystals by Ardley and Cottrell 1° and by Brown 1~, is discussed in detail in another 6o I

Specimen preparation and testing Sheet-type tensile specimens with gage section dimensions 0.57 in. thick x 0.250 in. wide x 1.0 in. long were machined from the recrystallized blanks and then were ultrasonically cleaned and heat treated for one hour in silicone oil at 130°Cto eliminate machining and quenching effects. Prior to testing in an Instron tensile machine, 0.002 inch was removed from each face of the specimens by electropolishing at 1.65 volts at room temperature in concentrated phosphoric acid. A strain rate of 0.02 min- ~ was used for most of the tests. Testing above room temperature was done in an atmosphere of argon by means of a sealed tube arrangement that fitted inside a movable resistance furnace. The specimen was centered in the hot zone

o45Zn o 48 Zn • 48 Z n - 2 M n • 48 Zn-2Ni A 4 5 Zn - 5 Mn v 44 Zn-7Mn o 45 Zn-5Ni • 45 Z n - I M n

•"~ so

% ~4o

-~ 3 o

I0

oi

zs

ioo

2oo

3oo

400

TEMPERATURE (*C)

Fig. 1. Temperature dependence of the yield stress for binary and ternary beta-brasses.

247

E M B R I T T L E M E N T OF BETA-BRASS A L L O Y S

publication 12. The important observation from Fig. 1 is that nickel and manganese both are solid solution strengtheners, with approximately equivalent effectiveness at room temperature. However, at temperatures over about 100°C, manganese is a more effective strengthener. For binary beta-brass, Cu-45%Zn is stronger than Cu-48~Zn, undoubtedly because of the strengthening effects of moving off stoichiometry.

I00

9O

~ z

ao

z

0 45% z n o

50 40

30

20

I

I

04

0,5

I

I

0.6

0.7

Internal voids formed in each alloy prior to or during fracture in air, as indicated by a dimpled fracture surface. The dimples were largest in the binary and nickel alloys while smaller dimples were characteristic of the manganese alloy fracture. Cleavage was not observed in any alloy tested at or above room temperature.

Liquid metal embrittlement of polycrystals Figure 3 shows the effect of gallium on the total elongation for four beta-brass alloys. (Reduction in area measurements were not made owing to the presence of liquid metals around the fracture surface.) Gallium induced a distinct ductile-brittle transition for each composition. Additions of manganese to the substrate increased the severity of embrittlement as well as the transition temperature while nickel additions had the opposite effect. The failure mode in gallium was primarily transgranular at all temperatures for binary and nickel ternary alloys, but there was also limited evidence for intergranular embrittlement. Secondary cracking was seldom observed in these alloys. Failures in the manganese

0,8

TIT c Polycrystolline/~-Brass

Fig. 2. Ductility in air as a function of beta-brasses.

T I T c for

Tested in Gallium

binary and ternary

o45Zn o n-

.~o-~° i

&45Zn-SMn

Since both manganese and nickel change T¢ (see Table 1), ductility data have been plotted versus T/T¢ in Fig. 2. This was done to minimize any effects of changes in the degree of order at elevated temperatures on the test results. Increasing the zinc content of binary beta-brass from 45 to 48~o increased the ductility at all temperatures. Additions of nickel slightly lowered ductility, while manganese additions decreased the ductility significantly at all temperatures*. Stable intergranular cracks were formed in each alloy during deformation in air at low temperatures, although the final failure mode was always transgranular. Additions of manganese increased both the degree of cracking and the maximum temperature of its occurrence. Intergranular cracking was observed up to 250°C in manganese-containing alloys while 200°C was the upper limit for cracking in binary and nickel alloys. These intergranular cracks at times propagated into the interior of grains. There is little doubt that the growth of these cracks initiates final failure. Plotting ductility data to the same conclusions. *

versus

temperature instead of

TIT c

led

50

g ~ 4o .

W 30

20

I0

z

~

25

tOO

I

I

I

200

TEMPERATURE °C

Fig. 3. Effect of gallium on temperature dependence of ductility for binary and ternary beta-brasses.

ternary alloys were strictly transgranular with substantial secondary cracking occurring. Examples of these failures at room temperature and 200°C for the Cu-45 ~oMn alloy are shown in Figs. 4(a) and 4(b) respectively. The environments of Ga-16.5 ~ I n (the eutectic composition for this system) and Ga-l.6%Hg slightly increased the severity of embrittlement but did not affect the transition temperatures, compared

248

M. M. SHEA, N. S. STOLOFF shortly after yielding at all temperatures at which embrittlement occurred. Manganese slightly increased the severity of the embrittlement but the transition temperature was not a function of composition. The transition temperature was between 225 ° and 250°C for all alloys. Each alloy was held at 200°C for 48 hours in the presence of indium or Hg-70 ~ I n and then tested at room temperature after the liquid metal had been either removed or solidified. This treatment had no effect on the tensile ductility of any of the alloys, indicating that stress is a necessary (but probably not sufficient) condition for intergranular penetration of these metals.

Liquid metal embrittlement of single crystals

Fig. 4. Transgranular crack paths in Cu-45~Zn-5~oMn fractured in gallium, x 100: (a) 25°C, (b) 200°C.

Fig. 5. Intergranularcrack paths in Cu-48 ~Zn=2 ~Ni tested in Hg-70~In at 200°C, × 100. with pure gallium. However, with these liquids a greater proportion of the fracture and secondary cracking were intergranular than was observed in pure gallium. The alloys also were tested in several other liquid metal environments. Failures of all alloys in indium or H g - 7 0 ~ I n were intergranular with profuse intergranular penetration to considerable distances from the fracture, as shown in Fig. 5 for C u ~ 8 ~ Z n - 2 ~Ni. All of the alloys failed at or

Single crystals embrittled with pure gallium or G a - 1 6 . 5 ~ I n showed the same effect of alloy composition on ductility and the transition temperature as the polycrystals. Failure occurred by cleavage on {1001 planes irrespective of alloy composition, crystal orientation or test temperature. The embrittlement phenomena were identical for these two liquid metal compositions. For tests in an environment of Hg-70 Kin, all beta-brasses failed in the liquid metal zone. The alloys were considerably more ductile in this environment than in pure gallium. Failure occurred at high strains after the initiation of double slip or the formation of deformation bands. The separation plane was close to either {321} or {320} except for orientations near (100) where {100} fracture was observed. No failures occurred in the liquid metal zone at temperatures above 50° C for any composition. Single crystals of each alloy were tested at room temperature in Hg-70 ~ I n by three-point bending. Failure did not occur even after the crystals were bent completely in half (radius approximately equal to 1/4 inch). However, all failed in the liquid metal zone when tested in reversed bending.

Stress corrosion of polycrystals Specimens of Cu-48 ~ Z n and Cu-43 ~oZn-7 Mn were tested in tension in 10M NH4OH + 0.12M Cu +. The ductility of both alloys was reduced by 5 6 ~ compared with tests in air. Failure was transgranular in both cases with much secondary transgranular cracking. Examples of this are shown in Fig. 6 for both alloys. Note the branch-like nature of the cracks in the manganese ternary alloy, Fig. 6(b), as compared with the straight single cracks in

EMBRITTLEMENT

OF BETA-BRASS ALLOYS

249

Fig. 6. Secondary stress corrosion cracks extending from side surface, 10M N H 4 O H + 0 . 1 2 M Cu ÷ at 25°C, ×400: (a) Cu 48 ~ Z n , (b) Cu~44 °/oZn 7 %Mn.

the binary alloy, Fig. 6(a). The stress axis was along the horizontal and the cracks are emanating from a side surface. There appears to have been surface attack in both alloys although a chemical-type attack is not believed responsible for the different crack morphology between the two alloys.

Deformation modes Previous investigations 13'~4 have shown that deformation bands and strain- or stress-induced martensite can be important deformation modes under certain conditions in beta-brass. However, most of the earlier work had been done at low temperatures, low zinc contents, large grain sizes (2mm), and high strain rates. Both banding and martensite were observed to a small extent in the binary and ternary alloys used in this investigation. With the exception of the observation that the manganese additions increased to 175°C the maximum temperature of occurrence of these modes, no correlation between martensite or deformation band

Fig. 7. Slip traces in polycrystalline beta-brasses, deformed 12 % in tension at 25 ° C, × 400 :(a) Cu-45 %Zn, (b) Cu 45 !'~;Zn 5 %Mn, (c) Cu 45 °/Zn-5 %Ni.

formation and any of the experimental results could be established. Surface slip traces were observed as a function of composition and temperature. Figure 7 shows the effect of nickel and manganese additions on the room temperature slip traces of alloys containing 45 ~ Z n . Manganese additions increased the occur-

250 rence of planar slip while nickel tended to enhance wavy glide. The effect on the slip traces of increasing the zinc concentration from 45 ~ Z n to 48 ~oZn was not as clear. However, in general it appeared as if the slip became more wavy in binary alloys with increasing zinc content. These results confirm those of Clark 1s.

M. M. SHEA, N. S. STOLOFF

Temperature had a pronounced effect on the nature of the slip traces for each composition. Up to approximately 100°C the slip bands appeared as in Fig. 7. However, between 100°C and 200°C the slip offsets coarsened and became more planar for all alloys. Examples of this are shown in Fig. 8 for binary and ternary alloys containing 48 ~oZn. This effect was much more pronounced in the alloys containing manganese. An important feature of the deformation of binary and ternary beta-brass is the occurrence of deformation near grain boundaries. This can be seen in an early stage in Figs. 8 (a) and 8 (b) respectively for the binary and nickel ternary alloys. This phenomenon has its beginning as grain boundary

Fig. 9. Grain boundary jogging in C u ~ 8 ~ Z n deformed to fracture at 300°C, x 200.

jogging by impinging slip lines. Figure 9 shows that as the temperature is increased above 200°C deformation became more severe in areas near grain boundaries, resulting in strain-induced grain boundary migration. When deformed at temperatures above 300°C all compositions exhibited recrystallization during testing. Addition of manganese increased the temperature of occurrence of all of the above phenomena while nickel again had the opposite effect. However, the changes were more pronounced for manganese additions.

DISCUSSION

Fig. 8. Slip traces in polycrystallinebeta-brasses, deformed25~o in tension at 200°C, x200: (a) Cu-48~oZn, (b) Cu-48~Zn2 ~Ni, (c) Cu-48~Zn-2 ~Mn.

Relation between slip character and embrittlement Clark is has shown that slip character in betabrass can be altered either through changing zinc content in binary alloys or by adding suitable ternary elements. The spacing, r, between the unit dislocations which comprise a superlattice dis-

EMBRITTLEMENT OF BETA-BRASS ALLOYS

251

location in the ordered CsCl type structure is the critical parameter governing the slip process. The value of r is obtained by balancing the mutual force of repulsion between two unit dislocations with the energy of the antiphase boundary connecting them; for a superlattice dislocation on plane {hkl} 16.1 v r-

Gb 2 aoN ~ 87~hVS 2

(1)

where N = h 2 d- k 2 + l 2, G is the shear modulus, S is the degree of order and V = ordering energy ~ k Tc/4. The separation, r, therefore depends inversely on ordering energy (which is itself proportional to To) and to S 2. Clark is suggested that the relative change in r from the value for 50 ~ Zn in binary C u - Z n may be expressed as rx rsozn

-

(rc)sozn'S2ozn

(2)

(T~),'S~

where r refers to the atom fraction of zinc. Since in binary alloys both S and Tc decrease with deviations from stoichiometry, r increases. Clark also suggested that this type of analysis could be applied to ternary Cu-Zn alloys. Consequently manganese, which lowers T~, see Table i, should make cross slip more difficult through an increase in r; superlattice dislocations are less likely to cross slip when their constituent unit dislocations are widely separated. However, in the limit of very low ordering energy, the two unit dislocations could cross slip independently. Nickel additions, on the other hand, which raise T~, should decrease r, and facilitate cross slip. This was observed, as has been shown in Fig. 7. Differences in ductility of binary and ternary alloys of beta-brass in air (Fig. 2) were consistent with the effects of composition on ease of cross slip and grain boundary accommodation as well as the effectiveness of manganese as a solid solution strengthener. Decreasing the ordering energy with manganese additions decreases the likelihood of cross slip and lowers the ductility. Additions of nickel, which raises ordering energy and consequently the ease of cross slip, did not increase the ductility over that of the binary alloy containing the same zinc content, presumably on account of the higher strength of the nickel ternary alloy, see Fig. 1. However, a slight improvement in ductility was noted for the nickel alloy when compared with the Cu-45 ~ Z n alloy. The results of the tests in gallium further substantiate the effects of alloying on ease of cross slip

and grain boundary accommodation. Manganese increases susceptibility to embrittlement, presumably by restricting cross slip, resulting in stress concentrations and subsequent crack nucleation. It should be pointed out, however, that the extent of embrittlement of the high-zinc binary alloy was similar to that of the nickel ternary alloy, indicating a similar ease of cross slip in these alloys. The ductile-brittle transitions of the several alloys in gallium occur at temperatures where grain boundary constraints to propagation of slip bands disappear. Once significant plastic compatibility occurs, preventing build up of stress concentrations, transgranular embrittlement is not possible. It is interesting to note that the failures in aqueous ammonia (Fig. 6) indicate a different crack morphology as a function of composition. Considering that crack propagation tends to occur normal to tensile stress, the branch-like nature of the cracks in the manganese alloy may reflect more effective sites for stress concentrations resulting from restrictive cross slip. The direction of the maximum tensile stress resulting from glide induced stress concentrations will be variable on a local scale. There was no tendency for branching in the binary, indicating easy cross slip. Intergranular embrittlement occurs in environments of mercury, indium and Hg-In amalgams, presumably owing to stress-assisted penetration along the boundaries. This is substantiated by the lack of composition and temperature dependences of the degree of embrittlement and the lack of embrittlement when substantial grain boundary jogging occurs, preventing build-up of normal stresses across boundaries. Also significant is the observation that single crystals are little influenced by these environments. Waterhouse and Grubb 18 showed that the stressinduced penetration of a liquid along a grain boundary was enhanced if one of the constituents of the alloy was soluble in the liquid metal. Zinc is highly soluble in mercury and indium, so that intergranular embrittlement of beta-brass is not surprising. However, zinc also has appreciable solubility in gallium over the temperature range studied, yet gallium embrittlement is primarily transgranular. Therefore, it appears that solubility considerations are not generally controlling. To differentiate between the embrittlement mode in gallium or mercury requires consideration of chemical effects and the nature of the adsorption process, as discussed below.

252 C o m p a r i s o n with f c . c . alloys

It is interesting to compare the behavior in liquid metal environments of beta-brass with alphabrasses and other alloys of f.c.c, structure 2'6. Crack propagation in alpha-brasses, Cu-A1 and C u - G e occurs along grain boundaries, independent of composition. However, the degree of embrittlement is a strong function of composition through the effects of solutes on stacking fault energy. Thus when av/ay is plotted against stacking fault energy (aF is fracture stress, ay is 0.2 ~o yield stress) a linear relation results 6. This indicates that the degree of plasticity in liquid metals increases linearly with increasing ease of cross slip. The results of the present work are entirely in accord with that hypothesis. Preece and Westwood 19 have shown that sharp ductile to brittle transitions m a y be induced in several f.c.c, alloys, the critical temperatures for which depend upon the composition of the liquid metal environments. Unlike beta-brasses, the temperature dependence of yielding in these f.c.c, metals (aluminum, silver, alpha-brass) is small in the vicinity of the respective ductility transitions, and in the case of the pure metals cross slip is easy at all temperatures. The ductility transition therefore was linked to a temperature dependent adsorption process such that as temperature is lowered, at some critical temperature the effective bond strength, ae, falls below a critical level relative to the flow stress. We believe, however, that the onset of brittleness in beta-brass alloys tested in gallium at low temperatures is too sensitive to composition of the solid (see Fig. 3) to suggest that 2-5 ~oMn can so drastically change the adsorption characteristics of gallium on beta-brass. With mercury as the environment, on the other hand, the onset of brittleness did not depend upon solid composition; consequently a temperature dependent adsorption mechanism might be more appropriate to explain those results. SUMMARY AND CONCLUSIONS 1. Manganese and nickel produce solid solution hardening in beta-brass ; manganese is more effective at temperatures above 100°C. 2. Manganese raises T c, the critical temperature for ordering ; nickel lowers T c. 3. The effects of solutes on cross slip correlate with changes in To. 4. Gallium induces a ductile to brittle transition in polycrystalline beta-brass, the temperature of

M. M. SHEA, N. S. STOLOFF which depends on solute content. Fractures in gallium are transcrystalline. 5. Mercury induces intergranular failure and a ductile to brittle transition, the temperature of which is independent of solute content. 6. Single crystals tested in gallium cleave along {100}. Mercury induces less severe embrittlement of single crystals, with fractures sometimes noted along {100}. 7. The nature and degree of embrittlement in each alloy can be related to the relative strengthening effects of manganese and nickel, and their influence on cross slip. ACKNOWLEDGEMENTS The authors are grateful to Mr. F r a n k Lang of International Nickel Co. for fabrication of the betabrass alloys. This research was supported by the National Aeronautics and Space Administration under G r a n t No. N G L 33-018-003. One of the authors (MMS) was supported by an N D E A Fellowship. REFERENCES 1 W. Rostoker, J. M. McCaughey and H. Markus, Embrittlement by Liquid Metals, Chapman and Hall, London, 1960, p. 152. 2 N. S. Stoloff,in Surfaces and Interfaces, V. II, Syracuse Univ. Press, Syracuse, N.Y., 1968, p. 157. 3 H. H. Uhlig, in H. Liebowitz (ed.), Fracture, V. 3, Academic Press, 1971, p. 645. 4 H. H. Uhlig, in T. Rhodin (ed.), Physical Metallurgy of Stress Corrosion Fracture, Wiley(Interscience),NewYork, 1959,p. 1. 5 E. G. Coleman, D. Weinstein and W. Rostoker, Acta Met., 9 (1961) 491. 6 N.S. Stoloff,R. G. Davies and T. L. Johnston, in EnvironmentSensitive Mechanical Behavior, Gordon and Breach, New York, 1966, p. 613. 7 P. Swann and J. N. Nutting, J. Inst. Metals, 88 (1959/60) 478. 8 M. M. Shea, PhD. Thesis, Rensselaer Polytechnic Institute, 1971. 9 D. E. Bradley, J. Inst. Metals, 83 (1954/55) 35. 10 G. W. Ardley and A. H. Cottrell, Proc. Roy. Soc. (London), 219 (1953) 328. 11 N. Brown, Phil. Mag., 4 (1959) 693. 12 M. M. Shea and N. S. Stoloff, to be published. 13 C. S. Barrett, Trans. AIME, 6 (1954) 1003. 14 T. B. Massalski and C. S. Barrett, Trans, AIME, 9 (1957)455. 15 H. McI. Clark, Phil. Mag., 16 (1967) 853. 16 N. S. Stoloffand R. G. Davies, Acta Met., 12 (1964) 473. 17 M. J. Marcinkowski, in Electron Microscopy and the Strength of Crystals, Wiley, New York, 1963, p. 333. 18 R.B. Waterhouse and D. Grubb, J. Inst. Metals, 91 (1962/63) 216. 19 C.M. Preece and A. R. C. Westwood, Trans. Am. Soc. Metals, 62 (1969) 418.

253

Fragilisation des laitons b~ta par les mOtaux liquides et par l'ammoniaque

VersprSdung yon Beta-Messinglegierungen durch flfissige Metalle und wdssrige Ammoniakl6sungen

La d6formation par traction en pr6sence de divers m6taux liquides d'une sdrie d'alliages binaires et ternaires de la famille des laitons b6ta a 6t6 6tudi6e dans l'intervalle de temp6rature compris entre 25 et 300 °C. Des essais compl6mentaires ont 6t6 effectu6s en pr6sence de solutions aqueuses d'ammoniaque. La m6tallographie optique montre que la sensibilit6 plus ou moins grande de chaque alliage au milieu environnant, ainsi que le trajet des fissures, sont li6s au caract6re du glissement et aux ph6nom6nes de relaxation de contraintes aux joints de grains. Les additions de mangan6se restreignent le glissement d6vi6 et la formation de crans aux joints de grains et accentuent ainsi la sensibilit6/l la fragilisation du laiton b~ta par le gallium liquide. L'addition de nickel produit un effet inverse sur les propri6t6s pr6c6dentes. Dans les monocristaux de laiton bata on observe 6galement une sdv6re fragilisation dans le gallium, la rupture se faisant par clivage suivant les plans {100}. Le mercure ne produit pas une fragilisation aussi prononcde des monocristaux, ce qui est en accord avec le caract6re intergranulaire des fissures observ6es dans les polycristaux de laiton b6ta mouill6s par le mercure.

Die Zugverformung einiger bin~irer und tern~irer beta-Messinglegierungen wurde im Temperaturbereich zwischen 250C und 300°C mit mehreren fliissigen Metallen als Versuchsumgebung untersucht~ Weitere Verformungsexperimente wurden in w~issrigen Ammoniakl6sungen durchgefiihrt. Das AusmaB der Suszeptibilit~it der Legierungen zu den Versuchsumgebungen und die beobachtete Ril3ausbreitung h~ingen mit dem Gleitcharakter und den Ph~inomenen der Spannungsrelaxation an Korngrenzen, wie sie durch die optische Metallographie sichtbar gemacht werden, zusammen. Manganbeimischung fiihrt zu verminderter Quergleitung und Sprungbildung in den Korngrenzen; sie erh6ht die Suszeptibilit~it des beta-Messings zu Galliuml6sungen. Nickelbeimengungen haben auf diese Eigenschaften den entgegengesetzten Effekt. Beta-Messing-Einkristalle zeigen in Gallium starke Verspr6dung und Spaltung auf {100}-Ebenen. Quecksilber hat keine so starke Verspr6dung der Einkristalle zur Folge. Dieses Ergebnis ist konsistent mit der in polykristallinem, mit Quecksilber benetztem beta-Messing beobachteten intergranularen Ril3ausbreitung.