Mechanical alloying of copper-BCC element mixtures

Scripta METALLURGICA et MATERIALIA

Vol.

24, pp. 1701-1706, 1990 Printed in the U.S.A.

Pergamon Press plc All rights reserved

MECHANICAL ALLOYING OF COPPER-BCC ELEMENT MIXTURES

Institute

of Structural

D.G. Morris and M,A. Morris, Metallurgy, University of Neuchitel,

2000 Neuch~tel, Switzerland

(Received May 14, 1990) (Revised July 2, 1990) Introduction The mechanical alloying of copper with insoluble phase additions is a potential means to the development of high strength alloys maintaining high electrical and thermal conductivity. Earlier studies have examined the mechanical alloying behaviour of copper with the relatively insoluble metals chromium or niobium (1,2,3), and in these cases it has been shown that the added elements can be distributed very finely t h r o u g h o u t the copper leading to strong materials. In cases where non-metallic additions are used, for example metal borides, boron or carbon, it has been seen that the rate of refinement and distribution of the additions can be very much slower (3,4). In the present study, a range of copper materials containing different bee elemental additions is prepared by mechanical alloying techniques in order to examine the mechanical, physical or chemical factors which affect the rate and extent of distribution of the bcc element addition within the copper matrix. Experimen~i~l Mechanical alloying has been carried out using the same methods as described previously (2-4.). Briefly, weighed amounts of elemental powders, about 1501.tin in size, were sealed under argon in a container together with hardened steel bails in a weight ratio of balls/powder of about 4.; the mixture was milled using a planetary ball mill (Fritsch, Pulverisette No7) for a period of 12h. Powders obtained in this way were examined by scanning electron microscopy and by x-ray diffraction. Subsequently, powders were compacted by first cold pressing to a billet of about 60-70% density, heating under inert gas to 700"C for 20 rains, and then extruding through a conical die at an extrusion ratio of 10/1. The bars obtained were examined to determine mechanical properties and microstructures. The alloys examined here each contain 5 volume percent addition of a given bcc element to copper. The elements considered are shown in Table I, together with values for the hardness of the pure elements and their melting temperatures (5). The hardness and temperature values are simple indications of the wide range of c h a r a c t e r i s t i c s of these bee elements (6). While c o n s i d e r i n g only the initial, u n d e f o r m e d hardness of the elements is certainly an o v e r s i m p l i f i c a t i o n of total mechanical behaviour, the trends of these hardness values are the same as for hardnesses in the work-hardened state as well as, inversely, the trends shown by ductilities and toughnesses or impact strengths, for the limited cases where all this information is available (5).

1701 0036-974B/90 $5.00 + .00 Copyright (c) 1990 Pergamon Press plc

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MECHANICAL

ALLOYING

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TABLE I Hardnesses and Melting Temperatures of Bcc Elemental Additions made to Copper compared with corresponding values for Copper Element

Cr

Mo

Nb

Ta

V

W

Cu

Elemental Hardness(Hv)

130

200

70

75

50

360

50

Melting Temperature(°C)

1857

2620

2467

2980

1902

3400

1085

The powders obtained after milling for 12 hours typically contained a uniform distribution of the bcc element throughout the copper matrix, Fig. 1. However, as seen in Fig. 1, the degree of refinement achieved varied greatly from case to case. The scanning electron micrographs of Fig. 1 were obtained using back-scattered electron contrast and show the dark, finely-distributed particles of the light atom vanadium and the bright, coarsely-distributed particles of the heavy atom tungsten. When the materials were examined in this way it was found that the molybdenum and tungsten additions were poorly refined, the vanadium and chromium additions were fairly well refined, and the tantalum and niobium additions were extremely finely distributed. ( It is clear that these comments refer to the degrees of refinement of the added bec elements in the copper matrix achieved after 12h milling and are unlikely to be ultimate degrees of refinement as could be possible by further milling. It would surely be possible to obtain a much better refinement of molybdenum and tungsten in copper by using longer milling times. However, on a practical note, it should be pointed out that when milling for somewhat longer than the 12 hours used here the powders all cold welded to the sides of the milling containers, thus effectively preventing the use of significantly longer milling periods. ) After extrusion, the fineness of the microstructures obtained depended directly on the degree of refinement of the bcc element in the copper. As seen in Fig. 2, the microstructure of the copper-vanadium material showed a fine grain size, about 100nm, with the grain boundaries pinned by vanadium particles 5-10nm in size. The coppermolybdenum material contained much coarser grains, about 200-300nm in size, with molybdenum particles ranging from about 20nm to 500nm. It has been shown that the grain size can be related fairly well to the value predicted by the Zener model of grain boundary pinning by fine particles (6). Mechanical properties of the powder, as-extruded bar, and the bar after further heat treatment at 1000°C are shown in Fig. 3. Hardness testing on mounted and polished powders was performed using a 25g load giving imprints of size 10-151.tin, much smaller than the 50-2001.tm particle size. For the consolidated materials a 2008 load was used. The properties after annealing are determined both by the initial refinement of the strengthening particles as well as by the resistance to coarsening of these particles (6). For the milled powders, as well as essentially for the materials after extrusion, it is the degree of refinement achieved during milling which determines the strength of the materials. The mechanical properties of the extruded materials, as well as after heat treatments at high temperatures are shown in Fig.4. In this figure it is clearly seen that the materials containing niobium and tantalum, where good particle refinement was achieved during milling, have the highest strengths; the materials containing

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MECHANICAL ALLOYING

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FIG. 1 Back-scattered electron images by scanning electron microscopy showing fairly good refinement of vanadium (a), and poor refinement of tungsten (b) in copper after milling for 12h.

Transmission electron micrographs extruded copper-vanadium material copper-molybdenum.

FIG. 2 showing the very fine microstructure of the and the much coarser microstructure of extruded

MECHANICAL ALLOYING

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FIG. 3 Hardness and yield strengths of milled powders,as-extruded materials, and after subsequent heat treatment at 1000°C.

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6 FIG. 5 Comparison of the degree of strengthening (refinement of elemental particles) with elemental hardness, maximum solid solubility in copper and the solubility in liquid copper at its melting point.

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FIG. 6 Comparison of the degree of mixing (refinement) with the heat of formation of a Cu-bcc element intermetallic phase.

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MECHANICAL

tungsten and m o l y b d e n u m , where milling, have the lowest strengths; are i n t e r m e d i a t e .

ALLOYING

1705

poor particle r e f i n e m e n t was achieved during the materials containing vanadium and chromium

Di~q:ussion Milling a range of bcc elements with copper under identical conditions has been shown to lead to widely differing degrees of refinement and mechanical properties according to the bcc element considered. Examination of Fig. 4, and reference to Table I, shows that this variation in behaviour cannot be explained in terms of the hardness of the element nor in terms of its melting point. The same conclusion is valid whether considering the hardnesses of the elements in the soft state (as in Table I), or when considering instead the w o r k - h a r d e n e d hardnesses. Fig. 5 further shows that the degree of strengthening, or of particle refinement, can neither be related to the solid solubility in copper nor to the solubility in liquid copper (5,7). For example, good refinement and strengthening by the tantalum and the niobium corresponds to cases of low to medium solid solubility in copper and also to medium solubility in liquid copper at its melting point. Some of the other elements studied have similarly low or higher solubilities in solid or liquid copper, and it must be concluded that these e q u i l i b r i u m solubilities are not important criteria for determining the degree of refinement of the bcc element during milling. Fig. 6 relates the degree of mixing or refinement during milling to the heat of formation of an (imaginary) Cu-bcc element i n t e r m e t a l l i c c o m p o u n d (8). This heat of f o r m a t i o n can be c o n s i d e r e d to be proportional to the chemical interaction of a copper atom with the bcc element atom. It is clearly seen that the degree of refinement during milling is related to the tendency to a negative heat of formation, that is to a preference for forming copper-bcc element bonds. A further clue to understanding the processes leading to the breakdown of the bcc element particles in the copper matrix is provided by x-ray diffraction analysis of the milled powder. An example for the copper-tan;alum powder is given in Fig. 7 in the form of a Hall plot. This plot shows the x-ray diffraction peak half-width (13) in terms of the diffraction Bragg angle (0): the intercept at 0 = 0 gives a measure of the particle size (grain size, subgrain size, etc.), whilst the slope is a measure of the accumulated strain. The experimental slope of the lines in Fig. 7 is evidence of severe strain in both the tantalum and the copper. The intercepts indicate a very large particle size for copper and a particle size of about 14nm for tantalum. The positions of the x-ray peaks are hardly displaced from the values expected for pure copper and tantalum, showing that the lattice parameters have changed only slightly as a consequence of the milling: the value for the copper matrix has increased by about 0.3% suggesting that the solubility of the large tantalum atom in copper has only been increased to a small extent. Conclusions An explanation of the mixing of copper with bcc elements during milling can be proposed based on: (i) the x-ray diffraction evidence, namely highly strained matrix with included bcc elemental particles and only minor dissolution of the bcc element in the copper matrix and (ii) the observed correlation between degree of refinement of the bcc element particles and the tendency to a negative energy of formation of an intermetallic compound with mixed Cu-bcc element bonds. Ball impacts during milling lead to extensive deformation of the matrix material. If there is only a weak chemical interaction across the matrix-particle interface there will be a tendency for the matrix

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to flow around the only-slightly deformed particles. If, however, there is a strong chemical interaction across the matrix-particle interface, the forces applied to the matrix .will be transferred across the interface and tend to deform the particle more effectively. An example of the first case is tungsten in copper; another example is that of oxide particles in superalloys, which are known to mix poorly when starting from single crystalline particles. An example of the second case is tantalum or niobium in copper. The selection of suitable dispersion strengthening particles, where there is a desire to obtain a very fine distribution, should consider such chemical factors affecting the rate of particle breakdown. References .

2.

3. 4.

. .

.

8.

A.N. Patel and S. Diamond, Mater. Sci. and Eng., A98, 329 (1988). D.G. Morris and M.A. Morris, Mater. Sci. and Eng., AI04. 201 (1988). M.A. Morris and D.G. Morris, Mater. Sci. and Eng., A I l I , 115 (1989). M.A. Morris and D.G. Morris, Proc. 7th Int. BNF Conf., "The Materials Revolution through the 90's", Iuly 1989, Oxford, BNF Publication, Wantage, England (1989). C.L Smithells, Metals Reference Book, 6th Edition, Ed. E.A. Brandes, Butterworths, London (1983). D.G. Morris and M.A. Morris, ASM Conf. on Structural Applications of Mechanical Alloying, Myrtle Beach, South Carolina, Eds. F.H. Froes and LL DeBarbadillo, ASM, Metals Park, Ohio, in press. Binary Phase Diagrams, Ed. T.B. Massalski, ASM, Metals Park, Ohio (1986). A.R. Miedema, Philips Technical Review, 36. 217 (1976).

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FIG. 7 Hall plot relating the half-widths (13) of x-ray diffraction peaks for both Ta and Cu to the Bragg angle (0): as-milled powder.