POWDER METALLURGICAL MATERIALS, PARTS, AND COATII{GS

PROPERTIES HARDENING D.

OF

SINTERED

COPPER

S. A r e n s b u r g e r

PRECIPITATION-

ALLOYS and

S. M.

Letunovich

UDC 621.762:669.018.5

H e a t - t r e a t a b l e copper alloys for e l e c t r i c a l contacts a r e produced m a i n l y by casting techniques, and t h e r e is r e l a t i v e l y little information on P / M m a t e r i a l s of this type. Now in s i n t e r e d m a t e r i a l s the p r o c e s s e s of alloy f o r m a t i o n and subsequent heat t r e a t m e n t as a rule take place differently than in c a s t alloys of the s a m e composition, and it is t h e r e f o r e of i n t e r e s t to study the technological p a r a m e t e r s of m a n u f a c t u r e of P / M alloys and c o m p a r e the l a t t e r ' s p r o p e r t i e s with those of c a s t m a t e r i a l s . In the m a n u f a c t u r e of p r e c i p i t a t i o n - h a r d e n i n g copper alloys the m o s t widely used is alloying with z i r conium, b e r y l l i u m , titanium, and c h r o m i u m [1] - e l e m e n t s which effectively strengthen a copper m a t r i x without e x c e s s i v e l y lowering its e l e c t r i c a l conductivity. The solubility of each of t h e s e e l e m e n t s in copper is v a r i a b l e , d e c r e a s i n g rapidly with fall in t e m p e r a t u r e , which m a k e s it possible to obtain a s u p e r s a t u r a t e d solid solution by quenching an alloy and then bring about its decomposition by aging. In the p r e s e n t work a study was m a d e of s o m e p r o p e r t i e s of s i n t e r e d p r e c i p i t a t i o n - h a r d e n i n g alloys of copper with c h r o m i u m and titanium. Specimens w e r e produced, using a s - s u p p l i e d standard copper, c h r o m i u m , and titanium powders, by m e c h a n i c a l mixing of the powders, p r e s s i n g , sintering in a vacuum furnace, r e - p r e s s i n g , and quenching f r o m t e m p e r a t u r e s of e x i s t e n c e of s i n g l e - p h a s e s - s o l i d solutions of the alloying e l e m e n t s in copper. Taey were then subjected to f r e e c o m p r e s s i o n and extrusion at r o o m t e m p e r a t u r e , the d e g r e e of plastic d e f o r m a t i o n in both c a s e s being the s a m e . The concluding technological operation was the aging of the s p e c i m e n s . In s o m e e x p e r i m e n t s the o r d e r of the quenching and plastic working o p e r a t i o n s was r e v e r s e d . Compre,~sion was followed by h a r d n e s s m e a s u r e m e n t s , and extrusion by e l e c t r i c a l r e s i s t a n c e m e a s u r e m e n t s , in which a single R316 bridge was employed, and also by ultimate tensile strength and elongation m e a s u r e m e n t s . D e t e r m i n a t i o n s w e r e m a d e also of the c r y s t a l lattice p a r a m e t e r s of the alloys, and t h e i r m i c r o s t r u c t u r e s w e r e examined. The m a x i m u m solubility of c h r o m i u m in copper at 1075~ is 0.65 [2], and that of titanium at 870~ wt. ~ [3], and the alloys i n v e s t i g a t e d t h e r e f o r e contained up to 1% Cr and up to 6% Ti.

7.4

The h a r d n e s s of the c h r o m i u m - c o n t a i n i n g alloys in the p r e l i m i n a r y o p e r a t i o n s depended only on t h e i r d e g r e e of w o r k - h a r d e n i n g , and was t h e r e f o r e p r a c t i c a l l y the s a m e a f t e r sintering and quenching (Fig. 1). The titanium-containing alloys w e r e s i n t e r e d at and quenched f r o m 820~ T h e i r h a r d n e s s a f t e r quenching, which was l o w e r c o m p a r e d with the worked condition, although the fall was l e s s than that with the c h r o m i u m - c o n taining alloys, was 55 at 2, 80 at 4, and 90-95 HRB at 6"~ Ti (in the l a s t - n a m e d c a s e it was no different f r o m the h a r d n e s s of the worked a l l o y s ) . The hardening of such an alloy is due to the f o r m a t i o n of an alloyed solid solution of titanium and copper. To d e t e r m i n e o p t i m u m conditions of aging, the operation was p e r f o r m e d in the t e m p e r a t u r e r a n g e 300500~ for 1 h. The t e m p e r a t u r e dependence of h a r d n e s s a f t e r aging is r e p r e s e n t e d by c u r v e s with m a x i m a , which a r e p a r t i c u l a r l y pronounced for the alloys with c h r o m i u m (Fig. 2). The g r e a t e s t i n c r e a s e in h a r d n e s s o c c u r r e d in the c h r o m i u m content r a n g e f r o m 0.2 to 0.6~ at a t e m p e r a t u r e of 400~ (Fig. 2a), which c o r r e sponded to its m a x i m u m solubility in copper. Raising the amount of c h r o m i u m in copper to 1% produced a s m a l l e r effect.. The h a r d n e s s of the t i t a n i u m - c o n t a i n i n g alloys g r e w with r i s e in titanium content in the r a n g e f r o m 0.5 to 2~o, but at any given composition the influence of aging t e m p e r a t u r e was l e s s pronounced (Fig. 2b). The p h a s e t r a n s f o r m a t i o n s induced in the C u - C r alloys by quenching and aging w e r e studied by x - r a y diffraction, by m e a s u r i n g t h e i r c r y s t a l lattice p a r a m e t e r s . This involved recording, using a DRON-2 d i f f r a c t o m e t e r and unfiltered cobalt radiation, the positions of the m a x i m a of the (400)/~, (331)~, and (400)~ lines of

Tallin Polytechnic Institute. Translated from Proshkovaya Metallurgiya, No. 7 (283), pp. 27-31, July, 1986. Original article submitted June 12, 1985.

0038-5735/86/2507- 0553 $12.50 9 1986 Plenum Publishing Corporation

553

ffRB 6 ~0'

o

0

c

~-

~

o

#o 2~ I

0,4

~,,/,~ I

o,s cr,

Fig. 1. H a r d n e s s of C u - C r alloys of various

compositions after sintering (1), re-pressing (2), quenching (3), and c o m p r e s s i o n (4).

HR8 ,~! /2

b

a, nm 0=36/6

oo~~ 300

400

Fig. 2

t, ~

0,3812

0,3608 O,2

~

0,8

o =

Cr, 70

Fig. 3

Fig. 2. Variation of h a r d n e s s of C u - C r (a) and C u Ti (b) alloys with aging t e m p e r a t u r e . Cr content: 1) 0.2; 2) 0.4; 3) 0.6; 4) 0.8; 5) 1.0~. Ti content: 6) 0.5; 7) 1; 8) 2; 9) 3; 10) 4; 11) 5; 12) 6~. Fig. 3. C r y s t a l lattice p a r a m e t e r s of C u - C r alloys of v a r i o u s c h r o m i u m contents a f t e r quenching (1) and aging (2). copper at glancing angles of 63.7, 77.7, and 81.7 ~, r e s p e c t i v e l y . The calculated c r y s t a l lattice p a r a m e t e r was g r a p h i c a l l y e x t r a p o l a t e d with the aid of the function 9 = ( t / 2 ) 9 (cos ~ ~/sin ~ + cos 2 ~ / ~ ) to a glancing angle of 90 ~ and the value obtained was c o r r e c t e d for a b s o r p t i o n by the s p e c i m e n and f o r the d i v e r g e n c e of the pencil of r a y s . The lattice p a r a m e t e r of the quenched alloys steadily g r e w f r o m 0.36148 at 0.2 to 0.36164 nm at 1~ Cr (Fig. 3). Decomposition of the s u p e r s a t u r a t e d solid solution during aging s h a r p l y d e c r e a s e d the lattice p a r a m e t e r s of the alloys with 0.2-0.4~ Cr, but had v e r y little effect on those of the alloys containing 0.81.0% of c h r o m i u m . Such changes a r e in good a c c o r d with the phase d i a g r a m . To attain m a x i m u m strengthening through aging, it ! was found n e c e s s a r y to add c h r o m i u m to c o p p e r in an amount slightly exceeding its solubility l i m i t in the d i a g r a m , 0.65%. This was a p p a r e n t l y a r e s u l t of insufficiently u n i f o r m mixing of the components and possibly a l s o of an e r r o r in the choice of sintering and quenching t e m p e r a t u r e , m a d e p o s s i b l e by the fact that at the limiting c h r o m i u m concentration the s i n g l e phase c~-solid solution region b e c o m e s v e r y n a r r o w . Now the strengthening effect induced by aging is g r e a t e r when s e c o n d - p h a s e p r e c i p i t a t e s a r e complex c h e m i c a l compounds which do not contain the main m e t a l and in which n e i t h e r diffusion p r o c e s s e s n o r r e a c -

554

a

2,~. /f/~B

b

7 " ~ ~ ~ z

3~/o

9 M

P

a

~

~

~

50~__~_~{_r

6

40

ot, MPa

~ 3oo

o

Fig. 4

~, o~

50[

~

/OO

400~

4'oo

r

o,z~

~

~ 4~5 ; ? , ~5

~'g'-,~ 20 0,50

o, zs Cr, g

o,z5

,

0,50

7

,

* ~,

,,_6 / ',,

o, z5 cr, %

Fig. 5

Fig. 4. Variation of hardness of Cu-Cr-0.2% Ti (1-3) and CuCr-0.1~o B (4-7) alloys with aging temperature: a) alloy after quenching and working; b) after working and quenching. Cr content: 1, 4) 0.25; 2, 5) 0.50; 3, 6) 0.75; 7) 1%. Fig. 5. Variation of electrical conductivity k, tensile strength ~t, and elongation 5 of C u - C r (a), C u - C r - 0 . 2 % Ti (b), and C u C r - 0 . 1 4 B (o) alloys with composition: 1) after extrusion; 2-5) after aging at 300, 350, 400, and 450~ respectively; 6-8) C u Cr-0.1% B, C u - C r , and C u - C r - 0 . 2 / o Ti alloys, respectively, after aging at 400~ ~:ions with the main metal take place. From this point of view it is preferable to have t hree- or four-component systems containing metals which a r e m o r e r e f r a c t o r y than the main metal. Because of this, c o p p e r chromium alloys additionally alloyed with titanium (0.2%) and boron (0.1%) were also included in the program. Titanium is one of the most effective age-strengtheners of copper alloys, while boron increases their ductility and strength by reducing their grain. A comparison was made of the hardness of alloys subjected to plastic working: (compression or extrusion) either after quenching or before it (Fig. 4). The aging of the C u - C r - T i alloys after quenching and plastic working increased their hardness to 80-83 HRB. The c h a r a c t e r of its variation was similar to that exhibited by the two-component C u - C r alloys, but the maximum was reached not at 400 but at 450~ (Fig. 4a, curves 1-3). When aging followed immediately after quenching, the hardness of the alloys steadily grew, without, however, attaining the values recorded with the alloys aged immediately after plastic working (Fig. 4b, curves 1-3). The addition of 0.14 of boron to the C u - C r alloy brought about a sharp fall in its hardness (Fig. 4a, curves 4-7), which was entirely a result of strain strengthening. The difference in the behavior of the alloys during aging after plastic working and after quenching was linked with g r e a t e r instability of the supersaturated solid solution and its fuller decomposition in the f o r m e r case. The electrical conductivities of the alloys were calculated relative to that of pure, annealed copper, whose electrical resistivity was taken to be equal to 0.0175 ~t f2.m [4]. For all specimens, the production schedule consisted of quenching, extrusion, and aging. The plots of strength and, particularly, electrical conductivity vs composition for the binary ( C u - C r ) aad t e r n a r y ( C u - C r - B ) alloys after quenching were found to exhibit pronounced maxima and minima (Fig. 5a and c). After aging at 400-450~ they became almost straight lines, with the strength of the alloys varying only slightly with composition. The maximum relative electrical conductivities were 92-93 for the C u - C r alloys and 92-95~ for the C u - C r - B alloys. As in the case of hardness, the maximum of electrical conducti[vity generally corresponded to a chromium content in the range 0.50-0.754. 555

With a 0.2~ of titanium addition to the binary C u - C r alloy, the curves of electrical conductivity and strength vs composition became practically straight lines both after quenching and after aging (Fig. 5b}. The strength of the C u - C r - T i alloys after aging attained 600 MPa, which was one and a half times higher than that of the C u - C r alloys, and their relative electrical conductivity was equal to 65~. Like that of their strength, the curve of the ductility of the C u - C r alloys passed through an extreme point (Fig. 5c}. The r e l a tive elongations of the C u - C r - T i and C u - C r - B alloys were virtually independent of composition. For purposes of comparison, determinations were made of the strength and ductility of standard cold-drawn 2.5-mmdiameter copper wire, which gave the values ~t = 240 MPa and 5 = 31~. The binary C u - T i alloys can be satisfactorily extruded at titanium contents of up to 1-1.5%. Exceeding this limit leads to their hardness after quenching rising to 50 HRB, as a result of which the extrusion p r e s sure grows to 3 GPa or more. Under these conditions tools, particularly rams made of Kh12M (12~ C r - M o ) and Khl2F (12% C r - V ) steels, frequently f r act ure (twin-tube dies made of these materials withstand p r e s sures of up to 4 GPa). Because of this, in our work electrical resistivity measurements were made on C u Ti alloys containing 0.25, 0.50, 0.75, and 1.5~ of titanium. The relative electrical conductivities of extruded specimens increased as a result of aging at 300-500~ by 10-12~, and were equal to 60, 50, 30, and 157o (~: 5/o), respectively. Thus, the electrical conductivity of C u - T i alloys is determined mainly by their composition, and is not greatly affected by aging conditions. It is much lower than the electrical conductivity of chromium alloys. Microsections of all alloys were examined, using a BS-300 Tesla scanning electron microscope at a magnification of 6000 diameters, with the aim of observing any relatively large particles which had not dissolved on quenching or had coagulated during aging. The m i c r o s t r u c t u r e s of the sintered alloys proved to be identical with the m i e r o s t r u c t u r e of Mallory-100 cast precipitation-hardening alloy (2.5~ Co and 0.5~ Be), and only their grain was finer. In view of both this and data on crystal lattice p a r a m e t e r variations (Fig. 3), it can be concluded that the mixing of the additions and their dissolution in copper during the sintering of the alloys investigated were satisfactory. CONCLUSIONS Small amounts (0.1-1%) of chromium, titanium, and boron added to copper enable precipitation-hardening alloys to be obtained characterized by wide ranges of variation of key properties. The required level of properties in chromium- and titanium-containing alloys can be achieved by cold working and aging. Alloys with small amounts (up to 0.1~) of boron exhibit only strain hardening, and soften on heating. LITERATURE 1. 2,

3. 4.

556

CITED

S. K. Sliozberg and P. L. Chuloshnikov, Electrodes for Contact Welding [in Russian], Mashinostroenie, Leningrad (1972}. M. Hansen and K. P. Anderko, Constitution of Binary Alloys, McGraw-Hill, New York (1957). V. N. Vigdorovich, A. I. Krestovnikov, and M. V. Mal'tsev, Izv. Akad. Nauk SSSR, No. 2, 149-153 (1958). G. G. Gnesin (ed.), Sintered Materials for Electrical Engineering and Electronics (Handbook) [in Russian], Metallurgiya, Moscow (1981).