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ISSN 10274510, Journal of Surface Investigation. Xray, Synchrotron and Neutron Techniques, 2013, Vol. 7, No. 3, pp. 531–535. © Pleiades Publishing, Ltd., 2013. Original Russian Text © V.S. Kovivchak, T.V. Panova, K.A. Mikhailov, E.V. Knyazev, 2013, published in Poverkhnost’. Rentgenovskie, Sinkhrotronnye i Neitronnye Issledovaniya, 2013, No. 6, pp. 34–38.

Features of the Surface Morphology of Brass and Bronze upon Irradiation with a High Power Ion Beam V. S. Kovivchak, T. V. Panova, K. A. Mikhailov, and E. V. Knyazev Omsk State University, Omsk, Russia Received July 20, 2012

Abstract—The change in the morphology and surface composition of copper alloys (brass LS 591, bronzes BrOS 1010 and BrAJ 94) upon irradiation with a high power ion beam of nanosecond duration is studied. It is shown that craters are mainly formed at the location of lead or sulfur inclusions. The reverse deposition of zinc onto the surface of brass is found. Possible mechanisms of these phenomena are considered. DOI: 10.1134/S1027451013020365

INTRODUCTION Most modern alloys used in industry consist of components with different (sometimes differing by orders of magnitude) volatilities. Studying the effect of volatile components on the formation of the surface morphology of alloys and the change in the composi tion under the action of a high power ion beam (HPIB) is of scientific and applied interest. It makes it possible to more accurately define the mechanisms of the formation of the surface relief of alloys with com plex composition and to optimize the parameters of HPIB action from the point of view of minimizing the surface roughness. It was shown earlier that the pres ence of highly volatile impurities in an alloy (metal) can favor the formation of craters on the surface upon intense ion beam action [1–3]. However, the role of highly volatile components (in a wide concentration range) and the homogeneity of their volume distribu tion in the formation of the surface relief upon HPIB action have been barely studied to date. Such studies should be better performed on model materials, i.e., alloys with a low melting temperature containing components with distinctly different vola tilities. This makes it possible to track the change in the surface morphology in the beam currentdensity

range of practical importance upon irradiation which causes weak to intense evaporation of the components. EXPERIMENTAL Copperbased alloys were chosen as the objects of the study, these included: brass LS 591 (37.35–42.2% Zn, 0.8–1.9% Pb, and a concentration of impurities of no more than 0.75%, GOST 1019–47), bronze BrOS 1010 (9–11% Sn, 8–11% Pb, and a concentra tion of impurities of no more than 0.9%, GOST 613– 79), bronze BrAJ 94 (8–10% Al, 2–4% Fe, and a concentration of impurities of no more than 1.7%, GOST 18175–78) with easily fusible and highly vola tile components in their composition, e.g., zinc, lead, tin, and aluminum. The melting temperatures Tmelt of these alloys were 900, 1024, and 1040°С, respectively. The main thermodynamic parameters of the compo nents and impurities in the studied alloys are given in Table 1 [4]. The studied alloy samples were disks polished to a mirror shine with a diameter of 12 mm and a thickness of 2 mm, greatly exceeding the path of ions of the used beam in the given material. Irradiation was performed on a Temp accelerator by a proton–carbon (30% Н+

Main thermodynamic parameters of the impurity components of the studied alloys nm

Specific heat of evaporation, J/g

Tmelt/Tboil, °C

Pressure of the saturated vapor, Pa (T, °C)

Cu Fe Sn Al Pb Zn S

4752.9 6267 2496.6 10859 857.6 1763.5 327.4

1083/2543 1538/2872 231.9/2620 660.2/2520 327.4/1745 419.4/906.2 112.8/444.6

10–2 (1018) 10–4 (990) 10–2 (988) 10–1 (1016) 2 ×102 (1011) 105 (909) 105 (433)

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2 µm Fig. 1. Initial surface of brass LS 591.

and 70% С+) beam with an average energy of 300 keV, a duration of 60 ns, in the current density range of 20– 150 A/cm2 with a variation in the number of irradia tion pulses from 1 to 5. The surface of the irradiated copper alloys was studied by means of scanning elec tron microscopy and Xray microanalysis. RESULTS AND DISCUSSION The typical initial surface of brass LS 591 is shown in Fig. 1. Since lead is not completely soluble in liquid copper, copper alloys with lead after solidification contain lead inclusions, which are located, as a rule, at grain boundaries. The light regions in Fig. 1 corre spond to lead inclusions. Their characteristic size is ~3 µm. The surface density of these inclusions is 2.5 × 107 cm–2. On the whole, the initial surface of bronze BrOS 1010 is analogous to that of the brass with the only difference being that, due to the higher lead con centration, the characteristic size of the lead inclu sions and the surface density are, respectively, ~10 µm and 5.1 × 105 cm–2. Unlike LS 591 and BrOS 1010, the initial surface of BrAJ 94 is more homogeneous and does not contain observable local inclusions of impurities. Since copper, as the basis of alloys, has low volatility, the element composition of the surface lay ers may be conveniently characterized by the ratio (R) of the weight percent of the component contents to copper, which were obtained by Xray microanalysis. We have RZn/Cu = 0.63 for nonirradiated brass LS 591, and in the region of lead inclusions (RPb/Cu = 9.33) the zinc content is also increased RZn/Cu = 0.82. The exposure of brass LS 591 to a single HPIB pulse with a current density of ~20 A/cm2 leads to the melt ing of lead inclusions localized on the surface, and to a small zinc depletion of the surface as indicated by RZn/Cu = 0.62. An increase in the current density to 50 A/cm2 leads to intense melting, evaporation and ejection of lead from its surface inclusions with the formation of

craters with a flat bottom and a deep hole at its center (Fig. 2a). The lead concentration in the region of the hole (RPb/Cu = 0.005) decreases sharply. The molten surface layer between the craters (RZn/Cu = 0.625) con tains droplets consisting mainly of lead (RPb/Cu = 6.89), a part of which is localized in hollows on the surface layer. In addition to the craters described above, there are a small number of shallow craters with a flat bottom. As a rule, there is a lead droplet at the center of such craters. The number of craters increases under exposure to a HPIB with a current density of 100 A/cm2, including those with a shallow central hole. A high content of lead (RPb/Cu = 1.04) and zinc (RZn/Cu = 0.89) in the region of the hole is characteristic for such craters. This indicates the important role of zinc vapors in the ejection of lead melt, which have a higher pressure than lead vapors. At the same time, the density of shal low flat craters with lead droplets located at their cen ters increases by nearly an order of magnitude (Fig. 2b). Arrays of smaller droplets with a size of 50– 100 nm are formed around these drops with sizes on the order of several micrometers. The surface between the craters is slightly zincenriched (RZn/Cu = 0.64). Single microcracks with a length of ~5 µm emerge near shallow craters. It is probable that their emer gence is associated with the action of stretching stresses arising at the surfacelayer cooling stage, when its temperature is in the range of 300–700°C, which corresponds to the brittleness zone of brass. An increase in the beam current density to 150 A/cm2 upon single irradiation leads to a consider able change in the surface morphology. In addition to craters with a flat bottom and a deep hole at the center, a developed surface relief is observed, which is proba bly formed by the unification of craters with different shapes and sizes and the complex hydrodynamic motion of the melt (Fig. 2c). Almost the entire irradi ated surface is zinc enriched (RZn/Cu = 0.666). The increase in the number of irradiation pulses to three for a constant beam current density leads to the emergence of ledges of different shapes with charac teristic sizes of ~10 µm and arrays of droplets with a size from 0.1 to 5 µm on the surface (Fig. 2d) Their density reaches 1.7 × 108 cm–2. The irradiated surface is strongly zinc enriched (RZn/Cu = 0.998) and gains a metallic hue characteristic of zinc. Droplets with a size of ~1 µm consist mainly of copper (RZn/Cu = 0.242). It is not possible to reliably determine the composition of droplets with a smaller size (~100 nm) under the given conditions because of the strong zinc enrichment of the surface, on which these droplets lie. Droplets with a size of ~1 µm are also zinc depleted under exposure of the brass to pulsed laser radiation [5]. This is associ ated with the formation of droplets from the molten brass layer depleted of zinc due to hydrodynamic detachment. The formation of smallsized droplets (~100 nm) consisting mainly of zinc under laser irra diation is linked with the condensation of zinc vapors

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Fig. 2. Surface of brass LS 591 after HPIB exposure with a current density of: (a) 50 A/cm2, 1 pulse, (b) 100 A/cm2, 1 pulse, (c) 150 A/cm2, 1 pulse, (d) 150 A/cm2, 3 pulses.

on nanoparticles injected from the melt. The forma tion of such a developed relief under repeated HPIB exposure is probably associated with the effect of the recoil momentum of the intensely evaporating zinc leading to the development of instability on the surface of the melt. Significant zinc enrichment of the surface is possibly associated with its reverse deposition onto the surface due to condensation, coagulation of the droplets as a result of cooling of the zinc vapors during expansion in vacuum, as was theoretically shown for the case of the evaporation of a metal target by an elec tron beam [6]. For nonirradiated bronze BrOS 1010 RSn/Cu = 0.105 and in the region of lead inclusions RPb/Cu = 10.06. The exposure of bronze BrOS 1010 to a pulse with a current density of ~50 A/cm2, the same as for the brass, leads to the formation of craters with differ ent shapes and depths in places of the localization of lead inclusions (Fig. 3a). The tin concentration on the surface between the craters barely changes. An increase in the current density to 100 A/cm2 leads to the emergence of clearly formed craters with a flat bot tom (Fig. 3b), the lead concentration in the central part decreases (RPb/Cu = 5.72) probably because of the

ejection of melt. The tin concentration increases on the ridges of a part of the craters (RSn/Cu = 0.126). The crater density increases at a current density of 150 A/cm2. A high lead concentration is still observed in the central part of craters and a high tin concentra tion, on the ridges of the craters. The internal crater walls have a more developed surface than that of the craters on the brass (Fig. 3c). A considerable area of the bronze surface between the craters under such irra diation modes is tin depleted (RSn/Cu = 0.085). An increase in the number of irradiation pulses to three at the same beam current density leads to an increase in the surface roughness due to the formation of craters with a size of ~20 µm and ledges with a char acteristic size of ~10 µm, and also individual particles of analogous size (Fig. 3d). The decrease in the lead concentration in the central part of the craters (RPb/Cu = 1.22) is probably associated with its signifi cant ejection when the surface temperature reaches that of boiling lead. Tin is also removed from this region as indicated by the decrease in its concentration (RSn/Cu = 0.089). The most part of the surface clearly shows regions of solidificated melt with different shapes and sizes, which consist mainly of lead (RPb/Cu = 0.552).


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20 µm (b)

10 µm

(c)

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Fig. 3. Surface of bronze BrOS 1010 after HPIB exposure with a current density of: (a) 50 A/cm2, 1 pulse, (b) 100 A/cm2, 1 pulse, (c) 150 A/cm2, 1 pulse, (d) 150 A/cm2, 3 pulses.

The lead concentration outside these regions is RPb/Cu = 0.163. Unlike the brass, this melt wets the bronze surface and individual spherical particles are not formed. The entire surface in the case of such an irradiation mode is tin enriched (RSn/Cu = 0.136) which is possibly linked to the higher volatility of cop per (in contrast to tin) at the temperatures reached on the surface. For nonirradiated bronze BrAJ 94 RAl/Cu = 0.082 and RFe/Cu = 0.025. The exposure of bronze BrAJ 94 to a pulse with a current density of ~50 A/cm2 leads to a decrease in the surface roughness due to the smooth ing of scratches and other defects of mechanical pol ishing during melting of the thin surface layer. No changes in the aluminum and iron concentration were observed in the surface layers. An increase in the current density to 100 A/cm2 causes the formation of smallsized flat craters on the surface. As a rule, they are situated at points of the localization of sulfur inclusions (RS/Cu = 0.025), creat ing considerable recoil momentum during evapora tion (Fig. 4a). The observed low iron enrichment of the irradiated region (RFe/Cu = 0.028) is due to its lower volatility in comparison with copper.

An increase in the current density to 150 A/cm2 leads to an increase in the crater surface density (Fig. 4b). Unlike brass LS 591 and bronze BrOS 1010, craters on BrAJ 94 are shallow, therefore the rough ness of its surface is less after PIB exposure. The sur face is still iron enriched (RFe/Cu = 0.029), and the alu minum concentration increases slightly (RAl/Cu = 0.083). An increase in the number of irradiation pulses to three at the same beam current density leads to a fur ther deepening of the craters containing sulfur inclu sions. The surface of the bronze between the craters becomes rougher than that under exposure to the irra diation a single pulse. The iron and aluminum con centrations slightly decrease which is probably associ ated with their more intense evaporation under repeated irradiation. In spite of the fact that BrAJ 94 contains aluminum (a more highly volatile element in comparison with copper and iron), its effect on the formation of craters on the surface of this alloy was not discovered. It is probable that the aluminum vapor pressure at the surface temperature reached during irradiation is not enough for the formation of signifi cant recoil momentum. The formation of craters on this alloy mainly occurs at places of the localization of


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Fig. 4. Surface of bronze BrAJ after HPIB exposure with a current density of: (a) 100 A/cm2, 1 pulse, (b) 150 A/cm2, 1 pulse.

a more volatile impurity, sulfur, not only under repeated irradiation as was found earlier for steel [2], but also under single exposure.

rule, are depleted of the most highly volatile compo nent of the alloy. REFERENCES

CONCLUSIONS Thereby, the formation of the morphology, first of all craters, on the surface of copper alloys containing elements with different volatilities under HPIB action depends not only on the vapor pressure during evapo ration of the most volatile component and their con centrations, but also on the homogeneity of the distri bution of these element over volume. The volatile ele ments localized in the surface layer with a thickness less than the projected path of beam ions most strongly affect the surface roughness under such exposure. Zinc enrichment of the surface observed for brass is probably due to condensation and the formation of zinc nanoparticles as a result of its cooling during expansion in vacuum. The micronsized particles of the alloy forming under repeated PIB irradiation, as a

1. V. A. Shulov, G. E. Remnev, N. A. Nochovnaya, et al., Poverkhnost, Fiz. Khim. Mekh., No. 7, 117 (1994). 2. T. J. Renk, P. P. Provencio, S. V. Pracad, et al., Proc. IEEE 92, 1057 (2004). 3. V. S. Kovivchak, T. V. Panova, V. I. Blinov, and R. B. Burlakov, Poverkhnost’, No. 4, 69 (2006). 4. Physical Values, The Handbook, Ed. by I. S. Grigor’ev and E. Z. Meilikhov (Energoatomizdat, Moscow, 1991) [in Russian]. 5. C. Liu, X. Mao, S. S. Mao, R. Greif, et al., Anal. Chem. 77, 6687 (2005). 6. N. B. Volkov, E. L. Fen’ko, and A. P. Yalovets, Tech. Phys. 55, 1389 (2010).

Translated by L. Mosina


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