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Effect of iron additions on the sub-and microstructure of vacuum copper condensates.

Introduction

Electron beam deposition of copper in certain conditions results in the formation of condensates with the grains having a twinned substructure (1). The thickness of the twinned domains made reach nanoscale values with the variation of the deposition conditions. With this nanotwinned substructure, the yield strength of the vacuum copper condensates may equal approximately 1500 MPa. The in-crease of the level of the strengths of copies is accompanied by a decrease of plasticity of this metal and, in contrast to the materials with the nanograined structure, nanotwinned copper retains the plasticity prior to fracture on the level of 10-12% (2)-(5).

Because of these properties of nanotwinned copper, the latter can be regarded as a promising structural material, combining high values of strength, plasticity, heat and electrical conductivity, typical of annealed copper (2).

The conditions of formation of the twinned substructure of the vacuum condensates of the FCC metals and the factors which may influence the thickness of the twinned interlayer is (density of the twinned boundaries) have been investigated in (6), (7). The experimental investigations and thermodynamic analysis of the process of crystal growth from the vapour phase shows that the substrate temperature and the stacking fault energy on the close-packed planes {111} have a strong effect on the density of the twinned boundaries (7). The twinned boundaries have been found mainly in the crystals stretched in the direction <111>, i.e., in the direction normal to the growth axis of the crystals.

This orientation of the twinned boundaries indicates that the twin-oriented domains form as a result of the formation of errors in the sequence of laying the atomic layers {111} of the FCC lattice of copper during grain growth. These errors at the solidification front may form as a result of the small difference of the free energies of the nucleation islands with the regular and irregular packing of the atoms in relation to the crystal surface (7).

Since the nucleation of the twin-oriented domains takes place at the growth front of the crystal, the formation of the twin substructure is possible only if the grain growth takes place in the <111>crystallographic direction. If the crystallographic direction of the growth of crystals differs for some reason, the formation of the growth will be more difficult.

Taking these considerations into account, it is assumed that the probability of formation of the nuclei with the regular packing of the atoms in relation to the atomic layer on which the nuclei form may influence, in addition to other factors, also the formation of the impurity atoms on the crystal surface. From the viewpoint, it is most interesting to examine the atoms of the components in-soluble in copper. Taking these considerations into account, in this work, investigations were carried out into the effect of introduction of the iron atoms into the copper vapour flow on the characteristics of the sub-and microstructure of the vacuum condensates of copper. Taking into account the equilibrium diagram of the Cu-Fe system, the solubility of iron in copper at T = 20[degrees]C is 0.1 at.% (8).

Preparation of the specimens and experimental procedure

Copper condensates with the iron content of 1.7, 4.2 and 7.0 wt.% with the thickness of 40-55 [micro]m were produced by the method of electron beam evaporation in vacuum of components followed by joint the position of the vapour flows on the substrate (9). In cooling, copper was evaporated from two independent sources - copper and iron ingots with the purity of respectively 99.6 and 99.8%. All the condensates were deposited in the same technological conditions, ensuring the formation of the nanotwinned substructure in the copper condensates (substrate temperature 260[degrees]C, the deposition rate of the condensates 2 [micro]m [min.sup.-1] (30 nm [s.sup.-1]); the pressure of the residual gases in the chamber did not exceed 1*[10.sup.-3] Pa; the substrate--target distance was 300 mm).

To investigate the structure and mechanical properties of the condensates, the specimens were prepared in the form of thin foils and the coating. The condensates were separated from the substrate by depositing a thin layer of sodium chloride.

The characteristics of the microstructure of the condensates and the chemical composition were investigated using a scanning electron microscope (CamScan-4), fitted with an energy-dispersing analyser (Energy 200) and a transmission electron microscope (Hitachi 800). The phase composition and texture were determined by the methods of x-ray diffractometry (DRON-4 diffraction meter with a texture attachment) in [CoK.sub.[alpha]]-radiation.

The microhardness of the condensates was measured on transverse sections of the specimens using a Polivar Met optical microscope fitted with an attachment for measuring hard-ness at a constant load on the indentor of 0.098N, duration 10 seconds. The accuracy of measurements was [+ or -]10%.

Experimental results and discussion

Structure of the condensates

The effect of the iron content of the condensates on the structure of the condensates is shown in Figure 1 and 2. The pure copper condensates are characterised by the columnar shape of the grains with the diameter of approximately 0.8 [micro]m, fully fragmented in the layers of the twinned domains, with the mean thickness of approximately 80 nm (Figure 1a and 2a). According to the micro-diffraction pattern, the twinning planes of the condensates are normal to the direction of crystal growth, i.e., atomic planes {111}.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

In the Cu-1.7% Fe condensates, a large proportion of the crystals have the columnar structure with fully fragmented twins, oriented in the direction normal to their growth <111>(Figure 2b). However, the mean thickness of the crystals in the Cu-1.7% Fe condensates decreases to 0.5 [micro]m (Figure 1b) and that of the twins to 30 nm, which is approximately 2.5 times smaller than in the pure copper condensates.

With the increase of the iron content of copper to 4.2 wt.%, the mean thickness of the crystals and the twins remains approximately the same as in the Cu-1.7% Fe condensates (Figure 1c); however, the number of these crystals in the nanotwinned substructure is approximately halved (Figure 3). A further increase of the iron content of 7.08% results in the formation of the nanograined structure and also in a large decrease of the number of crystals with the nanotwinned substructure (Fig. 1 and 3).

[FIGURE 3 OMITTED]

The investigations of the texture of the condensates with the different mass fraction of iron show the following relationship. The pure copper condensates are characterised by a distinctive maximum of density of the poles of the planes (111), situated in the vicinity of the centre of the pole figure, and by the relatively narrow radial distribution of the density of the poles of the plates (100) and (110) manifested in the form of continuous rings (Figure 1a) positioned at angular distances of respectively 55 and 35[degrees]. This configuration of the density of the poles corresponds to the texture of the 'fibre' of the type <111>; this indicates that the formation of the copper condensates takes place of the result of the growth of the copper grains mostly in the <111>crystallographic direction.

The addition of iron into copper results in the degeneration of the <111>fibrous texture. With increase of the iron content the maximum of pole density (101) in the centre of the pole figure greatly widens and weakens. This was accompanied by excessive the spreading of the distribution of the density of the poles (100) (110) and the appearance of the maximum of pole density in the vicinity of the centre of the pole figure (Figure 4b, c). The same distribution corresponds to the formation in the condensates of a fibrous multicomponent texture of the type <111 >+ <100>+ <110>.

[FIGURE 4 OMITTED]

Since the fibrous texture of the condensates forms as a result of the directional growth of columnar grains, on the basis of the analysis of the distribution of pole density may be assumed that in the case of pure copper grain growth takes place in the <111> direction, and when iron atoms added to copper, the direction of grain growth may coincide with both the <111> direction and <100>and <100>.

The experimental data shows that the effect of iron on the characteristics of the structure of the condensates is non-monotonic: the addition of a small amount of impurity atoms to copper results in the situation in which the crystallographic the orientation of the columnar grains remains the same as in pure copper, in the direction <111>, and the density of the twinned boundaries of these crystals greatly increases.

When the iron concentration is increased above some critical value (approximately 4 wt.%), the number of the columnar crystals, oriented along the <111> direction, greatly decrease. This was accompanied by the formation of grains without the nanotwinned structure, i.e., the microstructure of the vacuum condensates became bimodal.

The experimental results show that the formation of the atoms of the iron impurity at the growth front of the crystals relates to the formation of stacking faults, parallel to the (111) atomic layers. Consequently, the density of the twinned boundaries increases.

With the increase of the iron concentration, the density of the twinned boundaries should increase at a high iron content of copper, and under the condition that the addition of cop-per has no effect on the process of growth of the columnar crystals. However, when alloy in the material, the ratio between the surface images of the different crystallographic faces of the crystals may change (10).

Thus, it may be assumed that at some critical concentration of iron in copper, the ratio of the surface images of the phases of the crystal with low crystallographic indexes undergoes quantitative changes--the distinctive anisotropy, typical of the pure copper crystals, disappears. Consequently, the values of the growth rate of the crystals in the directions <100>, <110>and <111> differ only slightly.

As indicated by the texture analysis results, when the iron concentration is increased, the extent of the generation of the one-component axial texture <111> increases with the increase of the iron concentration, and columnar crystals with the crystallography growth directions other than <111>may form.

Since the formation of the twinned substructure requires that the twinning plane is situated parallel to the growth found, the crystal growth in the crystallographic directions other than <111> leads to almost complete disappearance of the twinned substructure. Consequently, the fraction of the crystals with the twinned substructure decreases with increasing iron concentration.

Thus, the increase of the iron content of the copper condensates leaves the quantitative changes of the characteristics of the micro-structure and substructure. The condensate with the iron content of up to 4 wt.% are characterised by the columnar crystals of the submicron size and by the polydomain twinned substructure, whereas with the increase of the iron content of copper the number of the crystals with the twinned substructure gradually decreases. As the iron content of the copper condensates of seven wt.%, the nanograined structure appears, mostly with the mono-domain substructure.

Microhardness of the condensates

The results of investigations of the characteristics of the microstructure of the copper condensates with different iron content indicate that the mechanical properties of the condensates differ. In fact, the addition of 1.7 wt.% of iron to copper leads to almost doubling of the microhardness of the condensates, by the microhardness values are not change within the measurement error range with a further increase of the iron content (Figure 5). This circumstance and the absence of the particles of FCC iron in matrix copper and the Cu--Fe condensates cause that the dispersion hardening factor can be ignored.

[FIGURE 5 OMITTED]

May also be assumed that doubling of the microhardness of the Cu--1.7% Fe condensates, in comparison with the copper condensates, is too large and is not characteristics of the solid solution hardening of the metals by interstitial elements, especially if it is taken into account that the atomic radii of copper and iron are almost the same, i.e., the strength properties of the investigated Cu--Fe condensates are determined not only by the condition of the solid solution of iron in FCC copper but mostly by the characteristics of their microstructure.

The controlling effect of the nanotwinned substructure on the increase of the micro-hardness of the vacuum copper condensates was shown in (1)-(5), (7), especially for the condensates deposited from the vapour phase: with a decrease of the thickness of the twins to 50 nm, the microhardness of the twins increased to 1.9 GPa. The hardening of the materials with the nanotwinned substructure is attributed mostly to the fact that the twinned boundaries represent effective barriers for the displacement of the dislocations in the body of the grain (7), (11).

The thickness of the twins in the investigated Cu--1.7% Fe condensates was 2.5 times smaller in comparison with the copper condensates; the fraction of these crystals was controlling, i.e., it may be assumed that the strength properties of this condensates are determined mainly by their twinned substructure.

With the increase of the iron content, the ratio of the number of crystals with the polydomain and mono domain substructure greatly changes, and the Cu--7% Fe condensates are characterised by the formation of mostly the mono-domain grains with the size of approximately 60 nm.

The strength properties of the material with these grain sizes should be higher in comparison with the condensates in which the grain size is of the submicron scale. However, as indicated by the results, the strength properties of this condensates are almost the same as those of the Cu--1.7% Fe condensates. This may be caused by the fact that the mechanisms of the formation of the materials with the nanotwinned substructure and the nanograined mono-domain microstructure differ and it should also be reflected in the mechanical properties.

Thus, the expected increase of the strengths of the Cu--7% Fe condensates as a result of decreasing the grain size is greatly restricted by the factors associated with the decrease of the volume fraction of the grains with the nanotwinned substructure.

Conclusions

1. The experimental results show that the effect of iron the characteristics of the microstructure of copper in combined electron beam deposition in vacuum can be divided into two types of concentration ranges. Low carbon concentrations (up to 1.7 wt.%) in-creased intensity of the twinned boundaries of the crystals.

2. Experimental results also show that at the iron concentration in the vicinity of 4 wt.%, the <111> fibrous structure, typical of copper with small in additions, is degenerated and this is accompanied by the formation of the fibrous texture of the type <111>+<100>+ <110> in which the fraction of the components <100> and <110> increases with increasing iron concentration.

3. It was shown that the columnar crystals, oriented along <100>and <110> has no twinned structure and, consequently, the microstructure of the vacuum condensates undergoes quantitative changes from the poly-domain the mono-domain substructure of the grain. Consequently, the changes of the mechanical properties of the copper condensates as a result of adding the iron atoms are determined, on the one side, by the increase of the density of the twinned boundaries of the crystals, growing in the <111> direction and on the other side by the decrease of the fraction of this crystals. As indicated by the experimental results, the increase of the density of the twinned boundaries in the crystal at a small decrease of the fraction of this crystals is accompanied by a large in-crease of the microhardness of the material. The reduction of the fraction of this crystals greatly restricts the hardening of the copper condensates with a further increase of the iron concentration.

References

(1.) Ustinov A.I., et al., Sovremen. Elektrometallurgiya, 2007, No. 4, 19-26.

(2.) Lu L., et al.,Science, 2004, volume 304, 422.

(3.) Ma E., et al., Appl. Phys. Lett., 2004 No. 21, 4932.

(4.) Lu L., et al., Scripta Materialia, 2005, vol. 52, 989-994.

(5.) Lu L., et al., Acta Materialia, 2005, No. 53, 2169-2179.

(6.) Zhou X.W., et al., ibid 1999, No. 3, 1063-1078.

(7.) Zhang X., et al., ibid, 2004, 52, 995-1002.

(8.) Shtremel' M.A., The strength of alloys. Deformation, Moscow Institute of Steel and Alloys, Moscow, 1997.

(9.) Paton B.E. and Movchan V.A., Sov. Technol. Review, 1991, No. 2, 43-64.

(10.) Honigman B., The growth and shape of crystals, IL, Moscow, 1961.

(11.) Lu L., et al., Science, 2009, 323, 607-610.

A.I. Ustinov, E.V. Fesyun, T.V. Mel'nichenko and A.A. Nekrasov

E.O. Paton Electric Welding Institute, Kiev G.V. Kurdyumov Institute of Physics of Metals,Kiev
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Title Annotation:ELECTRON BEAM PROCESSES
Author:Ustinov, A.I.; Fesyun, E.V.; Mel'nichenko, T.V.; Nekrasov, A.A.
Publication:Advances in Electrometallurgy
Date:Jul 1, 2010
Words:2789
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